Note 5 - Equity - Stock Option Activity (Details) - $ / shares		12 Months Ended
	Dec. 31, 2020	Dec. 31, 2020	Dec. 31, 2019
Outstanding, number of shares (in shares)		5,982,923	4,535,681
Outstanding, weighted average exercise price (in dollars per share)		$ 2.66	$ 3.31
Options granted, number of shares (in shares)	2,466,529	2,466,529	1,744,300
Options granted, weighted average exercise price (in dollars per share)		$ 2.87	$ 1.48
Options exercised, number of shares (in shares)	(1,277,743)	(1,277,743)
Options exercised, weighted average exercise price (in dollars per share)		$ 1.03
Options expired, number of shares (in shares)	(327,640)		(297,058)
Options expired, weighted average exercise price (in dollars per share)		$ 8.31
Outstanding, number of shares (in shares)	6,844,069	6,844,069	5,982,923
Outstanding, weighted average exercise price (in dollars per share)	$ 2.81	$ 2.81	$ 2.66
Exercisable, number of shares (in shares)	4,491,545	4,491,545
Exercisable, weighted average exercise price (in dollars per share)	$ 2.75	$ 2.75