Prepaid expenses (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Deferred Costs, Capitalized, Prepaid, and Other Assets Disclosure [Abstract]
Director compensation	$ 100	$ 66
Director, officer and other insurance	121	33
Marketing programs and conferences	53
Legal retainer	25	25
Inventory purchase deposits		20
Professional service retainer	8	8
Rent	19
Equipment service deposits	3	7
Engineering, software licenses and other	13	11
Total prepaid expenses	$ 342	$ 170