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Inventories (Tables)
3 Months Ended
Mar. 31, 2020
Inventories [Abstract]  
Schedule of inventories

   December 31,   March 31, 
   2019   2020 
Raw materials  $482,836   $598,039 
Work in progress   1,254,490    1,718,101 
Finished goods   6,929,388    5,113,489 
   $8,666,714   $7,429,629