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Intangible assets (Narrative) (Details) - USD ($)
12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Intangible Assets Narrative Details    
Gross value of intangible assets $ 951,000 $ 1,600,000
Amortization expense 105,000 116,000
Impairment of intangible assets $ 160,000 $ 0
Number of patents abandoned in prosecution 1 0