Condensed Consolidated Statements of Cash Flows - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Operating activities
Net loss	$ (14,703)	$ (13,426)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	694	529
Stock-based compensation expense	1,157	1,290
Non-cash interest expense	639	148
Deferred rent	(53)	(60)
Changes in operating assets and liabilities:
Accounts receivable	(82)	9
Prepaid expenses and other assets	(111)	(56)
Inventories, net	314	(576)
Accounts payable	199	679
Accrued expenses and other liabilities	(494)	36
Deferred revenue	(318)	(409)
Net cash used in operating activities	(12,758)	(11,836)
Investing activities
Purchases and manufacture of property and equipment	(1,594)	(2,158)
Net cash used in investing activities	(1,594)	(2,158)
Financing activities
Payment of offering costs for issuance of common stock in public offering	0	(370)
Proceeds from issuance of common stock and stock options exercises, net	339	301
Payments of issuance costs for notes payable	(354)	0
Repayments of note payable	(299)	(80)
Net cash used in financing activities	(314)	(149)
Net decrease in cash and cash equivalents	(14,666)	(14,143)
Cash and cash equivalents at beginning of period	73,488	73,662
Cash and cash equivalents at end of period	58,822	59,519
Supplemental disclosures of cash flow information
Cash paid for interest	993	517
Supplemental disclosures of noncash investing and financing activities
Accrued property and equipment	$ 61	$ 132