Condensed Statements of Cash Flows - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Cash flows from operating activities
Net loss	$ (10,694)	$ (9,941)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization expense	239	184
Stock-based compensation expense	3,533	1,293
Amortization of premiums on investments, net	186	79
Amortization of debt discount and debt issuance costs	41	12
Amortization of right-of-use asset	158	146
Non-cash interest expense		83
Change in provision for doubtful accounts receivable		(9)
Provision for excess and obsolete inventories	24	92
Changes in assets and liabilities:
Accounts receivable	(2,036)	(60)
Inventories	(1,790)	(1,871)
Prepaid expenses and other current assets	583	983
Other assets	(6)	55
Accounts payable	218	1,274
Accrued liabilities	(2,423)	(3,912)
Other liabilities	(5)	(203)
Net cash used in operating activities	(11,972)	(11,795)
Cash flows from investing activities
Purchases of property and equipment	(67)	(134)
Purchases of investments		(1,974)
Proceeds from maturity of investments	36,300	2,000
Net cash provided by (used in) investing activities	36,233	(108)
Cash flows from financing activities
Proceeds from issuance of common stock	848	274
Net cash provided by financing activities	848	274
Net change in cash, cash equivalents and restricted cash	25,109	(11,629)
Cash, cash equivalents and restricted cash, beginning of period	69,776	39,491
Cash, cash equivalents and restricted cash, end of period	94,885	27,862
Supplemental disclosure of cash flow information
Cash paid for interest	582	1,108
Noncash investing and financing activities:
Accounts payable and accrued liabilities for purchases of property and equipment	$ 106	$ 22