Condensed Consolidated Balance Sheets - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Current assets:
Cash and cash equivalents	$ 28,422	$ 50,805
Accounts receivable	1,179	1,786
Inventories	3,100	2,677
Prepaid expenses and other current assets	713	1,340
Total current assets	33,414	56,608
Property and equipment, net	7,262	7,315
Operating lease right-of-use assets	4,108
Restricted cash	180	180
Other assets	206	206
Total assets	45,170	64,309
Current liabilities:
Notes payable	42,885	42,373
Accounts payable	2,911	744
Accrued expenses and other current liabilities	8,823	6,073
Derivative liability	2,503	2,142
Deferred revenue	677	697
Current portion of lease incentives		268
Total current liabilities	57,799	52,297
Lease incentives, net of current portion		492
Operating lease liabilities, net of current portion	2,893
Deferred revenue, net of current portion	98	133
Commitments and contingencies (see Note 13)
Stockholders’ (deficit) equity:
Preferred stock, $0.001 par value; 10,000,000 shares authorized; no shares issued and outstanding at June 30, 2019 and December 31, 2018
Common stock, $0.001 par value; 200,000,000 shares authorized; 44,535,572 and 44,175,441 shares issued and outstanding at June 30, 2019 and December 31, 2018, respectively	44	44
Additional paid-in capital	332,301	328,514
Accumulated deficit	(347,965)	(317,171)
Total stockholders’ (deficit) equity	(15,620)	11,387
Total liabilities and stockholders’ (deficit) equity	$ 45,170	$ 64,309