Consolidated Statements of Cash Flows (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Cash flow from operating activities:
Net loss	$ (44,058)	$ (45,215)	$ (37,315)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	818	707	1,150
Provision for excess inventory	1,873	514	0
Stock-based compensation and 401(k) Plan employer match	3,923	3,236	3,270
Fair value adjustment of common stock warrants	(3,791)	(761)	(555)
Amortization of discount on long-term debt	1,948	534	0
Loss on disposal of equipment	0	0	42
Reduction in required restricted cash under lease agreement	100	75	0
Changes in:
Inventory	(1,788)	(431)	(195)
Accounts receivable	67	(67)	0
Prepaid expenses and other current assets	(44)	(58)	(277)
Accounts payable	(1,083)	267	55
Accrued expenses	1,331	626	1,187
Deferred revenue	(96)	139	0
Other assets	0	(115)	0
Other liabilities	(369)	95	(246)
Net cash used in operating activities	(41,169)	(40,454)	(32,884)
Cash flow from investing activities:
Purchase of property and equipment	(780)	(608)	(636)
Net cash used in investing activities	(780)	(608)	(636)
Cash flow from financing activities:
Proceeds from issuance of securities, net of expenses	0	70,774	43,551
Proceeds from issuance of long-term debt	0	30,000	0
Payment of debt issuance costs	0	(450)	0
Proceeds from exercise of common stock warrants and options	457	204	6,747
Principal payments under equipment loans	(80)	(75)	(75)
Net cash provided by financing activities	377	100,453	50,223
Net increase/ (decrease) in cash and cash equivalents	(41,572)	59,391	16,703
Cash and cash equivalents - beginning of year	86,283	26,892	10,189
Cash and cash equivalents - end of year	44,711	86,283	26,892
Supplementary disclosure of cash flows information:
Interest paid	$ 2,630	$ 920	$ 13