Prepaid expenses and other assets (Details) - USD ($) $ in Thousands	Mar. 31, 2022	Dec. 31, 2021
Prepaid expenses and other assets
Prepaid insurance	$ 1,422	$ 2,019
Research and development advance payments	636	586
Unbilled reimbursement receivable from DoD	795	1,638
Other prepaids and short term deposits	571	492
Reimbursement receivable from DoD		2,674
Prepaid expenses and other assets	$ 3,424	$ 7,409