Annual report pursuant to Section 13 and 15(d)

Summary of significant accounting policies - Additional Information (Details)

v3.8.0.1
Summary of significant accounting policies - Additional Information (Details) - USD ($)
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Mar. 31, 2018
New Accounting Pronouncements or Change in Accounting Principle [Line Items]      
Joint venture, percent owned 51.00%    
Allowance for doubtful accounts $ 22,400 $ 29,665  
Dilutive shares resulting from stock option exercises included in the calculation of EPS (shares) 171,594    
Bill and Hold Transactions $ 1,141,684 2,588,458  
Advertising expense 278,000 134,000  
Research and development $ 937,000 $ 677,100  
Forecast | Accumulated Deficit | Adjustments for New Accounting Pronouncement      
New Accounting Pronouncements or Change in Accounting Principle [Line Items]      
Cumulative effect of new accounting principle     $ 165,317
Forecast | Accumulated Other Comprehensive Loss | Adjustments for New Accounting Pronouncement      
New Accounting Pronouncements or Change in Accounting Principle [Line Items]      
Cumulative effect of new accounting principle     $ (165,317)