Note 12 - Stockholders' Equity - Shares of Common Stock Reserved for Issuance (Details) - shares	Dec. 31, 2020	Dec. 31, 2019
Common stock reserved for issuance (in shares)	9,288,870	45,476,994
Options Issued and Outstanding [Member]
Common stock reserved for issuance (in shares)	6,813,033	1,811,652
Options Available for Future Grants [Member]
Common stock reserved for issuance (in shares)	1,230,863	295,180
Common Warrants [Member]
Common stock reserved for issuance (in shares)	1,244,974	43,370,162