Note 7 - Stock Option Plans and Stock-based Compensation - Stock Option Activity (Details)	9 Months Ended
	Sep. 30, 2021 $ / shares shares
Beginning balance, number of shares available for grant (in shares)	1,384,761
Beginning balance, options outstanding (in shares)	65,471
Beginning balance, options outstanding, weighted average exercise price per share (in dollars per share) | $ / shares	$ 16.86
Granted, number of shares available for grant (in shares)	81,227
Options granted (in shares)	81,227
Options granted, weighted average exercise price per share (in dollars per share) | $ / shares	$ 4.38
Ending balance, number of shares available for grant (in shares)	1,303,534
Ending balance, options outstanding (in shares)	146,698
Ending balance, options outstanding, weighted average exercise price per share (in dollars per share) | $ / shares	$ 9.95
Options vested (in shares)	65,471
Options vested, weighted average exercise price per share (in dollars per share) | $ / shares	$ 16.86
Options vested and expected to vest (in shares)	146,698
Options vested and expected to vest, weighted average exercise price per share (in dollars per share) | $ / shares	$ 9.95