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Note 6 - Equity (Tables)
12 Months Ended
Dec. 31, 2022
Notes Tables  
Share-based Payment Arrangement, Expensed, Amount [Table Text Block]
  

Year Ended December 31,

 
  

2022

  

2021

 

General and administrative

 $1,467  $1,461 

Research and development

  808   912 

Total stock-based compensation

 $2,275  $2,373 
Schedule of Share-Based Payment Award, Stock Options, Valuation Assumptions [Table Text Block]
  

Year Ended December 31,

 
  

2022

  

2021

 

Risk-free interest rate

  1.6% to 3.3%   0.9% to 1.4% 

Volatility

  107.0% to 113.2%   114.8% to 118.8% 

Expected life (years)

  5.3 to 6.3   5.3 to 6.2 

Expected dividend yield

  

—%

   

—%

 
Share-Based Payment Arrangement, Option, Activity [Table Text Block]
  Number of Shares  Weighted Average Grant Date Fair Value  

Weighted Average Exercise Price

  

Weighted Average Remaining Contractual Term (years)

  Aggregate Intrinsic Value 

Outstanding, December 31, 2021

  1,438,352  $6.35  $8.45   7.8  $ 

Granted

  842,832  $1.26  $1.49         

Exercised

    $  $         

Forfeited

  (5,834) $15.57  $24.39         

Outstanding, December 31, 2022

  2,275,350  $4.44  $5.83   7.8  $ 

Exercisable, December 31, 2022

  927,723  $7.53  $10.38   6.3  $ 
Schedule of Nonvested Restricted Stock Units Activity [Table Text Block]
  

Number of Shares

  

Weighted Average Grant Date Fair Value

  

Weighted Average Remaining Contractual Term (years)

 

Unvested Shares, December 31, 2021

  220,559  $4.59   3.0 

Granted

  452,334  $1.49     

Vested

  (65,421) $4.98     

Unvested Shares, December 31, 2022

  607,472  $2.24   3.1