Artificial muscles – lighter, safer, more robust

Researchers at ETH Zurich have recently developed artificial muscles for robot motion. Their solution offers several advantages over previous technologies: it can be used wherever robots need to be soft rather than rigid or where they need more sensitivity when interacting with their environment.

Many roboticists dream of building robots not just from metal or other hard materials and motors, but of making them softer and more adaptable. Soft robots could interact with their environment in a completely different way; for example, they could cushion impacts like human limbs or grasp something with sensitivity. This would also be interesting from an energy point of view, as previous drives usually require a lot of energy to maintain a position, whereas soft systems can also store energy well. So what could be more obvious than to take the human muscle as a model and try to recreate this system?

The functioning of artificial muscles is therefore based on biology. Like their natural counterparts, the artificial muscles contract in response to an electrical impulse. However, the artificial muscles do not consist of cells and fibers, but of a pouch filled with a liquid - usually oil - whose shell contains electrodes. When they receive an electrical voltage, they contract and push the liquid into the rest of the bag. The bag stretches and can lift a weight, for example. A pouch is analogous to a short bundle of muscle fibers; if several of them are connected, a full drive element is created, which is also referred to as an actuator or artificial muscle.

Voltage too high

The idea of developing artificial muscles is not new, but until now there has been one major problem with the implementation: the actuators only worked with an enormously high voltage of around 6 to 10 thousand volts. This has several effects. Until now, these had to be connected to large, heavy voltage amplifiers, they did not work in water and were not entirely safe for people. Robert Katzschmann, professor of robotics at ETH Zurich, Stephan-Daniel Gravert and Elia Varini, together with a research team, have presented their version of an artificial muscle in Science Advances, which has several advantages.

Gravert, who works as a research assistant in Katzschmann's laboratory, has designed a new type of cover for the bag. The researchers call the new artificial muscles Halve actuators, an abbreviation for "hydraulically amplified low-voltage electrostatic". "In other actuators, the electrodes are on the outside of the casing. In ours, the sheath consists of different layers. We have combined a highly permittive ferroelectric material, i.e. one that can store relatively high amounts of electrical energy, with a layer of electrodes and then covered this with a polymer shell, which has very good mechanical properties and makes the bag more stable," explains Gravert. This also enabled the researchers to reduce the required voltage, because the much higher permittivity of the ferroelectric material allows large forces despite low voltage. Gravert and Varini not only co-developed the shell of the Halve actuators, but also produced them themselves in the laboratory for two specific robots.

Grippers and fish show what the muscle can do

The researchers illustrate the potential of the new development in the study using two robotic examples. An 11 centimeter high gripper has two fingers, each of which is moved by three bags of the actuator connected in series. It is supplied with 900 volts via a small, battery-operated power supply unit. The battery and power supply unit together weigh just 15 grams. The entire gripper, including the power and control electronics, weighs just 45 grams. The gripper can grip a smooth plastic object firmly enough to support its own weight when the object is lifted into the air with a cord. "This example shows very well how small, light and efficient these actuators are. It also means that we have come a big step closer to our goal of creating integrated muscle-operated systems," says a delighted Katzschmann.

The second object is a fish almost 30 centimeters long that swims smoothly through the water. The robotic fish consists of a head containing the electronics and a flexible body to which the Halve actuators are attached. These actuators move alternately and rhythmically, creating the swimming motion. The wireless fish reaches a speed of three centimeters per second from a standstill in 14 seconds - and that in normal tap water, mind you.

Waterproof and self-​sealing

This is important because it shows another new feature of the Halve actuators: as the electrodes are no longer unprotected on the outside of the sheath, the artificial muscles are now waterproof and can also be used in conductive liquids. "With the fish, we can also illustrate a general advantage of these actuators - the electrodes are protected from the environment and, conversely, the environment is also protected from the electrodes. You can therefore operate these electrostatic actuators in water or touch them, for example," explains Katzschmann. And the layered structure of the pouches has another advantage: the new actuators are much more robust than other artificial muscles.

Ideally, the bags should move a lot and quickly. Only the smallest production error - such as a speck of dust between the electrodes - can lead to an electrical breakdown - a kind of mini lightning strike. "With earlier models, this meant that the electrode burns, a hole is created in the casing, the liquid escapes and the actuator is defective," explains Gravert. This problem is solved with the Halve actuators, as a single hole virtually closes itself through the protective plastic outer layer. The bag usually remains fully functional even after an impact.

The two researchers are clearly delighted to have taken the development of artificial muscles a decisive step forward, but they are also realistic. Katzschmann says: "Now this technology has to be brought to industrial maturity, and we can't do that here in the ETH laboratory. But without giving too much away, I can say that there is already interest from companies that would like to work with us." It is possible, for example, that artificial muscles could one day be used in new types of robots, prostheses or wearables, i.e. technologies worn on the body.

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