Note 14 - Quarterly Financial Information (Unaudited) - Quarterly Financial Information (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017
Revenues	$ 111	$ 112	$ 109	$ 108	$ 110	$ 111	$ 108	$ 108	$ 440	$ 437
Gross profit	111	112	109	108	110	111	108	108
Operating loss	(3,992)	(2,488)	(2,255)	(2,486)	(4,008)	(2,414)	(2,105)	(4,113)	$ (11,220)	$ (12,640)
Net loss	$ (8,030)	$ (6,240)	$ (6,032)	$ (5,971)	$ (7,075)	$ (5,992)	$ (13,570)	$ (7,227)
Basic and diluted net loss per common share (in dollars per share)	$ (0.33)	$ (0.26)	$ (0.25)	$ (0.26)	$ (0.31)	$ (0.26)	$ (0.60)	$ (0.33)	$ (1.09)	$ (1.50)