The ElectroMechanical Film (EMFi) is a thin, cellular, biaxially oriented polypropylene film that can be used as an electret. Having a special voided internal structure and high resistivity, it is capable of storing large permanent charge. The charge is injected by a corona method using ∼10 kV cm−1 fields, thus creating internal electrical discharges inside the cellular structure. Films of different thickness and elasticity can be manufactured. The thickness in sensor and actuator applications is typically 30–70 μm. When metallized on both sides, EMFi is capable of measuring pressure and force changes offering large application potential in different fields of technology including microphones and also actuators. New loudspeaker panels based on EMFi are only a few millimeters thick.
Paajanen, M., Lekkala, J., & Kirjavainen, K. (2000). ElectroMechanical Film (EMFi): A new multipurpose electret material. Sensors and Actuators A: Physical, 84(1-2), 95 - 102. https://doi.org/10.1016/S0924-4247(99)00269-1