CONDENSED STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017
Operating activities
Net loss	$ (12,357)	$ (9,237)
Adjustments to reconcile net loss:
Depreciation and amortization	188	99
Accretion of debt discount	339	146
Stock-based compensation expense	324	108
Change in the fair value of preferred stock warrants	595
Changes in operating assets and liabilities:
Accounts receivable	(630)	(424)
Inventories	122	345
Prepaid expenses and other assets	(607)	(69)
Accounts payable	(753)	177
Accrued expenses	227	(532)
Net cash used in operating activities	(12,552)	(9,387)
Investing activities
Purchases of property and equipment	(59)	(226)
Purchases of short-term investments	(5,138)
Proceeds from sales or maturities of short-term investments	10,220
Net cash provided by (used in) investing activities	5,023	(226)
Financing activities
Proceeds from issuance of notes payable	8,000	458
Proceeds from the exercise of stock options	386	9
Proceeds from sale of common stock	112,041
Proceeds from sale of preferred stock		24,968
Net cash provided by financing activities	120,427	25,435
Increase in cash and cash equivalents	112,898	15,822
Cash and cash equivalents at beginning of period	8,955	6,685
Cash and cash equivalents at end of period	121,853	22,507
Supplemental cash flow information
Cash paid for interest	893	643
Issuance of preferred stock warrants	$ 103	$ 4