Robotics Artificial muscles – lighter, more robust, safer

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Until now, artificial muscles required an enormously high voltage. Researchers may have now solved this problem. Robotics in particular would benefit from this.

Many roboticists dream of building robots not just from metal or other hard materials and motors, but to make them softer and more adaptable. Soft robots could interact with their environment in a completely different way, for example, they could absorb shocks like human limbs or grasp something with finesse. From an energy perspective, this would also be interesting because current drives usually require a lot of energy to maintain a position, while soft systems can also store energy well. So, what could be more logical than taking the human muscle as a model and trying to replicate this system?

This is how artificial muscles work

The functioning of artificial muscles is therefore based on biology. Like their natural counterparts, artificial muscles contract upon receiving an electrical impulse. However, artificial muscles are not made of cells and fibers, but of a bag filled with a fluid – usually oil – whose shell contains electrodes. When these receive an electrical voltage, they contract and push the fluid into the rest of the bag. The bag tightens and can, for example, lift a weight. A bag thus analogously represents a short bundle of muscle fibers; by connecting several of these, a full drive element is created, which is also referred to as an actuator or indeed as an artificial muscle.

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Too high voltage required

The idea of developing artificial muscles is not new, but until now there was a significant problem in implementation: The actuators only worked with an enormously high voltage of about 6,000 to 10,000 volts. This has several implications.

• Thus, until now, they had to be connected to large, heavy voltage amplifiers.

• They did not work in water.

• They were also not entirely safe for humans.

Robert Katzschmann, robotics professor at ETH Zurich, Stephan-Daniel Gravert, and Elia Varini, together with a research team, have now presented their version of an artificial muscle that offers several advantages.

Gravert, who works as a scientific assistant in Katzschmann's lab, has designed a novel shell for the pouch. The researchers call these new artificial muscles Halve actuators, an acronym for hydraulically amplified low-voltage electrostatic, meaning a hydraulically amplified electrostatic low-voltage actuator in German.

In other actuators, the electrodes are on the outside of the casing. In ours, the casing 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 coated this with a polymer shell that has very good mechanical properties and makes the pouch more stable.

Stephan-Daniel Gravert

As a result, the researchers were also able to reduce the required voltage because the much higher permittivity of the ferroelectric material allows for large forces despite the low voltage.

Grippers and fish demonstrate what the muscle can do

The researchers illustrate the potential of the new development in the study with two robotic examples.

• A 11-centimeter-tall gripper has two fingers, each moved by three bags of the actuator connected in series. It is powered by a small, battery-operated power supply with 900 volts of voltage. The battery and power supply together weigh only 15 grams. The entire gripper, including power and control electronics, weighs only 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 string.

• The second object is an almost 30 centimeter long fish that swims smoothly through the water. The robotic fish consists of a head that contains the electronics, and a flexible body to which the halve actuators are attached. These actuators move rhythmically in alternation, which generates the swimming motion. Thus, the wireless fish reaches a speed of three centimeters per second from a standstill in 14 seconds – and that, notably, in normal tap water.

Waterproof and self-sealing

This is important because it demonstrates another innovation of the halve actuators: Since the electrodes are no longer exposed on the outside of the shell, the artificial muscles are now waterproof and can also be used in conductive liquids. The fish demonstrates a general advantage of the actuators: the electrodes are protected from the environment and, conversely, the environment is protected from the electrodes. Thus, these electrostatic actuators can be operated in water or touched, for example. Due to the layered structure of the bags, the new actuators are more robust than other artificial muscles.

Ideally, the bags should move a lot and quickly. Only the smallest production errors – such as a dust particle between the electrodes – can lead to an electrical discharge, a kind of mini lightning strike. "In earlier models, this meant: the electrode burns, a hole is created in the shell, the liquid leaks out, and the actuator is defective," explains Gravert. With the halve actuators, this problem is solved because a single hole is practically self-sealed by the protective plastic outer layer. The bag remains mostly fully functional even after a discharge.

For example, artificial muscles could be used in innovative robots, prostheses, or so-called wearables, i.e., technologies worn on the body.