Note 4 - Net Loss Per Share - Anti-dilutive Securities (Details) - shares		6 Months Ended
		Jun. 30, 2021	Jun. 30, 2020
Anti-dilutive securities (in shares)		7,798,606	8,685,945
Convertible Debt Securities [Member]
Anti-dilutive securities (in shares)		6,758,897	6,676,112
Vested Series A Warrants [Member]
Anti-dilutive securities (in shares)			40,441
Unvested Series A Warrants [Member]
Anti-dilutive securities (in shares)	[1]		69,853
Warrant, Other [Member]
Anti-dilutive securities (in shares)		653,248	1,006,190
Share-based Payment Arrangement, Option [Member]
Anti-dilutive securities (in shares)		386,461	893,349

[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 remained outstanding but unvested until they expired in February 2021.