Artificial tactile sensors are wonderful things. There are many types – resistive sensors, mechanical sensors, force sensors, capacitor sensors, magnetic and several kinds of optical sensors. All perform the same basic function – they work like mechanoreceptors, detecting the subtle or strong application of pressure and sending a signal to indicate the location, type, or intensity.

However, our artificial pressure sensors are inferior to their organic counterparts in one respect (other than size). The artificial type don't stretch, bend out of shape and still work. Most rely on mechanical precision, or reflection, and those that don't, don't have a lot of leeway in them. Certainly nothing to compare with a Pacinian corpuscle, which can be scrunched up and stretched to nearly twice its original length. It'll still work in that odd form. Maybe not near as well as normally, but it'll detect pressure, and successfully translate that to the nervous system.

If we had stretchable sensors like that, innumerable applications would open up to us. Sensors on car seats, sensors on normal comfy seating. Skin sensors on an artificial skin that can bend and flex like normal skin. Touch sensitive flexible displays and stretchable electronic systems. The list is nearly endless.

We are still some years away from that sort of capability, but for the first time, we have proof of concept that it can indeed be done – outside of the natural variant.

Researchers at the Fraunhofer Institute for Silicate Research ISC in Würzburg, Germany, have developed sensors capable of expanding, in extreme cases, to twice their original length and so supple as to blend with the weave of clothing. They are quite large of course, so not really of much practical use as of yet, but they are a working proof of concept.

“The sensor films can measure stretch, as well as pressure,” says Dr. Holger Böse, Scientific and Technical Manager of the ISC’s Center Smart Materials. “They are made of a highly stretchable  elastomeric 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.” In contrast to conventional, rather inelastic strain gauge strips, the new dielectric elastomeric sensors can stretch by up to 100 percent in extreme cases – in other words, 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 – similar to how a 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.

References

Sensors that can stretch