Balance Sheet Components - Schedule of Accrued Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Accrued liabilities
Accrued payroll-related expenses	$ 1,558	$ 1,331
Accrued joint development agreement expenses	661	749
Accrued inventory	1,678	897
Deferred rent	390	230
Accrued sales commissions payable to sales representatives	51	75
Other	663	466
Total accrued liabilities	$ 5,001	$ 3,748