Consolidated Statements of Stockholders' Equity - USD ($) $ in Thousands	Total	Preferred Stock	Common Stock	Additional Paid-in Capital	Accumulated Deficit	Healios License Agreement	Healios License Agreement Common Stock	Healios License Agreement Additional Paid-in Capital
Preferred stock shares, beginning balance (in shares) at Dec. 31, 2015		0
Common stock, beginning balance (in shares) at Dec. 31, 2015			83,720,154
Beginning balance at Dec. 31, 2015	$ 19,724	$ 0	$ 84	$ 322,582	$ (302,942)
Stock-based compensation	2,868			2,868
Issuance of common stock from warrant exercises (in shares)			161,366
Issuance of common stock from warrant exercises	163			163
Issuance of common stock, net of issuance costs (in shares)			2,191,418
Issuance of common stock, net of issuance costs	4,230		$ 2	4,228
Issuance of common stock under equity compensation plans (in shares)			556,364
Issuance of common stock under equity compensation plans	(467)		$ 1	(468)
Net and comprehensive loss	(15,337)				(15,337)
Preferred stock shares, ending balance (in shares) at Dec. 31, 2016		0
Common stock, ending balance (in shares) at Dec. 31, 2016			86,629,302
Ending balance at Dec. 31, 2016	11,181	$ 0	$ 87	329,373	(318,279)
Cumulative effect of accounting change	(110)				(110)
Stock-based compensation	3,154			3,154
Issuance of common stock from warrant exercises (in shares)			1,843,363
Issuance of common stock from warrant exercises	1,862		$ 2	1,860
Issuance of common stock, net of issuance costs (in shares)			33,172,300
Issuance of common stock, net of issuance costs	39,813		$ 33	39,780
Issuance of common stock under equity compensation plans (in shares)			432,488
Issuance of common stock under equity compensation plans	(283)			(283)
Net and comprehensive loss	$ (32,241)				(32,241)
Preferred stock shares, ending balance (in shares) at Dec. 31, 2017	0	0
Common stock, ending balance (in shares) at Dec. 31, 2017	122,077,453		122,077,453
Ending balance at Dec. 31, 2017	$ 23,376	$ 0	$ 122	373,884	(350,630)
Cumulative effect of accounting change	1,871				1,871
Stock-based compensation	3,849			3,849
Issuance of warrant to Healios at fair value	1,080			1,080
Issuance of common stock, net of issuance costs (in shares)			9,658,582				12,000,000
Issuance of common stock, net of issuance costs	$ 16,628		$ 9	16,619		$ 20,995	$ 12	$ 20,983
Issuance of common stock under equity compensation plans (in shares)	5,120,464		556,704
Issuance of common stock under equity compensation plans	$ (400)		$ 1	(401)
Net and comprehensive loss	$ (24,283)				(24,283)
Preferred stock shares, ending balance (in shares) at Dec. 31, 2018	0	0
Common stock, ending balance (in shares) at Dec. 31, 2018	144,292,739		144,292,739
Ending balance at Dec. 31, 2018	$ 43,116	$ 0	$ 144	$ 416,014	$ (373,042)