A device for forming at an ambient atmospheric pressure a gaseous plasma comprising active species for treatment of a treatment region. The device comprises a plasma cell for forming the gaseous plasma. The plasma cell comprises an inlet for receiving gas from a source and an outlet for discharging active species generated therein. A dielectric substrate preferably made of a polyimide encloses around a flow path for gas conveyed from the inlet to the outlet and an electrode is formed on the dielectric substrate for energising gas along the flow path to form active species. A protective lining is located on an inner surface of the dielectric substrate for resisting reaction of the active species with the material of the dielectric substrate. An earth electrode comprising an electrode formed on a dielectric substrate substantially surrounds and at least partially overlaps the plasma cell.
