Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Cash flows from operating activities:
Net loss	$ (22,425)	$ (21,092)	$ (21,334)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation expense	278	463	761
Noncash interest expense	523	712	2,113
Noncash loss on extinguishment of debt		1,469
Amortization of intangible assets	304	785
Inventory excess and obsolescence charge	1,591	2,224	452
Beneficial conversion feature upon conversion of promissory notes			1,408
Change in fair value of warrants	5	(244)	(54)
Stock-based compensation expense	457	216	197
Loss (gain) on sale of property and equipment		(2)	14
Change in operating assets and liabilities:
Accounts receivable	(1,528)	(541)	(578)
Inventory	(1,839)	(4,757)	(127)
Prepaid expenses and other assets	(1,977)	99	(58)
Restricted cash		24
Accounts payable	(773)	1,914	(748)
Accrued expenses and other liabilities	(118)	(1,194)	1,586
Foreign currency remeasurement gain	(21)
Net cash used in operating activities	(25,523)	(19,924)	(16,368)
Cash flows from investing activities:
Purchases of short-term investments	(9,284)
Payment for intangible asset	(2,500)	(1,500)
Purchase of property and equipment	(197)	(62)	(114)
Proceeds from sale of property and equipment		4	13
Net cash used in investing activities	(11,981)	(1,558)	(101)
Cash flows from financing activities:
Proceeds from initial public offering net of underwriting discounts, commissions and offering costs	51,151
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	5,000
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	14,731
Proceeds from issuance of convertible promissory notes and preferred stock warrants			7,386
Payment of deferred financing costs		(1,569)	(596)
Payment of capital lease obligations			(188)
Proceeds from exercise of stock options	14	5	2
Net cash provided by financing activities	65,532	27,414	26,335
Effect of exchange rate on cash	(4)
Net increase in cash and cash equivalents	28,024	5,932	9,866
Cash and cash equivalents, beginning of year	17,278	11,346	1,480
Cash and cash equivalents, end of year	45,302	17,278	11,346
Supplemental disclosure of cash flow information:
Cash paid during the period for interest	3,086	1,090	329
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	1,751
Accretion of redeemable convertible preferred stock	7,783	8,823	5,893
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	522
Conversion of convertible promissory notes and accrued interest to Series B redeemable convertible preferred stock			8,054
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	4	$ 5	$ 31
Unrealized loss on short-term investments	$ 4