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Stock-based compensation - Schedule of Nonvested Restricted Stock Awards Activity (Details) - Restricted Stock - 2015 Stock Plan
3 Months Ended
Mar. 31, 2021
$ / shares
shares
Outstanding shares  
Restricted stock awards, beginning balance (in shares) | shares 57,956
Granted (in shares) | shares 0
Canceled (in shares) | shares 0
Vested (in shares) | shares (8,279)
Restricted stock awards, ending balance (in shares) | shares 49,677
Weighted Average Grant Date Fair Value  
Beginning balance (in dollars per share) | $ / shares $ 0.67
Granted (in dollars per share) | $ / shares 0
Canceled (in dollars per share) | $ / shares 0
Vested (in dollars per share) | $ / shares 0.67
Ending balance (in dollars per share) | $ / shares $ 0.67