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Commitments and contingencies - Narrative (Details)
12 Months Ended
Dec. 31, 2018
USD ($)
Dec. 31, 2017
USD ($)
Dec. 31, 2016
USD ($)
May 31, 2018
USD ($)
Jul. 31, 2017
USD ($)
ft²
Operating Leased Assets [Line Items]          
Future minimum lease payments $ 338,414        
Rent expense 238,131 $ 214,634 $ 202,788    
Commitments due in 2019 122,858        
Commitments due in 2020 64,637        
Office Space Located in Maynard, Massachusetts          
Operating Leased Assets [Line Items]          
Square feet of office space | ft²         3,558
Future minimum lease payments 278,000       $ 332,824
Commitments due in 2019 63,000        
Panama Farm Site          
Operating Leased Assets [Line Items]          
Future minimum lease payments 60,000     $ 180,000  
Commitments due in 2019 $ 60,000