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Note 10 - Share-based Compensation - Valuation Assumptions for Employee Stock Options (Details) - Share-based Payment Arrangement, Option [Member]
12 Months Ended
Dec. 31, 2021
Dec. 31, 2020
Minimum [Member]    
Risk-free interest rate 0.60% 0.40%
Volatility 110.00% 107.00%
Expected term (Year) 6 years 3 months 6 years 3 months
Maximum [Member]    
Risk-free interest rate 1.60% 1.60%
Volatility 120.00% 112.00%
Expected term (Year) 10 years 10 years