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Advances to suppliers (Details) - USD ($)
Dec. 31, 2021
Dec. 31, 2020
Advances to suppliers    
Advances to suppliers $ 3,459,374 $ 7,033,556
Allowance for doubtful accounts (38,746) (179,095)
Including:    
Advances to suppliers, net $ 3,420,628 $ 6,854,461