Note 7 - Stockholders' Equity and Common Stock Warrants (Details Textual) - USD ($)	1 Months Ended				12 Months Ended
	May 31, 2020	Dec. 31, 2019	Nov. 30, 2019	May 31, 2019	Dec. 31, 2020	Dec. 31, 2019
Proceeds from Issuance of Common Stock					$ 10,827,100
Deemed Dividend From Warrant Exchange	$ 2,000,000				$ 1,950,378
Class of Warrant or Right, Exercised During Period (in shares)					19,106,504	17,415,859
Proceeds from Warrant Exercises					$ 8,046,103	$ 3,888,425
Class of Warrant or Rights, Expired (in shares)					0	1,767
The May 2020 Offering Warrants [Member]
Class of Warrant or Right, Number of Securities Called by Warrants or Rights (in shares)	571,429
Class of Warrant or Right, Exercise Price of Warrants or Rights (in dollars per share)	$ 1.3125				$ 1.31
Warrants and Rights Outstanding, Term (Year)	5 years
Prior Warrant [Member]
Class of Warrant or Right, Number of Securities Called by Warrants or Rights (in shares)	5,000,000
Class of Warrant or Right, Exercise Price of Warrants or Rights (in dollars per share)	$ 0.35
Class of Warrant or Right, Exchange, Additional Consideration Per Share (in dollars per share)	$ 0.125
New Warrants Issued May 8, 2020 [Member]
Class of Warrant or Right, Number of Securities Called by Warrants or Rights (in shares)	5,000,000
Class of Warrant or Right, Exercise Price of Warrants or Rights (in dollars per share)	$ 0.5263
The May 2020 PA Warrants [Member] | H.C Wainwright & Co., LLC [Member]
Class of Warrant or Right, Number of Securities Called by Warrants or Rights (in shares)	250,000
Class of Warrant or Right, Exercise Price of Warrants or Rights (in dollars per share)	$ 0.5938
Warrants Issued in Connection with the December 2019 Offering [Member]
Class of Warrant or Right, Number of Securities Called by Warrants or Rights (in shares)		6,266,787				6,266,787
Class of Warrant or Right, Exercise Price of Warrants or Rights (in dollars per share)		$ 0.4335				$ 0.4335
Placement Agent Warrant [Member]
Class of Warrant or Right, Number of Securities Called by Warrants or Rights (in shares)		313,339	571,429	65,853		313,339
Class of Warrant or Right, Exercise Price of Warrants or Rights (in dollars per share)		$ 0.6981	$ 0.4375	$ 6.11875		$ 0.6981
Warrants and Rights Outstanding, Term (Year)		5 years	5 years	5 years		5 years
Warrants Issued in Connection with the November 2019 Offering [Member]
Class of Warrant or Right, Exercise Price of Warrants or Rights (in dollars per share)			$ 0.35
Class of Warrant or Right, Issued During Period (in shares)			6,324,143
Class of Warrant or Right, Number of Securities Called by Each Warrant or Right (in shares)			1
Warrants with 18 Months Term Issued in Connection with the November 2019 Offering [Member]
Warrants and Rights Outstanding, Term (Year)			1 year 180 days
Class of Warrant or Right, Issued During Period (in shares)			11,428,572
Warrants with 5 Years Term Issued in Connection with the November 2019 Offering [Member]
Warrants and Rights Outstanding, Term (Year)			5 years
Class of Warrant or Right, Issued During Period (in shares)			11,428,572
Class of Warrant or Right, Exercised During Period (in shares)						11,091,716
Proceeds from Warrant Exercises						$ 3,900,000
The May 2020 Offering [Member]
Stock Issued During Period, Shares, New Issues (in shares)	11,428,572
Shares Issued, Price Per Share (in dollars per share)	$ 1.05
Proceeds from Issuance of Common Stock	$ 10,300,000
December 2019 Public Offering [Member]
Stock Issued During Period, Shares, New Issues (in shares)		6,266,787
Shares Issued, Price Per Share (in dollars per share)		$ 0.5585				$ 0.5585
Proceeds from Issuance or Sale of Equity, Total		$ 3,000,000
November 2019 Public Offering [Member]
Stock Issued During Period, Shares, New Issues (in shares)			5,104,429
Shares Issued, Price Per Share (in dollars per share)			$ 0.35
Proceeds from Issuance or Sale of Equity, Total			$ 3,300,000
Stock and Warrants Issued During Period, Number of Securities Called by Stock and Warrants (in shares)			22,857,144
May 2019 Public Offering [Member]
Stock Issued During Period, Shares, New Issues (in shares)				1,317,060
Shares Issued, Price Per Share (in dollars per share)				$ 4.895
Proceeds from Issuance or Sale of Equity, Total				$ 5,600,000
Warrants Issued in Connection with May 2019 Public Offering [Member]
Class of Warrant or Right, Number of Securities Called by Warrants or Rights (in shares)				1,317,060
Class of Warrant or Right, Exercise Price of Warrants or Rights (in dollars per share)				$ 5