CONDENSED CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018
Cash flows from operating activities
Net loss	$ (17,999)	$ (7,348)
Adjustments to reconcile net loss to net cash used in operating activities:
Stock-based compensation	3,685	754
Depreciation and amortization	395	356
Accretion on marketable securities	(940)	0
Amortization of debt discount	130	130
Change in fair value of redeemable convertible preferred stock warrants	0	224
Loss on sale and disposal of property and equipment	98	48
Changes in operating assets and liabilities:
Accounts receivable	(916)	194
Inventory	(1,092)	(320)
Prepaid expenses and other assets	102	237
Accounts payable	338	(462)
Accrued liabilities and other	725	514
Net cash used in operating activities	(15,474)	(5,673)
Cash flows from investing activities
Maturities of marketable securities	83,600	0
Purchases of marketable securities	(82,516)	0
Purchases of property and equipment	(893)	(715)
Net cash provided by (used in) investing activities	191	(715)
Cash flows from financing activities
Proceeds from the exercise of stock options	567	208
Proceeds from issuance of common stock under employee stock purchase plan	1,263	0
Payments of public offering costs	(167)	0
Net cash provided by financing activities	1,663	208
Effect of exchange rate changes on cash and cash equivalents	(4)	5
Net decrease in cash and cash equivalents	(13,624)	(6,175)
Cash and cash equivalents at
Beginning of period	25,120	22,408
End of period	11,496	16,233
Supplemental disclosure of non-cash information
Vesting of early exercised stock options	122	28
Purchases of property and equipment included in accounts payable and accrued liabilities	$ 57	$ 97