Note 10 - Stockholders' Equity - Shares of Common Stock Reserved for Issuance (Details) - shares	Sep. 30, 2019	Dec. 31, 2018
Common stock reserved for issuance (in shares)	47,614,992	1,099,454
Options Issued and Outstanding [Member]
Common stock reserved for issuance (in shares)	1,862,002	865,163
Options Available for Future Grants [Member]
Common stock reserved for issuance (in shares)	351,428	223,377
Common Warrants [Member]
Common stock reserved for issuance (in shares)	45,401,562	10,914