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Summary of Significant Accounting Policies - Property and Equipment (Details) - USD ($)
Jun. 30, 2019
Dec. 31, 2018
Dec. 31, 2017
Significant Accounting Policies [Line Items]      
Property and equipment, Gross   $ 7,796,135 $ 7,044,821
Less accumulated depreciation   (6,574,308) (6,209,284)
Property and equipment, net $ 197,502 1,221,827 835,537
Leasehold Improvements [Member]      
Significant Accounting Policies [Line Items]      
Property and equipment, Gross   745,800 729,504
Laboratory And Manufacturing Equipment [Member]      
Significant Accounting Policies [Line Items]      
Property and equipment, Gross   4,829,323 4,109,367
Computer Equipment [Member]      
Significant Accounting Policies [Line Items]      
Property and equipment, Gross   1,520,713 1,505,651
Office Equipment [Member]      
Significant Accounting Policies [Line Items]      
Property and equipment, Gross   $ 700,299 $ 700,299