Note 9 - Stockholders' Equity (Tables)

3 Months Ended

Mar. 31, 2021

Notes Tables

Schedule of Stockholders' Equity Note, Warrants or Rights [Table Text Block]

	Outstanding Warrants to Purchase Shares		Exercise Price		Expiration date
May 2018 warrants		1,068,892	$	4.05	May 30, 2022
December 2020 warrants		8,281,325	$	1.00	December 11, 2024-June 21, 2025
January 2021 warrants		5,293,163	$	1.055	July 8, 2025
March 2021 warrants		13,020,825	$	2.88	September 22, 2025
		27,664,205