Condensed Consolidated Statements of Cash Flows (Unaudited) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Cash flows from operating activities
Net loss	$ (6,443)	$ (7,298)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	304	80
Amortization of debt discount and deferred issuance cost	124	33
Inventory write-down and reserves	55	786
Stock-based compensation	210	123
Changes in operating assets and liabilities:
Accounts receivable	(96)	(72)
Inventory	(2,813)	(1,087)
Prepaid expenses and other assets	(119)	(188)
Accounts payable	982	533
Accrued expenses	(621)	518
Net cash used in operating activities	(8,417)	(6,572)
Cash flows from investing activities
Purchase of short-term investments		(24,953)
Maturities of short-term investments	5,987
Purchase of property and equipment	(550)	(370)
Net cash used in investing activities	(5,437)	(25,323)
Cash flows from financing activities
Proceeds from debt issuance, net of issuance costs
Principal payments on notes payable		(375)
Proceeds from issuance of common stock, net of issuance costs		32,649
Proceeds from exercise of stock options		36
Net cash provided by financing activities		32,310
Net decrease in cash and cash equivalents	(2,980)	415
Cash and cash equivalents — beginning of period	4,162	11,477
Cash and cash equivalents — end of period	1,182	11,892
Supplemental disclosure:
Cash paid for interest	423	14
Cash paid for taxes		$ 1