Note 10 - Stockholders' Equity - Shares of Common Stock Reserved for Issuance (Details) - shares	Jun. 30, 2019	Dec. 31, 2018
Common stock reserved for issuance (in shares)	15,185,398	1,099,454
Options Issued and Outstanding [Member]
Common stock reserved for issuance (in shares)	2,166,800	865,163
Options Available for Future Grants [Member]
Common stock reserved for issuance (in shares)	186,410	223,377
Common Warrants [Member]
Common stock reserved for issuance (in shares)	12,832,188	10,914