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Cover Page - shares	6 Months Ended
Cover Page - shares	Jun. 30, 2020	Jul. 31, 2020
Cover [Abstract]
Document Type	10-Q
Document Quarterly Report	true
Document Period End Date	Jun. 30, 2020
Document Transition Report	false
Entity File Number	001-38847
Entity Registrant Name	SILK ROAD MEDICAL, INC.
Entity Incorporation, State or Country Code	DE
Entity Tax Identification Number	20-8777622
Entity Address, Address Line One	1213 Innsbruck Dr.
Entity Address, City or Town	Sunnyvale
Entity Address, State or Province	CA
Entity Address, Postal Zip Code	94089
City Area Code	408
Local Phone Number	720-9002
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Entity Filer Category	Non-accelerated Filer
Entity Small Business	true
Entity Emerging Growth Company	false
Entity Shell Company	false
Title of 12(b) Security	Common Stock
Trading Symbol	SILK
Security Exchange Name	NASDAQ
Entity Common Stock, Shares Outstanding		33,679,635
Entity Central Index Key	0001397702
Current Fiscal Year End Date	--12-31
Document Fiscal Year Focus	2020
Document Fiscal Period Focus	Q2
Amendment Flag	false

Condensed Balance Sheets - USD ($) $ in Thousands	Jun. 30, 2020	Dec. 31, 2019
Current assets:
Cash and cash equivalents	$ 73,442	$ 39,181
Short-term investments	84,416	51,508
Accounts receivable, net	7,753	8,601
Inventories	11,621	10,322
Prepaid expenses and other current assets	3,863	2,878
Total current assets	181,095	112,490
Long-term investments	0	18,224
Property and equipment, net	2,885	2,734
Restricted cash	310	310
Other non-current assets	3,233	3,644
Total assets	187,523	137,402
Current liabilities:
Accounts payable	2,542	1,898
Accrued liabilities	9,142	15,034
Total current liabilities	11,684	16,932
Long-term debt	45,066	44,879
Other liabilities	3,693	3,700
Total liabilities	60,443	65,511
Commitments and contingencies (Note 7)
Preferred stock, $0.001 par value
Shares issued and outstanding: none	0	0
Common stock, $0.001 par value
Shares issued and outstanding: 33,610,099 and 31,255,267 at June 30, 2020 and December 31, 2019, respectively	34	31
Additional paid-in capital	338,550	263,384
Accumulated other comprehensive income	316	2
Accumulated deficit	(211,820)	(191,526)
Total stockholders' equity	127,080	71,891
Total liabilities and stockholders' equity	$ 187,523	$ 137,402

Condensed Balance Sheets (Parenthetical) - $ / shares	Jun. 30, 2020	Dec. 31, 2019
Statement of Financial Position [Abstract]
Preferred stock par value (in USD per share)	$ 0.001	$ 0.001
Preferred stock authorized (in shares)	5,000,000	5,000,000
Preferred stock issued (in shares)	0	0
Preferred stock outstanding (in shares)	0	0
Common stock, par value (in USD per share)	$ 0.001	$ 0.001
Common stock, shares authorized (in shares)	100,000,000	100,000,000
Common stock, shares issued (in shares)	33,610,099	31,255,267
Common stock, shares outstanding (in shares)	33,610,099	31,255,267

Condensed Statements of Operations and Comprehensive Loss - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Income Statement [Abstract]
Revenue	$ 15,094	$ 14,928	$ 34,027	$ 27,694
Cost of goods sold	5,336	3,697	10,586	7,035
Gross profit	9,758	11,231	23,441	20,659
Operating expenses:
Research and development	3,393	3,113	6,522	5,820
Selling, general and administrative	15,758	14,135	35,450	28,001
Total operating expenses	19,151	17,248	41,972	33,821
Loss from operations	(9,393)	(6,017)	(18,531)	(13,162)
Interest income	299	598	702	650
Interest expense	(1,203)	(1,207)	(2,406)	(2,560)
Other income (expense), net	(56)	(5,333)	(59)	(21,045)
Net loss	(10,353)	(11,959)	(20,294)	(36,117)
Other comprehensive loss:
Unrealized gain (loss) on investments, net	(126)	0	314	0
Net change in other comprehensive loss	(126)	0	314	0
Net loss and comprehensive loss	$ (10,479)	$ (11,959)	$ (19,980)	$ (36,117)
Net loss per share, basic and diluted (in USD per share)	$ (0.32)	$ (0.42)	$ (0.63)	$ (2.42)
Weighted average common shares used to compute net loss per share, basic and diluted (in shares)	32,682,360	28,458,793	32,010,335	14,905,052

Condensed Statements of Redeemable Convertible Preferred Stock and Stockholders' Equity (Deficit) - USD ($) $ in Thousands	Total	IPO	Common Stock	Common Stock IPO	Additional Paid-in Capital	Additional Paid-in Capital IPO	Accumulated Deficit	Accumulated Other Comprehensive Income
Temporary equity, beginning balance (in shares) at Dec. 31, 2018	21,233,190
Temporary equity, beginning balance at Dec. 31, 2018	$ 105,235
Redeemable Convertible Preferred Stock
Temporary equity, shares issued (in shares)	4,915
Net exercise of Series C preferred stock warrants in connection with IPO	$ 30
Temporary equity, ending balance (in shares) at Mar. 31, 2019	21,238,105
Temporary equity, ending balance at Mar. 31, 2019	$ 105,265
Beginning balance (in shares) at Dec. 31, 2018			1,135,310
Beginning balance at Dec. 31, 2018	(134,553)		$ 1		$ 4,557		$ (139,111)	$ 0
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Exercise of stock options (in shares)			251,305
Exercise of stock options	375				375
Stock-based compensation	262				262
Net loss	(24,158)						(24,158)
Comprehensive loss	(24,158)						(24,158)
Ending balance (in shares) at Mar. 31, 2019			1,386,615
Ending balance at Mar. 31, 2019	$ (158,074)		$ 1		5,194		(163,269)	0
Temporary equity, beginning balance (in shares) at Dec. 31, 2018	21,233,190
Temporary equity, beginning balance at Dec. 31, 2018	$ 105,235
Temporary equity, ending balance (in shares) at Jun. 30, 2019	0
Temporary equity, ending balance at Jun. 30, 2019	$ 0
Beginning balance (in shares) at Dec. 31, 2018			1,135,310
Beginning balance at Dec. 31, 2018	(134,553)		$ 1		4,557		(139,111)	0
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net loss	(36,117)
Unrealized gain (loss) on investments, net	0
Ending balance (in shares) at Jun. 30, 2019			30,747,714
Ending balance at Jun. 30, 2019	$ 84,377		$ 31		259,574		(175,228)	0
Temporary equity, beginning balance (in shares) at Mar. 31, 2019	21,238,105
Temporary equity, beginning balance at Mar. 31, 2019	$ 105,265
Redeemable Convertible Preferred Stock
Temporary equity, shares issued (in shares)	287,446	1,653,004
Conversion of preferred stock to common stock issued upon the conversion of preferred stock into common stock in IPO (in shares)		(23,178,555)
Conversion of preferred stock to common stock issued upon the conversion of preferred stock into common stock in IPO		$ (144,140)
Net exercise of Series C preferred stock warrants in connection with IPO	$ 1,754	37,121
Temporary equity, ending balance (in shares) at Jun. 30, 2019	0
Temporary equity, ending balance at Jun. 30, 2019	$ 0
Beginning balance (in shares) at Mar. 31, 2019			1,386,615
Beginning balance at Mar. 31, 2019	(158,074)		$ 1		5,194		(163,269)	0
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Exercise of common stock warrants (in shares)			3,764	2,204
Exercise of common stock warrants	31				31
Exercise of stock options (in shares)			176,576	6,000,000
Exercise of stock options	364	109,039	$ 1	$ 6	363	$ 109,033
Conversion of preferred stock to common stock issued upon the conversion of preferred stock into common stock in IPO (in shares)				23,178,555
Conversion of preferred stock to common stock issued upon the conversion of preferred stock into common stock in IPO		144,140		$ 23		144,117
Stock-based compensation	836				836
Net loss	(11,959)						(11,959)
Comprehensive loss	(11,959)						(11,959)
Unrealized gain (loss) on investments, net	0
Ending balance (in shares) at Jun. 30, 2019			30,747,714
Ending balance at Jun. 30, 2019	84,377		$ 31		259,574		(175,228)	0
Beginning balance (in shares) at Dec. 31, 2019			31,255,267
Beginning balance at Dec. 31, 2019	71,891		$ 31		263,384		(191,526)	2
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Exercise of stock options (in shares)			140,370
Exercise of stock options	274				274
Stock-based compensation	1,293				1,293
Net loss	(9,941)						(9,941)
Unrealized gain (loss) on investments, net	440							440
Ending balance (in shares) at Mar. 31, 2020			31,395,637
Ending balance at Mar. 31, 2020	63,957		$ 31		264,951		(201,467)	442
Beginning balance (in shares) at Dec. 31, 2019			31,255,267
Beginning balance at Dec. 31, 2019	$ 71,891		$ 31		263,384		(191,526)	2
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Exercise of stock options (in shares)	405,837
Net loss	$ (20,294)
Unrealized gain (loss) on investments, net	314
Ending balance (in shares) at Jun. 30, 2020			33,610,099
Ending balance at Jun. 30, 2020	127,080		$ 34		338,550		(211,820)	316
Beginning balance (in shares) at Mar. 31, 2020			31,395,637
Beginning balance at Mar. 31, 2020	63,957		$ 31		264,951		(201,467)	442
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Exercise of stock options (in shares)			265,467	1,923,076
Exercise of stock options	604	$ 70,543	$ 1	$ 2	603	$ 70,541
Issuance of common stock under employee stock purchase plan (in shares)			25,919
Issuance of common stock under employee stock purchase plan	801				801
Stock-based compensation	1,654				1,654
Net loss	(10,353)						(10,353)
Unrealized gain (loss) on investments, net	(126)							(126)
Ending balance (in shares) at Jun. 30, 2020			33,610,099
Ending balance at Jun. 30, 2020	$ 127,080		$ 34		$ 338,550		$ (211,820)	$ 316

Condensed Statements of Redeemable Convertible Preferred Stock and Stockholders' Equity (Deficit) (Parenthetical) - USD ($) $ in Thousands	3 Months Ended
	Jun. 30, 2020	Jun. 30, 2019
Statement of Stockholders' Equity [Abstract]
Payments of stock issuance costs	$ 4,457	$ 10,961

Condensed Statements of Cash Flows - USD ($) $ in Thousands	6 Months Ended
Condensed Statements of Cash Flows - USD ($) $ in Thousands	Jun. 30, 2020	Jun. 30, 2019
Cash flows from operating activities
Net loss	$ (20,294)	$ (36,117)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization expense	368	350
Stock-based compensation expense	2,947	1,099
Change in fair value of redeemable convertible preferred stock warrant liability	0	21,030
Amortization of premiums (accretion of discounts) on investments, net	57	0
Amortization of debt discount and debt issuance costs	23	22
Amortization of right-of-use asset	295	300
Non-cash interest expense	167	555
Loss on disposal of property and equipment	51	0
Change in provision for doubtful accounts receivable	(9)	123
Provision for excess and obsolete inventories	105	43
Changes in assets and liabilities
Accounts receivable	857	(1,622)
Inventories	(1,404)	(3,262)
Prepaid expenses and other current assets	(986)	(1,841)
Other assets	117	756
Accounts payable	262	(45)
Accrued liabilities	(5,925)	978
Other liabilities	(7)	(372)
Net cash used in operating activities	(23,376)	(18,003)
Cash flows from investing activities
Purchases of property and equipment	(479)	(206)
Purchases of investments	(23,606)	0
Proceeds from maturity of investments	9,180	0
Net cash used in investing activities	(14,905)	(206)
Cash flows from financing activities
Proceeds from public offerings, net of underwriting discount, commissions and offering costs paid	70,863	108,913
Proceeds from issuance of common stock	1,679	738
Net cash provided by financing activities	72,542	111,466
Net change in cash, cash equivalents and restricted cash	34,261	93,257
Cash, cash equivalents and restricted cash, beginning of period	39,491	25,300
Cash, cash equivalents and restricted cash, end of period	73,752	118,557
Supplemental disclosure of cash flow information
Cash paid for interest	2,216	1,982
Non-cash investing and financing activities:
Accounts payable and accrued liabilities for purchases of property and equipment	100	7
Offering costs in accounts payable and accrued liabilities	320	358
Right-of-use asset obtained in exchange for lease obligation	0	3,982
Net exercise of convertible preferred stock warrants to preferred stock	0	37,121
Conversion of convertible preferred stock to common stock upon initial public offering	0	144,140
Redeemable Convertible Preferred Stock
Cash flows from financing activities
Proceeds from exercise of redeemable convertible preferred stock warrants	0	1,784
Common Stock Warrant
Cash flows from financing activities
Proceeds from exercise of redeemable convertible preferred stock warrants	$ 0	$ 31

Formation and Business of the Company	6 Months Ended
Formation and Business of the Company	Jun. 30, 2020
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Formation and Business of the Company	Formation and Business of the Company The Company Silk Road Medical, Inc. (the “Company”) was incorporated in the state of Delaware on March 21, 2007. The Company has developed a technologically advanced, minimally-invasive solution for patients with carotid artery disease who are at risk for stroke. The Company's portfolio of TCAR products enable a new procedure, referred to as transcarotid artery revascularization, or TCAR, that combines the benefits of endovascular techniques and surgical principles. The Company manufactures and sells in the United States its portfolio of TCAR products which are designed to provide direct access to the carotid artery, effective reduction in stroke risk throughout the procedure, and long-term restraint of carotid plaque. The Company commercialized its products in the United States in April 2016. Reverse Stock Split On March 13, 2019, the Company’s Board of Directors approved an amendment to the Company’s amended and restated certificate of incorporation to effect a 1-for-2.7 reverse stock split of the Company’s common stock and redeemable convertible preferred stock to be consummated prior to the effectiveness of the Company’s planned initial public offering, or IPO. The reverse stock split was effected on March 27, 2019. The par values of the common stock and redeemable convertible preferred stock were not adjusted as a result of the reverse stock split. All the common stock, redeemable convertible preferred stock, stock options and warrants, and related per share amounts in the condensed financial statements have been retroactively adjusted for all periods presented to give effect to the reverse stock split. Public Offerings In April 2019, the Company issued and sold 6,000,000 shares of its common stock in its IPO at a public offering price of $20.00 per share, for net proceeds of approximately $109,119,000 after deducting underwriting discounts and commissions of approximately $8,400,000 and expenses of approximately $2,481,000. Upon the closing of the IPO, all shares of redeemable convertible preferred stock then outstanding converted into shares of common stock and the Company's outstanding warrants to purchase shares of common and redeemable convertible preferred stock were exercised, or automatically net exercised absent a prior election. The exercises resulted in the reclassification of the fair value of the related redeemable convertible preferred stock warrant liability to additional paid-in capital. In August 2019, the Company completed a secondary public offering of 4,200,000 shares of its common stock sold by certain selling stockholders, and the exercise in full of the underwriters' option to purchase 630,000 additional shares of its common stock from certain selling stockholders, at a public offering price of $39.50 per share. The Company did not receive any of the proceeds from the sale of the shares of its common stock by the selling stockholders. In May 2020, the Company completed an underwritten public offering of 6,808,154 shares of its common stock, of which 1,923,076 shares were offered for sale by the Company and the remaining 4,885,078 shares were offered for sale by certain selling stockholders, at a public offering price of $39.00 per share. The Company received cash proceeds of approximately $70,543,000 after deducting underwriting discounts and commissions of approximately $3,750,000 and expenses of approximately $707,000. Also in May 2020, the underwriters fully exercised their option to purchase 1,021,223 additional shares of common stock from the selling stockholders. The Company did not receive any of the proceeds from the sale of the shares of its common stock by the selling stockholders.

Summary of Significant Accounting Policies

6 Months Ended

Jun. 30, 2020

Accounting Policies [Abstract]

Summary of Significant Accounting Policies

Basis of Preparation

The accompanying financial statements have been prepared in accordance with generally accepted accounting principles in the United States of America, or U.S. GAAP, and applicable rules and regulations of the Securities and Exchange Commission, or SEC, regarding interim financial reporting. As permitted under those rules, certain footnotes or other financial information that are normally required by U.S. GAAP have been condensed or omitted, and accordingly the balance sheet as of December 31, 2019, and related disclosures, have been derived from the audited financial statements at that date but does not include all of the information required by U.S. GAAP for complete financial statements. These unaudited condensed financial statements have been prepared on the same basis as the Company’s annual financial statements and, in the opinion of management, reflect all adjustments (consisting only of normal recurring adjustments) that are necessary for the fair statement of the Company’s condensed financial information. The results of operations for the three and six months ended June 30, 2020 are not necessarily indicative of the results to be expected for the year ending December 31, 2020 or for any other interim period or for any other future year.

The accompanying interim unaudited condensed financial statements and related financial information should be read in conjunction with the audited financial statements and the related notes thereto for the year ended December 31, 2019 included in the Company's annual report on Form 10-K filed with the SEC on March 2, 2020.

Adjustment to Prior Period Financial Statements

The Company has adjusted the accompanying Condensed Statement of Cash Flows for the six months ended June 30, 2019 to increase each of the accounts receivable, net, and accrued liabilities amounts by $601,000 for an immaterial prior period error in the classification of provisions for returns from customers.

Use of Estimates

The preparation of financial statements in conformity with U.S. GAAP requires management to make estimates and assumptions that affect the amounts and disclosures reported in the financial statements. Management uses judgment when making estimates related to provisions for accounts receivable and excess and obsolete inventories, the valuation of deferred tax assets, the provision for sales returns, stock-based compensation, and for periods prior to the Company's IPO, the valuation of common stock and redeemable convertible preferred stock warrants. Management bases its estimates on historical experience and on various other assumptions that are believed to be reasonable under the circumstances, the results of which form the basis for making judgments about the carrying values of assets and liabilities that are not readily apparent from other sources. Although these estimates are based on the Company’s knowledge of current events and actions it may undertake in the future, actual results may ultimately materially differ from these estimates and assumptions.

Due to the coronavirus (“COVID-19”) pandemic, there has been continued uncertainty and disruption in the global economy and financial markets. The Company is not aware of any specific event or circumstance that would require an update to its estimates or judgments or a revision of the carrying value of its assets or liabilities as of June 30, 2020. The Company has also considered information available to it as of the date of issuance of these financial statements and is not aware of any specific events or circumstances that would require an update to its estimates or judgments, or a revision to the carrying value of its assets or liabilities. These estimates may change as new events occur and additional information becomes available. Actual results could differ materially from these estimates.

Fair Value of Financial Instruments

The Company has evaluated the estimated fair value of its financial instruments as of June 30, 2020 and December 31, 2019. The carrying amounts of certain of the Company’s financial instruments, which include cash equivalents, short-term investments, long-term investments, restricted cash, accounts receivable, accounts payable and accrued liabilities approximate their respective fair values because of the short-term nature of these instruments. Management believes that its long-term debt bears interest at the prevailing market rates for instruments with similar characteristics (Level 2 within the fair value hierarchy); accordingly, the carrying value of this instrument approximates its fair value. Prior to the Company's IPO, fair value accounting was applied to the redeemable convertible preferred stock warrant liability.

Cash, Cash Equivalents and Restricted Cash

The Company considers all highly liquid investments with an original maturity of three months or less at the time of purchase to be cash equivalents. Cash equivalents are considered available-for-sale marketable securities and are recorded at fair value, based on quoted market prices. As of June 30, 2020 and December 31, 2019, the Company’s cash equivalents are entirely comprised of investments in money market funds.

Restricted cash as of June 30, 2020 and December 31, 2019 consists of a letter of credit of $310,000 representing collateral for the Company's facility lease.

Investments

Short-term investments consist of debt securities classified as available-for-sale and have original maturities greater than 90 days, but less than one year as of the balance sheet date. Long-term investments have maturities greater than one year as of the balance sheet date. All investments are recorded at fair value based on the fair value hierarchy. Money market funds and United States treasury bills with an original maturity less than 90 days are classified within Level 1 of the fair value hierarchy, and commercial paper, corporate bonds/notes, United States Government securities, and asset-backed securities are classified within Level 2 of the fair value hierarchy. Unrealized gains and losses, deemed temporary in nature, are reported as a separate component of accumulated other comprehensive income (loss). The cost of available-for-sale investments sold is based on the specific-identification method. Realized gains and losses are included in earnings, and are derived for specific-identification method for determining the costs of investments sold. Amortization of premiums and accretion of discounts are reported as a component of interest income.

A decline in the fair value of any security below cost that is deemed other than temporary results in a charge to earnings and the corresponding establishment of a new cost basis for the investment.

Concentration of Credit Risk, and Other Risks and Uncertainties

The Company is subject to risks related to the public health crises such as the global pandemic associated with COVID-19, which has spread to most countries and all 50 states within the United States. The COVID-19 outbreak has negatively impacted, and may continue to negatively impact the Company’s operations, its revenue and overall financial condition by significantly decreasing the number of TCAR procedures performed. The number of TCAR procedures performed, similar to other surgical procedures, has significantly decreased as health care organizations globally have prioritized the treatment of patients with COVID-19. For example, in the United States, governmental authorities have recommended, and in certain cases required, that elective, specialty and other procedures and appointments, be suspended or canceled to avoid non-essential patient exposure to medical environments and potential infection with COVID-19 and to focus limited resources and personnel capacity toward the treatment of COVID-19.

These measures and challenges will likely continue for the duration of the pandemic, which is uncertain, and will continue to negatively impact the Company’s revenue while the pandemic continues.

Financial instruments that potentially subject the Company to credit risk consist of cash and cash equivalents, investments and accounts receivable to the extent of the amounts recorded on the balance sheet. Cash, cash equivalents, and investments are deposited in financial institutions which, at times, may be in excess of federally insured limits. Cash equivalents are invested in highly rated money market funds and United States treasury bills. The Company invests in a variety of financial instruments, such as, but not limited to, commercial paper, corporate bonds/notes, United States Government securities, asset-backed securities and, by policy, limits the amount of credit exposure with any one financial institution or commercial issuer. The Company has not experienced any material losses on its deposits of cash and cash equivalents or investments during the three and six months ended June 30, 2020 and 2019.

The Company’s accounts receivable are due from a variety of health care organizations in the United States. At June 30, 2020 and December 31, 2019, no customer represented 10% or more of the Company’s accounts receivable. For the three and six months ended June 30, 2020 and 2019, there were no customers that represented 10% or more of revenue.

The Company provides for uncollectible amounts when specific credit problems are identified. In doing so, the Company analyzes historical bad debt trends, customer credit worthiness, current economic trends and changes in customer payment patterns when evaluating the adequacy of the allowance for doubtful accounts.

The Company manufactures certain of its commercial products in-house. Certain of the Company’s product components and sub-assemblies continue to be manufactured by sole suppliers, the most significant of which is the ENROUTE stent. Disruption in component or sub-assembly supply from these manufacturers or from in-house production would have a negative impact on the Company’s financial position and results of operations.

The Company is subject to certain risks, including that its devices may not be approved or cleared for marketing by governmental authorities or be successfully marketed. There can be no assurance that the Company’s products will achieve widespread adoption in the marketplace, nor can there be any assurance that existing devices or any future devices can be developed or manufactured at an acceptable cost and with appropriate performance characteristics. The Company is also subject to risks common to companies in the medical device industry, including, but not limited to, new technological innovations, dependence upon government and third-party payers to provide adequate coverage and reimbursement, dependence on key personnel and suppliers, protection of proprietary technology, product liability claims, and compliance with government regulations.

Existing or future devices developed by the Company may require approvals or clearances from the FDA or international regulatory agencies. In addition, in order to continue the Company’s operations, compliance with various federal and state laws is required. If the Company were denied or delayed in receiving such approvals or clearances, it may be necessary to adjust operations to align with the Company’s currently approved portfolio of products. If clearance for the products in the current portfolio were withdrawn by the FDA, this would have a material adverse impact on the Company.

Leases

The Company adopted Accounting Standards Codification (“ASC”) 842, "Leases," on January 1, 2019 and used the modified retrospective method for all leases not substantially completed as of the date of adoption and the package of practical expedients available in the standard. The Company considers if an arrangement is a lease at inception if it obtains the right to control the use of an identified asset under a leasing arrangement with an initial term greater than twelve months. The Company determines whether a contract conveys the right to control the use of an identified asset for a period of time if the contract

contains both the right to obtain substantially all of the economic benefits from the use of the identified asset and the right to direct the use of the identified asset. The Company also evaluates the nature of each lease to determine whether it is an operating or financing lease and recognizes the right-of-use asset and lease liabilities based on the present value of future minimum lease payments over the expected lease term. The Company’s leases do not generally contain an implicit interest rate and therefore the Company uses the incremental borrowing rate it would expect to pay to borrow on a similar collateralized basis over a similar term in order to determine the present value of its lease payments. The Company’s considers renewal options in the determination of the lease term if the option to renew is reasonably certain. Variable lease costs represent payments that are dependent on usage, a rate or index. Variable lease costs, which consists primarily of taxes, insurance and common area maintenance costs, are expensed as incurred, as the Company has elected to account separately for contracts that contain lease and non-lease components, consistent with its historical practice. The Company does not have any finance leases.

Redeemable Convertible Preferred Stock Warrant Liability

Prior to its IPO, the Company accounted for its warrants for shares of redeemable convertible preferred stock as a liability based upon the characteristics and provisions of each instrument. Redeemable convertible preferred stock warrants classified as a liability were initially recorded at their fair value on the date of issuance and are subject to remeasurement at each subsequent balance sheet date. Any change in fair value as a result of a remeasurement was recognized as a component of other income (expense), net in the condensed statements of operations and comprehensive loss. The Company recorded adjustments to the estimated fair value of the redeemable convertible preferred stock warrants until they were exercised. Upon their exercise, the final fair value of the warrant liability was reclassified to stockholders’ equity. Subsequent to its IPO, the Company no longer recorded any related periodic fair value adjustments.

Redeemable Convertible Preferred Stock

Prior to its IPO, the Company recorded its redeemable convertible preferred stock at fair value on the dates of issuance, net of issuance costs, and classified the redeemable convertible preferred stock outside of stockholders’ equity on the condensed balance sheet as events triggering the liquidation preferences were not solely within the Company’s control. Upon the closing of the Company's IPO, all shares of convertible preferred stock then outstanding converted into an aggregate of 23,178,555 shares of common stock resulting in the reclassification of $144,140,000 from outside of stockholders’ equity to additional paid-in capital.

Revenue Recognition

The Company recognizes revenue in accordance with ASC Topic 606, "Revenue from Contracts with Customers." Under ASC 606, revenue is recognized when a customer obtains control of promised goods or services, in an amount that reflects the consideration which the entity expects to receive in exchange for those goods or services. To determine revenue recognition for arrangements that an entity determines are within the scope of ASC 606, the Company performs the following five steps:

(i) identify the contract(s) with a customer;

(ii) identify the performance obligations in the contract;

(iii) determine the transaction price;

(iv) allocate the transaction price to the performance obligations in the contract; and

(v) recognize revenue when (or as) the entity satisfies a performance obligation.

As of June 30, 2020 and December 31, 2019, the Company recorded $179,000 and $128,000, respectively, of unbilled receivables, which are included in accounts receivable, net on the condensed balance sheet, as the Company has an unconditional right to payment as of the end of the applicable period.

The Company’s revenue is generated from the sale of its products to hospitals and medical centers in the United States through direct sales representatives. Revenue is recognized when obligations under the terms of a contract with customers are satisfied, which occurs with the transfer of control of the Company’s products to its customers, either upon shipment of the product or delivery of the product to the customer under the Company’s standard terms and conditions. The Company’s products are readily available for usage as soon as the customer possesses it. Upon receipt, the customer controls the economic benefits of the product, has significant risks and rewards, and the legal title. The Company has present right to payment; therefore, the transfer of control is deemed to happen at a point in time. Revenue is measured as the amount of consideration the Company expects to receive in exchange for transferring the goods.

For sales where the Company’s sales representative hand delivers product directly to the hospital or medical center from the sales representative’s trunk stock inventory, the Company recognizes revenue upon delivery, which represents the point in time when control transfers to the customer. Upon delivery there are legally-enforceable rights and obligations between the parties which can be identified, commercial substance exists and collectability is probable. For sales which are sent directly from the Company to hospitals and medical centers, the transfer of control occurs at the time of shipment or delivery of the product. There are no further performance obligations by the Company or the sales representative to the customer after delivery under either method of sale. As allowed under the practical expedient, the Company does not disclose the value of unsatisfied performance obligations for (i) contracts with an original expected length of one year or less and (ii) contracts for which it recognizes revenue at the amount to which it has the right to invoice for services performed.

The Company is entitled to the total consideration for the products ordered by customers as product pricing is fixed according to the terms of customer contracts and payment terms are short. Payment terms fall within the one-year guidance for the practical expedient which allows the Company to forgo adjustment of the promised amount of consideration for the effects of a significant financing component. The Company excludes taxes assessed by governmental authorities on revenue-producing transactions from the measurement of the transaction price.

Costs associated with product sales include commissions and royalties. The Company applies the practical expedient and recognizes commissions and royalties as expense when incurred because the expense is incurred at a point in time and the amortization period is less than one year. Commissions are recorded as selling expense and royalties are recorded as cost of goods sold in the condensed statements of operations and comprehensive loss.

The Company accepts product returns at its discretion or if the product is defective as manufactured. The Company establishes estimated provisions for returns based on historical experience and considers other factors that it believes could significantly impact its expected returns, which provisions are classified within accrued liabilities on the condensed balance sheet. The Company elected to expense shipping and handling costs as incurred and includes them in the cost of goods sold. In those cases where the Company bills shipping and handling costs to customers, it will classify the amounts billed as a component of revenue.

Cost of Goods Sold

The Company manufactures certain of its portfolio of TCAR products at its facility and purchases other products from third party manufacturers. Cost of goods sold consists primarily of costs related to materials, components and subassemblies, manufacturing overhead costs, direct labor, reserves for

excess, obsolete and non-sellable inventories as well as distribution-related expenses. A significant portion of the Company’s cost of goods sold currently consists of manufacturing overhead costs. These overhead costs include the cost of quality assurance, material procurement, inventory control, facilities, equipment and operations supervision and management. Cost of goods sold also includes depreciation expense for production equipment and certain direct costs such as shipping costs and royalties.

Stock–Based Compensation

The Company accounts for stock-based compensation in accordance with Financial Accounting Standards Board (“FASB”) ASC 718, "Compensation-Stock Compensation." ASC 718 requires the recognition of compensation expense, using a fair-value based method, for costs related to all share-based payments including stock options, restricted stock units and shares issued under its employee stock purchase plan. ASC 718 requires companies to estimate the fair value of all share-based payment option awards on the date of grant using an option pricing model. The fair value of stock options is recognized over the period during which an optionee is required to provide services in exchange for the option award, known as the requisite service period (usually the vesting period), on a straight-line basis. For performance-based stock options, the Company will assess the probability of performance conditions being achieved in each reporting period. The amount of stock-based compensation expense recognized in any one period related to performance-based stock options can vary based on the achievement or anticipated achievement of the performance conditions. The Company accounts for option forfeitures as they occur.

The Company accounts for stock-based compensation for restricted stock units at their fair value, based on the closing market price of the Company's common stock on the date of grant. These costs are recognized on a straight-line basis over the requisite service period, which is usually the vesting period.

The Company accounts for stock-based compensation for its employee stock purchase plan based on the estimated fair value of the options on the date of grant. The Company estimates the grant date fair value using an option pricing model for each purchase period. These costs are recognized on a straight-line basis over the offering period.

Income Taxes

The Company accounts for income taxes under the liability method, whereby deferred tax assets and liabilities are determined based on the difference between the condensed financial statements and tax bases of assets and liabilities using the enacted tax rates in effect for the year in which the differences are expected to affect taxable income. A valuation allowance is established when necessary to reduce deferred tax assets to the amounts expected to be realized. As the Company has historically incurred operating losses, it has established a full valuation allowance against its net deferred tax assets, and there is no provision for income taxes.

The Company also follows the provisions of ASC 740-10, "Accounting for Uncertainty in Income Taxes." ASC 740-10 prescribes a comprehensive model for the recognition, measurement, presentation and disclosure in financial statements of any uncertain tax positions that have been taken or expected to be taken on a tax return. No liability related to uncertain tax positions is recorded on the condensed financial statements. It is the Company's policy to include penalties and interest expense related to income taxes as part of the provision for income taxes.

Comprehensive Loss

Comprehensive loss consists of net loss and changes in unrealized gains and losses on investments classified as available-for-sale. For the three and six months ended June 30, 2020, the Company’s unrealized gains and losses on available-for-sale investments represent the only component of other comprehensive loss that are excluded from the reported net loss and that are presented in the condensed

statements of operations and comprehensive loss. Accumulated other comprehensive income or loss is presented in the accompanying condensed balance sheet as a component of stockholders' equity. For the three and six months ended June 30, 2019, there was no difference between the Company's comprehensive loss and its net loss.

Net Loss per Share

Basic net loss per share is computed by dividing the net loss by the weighted average number of shares of common stock outstanding during the period, without consideration for potential dilutive common shares. Diluted net loss per share is computed by dividing the net loss by the weighted average number of shares of common stock and potentially dilutive securities outstanding for the period. For purposes of the diluted net loss per share calculation, redeemable convertible preferred stock and warrants, common stock options, and restricted stock units are considered to be potentially dilutive securities. Since the Company was in a loss position for all periods presented, basic net loss per share is the same as diluted net loss per share as the inclusion of all potential dilutive common shares would have been anti-dilutive.

The Company allocates no loss to participating securities because they have no contractual obligation to share in the losses of the Company. The shares of the Company’s redeemable convertible preferred stock participate in any dividends declared by the Company and were therefore considered to be participating securities.

Net loss per share was determined as follows (in thousands, except share and per share data):


	Three Months Ended June 30,						Six Months Ended June 30,
	2020		2019		2020		2019
Net loss	$	(10,353)	$	(11,959)	$	(20,294)	$	(36,117)
Weighted average common stock outstanding used to compute net loss per share, basic and diluted	32,682,360		28,458,793		32,010,335		14,905,052
Net loss, basic and diluted	$	(0.32)	$	(0.42)	$	(0.63)	$	(2.42)

The following potentially dilutive securities outstanding have been excluded from the computation of diluted weighted average shares outstanding because such securities have an antidilutive impact due to the Company's net loss:


	June 30,
	2020	2019
Common stock options	4,632,866	4,683,811
Restricted stock units	39,089	—
	4,671,955	4,683,811

Segment and Geographical Information

The Company operates and manages its business as one reportable and operating segment. The Company’s chief executive officer, who is the chief operating decision maker, reviews financial information on an aggregate basis for purposes of allocating resources and evaluating financial performance. All of the Company’s long-lived assets are based in the United States. Long-lived assets are comprised of property and equipment and the Company's right-of-use asset. All of the Company’s revenue was in the United States for the three and six months ended June 30, 2020 and 2019, based on the shipping location of the external customer.

Recent Accounting Pronouncements	6 Months Ended
Recent Accounting Pronouncements	Jun. 30, 2020
Accounting Changes and Error Corrections [Abstract]
Recent Accounting Pronouncements	Recent Accounting Pronouncements Recently Adopted Accounting Standards In August 2018, the FASB issued ASU 2018-13, "Fair Value Measurement," which changed the disclosure requirements for fair value measurements by removing, adding and modifying certain disclosures. The Company adopted the new standard effective January 1, 2020. The adoption did not have a material impact on the Company's financial statements and related disclosures. In August 2018, the FASB issued ASU 2018-15, "Cloud Computing Arrangements," which aligns the requirements for capitalizing implementation costs in a Cloud Computing Arrangement service contract with the requirements for capitalizing implementation costs incurred for an internal-use software license. The Company adopted the new standard effective January 1, 2020 on a prospective basis. The adoption did not have a material impact on the Company's financial statements and related disclosures. Recently Issued Accounting Standards In June 2016, the FASB issued ASU 2016-13, "Measurement of Credit Losses on Financial Statements." This update provides financial statement users with more decision-useful information about the expected credit losses on financial instruments and other commitments to extend credit held by a reporting entity at each reporting date. The update replaces the incurred loss impairment methodology in current U.S. GAAP with a methodology that reflects expected credit losses and requires consideration of a broader range of reasonable and supportable information to inform credit loss estimates. In October 2019, the FASB delayed the effective date of this standard for smaller reporting companies, as such ASU 2016-13 would originally be effective for the Company on January 1, 2023. As of June 30, 2020, the Company no longer qualifies as a smaller reporting company, accordingly ASU 2016-13 will now become effective for the Company on January 1, 2021. The Company is evaluating the impact of adopting this guidance to the Company's financial statements and related disclosures. In December 2019, the FASB issued ASU 2019-12, Income Taxes (Topic 740): Simplifying the Accounting for Income Taxes, which enhances and simplifies various aspects of the income tax accounting guidance related to intra-period tax allocation, interim period accounting for enacted changes in tax law, and the year-to-date loss limitation in interim period tax accounting. ASU 2019-12 also amends other aspects of the guidance to reduce complexity in certain areas. ASU 2019-12 will become effective for the Company on January 1, 2021. Early adoption is permitted. The Company is evaluating the impact of adopting this guidance to the Company's financial statements and related disclosures.

- Definition

The entire disclosure for change in accounting principle. Includes, but is not limited to, nature, reason, and method of adopting amendment to accounting standards or other change in accounting principle.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
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Reference 8: http://www.xbrl.org/2003/role/disclosureRef
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Reference 12: http://www.xbrl.org/2003/role/disclosureRef
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Reference 13: http://www.xbrl.org/2003/role/disclosureRef
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Reference 14: http://www.xbrl.org/2003/role/disclosureRef
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+ Details

Fair Value Measurements

6 Months Ended

Jun. 30, 2020

Fair Value Disclosures [Abstract]

Fair Value Measurements

The Company measures certain financial assets and liabilities at fair value on a recurring basis, including cash equivalents, investments, and the Company's previously outstanding preferred stock warrants. Fair value is an exit price, representing the amount that would be received to sell an asset or paid to transfer a liability in an orderly transaction between market participants. As such, fair value is a market-based measurement that should be determined based on assumptions that market participants would use in pricing an asset or a liability. A three-tier fair value hierarchy is established as a basis for considering such assumptions and for inputs used in the valuation methodologies in measuring fair value:

•Level 1 – quoted prices in active markets for identical assets or liabilities;

•Level 2 – observable inputs other than quoted prices in active markets for identical assets and liabilities;

•Level 3 – unobservable inputs.

Assets and liabilities measured at fair value are classified in their entirety based on the lowest level of input that is significant to the fair value measurement. The Company’s assessment of the significance of a particular input to the fair value measurement in its entirety requires management to make judgments and consider factors specific to the asset or liability. The Company’s cash equivalents are classified within Level 1 of the fair value hierarchy because they are valued using quoted market prices, broker or dealer quotations, or alternative pricing sources with reasonable levels of price transparency. The corporate bonds/notes, commercial paper, asset-backed securities and U.S. government securities are classified as Level 2 as they are valued based upon quoted market prices for similar instruments in active markets, quoted prices for identical or similar instruments in markets that are not active and model-based valuation techniques for which all significant inputs are observable in the market or can be corroborated by observable market data for substantially the full term of the assets.

The following tables sets forth by level within the fair value hierarchy the Company’s assets that are reported at fair value as of June 30, 2020 and December 31, 2019, using the inputs defined above (in thousands):


	June 30, 2020
	Level 1		Level 2		Level 3		Total
Assets:
Money market funds	$	72,924	$	—	$	—	$	72,924
U.S. treasury bills	—		9,993		—		9,993
Commercial paper	—		11,965		—		11,965
Corporate bonds/notes	—		10,742		—		10,742
U.S. government securities	—		49,714		—		49,714
Asset-backed securities	—		2,002		—		2,002
	$	72,924	$	84,416	$	—	$	157,340


	December 31, 2019
	Level 1		Level 2		Level 3		Total
Assets:
Money market funds	$	34,363	$	—	$	—	$	34,363
Commercial paper	—		9,919		—		9,919
Corporate bonds/notes	—		10,176		—		10,176
U.S. government securities	—		44,456		—		44,456
Asset-backed securities	—		5,181		—		5,181
	$	34,363	$	69,732	$	—	$	104,095

There were no transfers between fair value hierarchy levels during the three and six months ended June 30, 2020 and 2019.

- Definition

The entire disclosure for the fair value of financial instruments (as defined), including financial assets and financial liabilities (collectively, as defined), and the measurements of those instruments as well as disclosures related to the fair value of non-financial assets and liabilities. Such disclosures about the financial instruments, assets, and liabilities would include: (1) the fair value of the required items together with their carrying amounts (as appropriate); (2) for items for which it is not practicable to estimate fair value, disclosure would include: (a) information pertinent to estimating fair value (including, carrying amount, effective interest rate, and maturity, and (b) the reasons why it is not practicable to estimate fair value; (3) significant concentrations of credit risk including: (a) information about the activity, region, or economic characteristics identifying a concentration, (b) the maximum amount of loss the entity is exposed to based on the gross fair value of the related item, (c) policy for requiring collateral or other security and information as to accessing such collateral or security, and (d) the nature and brief description of such collateral or security; (4) quantitative information about market risks and how such risks are managed; (5) for items measured on both a recurring and nonrecurring basis information regarding the inputs used to develop the fair value measurement; and (6) for items presented in the financial statement for which fair value measurement is elected: (a) information necessary to understand the reasons for the election, (b) discussion of the effect of fair value changes on earnings, (c) a description of [similar groups] items for which the election is made and the relation thereof to the balance sheet, the aggregate carrying value of items included in the balance sheet that are not eligible for the election; (7) all other required (as defined) and desired information.

+ References + Details

Balance Sheet Components

6 Months Ended

Jun. 30, 2020

Organization, Consolidation and Presentation of Financial Statements [Abstract]

Balance Sheet Components

Investments

The fair value of the Company's available-for-sale investments as of June 30, 2020 and December 31, 2019 are as follows (in thousands):


	June 30, 2020
			Gross Unrealized						Estimated
	Amortized Cost		Gains		Losses		Fair Value
Money market funds	$	72,924	$	—	$	—	$	72,924
U.S. treasury bills	9,992		1		—		9,993
Commercial paper	11,965		—		—		11,965
Corporate bonds/notes	10,709		34		(1)		10,742
U.S. government securities	49,433		281		—		49,714
Asset-backed securities	2,001		1		—		2,002
	$	157,024	$	317	$	(1)	$	157,340
Classified as:
Cash equivalents										$	72,924
Short-term investments										84,416

							$	157,340


	December 31, 2019
			Gross Unrealized						Estimated
	Amortized Cost		Gains		Losses		Fair Value
Money market funds	$	34,363	$	—	$	—	$	34,363
Commercial paper	9,919		—		—		9,919
Corporate bonds/notes	10,180		—		(4)		10,176
U.S. government securities	44,450		9		(3)		44,456
Asset-backed securities	5,181		—		—		5,181
	$	104,093	$	9	$	(7)	$	104,095
Classified as:
Cash equivalents										$	34,363
Short-term investments										51,508
Long-term investments										18,224
							$	104,095

The following table summarizes the fair value of the Company’s cash equivalents, short-term and long-term investments classified by maturity as of June 30, 2020 and December 31, 2019 (in thousands):


	June 30,		December 31,
	2020		2019
Amounts maturing within one year	$	157,340	$	85,871
Amounts maturing after one year through two years	—		18,224
	$	157,340	$	104,095

Available-for-sale investments held as of June 30, 2020 had a weighted average days to maturity of 164 days.

The following table presents the Company's available-for-sale investments that were in an unrealized loss position as of June 30, 2020 and December 31, 2019 (in thousands):


	June 30, 2020						December 31, 2019
	Less than 12 months						Less than 12 months
Assets:	Fair Value		Unrealized Loss		Fair Value		Unrealized Loss
Corporate bonds/notes	$	2,558	$	(1)	$	10,128	$	(4)

U.S. government securities	—		—		19,067		(3)
	$	2,558	$	(1)	$	29,195	$	(7)

Inventories

Components of inventories were as follows:


(in thousands)	June 30,		December 31,
	2020		2019
Raw materials	$	1,736	$	1,203

Finished products	9,885		9,119
Total	$	11,621	$	10,322

As of June 30, 2020 and December 31, 2019, there were no work-in-process inventories.

Accrued Liabilities

Accrued liabilities consist of the following:


(in thousands)	June 30,		December 31,
	2020		2019
Accrued payroll and related expenses	$	5,352	$	9,151
Provision for sales returns	920		2,419
Accrued professional services	593		682
Operating lease liability	809		769
Accrued royalty expense	333		470
Deferred revenue	335		304
Accrued travel expenses	273		431
Accrued clinical expenses	181		241
Accrued other expenses	346		567
Total	$	9,142	$	15,034

Long-term Debt

6 Months Ended

Jun. 30, 2020

Debt Disclosure [Abstract]

Long-term Debt

In October 2015, the Company entered into a term loan agreement with CRG. The term loan agreement provides for up to $30,000,000 in term loans split into two tranches as follows: (i) the Tranche A Loans provided for $20,000,000 in term loans, and (ii) the Tranche B Loans provided for up to $10,000,000 in term loans. The Company drew down the Tranche A Loans on October 13, 2015. The Tranche B Loans were available to be drawn prior to March 29, 2017. In January 2017, the term loan agreement was amended to extend the commitment period of the Tranche B Loans to April 28, 2017. In April 2017, the Company drew down $5,000,000 of the available Tranche B Loans.

In September 2018, the Company entered into Amendment No. 5 to the term loan agreement with CRG. Under the amended terms of the amended loan agreement the maturity date was extended to December 31, 2022 and the repayment schedule of the existing term loans were changed to interest only so that the outstanding principal amount of the term loans will be payable in a single installment at maturity. The related fixed interest rate was changed to equal 10.75% per annum, due and payable quarterly in arrears. At the election of the Company, 2.75% of the interest due and payable may be “paid in kind” and added to the then outstanding principal and 8.0% of the interest due and payable paid in cash. All unpaid principal, and accrued and unpaid interest, is due and payable in full on December 31, 2022. The amended term loan agreement also provided for additional term loans in an aggregate principal amount of up to $25,000,000 and allowed for the conversion into shares of common stock, at the Company’s option, of up to 25% of the outstanding loans under the term loan agreement in connection with an initial public offering of the Company’s common stock which results in market capitalization of at least $250,000,000. In September 2018, the Company drew down an additional $15,000,000 under the term loan agreement with CRG. As provided for under the terms of the amended term loan agreement, the related fixed interest rate was further reduced to 10.0% upon the consummation of the Company's IPO in April 2019. Also, post consummation of the Company's IPO, 8.0% of the interest is due and payable in cash and at the election of the Company, 2.0% of the interest due and payable may be "paid in kind".

In June 2019, the Company entered into Amendment No. 7 to the term loan agreement with CRG to reflect flexibility with respect to permitted cash investments.

The Company may voluntarily prepay the borrowings in full. The Tranche A borrowing required a payment, on the borrowing date, of a financing fee equal to 1.75% of the borrowed loan principal, which is

recorded as a discount to the debt. In addition, a facility fee equal to 5.0% of the amounts borrowed plus any "paid in kind" is payable at the end of the term or when the borrowings are repaid in full. A long-term liability is being accreted using the effective interest method for the facility fee over the term of the loan agreement. The borrowings are collateralized by a security interest in substantially all of the Company’s assets.

The Company is subject to financial covenants related to liquidity and minimum trailing revenue targets that began in December 31, 2016 and are tested on an annual basis. The liquidity covenant requires the Company to maintain an amount which shall exceed the greater of (i) $3,000,000 and (ii) the minimum cash balance, if any, required of the Company by a creditor to the extent the Company has incurred permitted priority debt. The Company had to achieve minimum net revenue of $1,000,000 in 2016, $5,000,000 in 2017, $15,000,000 in 2018, $30,000,000 in 2019 and must achieve minimum net revenue of $40,000,000 in 2020. The liquidity financial covenant has a 90-day equity cure period following end of the calendar year to issue additional shares of equity interests in exchange for cash, or to borrow permitted cure debt. In addition, the term loan agreement prohibits the payment of cash dividends on the Company’s capital stock and also places restrictions on mergers, sales of assets, investments, incurrence of liens, incurrence of indebtedness and transactions with affiliates. CRG may accelerate the payment terms of the term loan agreement upon the occurrence of certain events of default set forth therein, which include the failure of the Company to make timely payments of amounts due under the term loan agreement, the failure of the Company to adhere to the covenants set forth in the term loan agreement, the insolvency of the Company or upon the occurrence of a material adverse change. As of June 30, 2020, the Company was in compliance with all applicable financial covenants. As of June 30, 2020, management does not believe that it is probable that the above clauses will be triggered within the next twelve months, and therefore, the debt is classified as long-term on the condensed balance sheet.

Future maturities under the term loan agreement as of June 30, 2020 are as follows (in thousands):


Period Ending December 31:	Amount
2020	$	2,240
2021	4,442
2022	48,255
	54,937
Add: Accretion of closing fees	1,369
	56,306
Less: Amount representing interest	(11,124)
Less: Amount representing debt discount and debt issuance costs	(116)
Present value of minimum payments	$	45,066

In October 2015, CRG purchased 327,759 shares of the Company’s Series C redeemable convertible preferred stock at $6.11 per share. In addition, CRG received warrants to purchase 163,877 shares of the Company’s Series C redeemable convertible preferred stock at an exercise price of $6.11 per share. Upon the closing of the IPO, the warrants were net exercised, based on the initial public offering price of $20.00 per share, into shares of common stock.

In July 2017, CRG purchased an additional 163,877 shares of the Company’s Series C convertible preferred stock at $6.11 per share. Upon the closing of the IPO, the Series C redeemable convertible preferred stock shares were converted into shares of common stock.

Commitments and Contingencies

6 Months Ended

Jun. 30, 2020

Commitments and Contingencies Disclosure [Abstract]

Commitments and Contingencies

Operating Lease and Rights-of-Use

The Company’s operating lease obligation consists of leased office, laboratory, and manufacturing space under a non-cancellable operating lease that expires in October 2024. The lease agreement includes a renewal provision allowing the Company to extend this lease for an additional period of five years. Operating lease costs were $217,000 for each of the three months ended June 30, 2020 and 2019 and $435,000 for each of the six months ended June 30, 2020 and 2019. Cash paid for amounts included in the measurement of operating lease liabilities was $188,000, $372,000, $169,000 and $363,000 for the three and six months ended June 30, 2020 and 2019, respectively. As of June 30, 2020, the weighted average discount rate was approximately 6.50% and the weighted average remaining lease term was 4.33 years. Balance sheet information as of June 30, 2020 consists of the following (in thousands):


Operating Lease:	June 30, 2020
Operating lease right-of-use asset in other non-current assets	$	3,105
Operating lease liability in accrued liabilities	$	809
Operating lease liability in other liabilities	3,288
Total operating lease liabilities	$	4,097

The following table summarizes the Company’s operating lease maturities as of June 30, 2020 (in thousands):


Period Ending December 31:	Amount
2020	$	525
2021	1,066
2022	1,096
2023	1,127
2024	904
Total lease payments	4,718
Less: imputed interest	(621)
Present value of lease liabilities	$	4,097

Purchase Obligations

Purchase obligations consist of agreements to purchase goods and services entered into in the ordinary course of business. As of June 30, 2020, the Company had non-cancellable purchase obligations to suppliers of $6,640,000.

Indemnification

In the normal course of business, the Company enters into contracts and agreements that contain a variety of representations and warranties and may provide for indemnification of the counterparty. The Company’s exposure under these agreements is unknown because it involves claims that may be made against it in the future but have not yet been made. To date, the Company has not been subject to any claims or been required to defend any action related to its indemnification obligations.

The Company indemnifies each of its directors and officers for certain events or occurrences, subject to certain limits, while the director is or was serving at the Company’s request in such capacity, as permitted under Delaware law and in accordance with its certificate of incorporation and bylaws. The term of the indemnification period lasts as long as a director may be subject to any proceeding arising out of acts or omissions of such director in such capacity. The maximum amount of potential future indemnification is unlimited; however, the Company currently holds director liability insurance. The Company believes that the fair value of these indemnification obligations is minimal. Accordingly, the Company has not recognized any liabilities relating to these obligations as of June 30, 2020.

Contingencies

The Company is not involved in any pending legal proceedings that it believes could have a material adverse effect on its financial condition, results of operations or cash flows. From time to time, the Company may pursue litigation to assert its legal right and such litigation may be costly and divert the efforts and attention of its management and technical personnel which could adversely affect its business. The Company accrues a liability for such matters when it is probable that future expenditures will be made and such expenditures can be reasonably estimated. There were no contingent liabilities requiring accrual at June 30, 2020 and December 31, 2019.

Redeemable Convertible Preferred Stock	6 Months Ended
Redeemable Convertible Preferred Stock	Jun. 30, 2020
Temporary Equity Disclosure [Abstract]
Redeemable Convertible Preferred Stock	Redeemable Convertible Preferred Stock Upon the closing of the IPO, all shares of convertible preferred stock then outstanding converted into shares of common stock. As of June 30, 2020 and December 31, 2019, the Company does not have any convertible preferred stock issued or outstanding. Preferred Stock Warrants Upon the closing of the Company's IPO, all of the outstanding convertible preferred stock warrants were exercised, or net exercised based on the IPO price of $20.00 per share, into 1,945,365 shares of common stock. As of June 30, 2020 and December 31, 2019, the Company does not have any convertible preferred stock warrants outstanding. Stockholders Equity Preferred Stock At June 30, 2020, the Company’s certificate of incorporation, as amended and restated, authorizes the Company to issue up to 5,000,000 shares of preferred stock with $0.001 par value per share, of which no shares were issued and outstanding. Common Stock At June 30, 2020, the Company’s certificate of incorporation, as amended and restated, authorizes the Company to issue up to 100,000,000 shares of common stock with $0.001 par value per share, of which 33,610,099 shares were issued and outstanding. The holders of common stock are also entitled to receive dividends whenever funds are legally available, when and if declared by the Board of Directors. As of June 30, 2020, no dividends have been declared to date. Each share of common stock is entitled to one vote. Common Stock Warrants In connection with the IPO, the common stock warrants were cash, or net exercised based on the IPO price of $20.00 per share, into 5,968 shares of common stock. As of June 30, 2020 and December 31, 2019, the Company does not have any common stock warrants outstanding.

Stockholders Equity	6 Months Ended
Stockholders Equity	Jun. 30, 2020
Equity [Abstract]
Stockholders Equity	Redeemable Convertible Preferred Stock Upon the closing of the IPO, all shares of convertible preferred stock then outstanding converted into shares of common stock. As of June 30, 2020 and December 31, 2019, the Company does not have any convertible preferred stock issued or outstanding. Preferred Stock Warrants Upon the closing of the Company's IPO, all of the outstanding convertible preferred stock warrants were exercised, or net exercised based on the IPO price of $20.00 per share, into 1,945,365 shares of common stock. As of June 30, 2020 and December 31, 2019, the Company does not have any convertible preferred stock warrants outstanding. Stockholders Equity Preferred Stock At June 30, 2020, the Company’s certificate of incorporation, as amended and restated, authorizes the Company to issue up to 5,000,000 shares of preferred stock with $0.001 par value per share, of which no shares were issued and outstanding. Common Stock At June 30, 2020, the Company’s certificate of incorporation, as amended and restated, authorizes the Company to issue up to 100,000,000 shares of common stock with $0.001 par value per share, of which 33,610,099 shares were issued and outstanding. The holders of common stock are also entitled to receive dividends whenever funds are legally available, when and if declared by the Board of Directors. As of June 30, 2020, no dividends have been declared to date. Each share of common stock is entitled to one vote. Common Stock Warrants In connection with the IPO, the common stock warrants were cash, or net exercised based on the IPO price of $20.00 per share, into 5,968 shares of common stock. As of June 30, 2020 and December 31, 2019, the Company does not have any common stock warrants outstanding.

Stock Option Plans

6 Months Ended

Jun. 30, 2020

Share-based Payment Arrangement [Abstract]

Stock Option Plans

In March 2019, the Company’s Board of Directors approved the adoption of the 2019 Equity Incentive Plan, or the 2019 Plan, which became effective immediately prior to the Company's IPO. The 2019 Plan replaced the 2007 Stock Option Plan which was terminated immediately prior to consummation of the Company’s IPO, collectively the “Plans.” The 2019 Plan provides for the grant of ISOs to employees and for the grant of NSOs, restricted stock, restricted stock units, stock appreciation rights, performance units and performance shares to employees, directors and consultants. A total of 2,317,000 shares of common stock were initially reserved for issuance pursuant to the 2019 Plan. In addition, the shares reserved for issuance under the 2019 Plan will also include shares reserved but not issued under the 2007 Stock Option Plan, plus any share awards granted under the 2007 Stock Option Plan that expire or terminate without having been exercised in full or that are forfeited or repurchased. In addition, the number of shares available for issuance under the 2019 Plan will also include an annual increase on the first day of each fiscal year beginning in fiscal 2020, equal to the lesser of (i) 3,000,000 shares; (ii) 4.0% of the outstanding shares of common stock as of the last day of the immediately preceding fiscal year; or (iii) an amount as determined by the Board of Directors. As of June 30, 2020, the Company has reserved 3,631,205 shares of common stock for issuance under the 2019 Plan.

The exercise price of ISOs and NSOs shall not be less than 100% and 85% of the estimated fair value of the shares on the date of grant, respectively, as determined by the Board of Directors. The exercise price of ISOs and NSOs granted to a 10% stockholder shall not be less than 110% of the estimated fair value of the shares on the date of grant as determined by the Board of Directors. To date, options have a term of 10 years and generally vest over 4 years from the date of grant.

Stock option activity under the Company’s Plans is set forth below:


		Options Outstanding
	Shares Available for Grant	Number of Shares	Weighted Average Exercise Price		Weighted Average Remaining Contractual Term (in Years)	Aggregate Intrinsic Value (in thousands)





Balances, December 31, 2019	1,554,690	4,310,790	$	7.91	7.27	$	140,234
Authorized	1,250,210
Options granted	(752,258)	752,258	$	33.86
Options exercised	—	(405,837)	$	2.16
Options cancelled	24,345	(24,345)	$	17.28
Balances, June 30, 2020	2,076,987	4,632,866	$	12.57	7.42	$	136,505
Vested and exercisable at June 30, 2020		2,554,809	$	5.95	6.40	$	91,917
Vested and expected to vest at June 30, 2020		4,632,866	$	12.57	7.42	$	136,505

The aggregate intrinsic value of options exercised during the six months ended June 30, 2020, was $15,048,000. The aggregate intrinsic value was calculated as the difference between the exercise prices of the underlying options and the estimated fair value of the common stock on the date of exercise.

Restricted Stock Units

In March 2020, the Company began granting restricted stock units, or RSUs, under the 2019 Plan. RSUs generally vest over four years in annual equal increments, no RSUs vested during the six months ended June 30, 2020. The fair value of RSUs is based on the Company’s closing stock price on the date of grant. A summary of RSUs activity for the six months ended June 30, 2020 is as follows:


	Number of Restricted Stock Units	Weighted Average Grant Date Fair Value
Balances, December 31, 2019	—	$	—
Restricted stock granted	39,089	$	31.18
Restricted stock vested	—	$	—
Restricted stock forfeited	—	$	—
Balances, June 30, 2020	39,089	$	31.18
Expected to vest at June 30, 2020	39,089	$	31.18

2019 Employee Stock Purchase Plan

In March 2019, the Company's Board of Directors adopted the 2019 Employee Stock Purchase Plan, or 2019 ESPP, under which eligible employees are permitted to purchase common stock at a discount through payroll deductions. A total of 434,000 shares of common stock were initially reserved for issuance and will be increased on the first day of each fiscal year, beginning in 2020, by an amount equal to the lesser of (i) 1,200,000 shares (ii) 1.0% of the outstanding shares of common stock as of the last day of the immediately preceding fiscal year; or (iii) an amount as determined by the Board of Directors. As of June 30, 2020, the Company has reserved 746,552 shares of common stock for issuance under the 2019 ESPP. The price of the common stock purchased will be the lower of 85% of the fair market value of the common stock at the beginning of an offering period or at the end of a purchase period. The 2019 ESPP was effective upon adoption by the Company's Board of Directors but was not in use until the completion of the Company's IPO in April 2019. The 2019 ESPP is intended to qualify as an "employee stock purchase plan" within the meaning of Section 423 of the Internal Revenue Code of 1986, as amended.

As of June 30, 2020, 87,567 shares of common stock have been issued to employees participating in the 2019 ESPP and 658,985 shares were available for future issuance under the 2019 ESPP.

Stock-Based Compensation

The Company estimated the fair value of stock options using the Black–Scholes option pricing model. The fair value of employee and nonemployee stock options is being amortized on a straight–line basis over the requisite service period of the awards. The fair value of employee and nonemployee stock options was estimated using the following assumptions for the three and six months ended June 30, 2020 and 2019:


	Three Months Ended June 30,			Six Months Ended June 30,
	2020	2019	2020	2019
Expected term (in years)	5.25 - 6.25	5.00 - 6.25	5.25 - 6.25	5.00 - 6.25
Expected volatility	46.6% - 47.7%	42.7% - 42.9%	42.9% - 47.7%	42.7% - 42.9%
Risk-free interest rate	0.42% - 0.56%	1.86% - 2.37%	0.42% - 1.41%	1.86% - 2.54%
Dividend yield	—%	—%	—%	—%

The fair value of the shares to be issued under the Company’s 2019 ESPP was estimated using the Black-Scholes valuation model with the following assumptions for the three and six months ended June 30, 2020:


	Three Months Ended June 30,			Six Months Ended June 30,
	2020	2019	2020	2019
Expected term (in years)	0.50	0.63	0.50	0.63
Expected volatility	76.4%	47.8%	44.4% - 76.4%	47.8%
Risk-free interest rate	0.14%	2.45%	0.14% - 1.58%	2.45%
Dividend yield	—%	—%	—%	—%

Total stock-based compensation expense relating to the Company's stock options, RSUs and 2019 ESPP during the three and six months ended June 30, 2020 and stock-based compensation expense related to the Company's stock options and 2019 ESPP during the three and six months ended June 30, 2019, is as follows (in thousands):


	Three Months Ended June 30,						Six Months Ended June 30,
	2020		2019		2020		2019
Cost of goods sold	$	89	$	54	$	153	$	69
Research and development expenses	246		127		423		155
Selling, general and administrative expenses	1,319		655		2,371		875
	$	1,654	$	836	$	2,947	$	1,099

As of June 30, 2020, there was total unrecognized compensation costs of $16,986,000 related to stock options expected to be recognized over a period of approximately 3.24 years, a total of $1,119,000 of unrecognized compensation costs related to unvested RSUs expected to be recognized over a period of approximately 3.68 years and $303,000 of unrecognized compensation costs related to the ESPP, which the Company will recognize over 0.4 years.

401(k) Plan	6 Months Ended
401(k) Plan	Jun. 30, 2020
Retirement Benefits [Abstract]
401(k) Plan	401(k) PlanThe Company has a qualified retirement plan under section 401(k) of the Internal Revenue Code (“IRC”) under which participants may contribute up to 90% of their eligible compensation, subject to maximum deferral limits specified by the IRC. Beginning in January 2020, the Company started matching employees' contributions to the 401(k) plan at 50% of the first 5% of compensation deferred to the 401(k) plan. The Company's matching contributions were $229,000 and $522,000 for the three and six months ended June 30, 2020, respectively.

Summary of Significant Accounting Policies (Policies)	6 Months Ended
Summary of Significant Accounting Policies (Policies)	Jun. 30, 2020
Accounting Policies [Abstract]
Basis of Preparation	Basis of Preparation The accompanying financial statements have been prepared in accordance with generally accepted accounting principles in the United States of America, or U.S. GAAP, and applicable rules and regulations of the Securities and Exchange Commission, or SEC, regarding interim financial reporting. As permitted under those rules, certain footnotes or other financial information that are normally required by U.S. GAAP have been condensed or omitted, and accordingly the balance sheet as of December 31, 2019, and related disclosures, have been derived from the audited financial statements at that date but does not include all of the information required by U.S. GAAP for complete financial statements. These unaudited condensed financial statements have been prepared on the same basis as the Company’s annual financial statements and, in the opinion of management, reflect all adjustments (consisting only of normal recurring adjustments) that are necessary for the fair statement of the Company’s condensed financial information. The results of operations for the three and six months ended June 30, 2020 are not necessarily indicative of the results to be expected for the year ending December 31, 2020 or for any other interim period or for any other future year. The accompanying interim unaudited condensed financial statements and related financial information should be read in conjunction with the audited financial statements and the related notes thereto for the year ended December 31, 2019 included in the Company's annual report on Form 10-K filed with the SEC on March 2, 2020. Adjustment to Prior Period Financial Statements The Company has adjusted the accompanying Condensed Statement of Cash Flows for the six months ended June 30, 2019 to increase each of the accounts receivable, net, and accrued liabilities amounts by $601,000 for an immaterial prior period error in the classification of provisions for returns from customers.
Use of Estimates	Use of Estimates The preparation of financial statements in conformity with U.S. GAAP requires management to make estimates and assumptions that affect the amounts and disclosures reported in the financial statements. Management uses judgment when making estimates related to provisions for accounts receivable and excess and obsolete inventories, the valuation of deferred tax assets, the provision for sales returns, stock-based compensation, and for periods prior to the Company's IPO, the valuation of common stock and redeemable convertible preferred stock warrants. Management bases its estimates on historical experience and on various other assumptions that are believed to be reasonable under the circumstances, the results of which form the basis for making judgments about the carrying values of assets and liabilities that are not readily apparent from other sources. Although these estimates are based on the Company’s knowledge of current events and actions it may undertake in the future, actual results may ultimately materially differ from these estimates and assumptions. Due to the coronavirus (“COVID-19”) pandemic, there has been continued uncertainty and disruption in the global economy and financial markets. The Company is not aware of any specific event or circumstance that would require an update to its estimates or judgments or a revision of the carrying value of its assets or liabilities as of June 30, 2020. The Company has also considered information available to it as of the date of issuance of these financial statements and is not aware of any specific events or circumstances that would require an update to its estimates or judgments, or a revision to the carrying value of its assets or liabilities. These estimates may change as new events occur and additional information becomes available. Actual results could differ materially from these estimates.
Fair Value of Financial Instruments	Fair Value of Financial Instruments The Company has evaluated the estimated fair value of its financial instruments as of June 30, 2020 and December 31, 2019. The carrying amounts of certain of the Company’s financial instruments, which include cash equivalents, short-term investments, long-term investments, restricted cash, accounts receivable, accounts payable and accrued liabilities approximate their respective fair values because of the short-term nature of these instruments. Management believes that its long-term debt bears interest at the prevailing market rates for instruments with similar characteristics (Level 2 within the fair value hierarchy); accordingly, the carrying value of this instrument approximates its fair value. Prior to the Company's IPO, fair value accounting was applied to the redeemable convertible preferred stock warrant liability.
Cash, Cash Equivalents and Restricted Cash	Cash, Cash Equivalents and Restricted Cash The Company considers all highly liquid investments with an original maturity of three months or less at the time of purchase to be cash equivalents. Cash equivalents are considered available-for-sale marketable securities and are recorded at fair value, based on quoted market prices. As of June 30, 2020 and December 31, 2019, the Company’s cash equivalents are entirely comprised of investments in money market funds.
Investments	Investments Short-term investments consist of debt securities classified as available-for-sale and have original maturities greater than 90 days, but less than one year as of the balance sheet date. Long-term investments have maturities greater than one year as of the balance sheet date. All investments are recorded at fair value based on the fair value hierarchy. Money market funds and United States treasury bills with an original maturity less than 90 days are classified within Level 1 of the fair value hierarchy, and commercial paper, corporate bonds/notes, United States Government securities, and asset-backed securities are classified within Level 2 of the fair value hierarchy. Unrealized gains and losses, deemed temporary in nature, are reported as a separate component of accumulated other comprehensive income (loss). The cost of available-for-sale investments sold is based on the specific-identification method. Realized gains and losses are included in earnings, and are derived for specific-identification method for determining the costs of investments sold. Amortization of premiums and accretion of discounts are reported as a component of interest income. A decline in the fair value of any security below cost that is deemed other than temporary results in a charge to earnings and the corresponding establishment of a new cost basis for the investment.
Concentration of Credit Risk, and Other Risks and Uncertainties	Concentration of Credit Risk, and Other Risks and Uncertainties The Company is subject to risks related to the public health crises such as the global pandemic associated with COVID-19, which has spread to most countries and all 50 states within the United States. The COVID-19 outbreak has negatively impacted, and may continue to negatively impact the Company’s operations, its revenue and overall financial condition by significantly decreasing the number of TCAR procedures performed. The number of TCAR procedures performed, similar to other surgical procedures, has significantly decreased as health care organizations globally have prioritized the treatment of patients with COVID-19. For example, in the United States, governmental authorities have recommended, and in certain cases required, that elective, specialty and other procedures and appointments, be suspended or canceled to avoid non-essential patient exposure to medical environments and potential infection with COVID-19 and to focus limited resources and personnel capacity toward the treatment of COVID-19. These measures and challenges will likely continue for the duration of the pandemic, which is uncertain, and will continue to negatively impact the Company’s revenue while the pandemic continues. Financial instruments that potentially subject the Company to credit risk consist of cash and cash equivalents, investments and accounts receivable to the extent of the amounts recorded on the balance sheet. Cash, cash equivalents, and investments are deposited in financial institutions which, at times, may be in excess of federally insured limits. Cash equivalents are invested in highly rated money market funds and United States treasury bills. The Company invests in a variety of financial instruments, such as, but not limited to, commercial paper, corporate bonds/notes, United States Government securities, asset-backed securities and, by policy, limits the amount of credit exposure with any one financial institution or commercial issuer. The Company has not experienced any material losses on its deposits of cash and cash equivalents or investments during the three and six months ended June 30, 2020 and 2019. The Company’s accounts receivable are due from a variety of health care organizations in the United States. At June 30, 2020 and December 31, 2019, no customer represented 10% or more of the Company’s accounts receivable. For the three and six months ended June 30, 2020 and 2019, there were no customers that represented 10% or more of revenue. The Company provides for uncollectible amounts when specific credit problems are identified. In doing so, the Company analyzes historical bad debt trends, customer credit worthiness, current economic trends and changes in customer payment patterns when evaluating the adequacy of the allowance for doubtful accounts. The Company manufactures certain of its commercial products in-house. Certain of the Company’s product components and sub-assemblies continue to be manufactured by sole suppliers, the most significant of which is the ENROUTE stent. Disruption in component or sub-assembly supply from these manufacturers or from in-house production would have a negative impact on the Company’s financial position and results of operations. The Company is subject to certain risks, including that its devices may not be approved or cleared for marketing by governmental authorities or be successfully marketed. There can be no assurance that the Company’s products will achieve widespread adoption in the marketplace, nor can there be any assurance that existing devices or any future devices can be developed or manufactured at an acceptable cost and with appropriate performance characteristics. The Company is also subject to risks common to companies in the medical device industry, including, but not limited to, new technological innovations, dependence upon government and third-party payers to provide adequate coverage and reimbursement, dependence on key personnel and suppliers, protection of proprietary technology, product liability claims, and compliance with government regulations. Existing or future devices developed by the Company may require approvals or clearances from the FDA or international regulatory agencies. In addition, in order to continue the Company’s operations, compliance with various federal and state laws is required. If the Company were denied or delayed in receiving such approvals or clearances, it may be necessary to adjust operations to align with the Company’s currently approved portfolio of products. If clearance for the products in the current portfolio were withdrawn by the FDA, this would have a material adverse impact on the Company.
Leases	Leases The Company adopted Accounting Standards Codification (“ASC”) 842, "Leases," on January 1, 2019 and used the modified retrospective method for all leases not substantially completed as of the date of adoption and the package of practical expedients available in the standard. The Company considers if an arrangement is a lease at inception if it obtains the right to control the use of an identified asset under a leasing arrangement with an initial term greater than twelve months. The Company determines whether a contract conveys the right to control the use of an identified asset for a period of time if the contract contains both the right to obtain substantially all of the economic benefits from the use of the identified asset and the right to direct the use of the identified asset. The Company also evaluates the nature of each lease to determine whether it is an operating or financing lease and recognizes the right-of-use asset and lease liabilities based on the present value of future minimum lease payments over the expected lease term. The Company’s leases do not generally contain an implicit interest rate and therefore the Company uses the incremental borrowing rate it would expect to pay to borrow on a similar collateralized basis over a similar term in order to determine the present value of its lease payments. The Company’s considers renewal options in the determination of the lease term if the option to renew is reasonably certain. Variable lease costs represent payments that are dependent on usage, a rate or index. Variable lease costs, which consists primarily of taxes, insurance and common area maintenance costs, are expensed as incurred, as the Company has elected to account separately for contracts that contain lease and non-lease components, consistent with its historical practice. The Company does not have any finance leases.
Redeemable Convertible Preferred Stock Warrant Liability	Redeemable Convertible Preferred Stock Warrant Liability Prior to its IPO, the Company accounted for its warrants for shares of redeemable convertible preferred stock as a liability based upon the characteristics and provisions of each instrument. Redeemable convertible preferred stock warrants classified as a liability were initially recorded at their fair value on the date of issuance and are subject to remeasurement at each subsequent balance sheet date. Any change in fair value as a result of a remeasurement was recognized as a component of other income (expense), net in the condensed statements of operations and comprehensive loss. The Company recorded adjustments to the estimated fair value of the redeemable convertible preferred stock warrants until they were exercised. Upon their exercise, the final fair value of the warrant liability was reclassified to stockholders’ equity. Subsequent to its IPO, the Company no longer recorded any related periodic fair value adjustments.
Redeemable Convertible Preferred Stock	Redeemable Convertible Preferred StockPrior to its IPO, the Company recorded its redeemable convertible preferred stock at fair value on the dates of issuance, net of issuance costs, and classified the redeemable convertible preferred stock outside of stockholders’ equity on the condensed balance sheet as events triggering the liquidation preferences were not solely within the Company’s control.
Revenue Recognition and Cost of Goods Sold	Revenue Recognition The Company recognizes revenue in accordance with ASC Topic 606, "Revenue from Contracts with Customers." Under ASC 606, revenue is recognized when a customer obtains control of promised goods or services, in an amount that reflects the consideration which the entity expects to receive in exchange for those goods or services. To determine revenue recognition for arrangements that an entity determines are within the scope of ASC 606, the Company performs the following five steps: (i) identify the contract(s) with a customer; (ii) identify the performance obligations in the contract; (iii) determine the transaction price; (iv) allocate the transaction price to the performance obligations in the contract; and (v) recognize revenue when (or as) the entity satisfies a performance obligation. As of June 30, 2020 and December 31, 2019, the Company recorded $179,000 and $128,000, respectively, of unbilled receivables, which are included in accounts receivable, net on the condensed balance sheet, as the Company has an unconditional right to payment as of the end of the applicable period. The Company’s revenue is generated from the sale of its products to hospitals and medical centers in the United States through direct sales representatives. Revenue is recognized when obligations under the terms of a contract with customers are satisfied, which occurs with the transfer of control of the Company’s products to its customers, either upon shipment of the product or delivery of the product to the customer under the Company’s standard terms and conditions. The Company’s products are readily available for usage as soon as the customer possesses it. Upon receipt, the customer controls the economic benefits of the product, has significant risks and rewards, and the legal title. The Company has present right to payment; therefore, the transfer of control is deemed to happen at a point in time. Revenue is measured as the amount of consideration the Company expects to receive in exchange for transferring the goods. For sales where the Company’s sales representative hand delivers product directly to the hospital or medical center from the sales representative’s trunk stock inventory, the Company recognizes revenue upon delivery, which represents the point in time when control transfers to the customer. Upon delivery there are legally-enforceable rights and obligations between the parties which can be identified, commercial substance exists and collectability is probable. For sales which are sent directly from the Company to hospitals and medical centers, the transfer of control occurs at the time of shipment or delivery of the product. There are no further performance obligations by the Company or the sales representative to the customer after delivery under either method of sale. As allowed under the practical expedient, the Company does not disclose the value of unsatisfied performance obligations for (i) contracts with an original expected length of one year or less and (ii) contracts for which it recognizes revenue at the amount to which it has the right to invoice for services performed. The Company is entitled to the total consideration for the products ordered by customers as product pricing is fixed according to the terms of customer contracts and payment terms are short. Payment terms fall within the one-year guidance for the practical expedient which allows the Company to forgo adjustment of the promised amount of consideration for the effects of a significant financing component. The Company excludes taxes assessed by governmental authorities on revenue-producing transactions from the measurement of the transaction price. Costs associated with product sales include commissions and royalties. The Company applies the practical expedient and recognizes commissions and royalties as expense when incurred because the expense is incurred at a point in time and the amortization period is less than one year. Commissions are recorded as selling expense and royalties are recorded as cost of goods sold in the condensed statements of operations and comprehensive loss. The Company accepts product returns at its discretion or if the product is defective as manufactured. The Company establishes estimated provisions for returns based on historical experience and considers other factors that it believes could significantly impact its expected returns, which provisions are classified within accrued liabilities on the condensed balance sheet. The Company elected to expense shipping and handling costs as incurred and includes them in the cost of goods sold. In those cases where the Company bills shipping and handling costs to customers, it will classify the amounts billed as a component of revenue. Cost of Goods Sold The Company manufactures certain of its portfolio of TCAR products at its facility and purchases other products from third party manufacturers. Cost of goods sold consists primarily of costs related to materials, components and subassemblies, manufacturing overhead costs, direct labor, reserves for excess, obsolete and non-sellable inventories as well as distribution-related expenses. A significant portion of the Company’s cost of goods sold currently consists of manufacturing overhead costs. These overhead costs include the cost of quality assurance, material procurement, inventory control, facilities, equipment and operations supervision and management. Cost of goods sold also includes depreciation expense for production equipment and certain direct costs such as shipping costs and royalties.
Stock-Based Compensation	Stock–Based Compensation The Company accounts for stock-based compensation in accordance with Financial Accounting Standards Board (“FASB”) ASC 718, "Compensation-Stock Compensation." ASC 718 requires the recognition of compensation expense, using a fair-value based method, for costs related to all share-based payments including stock options, restricted stock units and shares issued under its employee stock purchase plan. ASC 718 requires companies to estimate the fair value of all share-based payment option awards on the date of grant using an option pricing model. The fair value of stock options is recognized over the period during which an optionee is required to provide services in exchange for the option award, known as the requisite service period (usually the vesting period), on a straight-line basis. For performance-based stock options, the Company will assess the probability of performance conditions being achieved in each reporting period. The amount of stock-based compensation expense recognized in any one period related to performance-based stock options can vary based on the achievement or anticipated achievement of the performance conditions. The Company accounts for option forfeitures as they occur. The Company accounts for stock-based compensation for restricted stock units at their fair value, based on the closing market price of the Company's common stock on the date of grant. These costs are recognized on a straight-line basis over the requisite service period, which is usually the vesting period. The Company accounts for stock-based compensation for its employee stock purchase plan based on the estimated fair value of the options on the date of grant. The Company estimates the grant date fair value using an option pricing model for each purchase period. These costs are recognized on a straight-line basis over the offering period.
Income Taxes	Income Taxes The Company accounts for income taxes under the liability method, whereby deferred tax assets and liabilities are determined based on the difference between the condensed financial statements and tax bases of assets and liabilities using the enacted tax rates in effect for the year in which the differences are expected to affect taxable income. A valuation allowance is established when necessary to reduce deferred tax assets to the amounts expected to be realized. As the Company has historically incurred operating losses, it has established a full valuation allowance against its net deferred tax assets, and there is no provision for income taxes. The Company also follows the provisions of ASC 740-10, "Accounting for Uncertainty in Income Taxes." ASC 740-10 prescribes a comprehensive model for the recognition, measurement, presentation and disclosure in financial statements of any uncertain tax positions that have been taken or expected to be taken on a tax return. No liability related to uncertain tax positions is recorded on the condensed financial statements. It is the Company's policy to include penalties and interest expense related to income taxes as part of the provision for income taxes.
Comprehensive Loss	Comprehensive Loss Comprehensive loss consists of net loss and changes in unrealized gains and losses on investments classified as available-for-sale. For the three and six months ended June 30, 2020, the Company’s unrealized gains and losses on available-for-sale investments represent the only component of other comprehensive loss that are excluded from the reported net loss and that are presented in the condensed statements of operations and comprehensive loss. Accumulated other comprehensive income or loss is presented in the accompanying condensed balance sheet as a component of stockholders' equity.
Net Loss per Share	Net Loss per Share Basic net loss per share is computed by dividing the net loss by the weighted average number of shares of common stock outstanding during the period, without consideration for potential dilutive common shares. Diluted net loss per share is computed by dividing the net loss by the weighted average number of shares of common stock and potentially dilutive securities outstanding for the period. For purposes of the diluted net loss per share calculation, redeemable convertible preferred stock and warrants, common stock options, and restricted stock units are considered to be potentially dilutive securities. Since the Company was in a loss position for all periods presented, basic net loss per share is the same as diluted net loss per share as the inclusion of all potential dilutive common shares would have been anti-dilutive. The Company allocates no loss to participating securities because they have no contractual obligation to share in the losses of the Company. The shares of the Company’s redeemable convertible preferred stock participate in any dividends declared by the Company and were therefore considered to be participating securities.
Segment and Geographical Information	Segment and Geographical Information The Company operates and manages its business as one reportable and operating segment. The Company’s chief executive officer, who is the chief operating decision maker, reviews financial information on an aggregate basis for purposes of allocating resources and evaluating financial performance. All of the Company’s long-lived assets are based in the United States. Long-lived assets are comprised of property and equipment and the Company's right-of-use asset. All of the Company’s revenue was in the United States for the three and six months ended June 30, 2020 and 2019, based on the shipping location of the external customer.
Recent Accounting Pronouncements	Recent Accounting Pronouncements Recently Adopted Accounting Standards In August 2018, the FASB issued ASU 2018-13, "Fair Value Measurement," which changed the disclosure requirements for fair value measurements by removing, adding and modifying certain disclosures. The Company adopted the new standard effective January 1, 2020. The adoption did not have a material impact on the Company's financial statements and related disclosures. In August 2018, the FASB issued ASU 2018-15, "Cloud Computing Arrangements," which aligns the requirements for capitalizing implementation costs in a Cloud Computing Arrangement service contract with the requirements for capitalizing implementation costs incurred for an internal-use software license. The Company adopted the new standard effective January 1, 2020 on a prospective basis. The adoption did not have a material impact on the Company's financial statements and related disclosures. Recently Issued Accounting Standards In June 2016, the FASB issued ASU 2016-13, "Measurement of Credit Losses on Financial Statements." This update provides financial statement users with more decision-useful information about the expected credit losses on financial instruments and other commitments to extend credit held by a reporting entity at each reporting date. The update replaces the incurred loss impairment methodology in current U.S. GAAP with a methodology that reflects expected credit losses and requires consideration of a broader range of reasonable and supportable information to inform credit loss estimates. In October 2019, the FASB delayed the effective date of this standard for smaller reporting companies, as such ASU 2016-13 would originally be effective for the Company on January 1, 2023. As of June 30, 2020, the Company no longer qualifies as a smaller reporting company, accordingly ASU 2016-13 will now become effective for the Company on January 1, 2021. The Company is evaluating the impact of adopting this guidance to the Company's financial statements and related disclosures. In December 2019, the FASB issued ASU 2019-12, Income Taxes (Topic 740): Simplifying the Accounting for Income Taxes, which enhances and simplifies various aspects of the income tax accounting guidance related to intra-period tax allocation, interim period accounting for enacted changes in tax law, and the year-to-date loss limitation in interim period tax accounting. ASU 2019-12 also amends other aspects of the guidance to reduce complexity in certain areas. ASU 2019-12 will become effective for the Company on January 1, 2021. Early adoption is permitted. The Company is evaluating the impact of adopting this guidance to the Company's financial statements and related disclosures.

Summary of Significant Accounting Policies (Tables)

6 Months Ended

Jun. 30, 2020

Accounting Policies [Abstract]

Net Loss Per Share Determination

Net loss per share was determined as follows (in thousands, except share and per share data):


	Three Months Ended June 30,						Six Months Ended June 30,
	2020		2019		2020		2019
Net loss	$	(10,353)	$	(11,959)	$	(20,294)	$	(36,117)
Weighted average common stock outstanding used to compute net loss per share, basic and diluted	32,682,360		28,458,793		32,010,335		14,905,052
Net loss, basic and diluted	$	(0.32)	$	(0.42)	$	(0.63)	$	(2.42)

Schedule of Potentially Dilutive Securities Outstanding Excluded from Diluted Weighted Average Shares Outstanding


	June 30,
	2020	2019
Common stock options	4,632,866	4,683,811
Restricted stock units	39,089	—
	4,671,955	4,683,811

Fair Value Measurements (Tables)

6 Months Ended

Jun. 30, 2020

Fair Value Disclosures [Abstract]

Financial Liabilities Measure on a Recurring Basis


	June 30, 2020
	Level 1		Level 2		Level 3		Total
Assets:
Money market funds	$	72,924	$	—	$	—	$	72,924
U.S. treasury bills	—		9,993		—		9,993
Commercial paper	—		11,965		—		11,965
Corporate bonds/notes	—		10,742		—		10,742
U.S. government securities	—		49,714		—		49,714
Asset-backed securities	—		2,002		—		2,002
	$	72,924	$	84,416	$	—	$	157,340


	December 31, 2019
	Level 1		Level 2		Level 3		Total
Assets:
Money market funds	$	34,363	$	—	$	—	$	34,363
Commercial paper	—		9,919		—		9,919
Corporate bonds/notes	—		10,176		—		10,176
U.S. government securities	—		44,456		—		44,456
Asset-backed securities	—		5,181		—		5,181
	$	34,363	$	69,732	$	—	$	104,095

Balance Sheet Components (Tables)

6 Months Ended

Jun. 30, 2020

Organization, Consolidation and Presentation of Financial Statements [Abstract]

Fair Value of the Available-For-Sale Investments

The fair value of the Company's available-for-sale investments as of June 30, 2020 and December 31, 2019 are as follows (in thousands):


	June 30, 2020
			Gross Unrealized						Estimated
	Amortized Cost		Gains		Losses		Fair Value
Money market funds	$	72,924	$	—	$	—	$	72,924
U.S. treasury bills	9,992		1		—		9,993
Commercial paper	11,965		—		—		11,965
Corporate bonds/notes	10,709		34		(1)		10,742
U.S. government securities	49,433		281		—		49,714
Asset-backed securities	2,001		1		—		2,002
	$	157,024	$	317	$	(1)	$	157,340
Classified as:
Cash equivalents										$	72,924
Short-term investments										84,416

							$	157,340


	December 31, 2019
			Gross Unrealized						Estimated
	Amortized Cost		Gains		Losses		Fair Value
Money market funds	$	34,363	$	—	$	—	$	34,363
Commercial paper	9,919		—		—		9,919
Corporate bonds/notes	10,180		—		(4)		10,176
U.S. government securities	44,450		9		(3)		44,456
Asset-backed securities	5,181		—		—		5,181
	$	104,093	$	9	$	(7)	$	104,095
Classified as:
Cash equivalents										$	34,363
Short-term investments										51,508
Long-term investments										18,224
							$	104,095

Fair Value of Cash Equivalents, Short-Term and Long-Term Equivalents


	June 30,		December 31,
	2020		2019
Amounts maturing within one year	$	157,340	$	85,871
Amounts maturing after one year through two years	—		18,224
	$	157,340	$	104,095

Schedule of Unrealized Loss on Investments

The following table presents the Company's available-for-sale investments that were in an unrealized loss position as of June 30, 2020 and December 31, 2019 (in thousands):


	June 30, 2020						December 31, 2019
	Less than 12 months						Less than 12 months
Assets:	Fair Value		Unrealized Loss		Fair Value		Unrealized Loss
Corporate bonds/notes	$	2,558	$	(1)	$	10,128	$	(4)

U.S. government securities	—		—		19,067		(3)
	$	2,558	$	(1)	$	29,195	$	(7)

Schedule of Inventories

Components of inventories were as follows:


(in thousands)	June 30,		December 31,
	2020		2019
Raw materials	$	1,736	$	1,203

Finished products	9,885		9,119
Total	$	11,621	$	10,322

Schedule of Accrued Liabilities

Accrued liabilities consist of the following:


(in thousands)	June 30,		December 31,
	2020		2019
Accrued payroll and related expenses	$	5,352	$	9,151
Provision for sales returns	920		2,419
Accrued professional services	593		682
Operating lease liability	809		769
Accrued royalty expense	333		470
Deferred revenue	335		304
Accrued travel expenses	273		431
Accrued clinical expenses	181		241
Accrued other expenses	346		567
Total	$	9,142	$	15,034

Long-term Debt (Tables)

6 Months Ended

Jun. 30, 2020

Debt Disclosure [Abstract]

Future Maturities Under the Term Loan Agreement

Future maturities under the term loan agreement as of June 30, 2020 are as follows (in thousands):


Period Ending December 31:	Amount
2020	$	2,240
2021	4,442
2022	48,255
	54,937
Add: Accretion of closing fees	1,369
	56,306
Less: Amount representing interest	(11,124)
Less: Amount representing debt discount and debt issuance costs	(116)
Present value of minimum payments	$	45,066

Commitments and Contingencies (Tables)

6 Months Ended

Jun. 30, 2020

Commitments and Contingencies Disclosure [Abstract]

Balance Sheet Information

Balance sheet information as of June 30, 2020 consists of the following (in thousands):


Operating Lease:	June 30, 2020
Operating lease right-of-use asset in other non-current assets	$	3,105
Operating lease liability in accrued liabilities	$	809
Operating lease liability in other liabilities	3,288
Total operating lease liabilities	$	4,097

Operating Lease Maturities

The following table summarizes the Company’s operating lease maturities as of June 30, 2020 (in thousands):


Period Ending December 31:	Amount
2020	$	525
2021	1,066
2022	1,096
2023	1,127
2024	904
Total lease payments	4,718
Less: imputed interest	(621)
Present value of lease liabilities	$	4,097

Stock Option Plans (Tables)

6 Months Ended

Jun. 30, 2020

Share-based Payment Arrangement [Abstract]

Activity Under Stock Option Plans

Stock option activity under the Company’s Plans is set forth below:


		Options Outstanding
	Shares Available for Grant	Number of Shares	Weighted Average Exercise Price		Weighted Average Remaining Contractual Term (in Years)	Aggregate Intrinsic Value (in thousands)





Balances, December 31, 2019	1,554,690	4,310,790	$	7.91	7.27	$	140,234
Authorized	1,250,210
Options granted	(752,258)	752,258	$	33.86
Options exercised	—	(405,837)	$	2.16
Options cancelled	24,345	(24,345)	$	17.28
Balances, June 30, 2020	2,076,987	4,632,866	$	12.57	7.42	$	136,505
Vested and exercisable at June 30, 2020		2,554,809	$	5.95	6.40	$	91,917
Vested and expected to vest at June 30, 2020		4,632,866	$	12.57	7.42	$	136,505

Schedule of Share-based Compensation, Restricted Stock Units Award Activity

The fair value of RSUs is based on the Company’s closing stock price on the date of grant. A summary of RSUs activity for the six months ended June 30, 2020 is as follows:


	Number of Restricted Stock Units	Weighted Average Grant Date Fair Value
Balances, December 31, 2019	—	$	—
Restricted stock granted	39,089	$	31.18
Restricted stock vested	—	$	—
Restricted stock forfeited	—	$	—
Balances, June 30, 2020	39,089	$	31.18
Expected to vest at June 30, 2020	39,089	$	31.18

Assumptions Used for Estimating Fair Value of Employee Stock Options

The fair value of employee and nonemployee stock options was estimated using the following assumptions for the three and six months ended June 30, 2020 and 2019:


	Three Months Ended June 30,			Six Months Ended June 30,
	2020	2019	2020	2019
Expected term (in years)	5.25 - 6.25	5.00 - 6.25	5.25 - 6.25	5.00 - 6.25
Expected volatility	46.6% - 47.7%	42.7% - 42.9%	42.9% - 47.7%	42.7% - 42.9%
Risk-free interest rate	0.42% - 0.56%	1.86% - 2.37%	0.42% - 1.41%	1.86% - 2.54%
Dividend yield	—%	—%	—%	—%

Schedule of Fair Value Assumptions


	Three Months Ended June 30,			Six Months Ended June 30,
	2020	2019	2020	2019
Expected term (in years)	0.50	0.63	0.50	0.63
Expected volatility	76.4%	47.8%	44.4% - 76.4%	47.8%
Risk-free interest rate	0.14%	2.45%	0.14% - 1.58%	2.45%
Dividend yield	—%	—%	—%	—%

Stock-Based Compensation Expense Relating to Stock Options to Employees and Nonemployees


	Three Months Ended June 30,						Six Months Ended June 30,
	2020		2019		2020		2019
Cost of goods sold	$	89	$	54	$	153	$	69
Research and development expenses	246		127		423		155
Selling, general and administrative expenses	1,319		655		2,371		875
	$	1,654	$	836	$	2,947	$	1,099

Formation and Business of the Company (Details) $ / shares in Units, $ in Thousands		1 Months Ended			3 Months Ended
	Mar. 13, 2019	May 31, 2020 USD ($) $ / shares shares	Aug. 31, 2019 $ / shares shares	Apr. 30, 2019 USD ($) $ / shares shares	Jun. 30, 2020 USD ($) $ / shares	Jun. 30, 2019 USD ($)	Oct. 31, 2015 $ / shares
Subsequent Event [Line Items]
Reverse stock split	0.3704
Payments of stock issuance costs					$ 4,457	$ 10,961
IPO
Subsequent Event [Line Items]
Number of shares issued in transaction (in shares) \| shares				6,000,000
Stock issued, price per share (in USD per share) \| $ / shares				$ 20.00	$ 20.00		$ 20.00
Proceeds from stock issued, net of issuance costs				$ 109,119
Underwriting discounts and commissions				8,400
Payments of stock issuance costs				$ 2,481
Second Public Offering
Subsequent Event [Line Items]
Number of shares issued in transaction (in shares) \| shares			4,200,000
Stock issued, price per share (in USD per share) \| $ / shares			$ 39.50
Public Stock Offering
Subsequent Event [Line Items]
Number of shares issued in transaction (in shares) \| shares		6,808,154
Stock issued, price per share (in USD per share) \| $ / shares		$ 39.00
Public Stock Offering - Shares From Company
Subsequent Event [Line Items]
Number of shares issued in transaction (in shares) \| shares		1,923,076
Public Stock Offering - Shares From Existing Shareholders
Subsequent Event [Line Items]
Number of shares issued in transaction (in shares) \| shares		4,885,078
Over-Allotment Option
Subsequent Event [Line Items]
Number of shares issued in transaction (in shares) \| shares		1,021,223	630,000
Proceeds from stock issued, net of issuance costs		$ 70,543
Underwriting discounts and commissions		3,750
Payments of stock issuance costs		$ 707

Summary of Significant Accounting Policies - Adjustment to Prior Period Financial Statements (Details) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Increase in accounts receivable	$ (857)	$ 1,622
Increase in accrued liabilities	$ (5,925)	978
Restatement Adjustment
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Increase in accounts receivable		601
Increase in accrued liabilities		$ 601

Summary of Significant Accounting Policies - Cash, Cash Equivalents and Restricted Cash (Details) - USD ($) $ in Thousands	Jun. 30, 2020	Dec. 31, 2019
Accounting Policies [Abstract]
Restricted cash	$ 310	$ 310

Summary of Significant Accounting Policies - Redeemable Convertible Preferred Stock (Details) $ in Thousands	2 Months Ended
	Jun. 30, 2019 USD ($) shares
Class of Stock [Line Items]
Conversion of preferred stock to common stock upon IPO \| $	$ 144,140
Common Stock
Class of Stock [Line Items]
Conversion of preferred stock to common stock upon IPO (in shares) \| shares	23,178,555

Summary of Significant Accounting Policies - Revenue Recognition (Details) - USD ($) $ in Thousands	Jun. 30, 2020	Dec. 31, 2019
Accounting Policies [Abstract]
Unbilled receivables	$ 179	$ 128

Summary of Significant Accounting Policies - Schedule of Earnings Per Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Accounting Policies [Abstract]
Net loss	$ (10,353)	$ (11,959)	$ (20,294)	$ (36,117)
Weighted average common stock outstanding used to compute net loss per share, basic and diluted (in shares)	32,682,360	28,458,793	32,010,335	14,905,052
Net loss per share, basic and diluted (in USD per share)	$ (0.32)	$ (0.42)	$ (0.63)	$ (2.42)

- Definition

The portion of profit or loss for the period, net of income taxes, which is attributable to the parent.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 220
-SubTopic 10
-Section 45
-Paragraph 1A
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Reference 2: http://www.xbrl.org/2003/role/exampleRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 280
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-Section 50
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Reference 3: http://www.xbrl.org/2003/role/disclosureRef
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-SubTopic 10
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-Subparagraph (b)
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Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 220
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-04(22))
-URI http://asc.fasb.org/extlink&oid=120399700&loc=SL114874048-224260

Reference 5: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 250
-SubTopic 10
-Section 50
-Paragraph 11
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=109234566&loc=d3e22694-107794

Reference 6: http://www.xbrl.org/2003/role/disclosureRef
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-Name Accounting Standards Codification
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-URI http://asc.fasb.org/extlink&oid=120311839&loc=d3e8933-108599

Reference 7: http://www.xbrl.org/2003/role/disclosureRef
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-Name Accounting Standards Codification
-Topic 205
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-Section 50
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-URI http://asc.fasb.org/extlink&oid=109222650&loc=SL51721683-107760

Reference 8: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
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-URI http://asc.fasb.org/extlink&oid=109234566&loc=d3e22595-107794

Reference 9: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 220
-SubTopic 10
-Section 45
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+ Details

Summary of Significant Accounting Policies - Schedule of Potentially Dilutive Securities Not Included in Calculation of Earnings per Share (Details) - shares	6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of earnings per share (in shares)	4,671,955	4,683,811
Common stock options
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of earnings per share (in shares)	4,632,866	4,683,811
Restricted stock units
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of earnings per share (in shares)	39,089	0

Summary of Significant Accounting Policies - Segment and Geographical Information (Details)	6 Months Ended
	Jun. 30, 2020 segment
Accounting Policies [Abstract]
Number of operating segments	1
Number of reportable segments	1

Fair Value Measurements - Recurring Assets and Liabilities (Details) - USD ($) $ in Thousands	Jun. 30, 2020	Dec. 31, 2019
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets, gross	$ 157,340	$ 104,095
Money market funds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets, gross	72,924	34,363
U.S. treasury bills
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets, gross	9,993
Corporate bonds/notes
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets, gross	10,742	10,176
Asset-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets, gross	2,002	5,181
Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets	157,340	104,095
Recurring \| Money market funds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets, gross	72,924	34,363
Recurring \| U.S. treasury bills
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets, gross	9,993
Recurring \| Commercial paper
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets, gross	11,965	9,919
Recurring \| Corporate bonds/notes
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets, gross	10,742	10,176
Recurring \| U.S. government securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets, gross	49,714	44,456
Recurring \| Asset-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets, gross	2,002	5,181
Recurring \| Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets	72,924	34,363
Recurring \| Level 1 \| Money market funds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets, gross	72,924	34,363
Recurring \| Level 1 \| U.S. treasury bills
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets, gross	0
Recurring \| Level 1 \| Commercial paper
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets, gross	0	0
Recurring \| Level 1 \| Corporate bonds/notes
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets, gross	0	0
Recurring \| Level 1 \| U.S. government securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets, gross	0	0
Recurring \| Level 1 \| Asset-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets, gross	0	0
Recurring \| Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets	84,416	69,732
Recurring \| Level 2 \| Money market funds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets, gross	0	0
Recurring \| Level 2 \| U.S. treasury bills
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets, gross	9,993
Recurring \| Level 2 \| Commercial paper
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets, gross	11,965	9,919
Recurring \| Level 2 \| Corporate bonds/notes
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets, gross	10,742	10,176
Recurring \| Level 2 \| U.S. government securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets, gross	49,714	44,456
Recurring \| Level 2 \| Asset-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets, gross	2,002	5,181
Recurring \| Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets	0	0
Recurring \| Level 3 \| Money market funds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets, gross	0	0
Recurring \| Level 3 \| U.S. treasury bills
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets, gross	0
Recurring \| Level 3 \| Commercial paper
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets, gross	0	0
Recurring \| Level 3 \| Corporate bonds/notes
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets, gross	0	0
Recurring \| Level 3 \| U.S. government securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets, gross	0	0
Recurring \| Level 3 \| Asset-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets, gross	$ 0	$ 0

Balance Sheet Components - Fair Value of Available-For-Sale Investments (Details) - USD ($) $ in Thousands	Jun. 30, 2020	Dec. 31, 2019
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 157,024	$ 104,093
Gross Unrealized Gains	317	9
Gross Unrealized Losses	(1)	(7)
Estimated Fair Value	157,340	104,095
Money market funds
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	72,924	34,363
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Estimated Fair Value	72,924	34,363
U.S. treasury bills
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	9,992
Gross Unrealized Gains	1
Gross Unrealized Losses	0
Estimated Fair Value	9,993
Commercial paper
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	11,965	9,919
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Estimated Fair Value	11,965	9,919
Corporate bonds/notes
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	10,709	10,180
Gross Unrealized Gains	34	0
Gross Unrealized Losses	(1)	(4)
Estimated Fair Value	10,742	10,176
U.S. government securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	49,433	44,450
Gross Unrealized Gains	281	9
Gross Unrealized Losses	0	(3)
Estimated Fair Value	49,714	44,456
Asset-backed securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	2,001	5,181
Gross Unrealized Gains	1	0
Gross Unrealized Losses	0	0
Estimated Fair Value	2,002	5,181
Cash equivalents
Debt Securities, Available-for-sale [Line Items]
Estimated Fair Value	72,924	34,363
Short-term investments
Debt Securities, Available-for-sale [Line Items]
Estimated Fair Value	$ 84,416	51,508
Long-term investments
Debt Securities, Available-for-sale [Line Items]
Estimated Fair Value		$ 18,224

Balance Sheet Components - Fair Value of Cash Equivalents, Short-Term and Long-Term Investments (Details) - USD ($) $ in Thousands	Jun. 30, 2020	Dec. 31, 2019
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Amounts maturing within one year	$ 157,340	$ 85,871
Amounts maturing after one year through two years	0	18,224
Total	$ 157,340	$ 104,095

Balance Sheet Components - Narrative (Details) - USD ($)	Jun. 30, 2020	Dec. 31, 2019
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Weighted average days to maturity	164 days
Work-in-process	$ 0	$ 0

Balance Sheet Components - Available-for-Sale Investments in Unrealized Loss Position (Details) - USD ($) $ in Thousands	Jun. 30, 2020	Dec. 31, 2019
Debt Securities, Available-for-sale [Line Items]
Less than 12 months, Fair Value	$ 2,558	$ 29,195
Less than 12 months, Unrealized Loss	(1)	(7)
Corporate bonds/notes
Debt Securities, Available-for-sale [Line Items]
Less than 12 months, Fair Value	2,558	10,128
Less than 12 months, Unrealized Loss	(1)	(4)
U.S. government securities
Debt Securities, Available-for-sale [Line Items]
Less than 12 months, Fair Value	0	19,067
Less than 12 months, Unrealized Loss	$ 0	$ (3)

Balance Sheet Components - Schedule of Inventory (Details) - USD ($) $ in Thousands	Jun. 30, 2020	Dec. 31, 2019
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Raw materials	$ 1,736	$ 1,203
Finished products	9,885	9,119
Total	$ 11,621	$ 10,322

Balance Sheet Components - Accrued Liabilities (Details) - USD ($) $ in Thousands	Jun. 30, 2020	Dec. 31, 2019
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Accrued payroll and related expenses	$ 5,352	$ 9,151
Provision for sales returns	920	2,419
Accrued professional services	593	682
Operating lease liability	809	769
Accrued royalty expense	333	470
Deferred revenue	335	304
Accrued travel expenses	273	431
Accrued clinical expenses	181	241
Accrued other expenses	346	567
Total	$ 9,142	$ 15,034

Long-term Debt - Term Loan Narrative (Details) - USD ($)	1 Months Ended	6 Months Ended	12 Months Ended
	Sep. 30, 2018	Jun. 30, 2020	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	May 01, 2019	Apr. 30, 2017	Oct. 31, 2015
Debt Instrument [Line Items]
Term loan agreement amount	$ 15,000,000
Debt drawn down		$ 56,306,000
Minimum liquidity amount		$ 3,000,000
Minimum net revenue target				$ 30,000,000	$ 15,000,000	$ 5,000,000	$ 1,000,000
Equity cure period		90 days
Scenario, Forecast
Debt Instrument [Line Items]
Minimum net revenue target			$ 40,000,000
Term Loan
Debt Instrument [Line Items]
Term loan agreement amount	$ 25,000,000									$ 30,000,000
Stated interest rate	10.75%							10.00%
Interest rate paid-in-kind	2.75%							2.00%
Interest rate paid in cash	8.00%							8.00%
Percentage of outstanding loans	25.00%
Market capitalization (greater than)	$ 250,000,000
Term Loan, Tranche A
Debt Instrument [Line Items]
Term loan agreement amount										$ 20,000,000
Financing fee										1.75%
Facility fee										5.00%
Term Loan, Tranche B
Debt Instrument [Line Items]
Term loan agreement amount										$ 10,000,000
Debt drawn down									$ 5,000,000

Long-term Debt - Future Maturities Under the Term Loan (Details) $ in Thousands	Jun. 30, 2020 USD ($)
Debt Disclosure [Abstract]
2020	$ 2,240
2021	4,442
2022	48,255
Long-term debt, gross before accretion of closing fees	54,937
Add: Accretion of closing fees	1,369
Long-term debt, gross	56,306
Less: Amount representing interest	(11,124)
Less: Amount representing debt discount and debt issuance costs	(116)
Present value of minimum payments	$ 45,066

Long-term Debt - Series C Narrative (Details) - $ / shares	1 Months Ended
	Apr. 30, 2019	Jul. 31, 2017	Oct. 31, 2015	Jun. 30, 2020
IPO
Class of Stock [Line Items]
Temporary equity, shares issued (in shares)	6,000,000
Stock issued, price per share (in USD per share)	$ 20.00		$ 20.00	$ 20.00
Preferred Stock Warrants
Class of Stock [Line Items]
Warrants outstanding (in shares)			163,877
Series C Preferred Stock
Class of Stock [Line Items]
Temporary equity, shares issued (in shares)		163,877	327,759
Stock issued, price per share (in USD per share)		$ 6.11	$ 6.11

Commitments and Contingencies - Lease Narrative (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Commitments and Contingencies Disclosure [Abstract]
Renewal term	5 years		5 years
Operating lease costs	$ 217	$ 217	$ 435,000	$ 435,000
Operating lease liabilities	$ 188	$ 169,000	$ 372	$ 363
Weighted average discount rate	6.50%		6.50%
Weighted average remaining lease term	4 years 3 months 29 days		4 years 3 months 29 days

Commitments and Contingencies - Balance Sheet Information (Details) - USD ($) $ in Thousands	Jun. 30, 2020	Dec. 31, 2019
Commitments and Contingencies Disclosure [Abstract]
Operating lease right-of-use asset in other non-current assets	$ 3,105
Operating lease liability in accrued liabilities	809	$ 769
Operating lease liability in other liabilities	3,288
Total operating lease liabilities	$ 4,097

Commitments and Contingencies - Operating Lease Maturities (Details) $ in Thousands	Jun. 30, 2020 USD ($)
Commitments and Contingencies Disclosure [Abstract]
2020	$ 525
2021	1,066
2022	1,096
2023	1,127
2024	904
Total lease payments	4,718
Less: imputed interest	(621)
Present value of lease liabilities	$ 4,097

Commitments and Contingencies - Purchase Obligation Narrative (Details) $ in Thousands	Jun. 30, 2020 USD ($)
Commitments and Contingencies Disclosure [Abstract]
Purchase obligation	$ 6,640

Redeemable Convertible Preferred Stock - Narrative (Details) - $ / shares	6 Months Ended
	Jun. 30, 2020	Dec. 31, 2019	Jun. 30, 2019	Apr. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Oct. 31, 2015
Temporary Equity [Line Items]
Temporary equity, shares outstanding (in shares)			0		21,238,105	21,233,190
IPO
Temporary Equity [Line Items]
Stock issued, price per share (in USD per share)	$ 20.00			$ 20.00			$ 20.00
IPO \| Preferred Stock Warrants
Temporary Equity [Line Items]
Shares converted (in shares)	1,945,365
Convertible Preferred Stock
Temporary Equity [Line Items]
Temporary equity, shares issued (in shares)	0	0
Temporary equity, shares outstanding (in shares)	0	0

Stockholders Equity - Narrative (Details)	6 Months Ended
Stockholders Equity - Narrative (Details)	Jun. 30, 2020 USD ($) vote $ / shares shares	Dec. 31, 2019 $ / shares shares	Apr. 30, 2019 $ / shares	Oct. 31, 2015 $ / shares
Class of Warrant or Right [Line Items]
Preferred stock authorized (in shares)	5,000,000	5,000,000
Preferred stock par value (in USD per share) \| $ / shares	$ 0.001	$ 0.001
Preferred stock issued (in shares)	0	0
Preferred stock outstanding (in shares)	0	0
Shares authorized (in shares)	100,000,000	100,000,000
Par value (in USD per share) \| $ / shares	$ 0.001	$ 0.001
Shares issued (in shares)	33,610,099	31,255,267
Shares outstanding (in shares)	33,610,099	31,255,267
Dividends declared \| $	$ 0
Number of votes entitled per share of common stock \| vote	1
IPO
Class of Warrant or Right [Line Items]
Stock issued, price per share (in USD per share) \| $ / shares	$ 20.00		$ 20.00	$ 20.00
IPO \| Common Stock Warrant
Class of Warrant or Right [Line Items]
Shares converted (in shares)	5,968

Stock Option Plans - Narrative (Details) - USD ($) $ in Thousands	1 Months Ended	6 Months Ended
	Mar. 31, 2019	Jun. 30, 2020
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Aggregate intrinsic value of options exercised		$ 15,048
Compensation expensed not yet recognized		$ 16,986
ISO
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Exercise threshold as a percentage of fair value of shares		100.00%
NSO
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Exercise threshold as a percentage of fair value of shares		85.00%
ISO and NSO
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Option term		10 years
Vesting term		4 years
ISO and NSO \| 10% Stockholders
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Exercise threshold as a percentage of fair value of shares		110.00%
Restricted stock units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting term		4 years
Restricted stock vested (in shares)		0
Compensation expensed not yet recognized, period for recognition		3 years 8 months 4 days
Unrecognized compensation costs of unvested RSUs		$ 1,119
Stock Options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of additional shares allowable under the plan (in shares)	1,200,000
Percent of outstanding shares of common stock	1.00%
Common stock reserved for future issuance (in shares)	434,000	746,552
Percent of purchase of price of common stock		85.00%
Share of stock issued in ESPP (in shares)		87,567
Shares available for future issuance (in shares)		658,985
Compensation expensed not yet recognized, period for recognition		3 years 2 months 26 days
ESPP
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Compensation expensed not yet recognized, period for recognition		4 months 24 days
Unrecognized compensation costs related to the ESPP		$ 303
2019 Stock Option Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares of common stock reserved for issuance (in shares)	2,317,000	3,631,205
Percent of outstanding shares of common stock	4.00%
2019 Stock Option Plan \| Maximum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of additional shares allowable under the plan (in shares)	3,000,000

Stock Option Plans - Activity Under Compensation Plan (Details) $ / shares in Units, $ in Thousands	6 Months Ended	12 Months Ended
	Jun. 30, 2020 USD ($) $ / shares shares	Dec. 31, 2019 USD ($) $ / shares shares
Shares Available for Grant
Beginning balance (in shares)	1,554,690
Authorized (in shares)	1,250,210
Options granted (in shares)	(752,258)
Options cancelled (in shares)	24,345
Ending balance (in shares)	2,076,987	1,554,690
Number of Shares
Beginning balance (in shares)	4,310,790
Options granted (in shares)	752,258
Options exercised (in shares)	(405,837)
Options cancelled (in shares)	(24,345)
Ending balance (in shares)	4,632,866	4,310,790
Weighted Average Exercise Price
Beginning balance (in USD per share) \| $ / shares	$ 7.91
Options granted (in USD per share) \| $ / shares	33.86
Options exercised (in USD per share) \| $ / shares	2.16
Options cancelled (in USD per share) \| $ / shares	17.28
Ending balance (in USD per share) \| $ / shares	$ 12.57	$ 7.91
Weighted Average Remaining Contractual Term (in Years), awards outstanding	7 years 5 months 1 day	7 years 3 months 7 days
Aggregate Intrinsic Value, awards outstanding \| $	$ 136,505	$ 140,234
Number of Shares, vested and exercisable (in shares)	2,554,809
Weighted Average Exercise Price, vested and exercisable (in USD per share) \| $ / shares	$ 5.95
Weighted Average Remaining Contractual Term (in Years), vested and exercisable	6 years 4 months 24 days
Aggregate Intrinsic Value, vested and exercisable \| $	$ 91,917
Number of Shares, vested and expect to vest (in shares)	4,632,866
Weighted Average Exercise Price, vested and expected to vest (in USD per share) \| $ / shares	$ 12.57
Weighted Average Remaining Contractual Term (in Years), vested and expected to vest	7 years 5 months 1 day
Aggregate Intrinsic Value, vested and expected to vest \| $	$ 136,505

Stock Option Plans - RSUs (Details) - Restricted stock units	6 Months Ended
	Jun. 30, 2020 $ / shares shares
Number of Restricted Stock Units
Beginning balance (in shares) \| shares	0
Restricted stock granted (in shares) \| shares	39,089
Restricted stock vested (in shares) \| shares	0
Restricted stock forfeited (in shares) \| shares	0
Ending balance (in shares) \| shares	39,089
Weighted Average Grant Date Fair Value
Beginning balance (in USD per share) \| $ / shares	$ 0
Restricted stock granted (in USD per share) \| $ / shares	31.18
Restricted stock vested (in USD per share) \| $ / shares	0
Restricted stock forfeited (in USD per share) \| $ / shares	0
Ending balance (in USD per share) \| $ / shares	$ 31.18

Stock Option Plans - Fair Value of Stock Options (Details)	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected volatility, minimum	46.60%	42.70%	42.90%	42.70%
Expected volatility, maximum	47.70%	42.90%	47.70%	42.90%
Risk-free interest rate, minimum	0.42%	1.86%	0.42%	1.86%
Risk-free interest rate, maximum	0.56%	2.37%	1.41%	2.54%
Dividend yield	0.00%	0.00%	0.00%	0.00%
Minimum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected term (in years)	5 years 3 months	5 years	5 years 3 months	5 years
Maximum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected term (in years)	6 years 3 months	6 years 3 months	6 years 3 months	6 years 3 months
ESPP
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected term (in years)	6 months	7 months 17 days	6 months	7 months 17 days
Expected volatility	76.40%	47.80%		47.80%
Expected volatility, minimum			44.40%
Expected volatility, maximum			76.40%
Risk-free interest rate	0.14%	2.45%		2.45%
Risk-free interest rate, minimum			0.14%
Risk-free interest rate, maximum			1.58%
Dividend yield	0.00%	0.00%	0.00%	0.00%

Stock Option Plans - Stock-based Compensation (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Stock-based compensation expense	$ 1,654	$ 836	$ 2,947	$ 1,099
Cost of goods sold
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Stock-based compensation expense	89	54	153	69
Research and development expenses
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Stock-based compensation expense	246	127	423	155
Selling, general and administrative expenses
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Stock-based compensation expense	$ 1,319	$ 655	$ 2,371	$ 875

401(k) Plan (Details) - USD ($) $ in Thousands		3 Months Ended	6 Months Ended
401(k) Plan (Details) - USD ($) $ in Thousands	Jan. 01, 2020	Jun. 30, 2020	Jun. 30, 2020
Retirement Benefits [Abstract]
Percentage employee contribution			90.00%
Employer contribution match percentage	50.00%
Percent of matching contribution of the employee's pay	5.00%
Employer discretionary contribution		$ 229,000	$ 522,000