Consolidated Statements of Cash Flows - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019
Cash flows from operating activities:
Net loss	$ (58,080)	$ (41,577)
Adjustments to reconcile net loss to net cash used in operating activities
Stock-based compensation expense	3,491	3,637
Non-cash interest expense	2,156	1,418
Amortization of operating lease asset		330
Change in fair value of derivative liability	20,411	(1,617)
Depreciation and amortization expense	1,441	1,187
Changes in operating assets and liabilities:
Accounts receivable	(326)	(414)
Prepaid expenses and other current assets	173	(581)
Inventory	(113)	(545)
Right of use asset	386
Accounts payable	(786)	774
Accrued expenses	(1,866)	(869)
Operating lease liabilities	(537)	(310)
Net cash used in operating activities	(33,650)	(38,567)
Cash flows from investing activities:
Purchases of property and equipment	(374)	(1,218)
Net cash used in investing activities	(374)	(1,218)
Cash flows from financing activities:
Proceeds from issuance of 2026 convertible notes, net of issuance costs		37,275
Proceeds from exercise of stock options	506	1
Proceeds from the Paycheck Protection Program Loan	3,201
Repayment of the Paycheck Protection Program Loan	(3,201)
Proceeds from issuance of common stock pursuant to employee stock purchase plan	350	294
Proceeds from issuance of common stock upon public offering, net	63,025	10,068
Net cash provided by financing activities	63,881	47,638
Net increase in cash, cash equivalents and restricted cash	29,857	7,853
Cash, cash equivalents and restricted cash at beginning of period	56,201	60,676
Cash, cash equivalents and restricted cash at end of period	86,058	68,529
Supplemental disclosure of non-cash investing and financing activities:
Additional right of use asset and related lease liability		2,044
Additions to property and equipment included in accounts payable and accrued expenses at balance sheet dates	63	265
Derivative liability in connection with issuance of 2026 convertible notes		14,685
Public offering costs included in accounts payable and accrued expenses at balance sheet dates	$ 318	$ 40