Income Taxes - Deferred tax assets (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Deferred tax assets
Net operating loss carryforwards	$ 39,937	$ 32,704
Research and development credits	747	853
Depreciation and amortization	239	578
Accrued expenses and other	611	259
Inventory reserve	205	417
Gross deferred tax asset	41,739	34,811
Valuation allowance	(41,739)	(34,811)
Net deferred tax asset	$ 0	$ 0