ELECTRO MECHANICAL FILM or EMFi (previously known as electro thermo mechanical film(ETMF)) has a thin porous poly propylene structure. This cellular structure was developed for the first time by the Technical Research Institute of Finland.
By offering high resistance EMFi is an excellent material for low-current long term measurement applications.The sensitivity of EMFi is proved adequate for biomedical and computer applications.The sensitivity is strongly dependent on thermal environment during processing, storage and operation time. These sensors can be used for regular heartbeat monitoring and such diagnosis.By providing 4 sensors at the corners of a plate,it can be used as touch screens.
Electro Mechanical Film (EMFi) sensors are those sensors that can sense even tiny mechanical compressions. It acts as a transducer by converting force into its equivalent charge. It finds excellent application in biomedical field as a blood pressure pulse transducer and for heart beat monitoring. We have EMFi chairs fitted with EMFi sensors for heart beat monitoring. It is also possible to create robust touch sensitive screens using EMFi, thereby enabling easy computer interfacing. We use EMFi transducer arrays for the intended purpose.
The sensitivity of EMFi material is strongly dependent on thermal environment during processing, storage and operation time. Keeping EMFi sensors over a temperature of 50c can lead to lowering of charge thereby affecting its sensitivity .
GENERAL CONSTRUCTION AND WORKING
The film is produced by injecting gas bubbles into a molten plastic and then extracting a tube of this gas–plastic mixture with spherical bubbles. The tube is expanded into a thin film by blowing, which produces biaxially oriented bubbles in it. Next, the film is stretched, which transforms the bubbles into flat discs with m and a vertical dimension a lateral dimension of 10–100 m. The final thickness of the film is 37–70 m of about 3 depending on the type of processing it has undergone. After that , the film is permanently charged either by a plane electrode corona discharge system in a high electric field or by using electron beam charging. To provide electrodes, EMFi is metallized on both sides using one of three methods: sputtering, vacuum evaporation, or gluing a metallized thin polyester film.
The operation of the transducer is of capacitive nature: when
an external force or pressure introduces mechanical compression into the film, the spatial distribution of charges within the material changes with respect to the electrode’s layers. As a result, a mirror charge proportional to the force is induced at the electrodes. This charge can be measured with a charge amplifier or a voltage amplifier with very high input impedance. Biomedica lfield tests on EMFi includedetecting motor activity
and breathing movements in laboratory animals , , but it has also been used to measure respiration movements and blood pressure pulsations in humans. Furthermore, the material has been used in smarthouses to monitor demented elderly people, to recognize walkers on an EMFi floor , and to measure other physiological signals . The film can also be used as an actuator or a loudspeaker, because it undergoes
thickness alterations, when a high voltage is applied between the electrodes. With a useful spectrum extending as far as 500 kHz, it also serves as an ultrasonic transducer .
There are two different types of film,standard and highly sensitive,which have a sensitivity of about 40 and 200pC/N,respectively.This sensitivity is strongly dependent on the thermal environment during processing, storage, and operation time.
Thus, the charge tends to decrease, if the temperature remains same for a long period of time. This characteristic limits its application range, but the material can also be aged to stabilize its sensitivity. The properties of EMFi, including sensitivity and manufacturing process, have been widely researched and published in recent years. The results indicate that sensitivity can be enhanced by means of different gas atmospheres and pressures during corona charging and by using different gases for bubble filling. EMFi’s temper-
ature range can be extended by developing the manufacturing processes or changing the base material. As a result, EMFi is a highly interesting alternative for applications that call for a very sensitive and inexpensive transducer material that only requires a simple amplifier.
Due to the relatively large gas voids and local corona break owns, sensitivity may vary in different parts of the film. This may lead to problems when an application presupposes the use of a very small transducer. A case in point is a pressure transducer measuring blood pressure pulsations over the radial artery n the wrist. In this application, the transducer must be of the array type, because the correct pulse pressure waveform is obtained exactly from a single element sitting over the artery. Signals produced by edge elements may be decayed and noisy due to motion artifacts.