Note 12 - Quarterly Financial Information (Unaudited) - Quarterly Financial Information (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Revenues	$ 109	$ 120	$ 108	$ 75	$ 21	$ 227	$ 38	$ 18	$ 412	$ 304	$ 336
Gross profit	109	120	108	75	(22)	(64)	38	18
Operating loss	(2,321)	(1,930)	(2,606)	(2,353)	(3,889)	(3,651)	(3,751)	(2,718)	(9,210)	(14,009)	(10,359)
Net loss	$ (6,724)	$ (5,175)	$ (5,645)	$ (8,794)	$ (7,216)	$ (5,965)	$ (5,990)	$ (4,842)	$ (26,339)	$ (24,013)	$ (18,881)
Basic and diluted net loss per common share (in dollars per share)	$ (0.34)	$ (0.28)	$ (0.31)	$ (0.49)	$ (0.40)	$ (0.33)	$ (0.34)	$ (0.27)	$ (1.41)	$ (1.35)	$ (1.15)