The innovative artificial muscles can provide this fish robot with wireless drive. © Gravert et al. Science Advances 2024
Until now, problematic high voltages were necessary in order to imitate the function of their natural models. But now researchers have developed artificial muscles that can develop strength in a practical way. Thanks to a new type of covering material, only relatively low tension is required for the contraction of the oil-filled bags. This means they can be designed to be lighter, more robust and less likely to cause electric shock, as initial example applications make clear. In the future, the technology could lead to the development of optimized robots, prostheses or so-called wearables, say the scientists.
Hard and rigid structures, driven by motors – in addition to these conventional systems, more flexible concepts have now found their way into robotics: The creations of so-called soft robotics are based on the materials and processes of nature. Thanks to their flexible features, these developments offer a wide range of application potential in industry, medicine or research. The developers also draw inspiration from nature when it comes to driving the moving parts of soft robots. Instead of conventional motors, concepts are used that mimic the function of muscles. In addition to the gentle movement, such units could also offer energetic benefits, it is hoped.
The most powerful system of artificial muscles to date is based closely on the biological model: they are structures that contract when tension is applied – similar to muscle fibers caused by nerve impulses. The artificial muscle consists of a bag filled with a fluid. The special covering material allows a shortening deformation of the bag to be triggered by applying tension – this creates tensile stress. By combining several bags, a so-called actuator can be formed, which can provide movement similar to a bundle of muscle fibers.
So far too high voltages have been required
Until now, however, the concept had a catch: the actuators only contract effectively when high voltages of around 6,000 to 10,000 volts are applied. This meant that the artificial muscles had to be connected to large and heavy voltage amplifiers, which made the concept hardly practical for use in smaller and wireless robots. In addition, good insulation of the units had to be ensured and the systems carried the risk of dangerous electric shocks during handling. That’s why the research team led by Stephan-Daniel Gravert from the Swiss Federal Institute of Technology in Zurich (ETH Zurich) dedicated itself to optimizing the art-muscle concept.
The central advantage of their new development is the significantly lower charging requirement. This is made possible by a layer of a special ferroelectric material in the innovative shell structure of the bags. It has a particularly high permittivity. This refers to the ability of a material to polarize through electric fields. Due to its high permittivity, the material used can absorb electrical energy very effectively and thus generate strong attractive forces at a relatively low voltage, the researchers explain. “With other actuators, the electrodes are on the outside of the casing. Our shell consists of different layers. “We combined the high-permittivity ferroelectric material with a layer of electrodes and then covered them with a polymer shell that has very good mechanical properties and makes the bag more stable,” says Gravert.
Apparently, less than 1000 volts are now sufficient to ensure effective activation of the actuators.
At this tension, the differently polarized walls in the upper part of the bags contract. In doing so, they push oil into the lower area of the artificial muscle, causing it to inflate. This is associated with a shortening of the entire unit, which can cause tensile stress.
Application potential demonstrated
To illustrate the potential of their system, the developers used it for two application examples. One is an eleven centimeter high gripping robot in which two fingers are moved through three bags of the new actuator system connected in series. A built-in battery and a power supply provide the necessary voltage of 900 volts. The entire gripper, including the power and control electronics, weighs 45 grams. With this device, the team was able to demonstrate that the system is capable of lifting its own weight on a cord. “This example shows very well how small, light and efficient these actuators are. This also means that we have come a big step closer to the goal of creating integrated muscle-powered systems,” says senior author Robert Katzschmann from ETH Zurich.
The second application example is a fish robot about 30 centimeters long. There is control electronics in its head, which is linked to two actuators integrated on the sides. Their alternating activation creates swimming movements that can bring the wirelessly operated fish robot from a standstill to a speed of three centimeters per second in 14 seconds. This works in normal tap water, which would have caused insulation problems in earlier versions of artificial muscles. “With the fish we can illustrate that the electrodes are protected from the environment and, conversely, the environment also remains shielded from the voltage. “You can operate and touch these electrostatic actuators in water,” says Katzschmann. The developers emphasize that the new actuators are also significantly more robust than other artificial muscle systems.
According to them, the concept now has considerable potential for the development of new types of robots, prostheses or body-worn technologies. Finally, Katzschmann says: “Now this technology must be brought to industrial maturity. Without wanting to give too much away, I can say that there is already interest from companies that want to work with us,” says the scientist happily.
Source: Swiss Federal Institute of Technology Zurich, specialist article: Science Advances, doi: 10.1126/sciadv.adi9319