Note 5 - Equity - Common Stock Purchase Warrant Activity (Details) - $ / shares	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Outstanding, number of warrants (in shares)	7,476,056	3,864,552
Outstanding, weighted average exercise price (in dollars per share)	$ 1.45	$ 2.36
Issued, number of warrants (in shares)	550,000	3,611,504
Issued, weighted average exercise price (in dollars per share)	$ 2.41	$ 0.47
Cancelled or expired, number of warrants (in shares)	(44,528)
Cancelled or expired, weighted average exercise price (in dollars per share)	$ 0.50
Exercised, number of warrants (in shares)	(5,826,781)
Exercised, weighted average exercise price (in dollars per share)	$ 0.47
Outstanding, number of warrants (in shares)	2,154,747	7,476,056
Outstanding, weighted average exercise price (in dollars per share)	$ 4.37	$ 1.45
Exercisable at December 31, 2020 (in shares)	1,904,747
Exercisable at December 31, 2020 (in dollars per share)	$ 4.61