Note 7 - Stockholders' Equity and Common Stock Warrants - Warrants Outstanding to Acquire Shares of Its Common Stock (Details) - $ / shares	Dec. 31, 2020	May 31, 2020	Dec. 31, 2019	Nov. 30, 2019
Common stock warrants, outstanding (in shares)	9,100,112
Warrants Attached to Series A Convertible Preferred Stock [Member]
Common stock warrants, outstanding (in shares)	903,870
Range of exercise price (in dollars per share)	$ 33.30
Warrants Issued in Connection with the January 2018 Offering [Member]
Common stock warrants, outstanding (in shares)	1,181,421
Range of exercise price (in dollars per share)
Warrants Issued in Connection with the January 2018 Offering [Member] | Minimum [Member]
Range of exercise price (in dollars per share)	12
Warrants Issued in Connection with the January 2018 Offering [Member] | Maximum [Member]
Range of exercise price (in dollars per share)	$ 15
Warrants Issued in Connection with May 2019 Public Offering [Member]
Common stock warrants, outstanding (in shares)	1,382,913
Warrants Issued in Connection with May 2019 Public Offering [Member] | Minimum [Member]
Range of exercise price (in dollars per share)	$ 5
Warrants Issued in Connection with May 2019 Public Offering [Member] | Maximum [Member]
Range of exercise price (in dollars per share)	$ 6.11875
Warrants Issued in Connection with the November 2019 Offering [Member]
Common stock warrants, outstanding (in shares)	497,140
Range of exercise price (in dollars per share)				$ 0.35
Warrants Issued in Connection with the November 2019 Offering [Member] | Minimum [Member]
Range of exercise price (in dollars per share)	$ 0.35
Warrants Issued in Connection with the November 2019 Offering [Member] | Maximum [Member]
Range of exercise price (in dollars per share)	$ 0.4375
Warrants Issued in Connection with the December 2019 Offering [Member]
Common stock warrants, outstanding (in shares)	313,339
Range of exercise price (in dollars per share)			$ 0.4335
Warrants Issued in Connection with the December 2019 Offering [Member] | Minimum [Member]
Range of exercise price (in dollars per share)	0.4335
Warrants Issued in Connection with the December 2019 Offering [Member] | Maximum [Member]
Range of exercise price (in dollars per share)	$ 0.6981
The May 2020 Offering Warrants [Member]
Common stock warrants, outstanding (in shares)	571,429
Range of exercise price (in dollars per share)	$ 1.31	$ 1.3125
The May 2020 Investor Warrant Exercise [Member]
Common stock warrants, outstanding (in shares)	4,250,000
The May 2020 Investor Warrant Exercise [Member] | Minimum [Member]
Range of exercise price (in dollars per share)	$ 0.5263
The May 2020 Investor Warrant Exercise [Member] | Maximum [Member]
Range of exercise price (in dollars per share)	$ 0.5938