Prepaid expenses and other assets (Details) - USD ($)	Jun. 30, 2022	Dec. 31, 2021
Prepaid expenses and other assets
CARES Act employee retention credit receivable	$ 847,000	$ 0
Prepaid insurance	830,000	2,019,000
Research and development advance payments	501,000	586,000
Other prepaids and short-term deposits	528,000	492,000
Reimbursement receivable from DoD	243,000	2,674,000
Unbilled reimbursement receivable from DoD		1,638,000
Prepaid expenses and other assets	$ 2,949,000	$ 7,409,000