Note 13 - Quarterly Financial Information (Unaudited) - Quarterly Financial Information (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Revenues	$ 110	$ 111	$ 108	$ 108	$ 109	$ 120	$ 108	$ 75	$ 437	$ 412	$ 304
Gross profit	110	111	108	108	109	120	108	75
Operating loss	(4,008)	(2,414)	(2,105)	(4,113)	(2,321)	(1,930)	(2,606)	(2,353)	$ (12,640)	$ (9,210)	$ (14,009)
Net loss	$ (7,075)	$ (5,992)	$ (13,570)	$ (7,227)	$ (6,724)	$ (5,175)	$ (5,645)	$ (8,795)
Basic and diluted net loss per common share (in dollars per share)	$ (0.31)	$ (0.26)	$ (0.60)	$ (0.33)	$ (0.34)	$ (0.28)	$ (0.31)	$ (0.49)	$ (1.50)	$ (1.41)	$ (1.35)