Sensitive ceramics

Empa’s Laboratory for High-Performance Ceramics’ Research Group Leader Frank Clemens, along with his research team, are developing soft sensor materials based on ceramics that can feel temperature, strain, pressure, or humidity.

While ceramics are an inorganic, non-metallic material produced from a collection of loose particles via sintering, composition of ceramics can vary, and their properties change as a result. Empa’s researchers work with materials such as potassium sodium niobate and zinc oxide, but also with carbon particles. None of these materials are soft, so to fashion them into flexible sensors the researchers embed ceramic particles in stretchable plastics.

“We work with so-called highly filled systems,” Clemens says. “We take a matrix made of a thermoplastic and fill it with as many ceramic particles as possible without compromising the elasticity of the matrix.”

If this highly filled matrix is then stretched, compressed, or exposed to temperature fluctuations, the distance between the ceramic particles changes, and with it the electrical conductivity of the sensor. It’s not necessary to fill the entire matrix with ceramic, Clemens notes, because using 3D printing allows researchers to embed the ceramic sensors as nerves in flexible components.

Selective and intelligent

The production of soft ceramic sensors isn’t trivial. Usually, soft sensors are sensitive to different environmental influences at the same time, such as temperature, strain, and humidity. “If you want to use them in practice, you need to know what you’re measuring,” Clemens says.

His research group has succeeded in producing soft sensors that react very selectively only to pressure or only to temperature. The researchers integrated these sensors into a prosthetic hand. The prosthesis senses the flexion of its fingers and notices when it touches a hot surface. Such sensitivity would be an advantage for robotic gripping tools and for human prostheses.

The Empa team even went one step further, developing a soft robot skin. Similar to human skin, the multi-layered plastic skin reacts to touch and temperature differences. To evaluate the complex data, Empa researchers developed an artificial intelligence (AI) model with researchers from the University of Cambridge and trained it using data around 4,500 measurements – reminiscent of human perception, as the nerve impulses from our skin are evaluated and extrapolated in the brain.

In their most recent project, the researchers combined ceramic sensors with artificial muscles. Together with researchers from ETH Zurich and the University of Tokyo, they have developed a bio-hybrid robot that recognizes its contraction state with the help of a soft, biocompatible, tissue-integrated piezoresistive sensor.

Safe collaboration

The aim, Clemens says, is for humans and machines to work together safely and harmoniously. “Today’s robotic systems are big, clunky, and very strong. They can be dangerous for humans. If you accidentally touch another person, you automatically pull away. We want to give robots the same reflex.”

Researchers are now looking for industrial partners in the field of robotic gripping systems. But soft sensors are also in demand in medicine – the team recently completed an Innosuisse project with the company IDUN Technologies, where they produced flexible electrodes for brain wave measurements.

The researchers want to make their soft ceramic sensors even more sensitive and intelligent, which involves combining new ceramic materials and soft polymers and optimizing their sensor properties. The secret to success lies in the interaction of these two components.

Empa Laboratory for High-Performance Ceramics
https://www.empa.ch/web/s201