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Note 11 - Stockholders' Equity (Tables)
12 Months Ended
Dec. 31, 2021
Notes Tables  
Schedule of Common Stock Reserved for Issuance [Table Text Block]
  

December 31, 2021

  

December 31, 2020

 
         

Options issued and outstanding

  10,216,106   6,813,033 

Available for future grants of equity awards

  5,582,742   1,230,863 

Common stock warrants

  232,434   1,244,974 

Total

  16,031,282   9,288,870 
Schedule of Stockholders' Equity Note, Warrants or Rights [Table Text Block]

Securities into which warrants are convertible

 

Warrants outstanding

  

Exercise Price

 

Expiration Date

          

Common Stock

  5,000  $0.30 

September 2024

Common Stock

  44,148  $1.10 

April 2024

Common Stock

  26,515  $1.375 

April 2024

Common Stock

  29,150  $2.50 

March 2025

Common Stock

  100,532  $3.125 

February 2025

Common Stock

  16,175  $3.125 

March 2024

Common Stock

  10,914  $22.99 

December 2026

Total

  232,434      
Fair Value of Warrants, Assumptions [Table Text Block]

Offering

 

March 2019

  

April 2019

  

September 2019

  

March 2020

 

Aggregate valuation on issuance date

 $100,000  $333,000  $497,000  $453,000 

Number of warrants issued

  84,000   636,364   2,115,738   280,000 

Exercise price

 $3.125  $1.375  $0.375  $3.125 

Closing stock price

 $2.08  $0.89  $0.36  $2.34 

Risk-free interest rate

  2.34%  2.31%  1.55%  0.88%

Expected term (in years)

  5.00   5.00   5.00   4.99 

Expected volatility

  80%  83%  83%  98%

Dividend yield

  %  %  %  %