Income Taxes - Deferred Tax Assets and Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Deferred tax assets:
Research and development credits	$ 2,486	$ 2,063
Operating loss carry forwards	7,085	7,320
Interest	315	460
Inventories	240	289
Allowance for doubtful accounts	88	129
Depreciation	316	353
Non-cash compensation	739	946
Other	533	929
Total gross deferred tax assets	11,802	12,489
Valuation allowance	(10,874)	(11,441)
Net deferred tax assets	928	1,048
Deferred tax liabilities:
Other identifiable intangible assets	(284)	(494)
Total gross deferred tax liabilities	(284)	(494)
Deferred tax assets, net	$ 644	$ 554