Note 9 - Stockholders' Equity - Outstanding Warrants (Details)	Mar. 31, 2021 $ / shares shares
Outstanding Warrants to Purchase Shares (in shares)	27,664,205
The 2018 Warrants [Member]
Outstanding Warrants to Purchase Shares (in shares)	1,068,892
Exercise Price (in dollars per share) | $ / shares	$ 4.05
Expiration date	May 30, 2022
The 2020 Warrants [Member]
Outstanding Warrants to Purchase Shares (in shares)	8,281,325
Exercise Price (in dollars per share) | $ / shares	$ 1
January 2021 Warrants [Member]
Outstanding Warrants to Purchase Shares (in shares)	5,293,163
Exercise Price (in dollars per share) | $ / shares	$ 1.055
Expiration date	Jul. 08, 2025
March 2021 Warrants [Member]
Outstanding Warrants to Purchase Shares (in shares)	13,020,825
Exercise Price (in dollars per share) | $ / shares	$ 2.88
Expiration date	Sep. 22, 2025