Balance Sheet Components - Property and Equipment (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Property Plant And Equipment [Line Items]
Property and equipment, gross	$ 7,335	$ 7,296
Less: accumulated depreciation and amortization	(7,008)	(6,880)
Property and equipment, net	327	416
Leasehold Improvements
Property Plant And Equipment [Line Items]
Property and equipment, gross	3,201	3,174
Research Equipment
Property Plant And Equipment [Line Items]
Property and equipment, gross	3,499	3,499
Furniture and Fixtures
Property Plant And Equipment [Line Items]
Property and equipment, gross	350	350
Computer Equipment
Property Plant And Equipment [Line Items]
Property and equipment, gross	$ 285	$ 273

Amount of accumulated depreciation, depletion and amortization for physical assets used in the normal conduct of business to produce goods and services.

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Amount before accumulated depreciation, depletion and amortization of physical assets used in the normal conduct of business and not intended for resale. Examples include, but are not limited to, land, buildings, machinery and equipment, office equipment, and furniture and fixtures.

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