Note 2 - Summary of Significant Accounting Policies - Stock-based Compensation Expense (Details) - USD ($) $ in Thousands	3 Months Ended	6 Months Ended
Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Stock-based compensation expense	$ 540	$ 488	$ 1,045	$ 1,205
Research and Development Expense [Member]
Stock-based compensation expense	245	135	463	256
General and Administrative Expense [Member]
Stock-based compensation expense	$ 295	$ 353	$ 582	$ 949

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

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