Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Cash flows from operating activities:
Net loss	$ (33,179)	$ (20,018)	$ (35,201)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization of property and equipment	432	335	279
Loss on disposal of property and equipment	2
Stock-based compensation	3,367	2,687	1,475
Stock grant in connection with services by a director			175
Forgiveness of notes receivable related to stock option exercise			423
Amortization of discount on convertible notes payable			5,077
Non-cash interest accrued on convertible notes payable			1,952
Write off of unamortized discount on conversion of convertible promissory notes			6,955
Common stock issued for research and development agreement			9
Bad debt expense	258
Excess inventory reserve	4,728
Common stock issued for services	272	324	166
Changes in operating assets and liabilities:
Accounts receivable	955	(793)	(239)
Inventories	10	(2,488)	(3,375)
Prepaid expenses and other assets	378	(127)	(556)
Accounts payable	446	197	199
Accrued expenses	44	656	749
Accrued compensation expenses	(469)	707	216
Accrued clinical trial expenses	13	127	(2)
Deferred revenue	(234)	(278)	531
Deferred grant revenue	(2,093)	(1,878)	4,075
Net cash used in operating activities	(25,070)	(20,549)	(17,092)
Cash flows from investing activities:
Purchases of property and equipment	(490)	(762)	(560)
Proceeds from (investment in) money market funds	5,378	18,279	(25,388)
Net cash provided (used) in investing activities	4,888	17,517	(25,948)
Cash flows from financing activities:
Net proceeds from sale of common stock in rights and initial public offering	19,483		43,295
Proceeds from exercise of options, warrants and employee stock purchase plan options	969	2,883	509
Payment of employment taxes related to stock option exercises		(124)
Net cash provided by financing activities	20,452	2,759	43,804
Effect of exchange rate changes on cash	30	(107)	(207)
Cash:
Net increase (decrease)	300	(380)	557
Balance at beginning of year	239	619	62
Balance at end of year	539	239	619
Non-cash financing and investing activities:
Fair value of stock options issued for services rendered in connection with rights offering	$ 53
Fair value of warrant issued as part of underwriting fee for the Company's initial public offering			2,772
Principal and accrued interest on notes payable converted to common stock			33,196
Common stock issued in connection with finder fees paid on private placements			451
Common stock issued for professional services rendered in connection with initial public offering			$ 170