Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Operating activities
Net loss	$ (24,283)	$ (32,241)	$ (15,337)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation	855	684	382
Gain from forgiveness of note payable	0	0	(190)
Stock-based compensation	3,849	3,044	2,868
Discount on revenue from issuance of warrant	1,080	0	0
Other	0	(22)	2
Stock–based patent license and settlement expense	315	3,150	0
Gain from insurance proceeds, net	(617)	0	(682)
Change in fair value of warrant liabilities	0	(728)	557
Changes in operating assets and liabilities:
Accounts receivable	324	12	(237)
Prepaid expenses and other	(1,346)	(206)	(462)
Accounts payable and accrued expenses	4,269	1,537	2,413
Advances and deposits from Healios	4,889	134	0
Deferred revenue	(250)	771	(245)
Net cash used in operating activities	(13,350)	(24,018)	(10,931)
Investing activities
Purchase of available-for-sale securities	0	0	(16,343)
Sales of available-for-sale securities	0	0	16,305
Proceeds from insurance, net	617	0	682
Purchases of equipment	(1,532)	(285)	(1,711)
Net cash used in investing activities	(915)	(285)	(1,067)
Financing activities
Proceeds from issuance of common stock, net	15,415	37,287	4,028
Proceeds from exercise of warrants	0	1,862	163
Shares retained for withholding tax payments on stock-based awards	(402)	(283)	(467)
Net cash provided by financing activities	36,008	38,866	3,724
Increase (decrease) in cash and cash equivalents	21,743	14,563	(8,274)
Cash and cash equivalents at beginning of year	29,316	14,753	23,027
Cash and cash equivalents at end of year	51,059	29,316	14,753
Healios License Agreement
Changes in operating assets and liabilities:
Accounts receivable	(4,545)	(153)	0
Deferred revenue	(2,110)	0	0
Financing activities
Proceeds from issuance of common stock, net	$ 20,995	$ 0	$ 0