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Stockholders' equity - Compensation Costs Recognized on Financial Statements (Details) - USD ($)
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]      
Total share-based compensation $ 263,396 $ 122,134 $ 218,294
Research and development      
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]      
Total share-based compensation 3,238 3,168 2,115
Sales and marketing      
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]      
Total share-based compensation 0 9,315 65,517
General and administrative      
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]      
Total share-based compensation $ 260,158 $ 109,651 $ 150,662