Note 11 - Equity Incentive Plans - Total Stock-based Compensation Recognized for Options (Details) - USD ($) $ in Thousands	3 Months Ended	6 Months Ended
Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Stock-based compensation	$ 180	$ 118	$ 344	$ 204
Research and Development Expense [Member]
Stock-based compensation	80	59	159	103
General and Administrative Expense [Member]
Stock-based compensation	$ 100	$ 59	$ 185	$ 101

Amount of expense for award under share-based payment arrangement. Excludes amount capitalized.

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