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Summary of Significant Accounting Policies - Accounts receivable, net (Details) - USD ($)
Dec. 31, 2018
Dec. 31, 2017
Accounts receivable    
Allowance for doubtful accounts $ 0 $ 0
Allowance for product returns 144,000 147,000
Allowance for price concessions 569,000 0
Trade accounts receivable 7,297,000 4,188,000
Unbilled accounts receivable 938,000  
Allowance for accounts receivable (713,000) (147,000)
Accounts receivable, net, Current, Total $ 7,522,000 $ 4,041,000