CONSOLIDATED STATEMENT OF CASH FLOWS - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Cash flows from operating activities
Net loss	$ (50,500)	$ (61,850)	$ (50,601)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	7,088	5,317	3,916
Net amortization/(accretion) of premiums/discounts on investments	(142)	(39)	89
Gain on sale of investment in preferred stock	0	0	(9)
Net amortization/(accretion) of premiums/discounts on investments	938	1,132	388
Stock-based compensation	11,697	12,170	9,236
Provision for Doubtful Accounts	23	14	13
Non-cash inventory adjustments	1,426	1,323	134
Other non-cash adjustments	15	(224)	145
Changes in operating assets and liabilities:
Accounts receivable	(878)	(1,555)	(2,250)
Inventories	(2,414)	(10,512)	(3,450)
Prepaid expenses and other assets	854	(599)	(613)
Accounts payable	(1,389)	2,557	4,105
Accrued compensation	1,059	(263)	2,172
Other current and non-current liabilities	(289)	(893)	1,088
Net cash used in operating activities	(32,512)	(53,422)	(35,637)
Investing activities:
Payments for intellectual property licenses	0	(500)	(1,500)
Purchases of property and equipment	(2,575)	(4,815)	(7,000)
Purchases of marketable securities	(29,778)	(70,989)	(33,688)
Proceeds from sales of marketable securities	0	13,896	8,015
Maturities of marketable securities	66,300	37,500	10,050
Net cash provided by (used in) investing activities	33,947	(24,908)	(24,123)
Financing activities:
Proceeds from issuance of common stock	1,061	87,267	30,920
Costs incurred in conjunction with public offering	0	(5,469)	(1,143)
Principal repayment of borrowings	92	7,848	40
Proceeds from borrowings	7,098	15,000	10,000
Costs associated with debt issuance	(20)	(187)	(90)
Proceeds from stock option exercises	22	287	712
Net cash provided by financing activities	8,069	89,050	40,359
Effect of exchange rate changes on cash	28	75	(25)
Net increase (decrease) in cash and cash equivalents	9,532	10,795	(19,426)
Cash and cash equivalents at beginning of year	27,512	16,717	36,143
Cash and cash equivalents at end of year	37,044	27,512	16,717
Non-cash investing and financing activities:
Transfer of systems from property and equipment into inventory	1,689	4,885	263
Property and equipment costs incurred but not paid included in accounts payable	372	227	1,159
Supplemental cash flow information:
Cash paid for interest	2,028	1,643	1,130
Cash paid for income taxes, net	$ 165	$ 61	$ 65