Accounts receivable, net - Allowance for doubtful accounts (Details) - USD ($)	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020
Accounts receivable, net.
Opening allowance for doubtful accounts	$ 270,110	$ 142,236
Provision for doubtful accounts	0	129,107
Accounts written off during the year	0	(1,233)
Recovery of accounts written off	10,751	0
Ending allowance for doubtful accounts	$ 280,861	$ 270,110