Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Cash flows from operating activities:
Net loss	$ (28,794)	$ (22,425)	$ (21,092)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation expense	221	278	463
Noncash interest expense	584	523	712
Noncash loss on extinguishment of debt			1,469
Amortization of intangible assets	304	304	785
Inventory excess and obsolescence charge	1,327	1,591	2,224
Change in fair value of warrants		5	(244)
Stock‑based compensation expense	2,058	457	216
Gain on sale of equipment			(2)
Change in operating assets and liabilities:
Accounts receivable	149	(1,528)	(541)
Inventory	(620)	(1,839)	(4,757)
Prepaid expenses and other assets	66	(1,977)	99
Restricted cash			(24)
Accounts payable	(2,002)	(773)	1,914
Accrued expenses and other liabilities	2,321	(118)	(1,194)
Foreign currency remeasurement loss	(70)	(21)
Net cash used in operating activities	(24,456)	(25,523)	(19,924)
Cash flows from investing activities:
Purchases of short-term investments		(9,284)
Proceeds from the sale and maturity of short-term investments	9,289
Payment for intangible asset		(2,500)	(1,500)
Purchase of property and equipment	(167)	(197)	(62)
Proceeds from the sale of property and equipment			4
Net cash provided by (used in) investing activities	9,122	(11,981)	(1,558)
Cash flows from financing activities:
Proceeds from underwritten public offering, net of underwriting discounts, commissions and offering costs		51,151
Proceeds from initial public offering, net of underwriting discounts, commissions and offering costs	44,722
Proceeds from issuance of long-term debt with related party			30,000
Proceeds from issuance of long‑term debt and preferred stock warrants			8,000
Repayment of long-term debt			(13,000)
Borrowings under revolving credit facility			5,732
Repayments of revolving credit facility			(5,732)
Proceeds from issuance of Series B redeemable convertible preferred stock, net of offering costs		14,367	3,978
Payment of initial public offering costs	(522)
Payment of deferred financing costs			(1,569)
Proceeds from exercise of stock options	175	14	5
Proceeds from issuance of common stock under the employee stock purchase plan	34
Net cash provided by financing activities	44,409	65,532	27,414
Effect of exchange rate on cash	17	(4)
Net increase (decrease) in cash and cash equivalents	29,092	28,024	5,932
Cash and cash equivalents, beginning of period	45,302	17,278	11,346
Cash and cash equivalents, end of period	74,394	45,302	17,278
Supplemental disclosure of cash flow information:
Cash paid during the period for interest	2,980	3,086	1,090
Cash paid on loss on extinguishment of debt			353
Supplemental disclosures of noncash investing and financing activities:
Fair value of warrants issued in connection with equity and debt financing			187
Accretion of redeemable convertible preferred stock		7,783	8,823
Conversion of convertible preferred stock to common stock in connection with the initial public offering		146,300
Conversion of outstanding preferred stock warrants		1,645
Offering costs in accounts payable and accrued expense and other current liabilities		522
Property and equipment in accounts payable	3
Intangible assets in accrued expenses and other liabilities			2,500
Recognition of exit fee for debt discount			3,400
Issuance of common stock for early exercised stock options	3	4	$ 5
Unrealized loss on short-term investments		$ 4
Liability classified stock-based compensation in accrued expenses and other current liabilities	$ 82