Note 2 - Summary of Significant Accounting Policies - Anti-dilutive Securities (Details) - shares		3 Months Ended
		Mar. 31, 2020	Mar. 31, 2019
Anti-dilutive securities (in shares)		6,831,100	7,055,832
Convertible Debt Securities [Member]
Anti-dilutive securities (in shares)		5,159,496	5,096,721
Vested Series A Warrants [Member]
Anti-dilutive securities (in shares)		40,441	40,441
Unvested Series A Warrants [Member]
Anti-dilutive securities (in shares)	[1]	69,853	69,853
Warrant, Other [Member]
Anti-dilutive securities (in shares)		1,273,461	1,557,884
Share-based Payment Arrangement, Option [Member]
Anti-dilutive securities (in shares)		287,849	290,933

[1] The unvested Series A warrants were subject to vesting based upon the amount of funds actually received by the Company in the second close of the August 2015 financing which never occurred. The warrants will remain outstanding but unvested until they expire in February 2021.