Accrued expenses (Details) - USD ($) $ in Thousands	Jun. 30, 2016	Dec. 31, 2015
Accrued expenses
Research and development costs	$ 3,245	$ 1,329
Employee compensation costs	1,096	1,338
Professional services	435	350
Other	134	235
Patent costs	139	178
Total	$ 5,049	$ 3,430