Multifunctional steering wheels are today standard in many vehicles. Drivers can set the functions of cruise control or infotainment system with his hands constantly at the wheel. These switches however are rather rigid and allow only two positions – “on” and “off” of “forward” and “back” etc. “The reason is that they are made of rigid materials such as plastic, metal or ceramics”, explains Holger Böse from the Fraunhofer Institute for silicate research (Fraunhofer ISC, Würzburg, Germany). Being the scientific manager of the institute’s Center of Smart Material (CeSMa), he gets granular with intelligent materials whose mechanical properties can be controlled through electric or magnetic fields.
The CeSMa has developed sensors that make use of these properties. They transmit electric signals to control things. They are structured like a capacitor: Two electrode layers made of conductive silicone and an isolating film in between. If one exerts pressure onto the capacitor, initially nothing happens. To amplify the effect, the researchers added more silicone layers on top of the film. Thus, two additional films exert pressure to the film in the middle. Both are not completely even but carry a particular profile. This specific capacitor design enabled the researchers to utilize a physical property of the silicone that hitherto only could be observed if a silicone film with elastic electrode layers on top is protracted: Its geometry is changing, the area becomes larger, the silicone layer thinner. This principle, described here for tensile forces, can be applied to pressure forces as well, explains Böse.