A research team led by smart materials experts Professor Stefan Seelecke and Junior Professor Gianluca Rizzello, from Saarland University, is exploring the fundamental aspects of this exciting field of soft robotics.

In the factory of the future, humans and machines will work side by side, in harmony, as a team, joining forces whenever necessary, as if the collaborative robot were flesh and blood. Although collaborative robots ("cobots") are already being implemented on industrial production lines, true hand-to-hand teamwork between robots and their human counterparts is still a long way off. The problem lies in the physical proximity of human collaborators, whose actions—unlike those of a robot—don't follow predictable algorithms. A human worker can become tired or distracted and act suddenly or even illogically. This has clear safety implications and explains why the robotic arms currently used on production lines are usually housed in cages. For anyone who gets too close, things can get dangerous. Industrial robots are typically large and heavy machines. But they are also powerful, fast, and agile, and are used for a wide range of operations, such as welding, assembling, painting, stacking, and lifting. However, their movements are dictated entirely by the programs that control them. And if someone gets in their way or gets too close, the consequences can be serious.

The team led by Professor Stefan Seelecke and Junior Professor Gianluca Rizzello from Saarland University and the Centre for Mechatronics and Automation Technology (ZeMA) in Saarbrücken is working on new types of intelligent robotic arms. “Our technology is based on smart polymer systems and allows us to create new soft robotic tools that are lighter, more maneuverable, and more flexible than the rigid components currently in use,” explains Stefan Seelecke. An accidental nudge from one of these robotic arms of the future would be more like a shove from a human coworker (and less likely to land you in the hospital).

The material used for these new soft robotic arms is a special type of polymer known as a “dielectric elastomer.” Researchers in Saarbrücken use this composite material to create artificial muscles and nerves. The special properties of dielectric elastomers allow for the development of systems inspired by nature’s ingenious designs. These elastomers can be compressed, but then stretched back to their original shape. “We print electrodes on both sides of the elastomer material. When we apply a voltage, the two electrodes attract each other, compressing the polymer and causing it to expand laterally,” explains Dr. Gianluca Rizzello, junior professor of adaptive systems based on in polymers. The Italian researcher has been working on Seelecke's team since 2016. Thus, the elastomer can contract and relax, like muscle tissue. “We take advantage of this property to design our actuators,” explains Rizzello. By precisely varying the electric field, engineers can make the elastomer perform high-frequency vibrations or continuously variable bending movements, or even remain motionless in a specific desired intermediate position.

The researchers then combine a large number of these tiny “muscles” to create a flexible robotic arm. When combined in this way to form a robotic tentacle, the interaction between the muscles produces movements that mimic those of an octopus arm, which can rotate in all directions. Unlike the heavy, rigid robotic limbs currently in use, which, like humans, can only move in certain directions, these new robotic tentacles are free to move in almost any direction. Gianluca Rizzello, along with his PhD student Johannes Prechtl, recently won the Best Paper award at the RoboSoft 2021 conference for their work developing a prototype tentacle based on the dielectric elastomer—one of numerous accolades. obtained by Professor Seelecke's research team. The team expects to have the fully developed tentacle prototype within about a year.
When it comes to giving polymer materials intelligence, Gianluca Rizzello is a true expert. He provides the control unit (i.e., the robot's "brain") with the data needed to move the arm intelligently—a very complex and ambitious task. These systems are far more complex than current robotic arms. "Using artificial intelligence to control polymer-based components is much more difficult than controlling conventional mechatronic systems," explains Rizzello. Because the elastomer muscles also have sensory properties, they can act as the system's nerves, meaning the robotic arm doesn't need to be equipped with additional sensors. Every distortion of the elastomer, every change in its geometry, causes a change in the material's capacitance, allowing the team to assign a precise electrical capacitance value to any specific deformation of the elastomer. “By measuring capacitance, we know exactly what shape the elastomer has taken, allowing us to extract data from the sensor,” explains Rizzello. “

This quantitative data can then be used to precisely model and program the movement of the elastomer arm.” Rizzello’s research focuses on developing intelligent algorithms that can train these novel robotic tentacles to move and respond as required. “We are trying to discover which physical properties are responsible for the behavior of these polymers.” “The more we know, the more precisely we can design the algorithms to control the elastomer muscles,” says Dr. Rizzello.

The technology being developed in Saarland will be scalable. It can be used to create miniature tentacles for medical instruments or to manufacture large robotic arms for industrial applications. But unlike the heavy robotic arms used today, robotic limbs built with smart elastomers will be much lighter. Our robotic arms don’t need to be powered by motors or hydraulic or pneumatic systems; they can operate simply by applying an electric current. Furthermore, the elastomer muscles can be manufactured in shapes that meet the requirements of a specific application. And they consume very little electrical energy. Depending on the capacitance, the electrical currents flowing are in the microampere range. “This type of soft robot technology is very promising for the future, as its manufacture is energy-efficient and cost-effective,” summarizes Stefan Seelecke.

This research is supported by the German Research Foundation (DFG) through the priority program SPP2100 "Soft Material Robotic Systems".