Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Operating activities:
Net loss	$ (12,308)	$ (10,310)	$ (6,697)
Adjustments to reconcile net loss to net cash used in operating activities:
Write-off of inventory		597
Depreciation and amortization	78	39	47
Change in fair value of convertible preferred stock warrant liability		(250)	(274)
Change in fair value of maturity date preferred stock warrant liability		(10)	(87)
Change in fair value of convertible shareholder notes derivative liability		1,224	(1,373)
Share-based compensation	2,707	942	283
Non-cash interest expense on convertible shareholder notes		1,736	1,387
Changes in operating assets and liabilities:
Accounts receivable	(21)	33	65
Inventory	(56)	27	(82)
Prepaid expenses and other current assets	16	(110)	(129)
Deferred financing costs			48
Other assets		(11)
Accounts payable	377	(17)	228
Accrued liabilities excluding accrued interest on convertible note	415	530	(416)
Deferred revenue	96	32
Deferred rent	25	26	(23)
Net cash used in operating activities	(8,671)	(5,522)	(7,023)
Investing activities:
Purchase of property and equipment	(136)		(125)
Cash acquired in reverse merger		19,017
Payment of transaction costs of reverse merger		(96)
Purchase of short-term investments	(1,800)
Maturity of short-term investments	1,800
Net provided by (used in) in investing activities	(136)	18,921	(125)
Financing activities:
Proceeds from the exercise of convertible preferred stock warrants			27
Proceeds from issuance of convertible notes and warrants		4,374	7,435
Proceeds from the exercise of common stock options	144	22	59
Net cash provided by financing activities	144	4,396	7,521
Net increase (decrease) in cash and cash equivalents	(8,663)	17,795	373
Cash and cash equivalents at beginning of period	21,352	3,557	3,184
Cash and cash equivalents at end of period	12,689	21,352	3,557
Supplemental disclosure of noncash investing and financing activities:
Exchange of convertible preferred stock warrants for common stock		25
Conversion of convertible shareholder notes and related interest payable		12,156	7,781
Reclassification of convertible shareholder notes derivative liability		2,268
Conversion of preferred stock		$ 46,030