Note 9 - Stockholders' Equity - Shares of Common Stock Reserved for Issuance (Details) - shares	Mar. 31, 2021	Dec. 31, 2020
Common stock reserved for issuance (in shares)	8,246,279	9,288,870
Options Issued and Outstanding [Member]
Common stock reserved for issuance (in shares)	7,764,164	6,813,033
Options Available for Future Grants [Member]
Common stock reserved for issuance (in shares)	67,863	1,230,863
Common Warrants [Member]
Common stock reserved for issuance (in shares)	414,252	1,244,974