Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
CASH FLOWS FROM OPERATING ACTIVITIES
Net loss	$ (28,414)	$ (29,928)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	1,000	985
Stock-based compensation	5,824	5,806
Patent write-offs	383	145
Other	6	4
Operating lease right-of-use asset amortization	602	586
Changes in assets and liabilities:
Accounts receivable	(130)	87
Prepaids and other current assets	(134)	(10)
Other assets	(44)	1
Accounts payable	(245)	249
Accrued expenses	19	210
Accrued salaries and payroll related expenses	(55)	191
Operating lease liabilities	(501)	(492)
Deferred revenue	52	1,460
Net cash used in operating activities	(21,637)	(20,706)
CASH FLOWS FROM INVESTING ACTIVITIES
Purchases of property and equipment	(705)	(806)
Expenditures for patents	(881)	(424)
Redemptions of investments held-to-maturity	0	29,295
Purchases of investments held-to-maturity	0	(12,432)
Net cash provided by (used in) investing activities	(1,586)	15,633
CASH FLOWS FROM FINANCING ACTIVITIES
Gross proceeds from the sale of common stock	40,173	10,020
Offering costs in connection with the sale of common stock	(2,726)	(101)
Proceeds from exercises of warrants	0	1,387
Proceeds from exercises of stock options	10	0
Net cash provided by financing activities	37,457	11,306
Effects of currency translation on cash, cash equivalents and restricted cash	1	0
NET INCREASE IN CASH, CASH EQUIVALENTS AND RESTRICTED CASH	14,235	6,233
CASH, CASH EQUIVALENTS AND RESTRICTED CASH — Beginning of year	10,838	4,605
CASH, CASH EQUIVALENTS AND RESTRICTED CASH — End of year	25,073	10,838
SUPPLEMENTAL DISCLOSURE OF CASH FLOW INFORMATION
Cash paid for taxes	2	1
SUPPLEMENTAL DISCLOSURE OF NON-CASH ACTIVITIES
Common stock issued in settlement of accrued salaries and payroll related expenses	343	342
Property and equipment included in accounts payable	193	206
Property and equipment included in accrued liabilities	68	159
Patents included in accounts payable	224	83
Property acquired through finance lease obligation	204	0
Operating lease assets obtained in exchange for new lease liabilities	114	0
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents
Total cash, cash equivalents and restricted cash	$ 10,838	$ 10,838