Note 6 - Share-based Compensation - Stock Options Issued Outside of the 2007 Equity Incentive Plan (Details) - Outside of the 2007 Plan [Member] - $ / shares	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Options outstanding, Balance (in shares)	20,018,408	20,148,766
Exercise price per share, Balance (in dollars per share)
Weighted average price per share, Balance (in dollars per share)	$ 0.51	$ 0.40
Options outstanding, granted (in shares)	619,435	583,526
Exercise price per share, granted (in dollars per share)
Weighted average price per share, granted (in dollars per share)	$ 0.26	$ 0.59
Options outstanding, expired (in shares)	(2,400,000)
Exercise price per share,Expired (in dollars per share)	$ 0.99
Weighted average price per share, expired (in dollars per share)	$ 0.99
Options outstanding, Balance (in shares)	18,237,843	20,732,292
Exercise price per share, Balance (in dollars per share)
Weighted average price per share, Balance (in dollars per share)	0.45	0.41
Minimum [Member]
Exercise price per share, Balance (in dollars per share)	0.25	0.18
Exercise price per share, granted (in dollars per share)	0.26	0.50
Exercise price per share,Expired (in dollars per share)
Exercise price per share, Balance (in dollars per share)	0.25	0.18
Maximum [Member]
Exercise price per share, Balance (in dollars per share)	1	1
Exercise price per share, granted (in dollars per share)	0.28	0.67
Exercise price per share,Expired (in dollars per share)
Exercise price per share, Balance (in dollars per share)	$ 1	$ 1