Electromechanical film

Electromechanical Film (EMFi) is a thin, flexible film that can function as a sensor or actuator. It is composed of a charged polymer coated with two conductive layers, making it an electret. It was first made by Finnish inventor Kari Kirjavainen. EMFi’s voided internal structure and high resistivity allows it to hold a high electric charge and makes the film very sensitive to force. Through either measuring the film’s internal electric field generated from an electric charge, or changes in the film’s thickness in the when an electric voltage is applied, EMFi can function either as a sensor, or an actuator. This gives the film applications in different fields of technology, including, but not limited to, microphones, loudspeaker panels, keyboards, and physiological sensors.[1]Other than being cheap, its main advantage is its versatility; it can be cut, reshaped, and resized depending on its surface of application.[2]

Manufacturing and structure

EMFi's base film is first made from bi-axially orienting a polypropylene film. It is created through a "film-blowing" process, in which the plastic is extruded using a film blowing machine in the shape of a tube. Through the process of foaming, gaseous bubbles can be formed at a fixed density in the tube, which would give rise to EMFi's "voided internal structure". It is then expanded into two different directions depending on the desired thickness and orientation(bi-axial orientation). The tube is then coated with some electrically conductive material, and then cut open into a film. This film is then charged using the Corona Treatment, and the electrically conductive layers create electrodes.

EMFi has three layers, two of which that are homogeneous and act as electrodes as mentioned above, and a middle layer that is filled with flat, disk-shaped voids. Upon receiving charge from the Corona method, electrical breakdowns occur and the surfaces of the voids are permanently charged. There are two basic types of EMFi films that are manufactured currently, O01 and HS01, their thicknesses being 37 μm and 70 μm respectively.[3][4][2]

Operation

Sensor

EMFi can be used as a sensor. As the film is charged, it creates an electric field. When pressure is applied to the film, the film's thickness is reduced and changes in the shapes of the individual voids in its structure occur. Any electric charges residing in these voids will move and create mirror charges at the electrode surfaces of the film. These charges are proportional to the force applied to the film, which is given by the equation:

Δq = kΔF

where ΔF is the dynamic force, Δq is the charge generated, and k is the sensitivity factor. [1]

Actuator

EMFi can also be used as an actuator. Changes in thickness can be induced by applying a voltage on the film; compression and expansion of the film depends on the polarity of the voltage, and it occurs when both the outer surfaces of the film either attract or repel from each other.[4]The attractive force between the surfaces while the film is uncharged is given by the equation:

F = 12CU2x

where C is the capacitance of the film and x is the film's thickness.[1]

Applications

EMFi has a diverse range of applications due to it being flexible, durable, and sensitive to a wide range of frequencies. These properties are attributed to EMFi's base material: a polypropylene film. Due to these properties, in conjunction with the two modes of operation, EMFi is has already seen use in keyboards, speakers, and microphones.

  • In active noise cancellation, a part of EMFi can be used in the sensor mode to identify sound signals, and an actuator film can then be used to produce sound signals that cancel out the first.
  • EMFi has been implemented in physiological sensors where contact with the skin is required, such as a BCG, as its application is painless and non-invasive.[5][6][1]

Drawbacks

Due to the constraints faced by using a polypropylene base material, EMFi can only be used in temperatures below 50°C, which limits its scope in terms of potential applications.

The air voids present in EMFi's structure become smaller and higher in pressure as force is applied to the film. This means that the film becomes harder to compress as it goes under more load, meaning that in the sensor mode, the charge output is non-linear, which can make calibrating the sensor difficult.[6]

References

  1. Paajanen, Mika; Lekkala, Jukka; Kirjavainen, Kari (2000-08-01). "ElectroMechanical Film (EMFi) — a new multipurpose electret material". Sensors and Actuators A: Physical. 84 (1): 95–102. doi:10.1016/S0924-4247(99)00269-1. ISSN 0924-4247.
  2. , "Electromechanical film and procedure for manufacturing same", issued 1984-11-20
  3. Savolainen, A.; Kirjavainen, K. (1989-02-01). "Electrothermomechanical Film. Part I. Design and Characteristics". Journal of Macromolecular Science: Part A - Chemistry. 26 (2–3): 583–591. doi:10.1080/00222338908051994. ISSN 0022-233X.
  4. Paajanen, Mika; Välimäki, Hannu; Lekkala, Jukka (2000-03-01). "Modelling the electromechanical film (EMFi)". Journal of Electrostatics. 48 (3): 193–204. doi:10.1016/S0304-3886(99)00065-0. ISSN 0304-3886.
  5. Albert Manninen, Johan Sand, Jaakko Saarela, Tapio Sorvajärvi, Juha Toivonen, and Rolf Hernberg, "Electromechanical film as a photoacoustic transducer," Opt. Express 17, 16994-16999 (2009)
  6. Junnila, Sakari; Akhbardeh, Alireza; Värri, Alpo (2009-12-01). "An Electromechanical Film Sensor Based Wireless Ballistocardiographic Chair: Implementation and Performance". Journal of Signal Processing Systems. 57 (3): 305–320. doi:10.1007/s11265-008-0307-2. ISSN 1939-8115.
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