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Intangible Assets - Schedule of Intangible Assets, Net (Details) - USD ($)
$ in Thousands
Mar. 31, 2019
Dec. 31, 2018
Intangible assets    
Trademarks $ 40 $ 40
Total intangible assets 2,878 2,976
Less patent amortization (899) (901)
Intangible assets, net 1,979 2,075
Patents issued    
Intangible assets    
Finite-lived, gross 2,791 2,847
Patents pending    
Intangible assets    
Finite-lived, gross $ 47 $ 89