Balance Sheet Components - Schedule of Accrued Liabilities (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Accrued liabilities
Accrued payroll-related expenses	$ 1,267	$ 1,331
Accrued joint development agreement expenses	3,446	749
Accrued inventory	690	897
Deferred rent	322	230
Accrued sales commissions payable to sales representatives	133	75
Other	481	466
Total accrued liabilities	$ 6,339	$ 3,748