Condensed Statements of Cash Flows (unaudited) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Cash flows from operating activities
Net loss	$ (9,941)	$ (24,158)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization expense	184	174
Stock-based compensation expense	1,293	262
Change in fair value of redeemable convertible preferred stock warrant liability	0	15,712
Amortization of premium on investments	79	0
Amortization of debt discount and debt issuance costs	12	11
Amortization of right-of-use asset	146	162
Non-cash interest expense	83	471
Change in provision for doubtful accounts receivable	(9)	(3)
Provision for excess and obsolete inventories	92	27
Changes in assets and liabilities
Accounts receivable	(60)	157
Inventories	(1,871)	(2,339)
Prepaid expenses and other current assets	983	16
Other assets	55	(651)
Accounts payable	1,274	2,287
Accrued liabilities	(3,912)	321
Other liabilities	(203)	(1,069)
Net cash used in operating activities	(11,795)	(8,620)
Cash flows from investing activities
Purchases of property and equipment	(134)	(117)
Purchases of investments	(1,974)	0
Proceeds from maturity of investments	2,000	0
Net cash used in investing activities	(108)	(117)
Cash flows from financing activities
Payment of deferred offering costs	0	1,149
Proceeds from issuance of common stock	274	375
Net cash provided by (used in) financing activities	274	(744)
Net change in cash, cash equivalents and restricted cash	(11,629)	(9,481)
Cash, cash equivalents and restricted cash, beginning of period	39,491	25,300
Cash, cash equivalents and restricted cash, end of period	27,862	15,819
Supplemental disclosure of cash flow information
Cash paid for interest	1,108	870
Non-cash investing and financing activities:
Accounts payable and accrued liabilities for purchases of property and equipment	22	22
Offering costs in accounts payable and accrued liabilities	0	1,204
Right-of-use asset obtained in exchange for lease obligation	0	3,982
Redeemable Convertible Preferred Stock
Cash flows from financing activities
Proceeds from exercise of redeemable convertible preferred stock warrants	$ 0	$ 30