Note 6 - Stockholders' Equity, Stock Options and Warrants - Stock Offerings (Details) - USD ($)		2 Months Ended	6 Months Ended
		Feb. 28, 2021	Jun. 30, 2021	Jun. 30, 2020
Shares issued (in shares)		22,531,864
Gross proceeds		$ 34,396,319
Net proceeds		$ 31,077,231	$ 50,515,810	$ 5,057,919
Investor Warrants [Member]
Warrants issued (in shares)		11,265,932
Warrants issued, exercise price (in dollars per share)			$ 0.86
Placement Agent Warrants [Member]
Warrants issued (in shares)		1,689,890
Warrants issued, exercise price (in dollars per share)			$ 1.71875
January 12, 2021 Registered Direct Offering [Member]
Shares issued (in shares)		3,650,840
Shares issued, price per share (in dollars per share)	[1]	$ 0.842
Gross proceeds		$ 3,074,007
Net proceeds		$ 2,731,766
January 12, 2021 Registered Direct Offering [Member] | Investor Warrants [Member]
Warrants issued (in shares)		1,825,420
Warrants issued, exercise price (in dollars per share)		$ 0.80
January 12, 2021 Registered Direct Offering [Member] | Placement Agent Warrants [Member]
Warrants issued (in shares)		273,813
Warrants issued, exercise price (in dollars per share)		$ 1.0525
January 21, 2021 Registered Direct Offering [Member]
Shares issued (in shares)		2,200,000
Shares issued, price per share (in dollars per share)	[1]	$ 1.00
Gross proceeds		$ 2,200,000
Net proceeds		$ 1,932,050
January 21, 2021 Registered Direct Offering [Member] | Investor Warrants [Member]
Warrants issued (in shares)		1,100,000
Warrants issued, exercise price (in dollars per share)		$ 1.00
January 21, 2021 Registered Direct Offering [Member] | Placement Agent Warrants [Member]
Warrants issued (in shares)		165,000
Warrants issued, exercise price (in dollars per share)		$ 1.25
January 26, 2021 Registered Direct Offering [Member]
Shares issued (in shares)		3,414,970
Shares issued, price per share (in dollars per share)	[1]	$ 1.20
Gross proceeds		$ 4,097,964
Net proceeds		$ 3,668,687
January 26, 2021 Registered Direct Offering [Member] | Investor Warrants [Member]
Warrants issued (in shares)		1,707,485
Warrants issued, exercise price (in dollars per share)		$ 1.37
January 26, 2021 Registered Direct Offering [Member] | Placement Agent Warrants [Member]
Warrants issued (in shares)		256,123
Warrants issued, exercise price (in dollars per share)		$ 1.50
February 16, 2021 Registered Direct Offering [Member]
Shares issued (in shares)		4,222,288
Shares issued, price per share (in dollars per share)	[1]	$ 1.75
Gross proceeds		$ 7,389,004
Net proceeds		$ 6,679,989
February 16, 2021 Registered Direct Offering [Member] | Investor Warrants [Member]
Warrants issued (in shares)		2,111,144
Warrants issued, exercise price (in dollars per share)		$ 2.00
February 16, 2021 Registered Direct Offering [Member] | Placement Agent Warrants [Member]
Warrants issued (in shares)		316,672
Warrants issued, exercise price (in dollars per share)		$ 2.1875
Febuary 23, 2021 Private Placement [Member]
Shares issued (in shares)		9,043,766
Shares issued, price per share (in dollars per share)	[1]	$ 1.95
Gross proceeds		$ 17,635,344
Net proceeds		$ 16,064,739
Febuary 23, 2021 Private Placement [Member] | Investor Warrants [Member]
Warrants issued (in shares)		4,521,883
Warrants issued, exercise price (in dollars per share)		$ 2.00
Febuary 23, 2021 Private Placement [Member] | Placement Agent Warrants [Member]
Warrants issued (in shares)		678,282
Warrants issued, exercise price (in dollars per share)		$ 2.4375

[1] Sale price includes one share and a warrant to purchase one-half share.