Income Taxes - Summary of Deferred Tax Assets (Liabilities) (Detail) - USD ($)	Dec. 31, 2017	Dec. 31, 2016
Gross deferred tax assets:
Net operating loss carryforward – U.S.	$ 43,046,000	$ 58,206,000
Net operating loss carryforward – Switzerland	67,000	65,000
Research and development tax credit carryforward	6,153,000	5,477,000
Deferred revenue	339,000	438,000
Stock-based compensation	1,882,000	2,678,000
Inventory reserve	140,000	364,000
Compensation accruals	662,000	996,000
Other	575,000	378,000
Deferred tax asset, Total	52,864,000	68,602,000
Gross deferred tax liabilities – depreciation and amortization	(826,000)	(844,000)
Less valuation allowance	(52,038,000)	(67,758,000)
Net deferred tax asset	$ 0	$ 0