Other current liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Other current liabilities
Accrued royalties	$ 534	$ 605
Accrued warranties	330	469
Deferred revenue	245	448
Deferred Rent Credit, Current	520	593
Other Accrued Liabilities, Current	1,414	1,111
Total	$ 3,043	$ 3,226