Note 7 - Stockholders' Equity and Common Stock Warrants - Warrants Outstanding to Acquire Shares of Its Common Stock (Details) - $ / shares	Dec. 31, 2022	Dec. 31, 2021
Common stock warrants, outstanding (in shares)	111,891
Warrants Issued in Connection with the January 2018 Offering [Member]
Common stock warrants, outstanding (in shares)	23,639
Warrants Issued in Connection with the January 2018 Offering [Member] | Minimum [Member]
Range of exercise price (in dollars per share)		$ 599.71
Warrants Issued in Connection with the January 2018 Offering [Member] | Maximum [Member]
Range of exercise price (in dollars per share)		749.76
Warrants Issued in Connection with May 2019 Public Offering [Member]
Common stock warrants, outstanding (in shares)	27,648
Warrants Issued in Connection with May 2019 Public Offering [Member] | Minimum [Member]
Range of exercise price (in dollars per share)		250.09
Warrants Issued in Connection with May 2019 Public Offering [Member] | Maximum [Member]
Range of exercise price (in dollars per share)		306.04
Warrants Issued in Connection with the November 2019 Offering [Member]
Common stock warrants, outstanding (in shares)	4,269
Range of exercise price (in dollars per share)	$ 17.51
Warrants Issued in Connection with the December 2019 Offering [Member]
Common stock warrants, outstanding (in shares)	6,264
Warrants Issued in Connection with the December 2019 Offering [Member] | Minimum [Member]
Range of exercise price (in dollars per share)		$ 21.68
Warrants Issued in Connection with the December 2019 Offering [Member] | Maximum [Member]
Range of exercise price (in dollars per share)	$ 34.92
The May 2020 Offering Warrants [Member]
Common stock warrants, outstanding (in shares)	11,424
Range of exercise price (in dollars per share)	$ 65.65
The May 2020 Investor Warrant Exercise [Member]
Common stock warrants, outstanding (in shares)	4,998
Range of exercise price (in dollars per share)	$ 29.70
Underwriter Warrants in Connection with the February 2021 Common Stock Offering [Member]
Common stock warrants, outstanding (in shares)	33,649
Range of exercise price (in dollars per share)	$ 64.08