Note 15 - Quarterly Financial Information (Unaudited) - Quarterly Financial Information (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018
Revenues	$ 111	$ 110	$ 111	$ 109	$ 111	$ 112	$ 109	$ 108	$ 441	$ 440
Gross profit	111	110	111	109	111	112	109	108
Operating loss	(2,912)	(2,956)	(3,240)	(2,881)	(3,992)	(2,488)	(2,255)	(2,486)	$ (11,989)	$ (11,220)
Net loss	$ (7,348)	$ (7,444)	$ (7,476)	$ (7,260)	$ (8,030)	$ (6,240)	$ (6,032)	$ (5,971)
Basic and diluted net loss per common share (in dollars per share)	$ (0.27)	$ (0.28)	$ (0.28)	$ (0.29)	$ (0.33)	$ (0.26)	$ (0.25)	$ (0.26)	$ (1.11)	$ (1.09)