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Note 2 - Significant Accounting Policies (Tables)
12 Months Ended
Dec. 31, 2021
Notes Tables  
Share-based Payment Arrangement, Expensed and Capitalized, Amount [Table Text Block]
  

Year Ended

December 31,

 
  

2021

  

2020

  

2019

 
             

Research and development

 $2,020  $3,558  $1,635 

General and administrative

  2,162   7,537   1,723 

Cost of goods sold

  282   1,486   - 

Total share-based compensation expense

 $4,464  $12,581  $3,358 
Schedule of Share-based Payment Award, Stock Options, Valuation Assumptions [Table Text Block]
  

Year Ended

December 31,

 
   2021   2020   2019 
             

Weighted-average grant date fair value per share

   $9.29     $12.23     $9.36  

Expected volatility

  83%-91%   87%-97%   80%-97% 

Risk-free interest rate

  0.5%-1.4%   0.3%-1.9%   1.4%-2.6% 

Expected life (in years)

  5.38-6.25   5.38-10   5.38-10 

Dividend yield