Note 9 - Stockholders' Deficit - Common Stock Reserved for Future Issuance (Details) - shares	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Stock options outstanding (in shares)	1,032,211
Shares available for grant under equity incentive plan (in shares)	732,149	1,060,400	1,114,400
Estimated common shares issuable upon conversion of notes payable and accrued interest (in shares)	383,947
Class of Warrant or Right, Outstanding	2,035,197
(in shares)	4,183,504