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	$ (47,365)	$ (52,415)	$ (37,630)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization expense	789	712	517
Stock-based compensation expense	7,226	2,977	911
Change in fair value of redeemable convertible preferred stock warrant liability	0	21,030	11,906
Amortization of premiums (accretion of discounts) on investments, net	304	(309)	0
Amortization of debt discount and debt issuance costs	66	46	68
Amortization of right-of-use asset	602	582
Non-cash interest expense	241	672	1,555
Loss on debt extinguishment	1,119	0	0
Loss on disposal of property and equipment	52	0	159
Change in provision for doubtful accounts receivable	(32)	23	(123)
Provision for excess and obsolete inventories	117	118	23
Changes in assets and liabilities
Accounts receivable	(437)	(2,241)	(446)
Inventories	(2,161)	(4,696)	(2,565)
Prepaid expenses and other current assets	250	(1,471)	(1,128)
Other assets	210	552	(62)
Accounts payable	592	615	(309)
Accrued liabilities	146	4,964	5,023
Other liabilities	26	(769)	406
Repayment of interest paid in kind	(3,813)	0	0
Net cash used in operating activities	(42,068)	(29,610)	(21,695)
Cash flows from investing activities
Purchase of property and equipment	(842)	(535)	(2,276)
Proceeds from sale of property and equipment	0	0	6
Purchases of investments	(79,906)	(69,421)	0
Proceeds from maturity of investments	71,355	0	0
Net cash used in investing activities	(9,393)	(69,956)	(2,270)
Cash flows from financing activities
Proceeds from public offerings, net of underwriting discount, commissions and offering costs paid	70,568	109,352	(233)
Proceeds from long-term debt, net	48,506	0	15,000
Proceeds from issuance of common stock	5,168	2,590	656
Principal repayment of long-term debt	(40,000)	0	0
Payments of prepayment penalty and lender fees	(2,496)	0	0
Non-controlling interest	0	0	1
Net cash provided by financing activities	81,746	113,757	15,424
Net change in cash, cash equivalents and restricted cash	30,285	14,191	(8,541)
Cash, cash equivalents and restricted cash, beginning of year	39,491	25,300	33,841
Cash, cash equivalents and restricted cash, end of year	69,776	39,491	25,300
Supplemental disclosure of cash flow information
Cash paid for interest	7,917	4,234	2,738
Non-cash investing and financing activities:
Accounts payable and accrued liabilities for purchases of property and equipment	108	32	6
Landlord paid tenant improvements	0	0	794
Unpaid deferred offering costs	25	0	717
Right-of-use asset obtained in exchange for lease obligation	0	3,982
Net exercise of redeemable convertible preferred stock warrants to preferred stock	0	37,121	0
Conversion of redeemable convertible preferred stock to common stock upon initial public offering	0	144,140	0
Redeemable convertible preferred stock warrants outstanding
Cash flows from financing activities
Proceeds from exercise of warrants	0	1,784	0
Common stock warrants outstanding
Cash flows from financing activities
Proceeds from exercise of warrants	$ 0	$ 31	$ 0