Note 5 - Stockholders' Equity and Common Stock Warrants - Warrants Outstanding to Acquire Shares of Its Common Stock (Details) - $ / shares	Mar. 31, 2020	Dec. 31, 2019	Nov. 30, 2019
Common stock warrants, outstanding (in shares)	21,261,070
Range of exercise price (in dollars per share)	$ 0.35
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)	1,181,375
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)	11,212,786
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)	6,580,126
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