Condensed Statement of Cash Flows (Unaudited) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2022	Mar. 31, 2021
Cash flows from operating activities:
Net loss	$ (36,892)	$ (30,025)
Adjustments to reconcile net loss to net cash used in operating activities:
Stock-based compensation expense	5,006	5,852
Depreciation and amortization	1,262	1,246
Non-cash lease expense	269	249
Non-cash interest expense	140	140
Accretion of discount on investments, net	(667)	(60)
Other non-cash items	3	5
Provision for inventories	496	115
Changes in operating assets and liabilities:
Accounts receivable	(1,299)	(5,646)
Inventories	(4,184)	(7,247)
Prepaid expenses and other assets	137	(981)
Accounts payable	3,276	1,613
Accrued payroll and related benefits	(11,470)	(4,244)
Accrued expenses and other current liabilities	4,226	164
Accrued warranty liability	(76)	(53)
Deferred revenue	375	305
Operating lease liabilities	(271)	(169)
Net cash used in operating activities	(38,335)	(38,616)
Cash flows from investing activities:
Purchases of property and equipment	(1,478)	(919)
Purchases of investment securities	(64,712)	(25,788)
Sales and maturities of investment securities	32,800	19,900
Net cash used in investing activities	(33,390)	(6,807)
Cash flows from financing activities:
Proceeds from stock option exercises and employee stock purchase plan purchases	3,723	2,218
Net cash provided by financing activities	3,723	2,218
Net decrease in cash, cash equivalents and restricted cash	(68,002)	(43,205)
Cash, cash equivalents and restricted cash as of beginning of period	215,659	328,283
Cash, cash equivalents and restricted cash as of end of period	147,657	285,078
Supplemental cash flow disclosures:
Cash paid for income taxes	205
Cash paid for interest	282	282
Cash paid for amounts included in the measurement of operating lease liabilities	271	169
Supplemental non-cash investing and financing activities:
Capital expenditures included in accounts payable and accrued expenses	907	238
Transfer of inventories to property and equipment		$ 962