Note 4 - Net Loss Per Share - Anti-dilutive Securities (Details) - shares		3 Months Ended
		Mar. 31, 2021	Mar. 31, 2020
Anti-dilutive securities (in shares)		7,992,834	6,831,100
Convertible Debt Securities [Member]
Anti-dilutive securities (in shares)		6,449,950	5,159,496
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	1,273,461
Share-based Payment Arrangement, Option [Member]
Anti-dilutive securities (in shares)		889,636	287,849

[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.