Note 6 - Stock-based Compensation - Stock Option Activity (Details)	6 Months Ended
	Jun. 30, 2021 USD ($) $ / shares shares
Options outstanding, number (in shares) | shares	2,240,204
Options outstanding, weighted-average exercise price (in dollars per share) | $ / shares	$ 8.28
Granted (in shares) | shares	924,115
Granted, weighted-average exercise price (in dollars per share) | $ / shares	$ 1.10
Forfeited (in shares) | shares	(111,432)
Forfeited, weighted-average exercise price (in dollars per share) | $ / shares	$ 0.91
Options outstanding, number (in shares) | shares	3,052,887
Options outstanding, weighted-average exercise price (in dollars per share) | $ / shares	$ 6.37
Options outstanding, weighted-average remaining contractual life (Year)	8 years 9 months 18 days
Options outstanding, aggregate intrinsic value | $	$ 219,228
Exercisable (in shares) | shares	1,563,224
Exercisable, weighted-average exercise price (in dollars per share) | $ / shares	$ 11.54
Exercisable, weighted-average remaining contractual life (Year)	8 years 1 month 6 days
Exercisable, aggregate intrinsic value | $	$ 195,252
Vested and expected to vest (in shares) | shares	3,052,887
Vested and expected to vest, weighted-average exercise price (in dollars per share) | $ / shares	$ 6.37
Vested and expected to vest, weighted-average remaining contractual life (Year)	8 years 9 months 18 days
Vested and expected to vest, aggregate intrinsic value | $	$ 219,228