Stretchable sensors enable safety applications

Researchers at the Fraunhofer Institute for Silicate Research ISC in Würzburg have developed sensors that can be integrated into the car seat, for example, where they detect not only if the seat is occupied but the position of the occupant such as when the person is leaning over or sitting back in the seat. It can also detect if a child or an adult is sitting into the car seat.

This is to prevent scenarios such as when a person in the passenger seat is leaning too far forward, perhaps looking for something in a bag in the foot space and can be injured by the force of the airbag in a crash.

"The sensor films can measure stretch, as well as pressure," says Holder Böse, scientific and technical manager of the ISC's Centre Smart Materials. "They are made of a highly stretchableÿelastomeric film, coated on both sides with flexible electrodes. Whenever the sensor is stretched by changes in the shape of the seat, the sensor's thickness and, as a result, its electrical capacitance also change, which we can measure." Reports said that in contrast to conventional, rather inelastic strain gauge strips, the new dielectric elastomeric sensors can stretch by up to 100 per cent in extreme cases. This means that they can be drawn out to twice their size.

"Depending on the field in which the smart materials are applied, it might be necessary to coat the elastomer film with multiple electrode pairs. This is the case, for example, when measuring the distribution of body pressure to determine a person's posture in a seat. Each pair of electrodes serves, in effect, as an independent sensor, measuring the local strain. This is how we can say precisely where and to what degree the pressure has changed," explains Böse.

In making the sensors, the researchers choose the material that best meets the specific requirements of each application. The elastomer film consists of a polymer in which the individual molecules are chemically bonded with one another. The better the network of molecules, the sturdier the material becomes. This is similar to how fine-mesh fishing net is stronger than one with a larger mesh. The degree of bonding in the polymer can be controlled by the scientists.

"If the sensor is being used to measure high pressures, we produce a sturdier elastomer film as substrate; for measuring lower pressures, we use more pliant films," says Böse.

These sensors have numerous applications. For instance, they can be used to measure the pressure of gases. To do this, the elastomer film is stretched like a membrane across a ring. If gas exerts pressure on the sensor membrane, it deforms – which is detected by the sensor. Pressure sensors are also useful in safety technology: If someone enters an area too close to a hazardous machine, sensors embedded in the floor can detect this and set off a warning. These intelligent materials could even be integrated into clothing: Here, they might be used to analyse sequences of movement, thereby helping athletes to optimise their training. Because they are so flexible, sensors that are part of clothing are hardly noticeable at all.