STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Operating activities
Net loss	$ (21,828)	$ (17,511)	$ (18,528)
Adjustments to reconcile net loss:
Depreciation and amortization	393	285	103
Accretion of investment discount	(648)	0	0
Accretion of debt discount	568	315	180
Stock-based compensation expense	1,219	243	248
Non-cash stock issuance for services rendered	95	0	0
Change in the fair value of preferred stock warrants	595	100	(195)
Other, net	51	0	0
Changes in operating assets and liabilities:
Accounts receivable	(2,836)	(1,767)	(760)
Inventories	1,003	(315)	612
Prepaid expenses and other assets	(1,326)	(308)	88
Accounts payable	439	1,825	(480)
Accrued expenses	3,581	1,342	783
Net cash used in operating activities	(18,694)	(15,791)	(17,949)
Investing activities
Purchases of property and equipment	(250)	(412)	(306)
Purchases of short-term investments	(115,481)	(8,969)	0
Proceeds from sales or maturities of short-term investments	32,342	1,781	0
Net cash used in investing activities	(83,389)	(7,600)	(306)
Financing activities
Proceeds from issuance of notes payable	8,000	458	0
Proceeds from the exercise of stock options and warrants	558	235	466
Proceeds from sale of common stock	181,825	0	0
Proceeds from sale of preferred stock	0	24,968	12,348
Net cash provided by financing activities	190,383	25,661	12,814
Effect of exchange rate on cash	33	0	0
Increase (decrease) in cash and cash equivalents	88,333	2,270	(5,441)
Cash and cash equivalents at beginning of year	8,955	6,685	12,126
Cash and cash equivalents at end of year	97,288	8,955	6,685
Supplemental cash flow information
Cash paid for interest	2,019	1,323	1,232
Issuance of preferred stock warrants	$ 103	$ 4	$ 0