Electro-responsive Polymer and its Sensor Applications
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Electro-responsive Polymer and its Sensor Applications Lasse Räisänen and Heikki Räisänen Emfit Ltd Konttisentie 8 B, FIN-40800 Vaajakoski, FINLAND ABSTRACT Emfit Ltd is manufacturer of electro-responsive ferroelectret sensors. These film- like sensors can be used for various applications, for example non-body-contact vitals signs monitoring, dementia care, actigraphs, visitor count and keypads. The sensor material is based on a polyolefin material manufactured in a continuous biaxial orientation process that stretches the film in two perpendicular directions (machine direction and the transverse direction). The biaxially stretched material is further expanded to more than twice its original thickness in high-pressure gas-diffusion process. The structure of Emfit material consists of flat voids separated by thin polyolefin layers. The voids are made by compounding small particles that function as rupture nuclei, and form closed lens like cavities to the film during the biaxial orientation. A permanent electric charge is injected into the material using a corona charging process. The voided internal structure makes the film soft and elastic in the thickness direction. This means that the voids expand and contract in relation to the orientation of the film. The excellent sensor and actuator operations of Emfit film are based on the fact that reduction in the thickness of the film caused by an external force takes place in the gas voids and these are much softer than the polymer layers. INRODUCTION Emfit material was originally invented and patented in the 1980’s by Mr. Kari Kirjavainen, a wellknown Finnish innovator [1]. Since then, a lot of work has been done in developing manufacturing machinery and research and development of the film’s properties as a sensor material. Emfit’s role has been development of production facilities for the production of both the film material and sensors, and commercialization of the sensor applications. Research of the material’s properties has been done in many European research institutes and especially the role of VTT (Technical Research Centre of Finland) has been significant. Co-operation between Emfit and VTT started 1993 and this mutually beneficial partnership has generated extensive resources to enhance Emfit's techno logical product development and research. The technical cornerstone for Emfit was development of the patented gas diffusion expansion (GDE) technology together with VTT in the late 1990's [2]. Depending on the film type and expansion parameters GDE typically increases the void volume by 100 % and increases the sensitivity value (d33) by a factor of 5-10. Figure 1 shows a SEM-picture of typical Emfit sensor material expanded with GDE.
Figure 1. SEM-picture of standard Emfit film expanded with GDE, thickness 70 µm
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Emfit sensor film consists of thin polymer layers separated by air voids. The film has an internal electric field produced by permanent charges inside the film. Charging is done by placing film into high electric field (co
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