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Note 6 - Stockholders' Equity and Common Stock Warrants (Tables)
12 Months Ended
Dec. 31, 2021
Notes Tables  
Schedule of Stockholders' Equity Note, Warrants or Rights [Table Text Block]
  

Outstanding

  

Range of exercise

price per share

 

Expiration dates

Common stock warrants issued in 2017 related to Series A convertible preferred stock offering

  903,870   $33.30  

March 2022

Common stock warrants issued in 2018 related to the January 2018 common stock offering

  1,181,421  $12.00-$15.00 

January 2023

Common stock warrants issued related to the May 2019 Offering

  1,382,913  $5.00-$6.11875 

May and December 2024

Common stock warrants issued related to the November 2019 Offering

  213,570   $0.35  

May 2024

Common stock warrants issued related to the December 2019 Offering

  313,339   $0.6981  

December 2024

Common stock warrants issued related to the May 2020 Offering

  571,429   $1.3125  

March 2025

Common stock warrants issued related to the May 2020 Investor Warrant Exercise

  250,000   $0.5938  

November 2025

Common stock warrants issued related to the February 2021 Offering

  1,682,927   $1.28  

February 2026

   6,499,469