Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Cash flows from operating activities
Net loss	$ (52,415)	$ (37,630)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization expense	712	517
Stock-based compensation expense	2,977	911
Change in fair value of redeemable convertible preferred stock warrant liability	21,030	11,906
Accretion of discount on investments	(309)	0
Amortization of debt discount and debt issuance costs	46	68
Amortization of right-of-use asset	582
Non-cash interest expense	672	1,555
Loss on disposal of property and equipment	0	159
Provision for doubtful accounts receivable	23	(123)
Provision for excess and obsolete inventories	118	23
Changes in assets and liabilities
Accounts receivable	(2,241)	(446)
Inventories	(4,696)	(2,565)
Prepaid expenses and other current assets	(1,471)	(1,128)
Other assets	552	(62)
Accounts payable	615	(309)
Accrued liabilities	4,964	5,023
Other liabilities	(769)	406
Net cash used in operating activities	(29,610)	(21,695)
Cash flows from investing activities
Purchase of property and equipment	(535)	(2,276)
Proceeds from sale of property and equipment	0	6
Purchases of investments	(69,421)	0
Net cash used in investing activities	(69,956)	(2,270)
Cash flows from financing activities
Proceeds from initial public offering, net of underwriting discount, commissions and offering costs paid	109,352	(233)
Proceeds from long-term debt	0	15,000
Proceeds from issuance of common stock	2,590	656
Non-controlling interest	0	1
Net cash provided by financing activities	113,757	15,424
Net change in cash, cash equivalents and restricted cash	14,191	(8,541)
Cash, cash equivalents and restricted cash, beginning of year	25,300	33,841
Cash, cash equivalents and restricted cash, end of year	39,491	25,300
Supplemental disclosure of cash flow information
Cash paid for interest	4,234	2,738
Non-cash investing and financing activities:
Accounts payable and accrued liabilities for purchases of property and equipment	32	6
Landlord paid tenant improvements	0	794
Unpaid deferred offering costs	0	717
Right-of-use asset obtained in exchange for lease obligation	3,982
Net exercise of redeemable convertible preferred stock warrants to preferred stock	37,121	0
Conversion of redeemable convertible preferred stock to common stock upon initial public offering	144,140	0
Redeemable convertible preferred stock warrants outstanding
Cash flows from financing activities
Proceeds from exercise of warrants	1,784	0
Common stock warrants outstanding
Cash flows from financing activities
Proceeds from exercise of warrants	$ 31	$ 0