A research team led by Professors Stefan Seelecke and Paul Motzki at Saarland University is using a highly versatile film, not much thicker than household cling film, to give objects new capabilities and save energy in the process. When used in wearable textiles, these films can move and press against the skin, providing haptic feedback that can enhance the virtual reality gaming experience by allowing players to feel textures, impacts, and other physical sensations. When the thin polymer film is integrated into an industrial glove, it can respond to the operator's hand and finger movements, allowing a computer to "understand" specific hand movements and gestures. Applied to the top of a flat glass screen, the film can create the transient sensation of a touch button, switch, or slider under the user's finger. Lightweight speakers that use far less energy than their conventional counterparts, novel signal generators, and noise-canceling fabrics are just some of the other prototypes being developed by smart materials systems experts at Saarland University and the Saarbrücken Center for Mechatronics and Automation Technology (ZeMA).
But how do they bring these films to life? “Each side of the film is coated with an electrically conductive layer,” explains Paul Motzki, professor of Intelligent Materials Systems for Innovative Production at Saarland University and chief scientist and CEO of ZeMA. When researchers apply an electrical voltage to the polymer film, these conductive layers attract each other, compressing the polymer and causing it to expand laterally, thus increasing its surface area. “By varying the applied electric field, we can control the film’s movement, essentially creating a lightweight but highly efficient actuator,” says Motzki. The researchers are able to precisely control the movement of these coated films, known as dielectric elastomers (DEs), and can make them perform slow or rapid bending motions or vibrate at a desired frequency. Or they can make the film hold a fixed, stationary position without requiring a continuous supply of electrical power.
Using these electroactive polymer films, the research team is developing novel drive systems (actuators) that can be controlled without additional sensors. “A precise electrical capacitance value can be assigned to each deformation or change in position of the film. The capacitance data can show us how the film deforms, for example, when it is stroked or tapped with a finger. These dielectric elastomer films are self-sensitive and can act as their own position sensors,” explains Paul Motzki. The measurement data is also used to train AI models that allow researchers to program the film’s movement to flex, maintain a desired shape, or oscillate at a required frequency.
The team in Saarbrücken is now taking these DE actuators to a new level and opening up a whole host of new applications. The new generation of films can be controlled with greater precision and can vibrate at even higher frequencies. One of the team's goals is to enable these flexible films to oscillate at ultrasonic frequencies. A new project, TransDES (transistor structures based on flexible dielectric elastomeric systems), funded by the Saarland State through the EU's ERDF investment fund, also aims to break new ground by developing elastomeric circuits for high-voltage applications. Printed circuit boards (PCBs) are found in most electrical devices. The technology that controls these devices, whether a blender or a smartphone, is usually soldered onto flat, rigid PCBs. The research team in Saarbrücken is trying to develop lightweight, flexible PCBs that can be used in the future as low-cost, flexible alternatives to conventional PCBs. The flexible circuits being developed in Saarbrücken would also come with integrated miniature actuators and their own sensors.
The TransDES project is a collaboration between Paul Motzki's team at ZeMA and Professor John Heppe's team at htw saar (University of Applied Sciences). The technology being developed in Saarland is unique worldwide and involves creating elastomeric films with novel electrode layers.
Until now, electrically conductive layers were manufactured by applying amorphous carbon powder (“carbon black”) to the top and bottom of the polymer film using a screen-printing process. However, the electrical resistance of the carbon black layer is about 10,000 ohms, which is too high if the film is going to vibrate in the ultrasonic frequency range.
By replacing the carbon black layer with an ultrathin, more conductive metallic coating, the researchers aim to create a DE actuator that can switch on and off at ultrafast speeds. “This will allow us to get even better performance from the film,” explains PhD student Sebastian Gratz-Kelly. “Even at very high frequencies, we can control the entire surface of the film, not just parts of it.” But the new ultrathin metallic coating also means the film actuator is more energy-efficient and power losses are lower, partly due to the reduced contact resistance between the wire and the film. “And we’re using a special laser technique that allows us to achieve a much smaller structure size within the coating,” explains Gratz-Kelly. “Previously, when we used screen printing, the distance between the electrodes was about one centimeter. Now, the gap between the electrodes is only a few micrometers. And this is what allows us to design flexible elastomeric circuits,” adds Professor Motzki.
The challenge is that the entire film has to undergo significant stretching, but the film's deformation is hindered by the new metallic coating. This is where John Heppe's team comes in. Heppe, a professor of Physical Sensor Technology and Mechatronics at htw saar, also leads a research group at ZeMA. The two universities are working together to translate their findings into practical industrial and commercial solutions. To balance the properties of a solid metal conductive layer with those of a flexible polymer substrate, Heppe's team uses a special process to deposit the metal layer onto the elastomer. "We use a material deposition technique known as sputtering. The conductive layer we deposit on the elastomer's surface is only ten nanometers thick—more than a thousand times thinner than a human hair," says Mario Cerino, a research scientist on John Heppe's team.
The team's "trick" is to stretch the elastomer before depositing the ultrathin metallic layer. Anyone who has ever stuck adhesive tape to an inflated balloon will know the effect. When the air is released from the balloon, the strip of tape wrinkles. The same thing happens with the elastomer film. When the elastomer relaxes, the metallic layer contracts, forming wrinkles. "When we do this, we achieve a resistance of between 50 and 100 ohms in an area of one square centimeter, which is significantly lower than before," says Mario Cerino.
Researchers are currently using metal-coated films to develop energy-efficient and cost-effective silicon transistors. Transistors are electronic components that can be used to switch electrical voltages and signals on and off, or to amplify them, but in this case, the team wants to develop film-based transistors to switch high voltages. “By reducing electrical resistance, we can make more current flow, just like a kitchen tap lets more water through the wider it is opened. This allows us to achieve high-voltage switching with extremely short cycle times, which can be used to control valves, pumps, or loudspeakers,” explains Mario Cerino. “We make use of a rather special property of these systems,” explains Professor John Heppe. “If the metal-coated film is stretched more than it was during sputtering, cracks will appear in the electrode layer, leading to a large increase in electrical resistance. So, if we stretch the film, cracks appear. If we let the film relax, the cracks close again, and the wrinkled structure returns.” "This allows us to go from very low resistances to very high resistances, comparable to using a transistor as an electrical switch," explains Professor Heppe.
A demonstration of the technology showcases a new sensor element with a metal-coated film on a textile wristband. This wearable touch panel is a touch-sensitive fabric surface that can recognize shapes drawn on it. When someone slides their finger across this smart fabric, the pressure and direction of movement are recorded. Then, integrated advanced machine learning algorithms allow the system to recognize the letters or patterns drawn.
The Saarbrücken-based experts in smart materials systems will also present other innovative developments that make use of these dielectric elastomers with their own sensors, such as smart textiles and actuators that provide haptic feedback, energy-efficient valves and pumps, and high-performance actuators.
The state of Saarland is funding the TransDES project with approximately €500,000 from state funds and the European Regional Development Fund (ERDF).
Read the team's latest publication: https://doi.org/10.3390/ma17235993
Background:
The technology of dielectric elastomers is being further developed by PhD students conducting research as part of their doctoral dissertation projects under the supervision of Professors Stefan Seelecke, Paul Motzki, and John Heppe. The results have been published as articles in various scientific journals. The research has also received support from numerous sources. EU funding was provided through a Marie Curie research grant, and funding from the German Research Foundation (DFG) was provided through the DFG's SPP KOMMMA Priority Programme. The Saar State Government has provided financial support through the FEDER iSMAT and Multi-Immerse projects, and ME Saar (the Association of Metallurgical and Electrical Industries of Saar) has funded a doctoral research program. To facilitate the transfer of their smart materials technology to the commercial and industrial sectors, the researchers established the company mateligent GmbH, which will also be exhibiting at the same stand at this year's Hannover Fair.
