Note 11 - Shareholders' Equity - Stock Option Activity (Details) - USD ($)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Balance, number of shares (in shares)	1,544,826	1,390,012
Balance, weighted average exercise price (in dollars per share)	$ 2.74	$ 4.77
Balance, weighted average remaining contractual life (Year)	7 years 273 days	7 years 171 days	6 years 302 days
Balance, aggregate intrinsic value	$ 330,831	$ 46,582
Granted, number of shares (in shares)	683,900	958,798
Granted, weighted average exercise price (in dollars per share)	$ 1.08	$ 1.69
Exercised, number of shares (in shares)	(26,250)
Exercised, weighted average exercise price (in dollars per share)	$ 1.34
Forfeited, number of shares (in shares)	(215,218)	(251,213)
Forfeited, weighted average exercise price (in dollars per share)	$ 1.50	$ 4.81
Expired, number of shares (in shares)	(200,367)	(552,771)
Expired, weighted average exercise price (in dollars per share)	$ 4.82	$ 5.07
Balance, number of shares (in shares)	1,786,891	1,544,826	1,390,012
Balance, weighted average exercise price (in dollars per share)	$ 3.10	$ 2.74	$ 4.77
Vested and expected to vest, number of shares (in shares)	1,593,216
Vested and expected to vest, weighted average exercise price (in dollars per share)	$ 2.12
Vested and expected to vest, weighted average remaining contractual life (Year)	7 years 229 days
Vested and expected to vest, aggregate intrinsic value	$ 278,883
Exercisable, number of shares (in shares)	609,781
Exercisable, weighted average exercise price (in dollars per share)	$ 3.26
Exercisable, weighted average remaining contractual life (Year)	6 years 10 days
Exercisable, aggregate intrinsic value	$ 19,668