"Soft Robotics" Ceramics with finesse

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Robots that feel touches and perceive temperature differences? An unexpected material makes this possible. In the Empa Laboratory for High-Performance Ceramics, researchers are developing soft and intelligent sensor materials based on ceramic particles.

When people hear the word "ceramics," most think of coffee cups, bathroom tiles, or flower pots. Not so for Frank Clemens. For the research group leader in the "High-Performance Ceramics" lab at Empa, ceramics can conduct electricity, be intelligent, and even feel. Clemens, together with his team, develops soft sensor materials based on ceramics. Such sensors "sense" temperature, strain, pressure, or humidity, making them interesting for applications in medicine as well as in the field of "soft robotics."

Soft ceramics—how is that possible? For material researchers like Clemens, ceramics are understood to be an inorganic non-metallic material produced in a so-called sintering process at high temperatures from a collection of loose particles. The composition of the ceramics can vary—thereby changing their properties. Stoneware and porcelain are not to be found in Clemens' lab. The researchers work with materials like potassium sodium niobate, zinc oxide, and also carbon particles.

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None of these materials are soft. To create soft sensors from them, the researchers embed them in stretchable plastics. "We work with so-called highly filled systems," says Clemens. "We take a matrix of a thermoplastic polymer and fill it with as many ceramic particles as possible without impairing the stretchability of the matrix." When this highly filled matrix is then stretched, compressed, or exposed to temperature fluctuations, the distance between the ceramic particles changes, and thus the electrical conductivity of the sensor changes. The entire matrix does not need to be filled with ceramics, Clemens emphasizes: using 3D printing, the researchers can also embed the ceramic sensors as a kind of "nerve pathways" in flexible components.

Selective and intelligent

However, the production of soft ceramic sensors is not trivial. Soft sensors are generally sensitive to multiple environmental influences simultaneously, such as temperature, strain, and moisture. "If you want to use them in practice, you should know what you're measuring," Clemens explains. His research group has succeeded in manufacturing soft sensors that very selectively react only to pressure or only to temperature. The researchers integrated these sensors into a prosthetic hand. The prosthesis "feels" the bending of its fingers and notices when it touches a hot surface. Such "sensitivity" would be advantageous for both robotic grippers and prosthetics for humans.

The Empa team went a step further in developing a soft "robot skin." Similar to human skin, the multilayered polymer skin reacts to touch and temperature differences. To evaluate the complex data, the Empa researchers, together with researchers from the University of Cambridge, developed an AI model and trained it using data from around 4500 measurements. This also resembles human perception, as nerve impulses from our skin are also evaluated and "extrapolated" in the brain.

In their latest project, the researchers were able to combine ceramic sensors with artificial muscles. Together with researchers from ETH Zurich and the University of Tokyo, they have developed a bio-hybrid robot that detects its contraction state using a soft, biocompatible, tissue-integrated piezoresistive sensor. This work was published in the journal "Advanced Intelligent Systems."

Safe collaboration

The goal, says Frank Clemens, is the safe and harmonious collaboration between humans and machines. "Today's robotic systems are large, bulky, and very strong. They can become dangerous to humans," the researcher explains. If we are to share our workplaces increasingly with robots in the future, they should react quickly and sensitively to touches. "When you accidentally touch another person, you automatically pull back immediately," says Clemens. "We want to give robots the same reflex." For this, the researchers are now seeking industry partners in the field of robotic gripping systems. Soft sensors are also in demand in medicine—recently, the team completed an Innosuisse project with the company IDUN Technologies, where they manufactured flexible electrodes for brainwave measurements.

The work is far from finished: the researchers want to make their soft ceramic sensors even more sensitive and intelligent. To achieve this, the aim is to combine new ceramic materials and soft polymers and optimize their sensor properties – because the secret of success lies in the interplay of these two components.

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