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Summary of significant accounting policies - Additional Information (Details) (USD $)
3 Months Ended
Mar. 31, 2014
Mar. 31, 2013
Dec. 31, 2013
Mar. 31, 2014
Chief Executive Officer (John N. Hatsopoulos)
Mar. 26, 2014
Chief Executive Officer (John N. Hatsopoulos)
Mar. 25, 2013
Revolving credit facility
Chief Executive Officer (John N. Hatsopoulos)
Debt Instrument [Line Items]            
Cash insured $ 250,000          
Cash, Uninsured Amount 2,248,969          
Allowance for doubtful accounts 103,800   154,400      
Advertising expense 48,439 29,000        
Research and development costs reimbursed by government agencies   67,000        
Bank credit facility       1,500,000 3,500,000 1,500,000
Research and development expenditures $ 203,000 $ 161,000