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Note 10 - Net Loss Per Share (Tables)
3 Months Ended
Mar. 31, 2019
Notes Tables  
Schedule of Antidilutive Securities Excluded from Computation of Earnings Per Share [Table Text Block]
   
March 31,
 
   
2019
   
2018
 
Stock options to purchase common stock
   
5,477,364
     
3,816,927
 
Unvested restricted stock units
   
246,915
     
61,333
 
Common stock warrants
   
2,666,666
     
-
 
Total
   
8,390,945
     
3,878,260