Note 10 - Stock-based Compensation - Stock Option Activity (Details) - $ / shares	3 Months Ended	12 Months Ended
	Mar. 31, 2022	Dec. 31, 2020
Options, Outstanding, Number of Shares (in shares)	4,640,660
Options, Outstanding, Weighted Average Exercise Price (in dollars per share)	$ 6.64
Options, Outstanding, Weighted Average Remaining Contractual Term (Year)	5 years 10 months 28 days	5 years 7 months 28 days
Options, Granted, Number of Shares (in shares)	2,173,832
Options, Grants in Period, Weighted Average Exercise Price (in dollars per share)	$ 0.82
Options, Forfeited, Number of Shares (in shares)	(4,155)
Options, Forfeitures in Period, Weighted Average Exercise Price (in dollars per share)	$ 1.71
Options, Cancelled, Number of Shares (in shares)	(13,816)
Options, Expirations in Period, Weighted Average Exercise Price (in dollars per share)	$ 38.80
Options, Exercised, Number of Shares (in shares)	(30,001)
Options, Exercises in Period, Weighted Average Exercise Price (in dollars per share)	$ 0.41
Options, Outstanding, Number of Shares (in shares)	6,766,520
Options, Outstanding, Weighted Average Exercise Price (in dollars per share)	$ 4.74