Note 10 - Stockholders' Equity - Warrant Activity (Details) - $ / shares	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Outstanding (in shares)	1,726,522	482,873
Outstanding, weighted average exercise price (in dollars per share)	$ 29.88	$ 93.70
Warrants granted (in shares)	444,445	1,512,816
Warrants granted, weighted average exercise price (in dollars per share)	$ 0.10	$ 4.20
Warrants exercised (in shares)	(435,264)	(269,167)
Warrants exercised, weighted average exercise price (in dollars per share)	$ 0.35	$ 0.10
Warrants expired/canceled (in shares)	(19,637)
Warrants expired/canceled, weighted average exercise price (in dollars per share)
Outstanding (in shares)	1,716,066	1,726,522
Outstanding, weighted average exercise price (in dollars per share)	$ 25.23	$ 29.88
Exercisable (in shares)	1,646,214	1,656,668
Exercisable, weighted average exercise price (in dollars per share)	$ 22.91	$ 27.80