Condensed Statements of Cash Flows - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019
Cash flows from operating activities
Net loss	$ (30,608)	$ (44,124)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization expense	563	530
Stock-based compensation expense	4,953	2,023
Change in fair value of redeemable convertible preferred stock warrant liability	0	21,030
Amortization of premiums (accretion of discounts) on investments, net	145	0
Amortization of debt discount and debt issuance costs	35	34
Amortization of right-of-use asset	447	440
Non-cash interest expense	249	595
Loss on disposal of property and equipment	51	0
Change in provision for doubtful accounts receivable	(26)	641
Provision for excess and obsolete inventories	109	78
Changes in assets and liabilities
Accounts receivable	(771)	(3,575)
Inventories	(732)	(3,609)
Prepaid expenses and other current assets	(565)	(2,096)
Other assets	200	764
Accounts payable	29	712
Accrued liabilities	(2,054)	2,777
Other liabilities	176	(569)
Net cash used in operating activities	(27,799)	(24,349)
Cash flows from investing activities
Purchases of property and equipment	(587)	(337)
Purchases of investments	(58,884)	0
Proceeds from maturity of investments	37,180	0
Net cash used in investing activities	(22,291)	(337)
Cash flows from financing activities
Proceeds from public offerings, net of underwriting discount, commissions and offering costs paid	70,568	109,352
Proceeds from issuance of common stock	2,809	806
Net cash provided by financing activities	73,377	111,973
Net change in cash, cash equivalents and restricted cash	23,287	87,287
Cash, cash equivalents and restricted cash, beginning of period	39,491	25,300
Cash, cash equivalents and restricted cash, end of period	62,778	112,587
Supplemental disclosure of cash flow information
Cash paid for interest	3,336	3,107
Non-cash investing and financing activities:
Accounts payable and accrued liabilities for purchases of property and equipment	118	9
Offering costs in accounts payable and accrued liabilities	25	0
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 stock warrants	0	1,784
Common Stock Warrant
Cash flows from financing activities
Proceeds from exercise of stock warrants	$ 0	$ 31