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Note 9 - Net Loss Per Share (Tables)
6 Months Ended
Jun. 30, 2021
Notes Tables  
Schedule of Antidilutive Securities Excluded from Computation of Earnings Per Share [Table Text Block]
   

June 30,

 
   

2021

   

2020

 
                 

Stock options to purchase common stock

    1,685,059       1,055,203  

Unvested restricted stock units

    5,468       78,323  

Common stock warrants

    2,424,724       2,435,807  

Total

    4,115,251       3,569,333