Strong and Gentle, the Latest Generation of Robot Muscles

In 1967 Dr. Christiaan Barnard carried out the first successful heart transplant. His skillful use of a precision scalpel made him a medical legend. Fifty years later, Eddie Hall became the first person in the world to ever deadlift a thousand pounds. Both examples are human exploits at the limits of the muscular spectrum. However, while industrial robots can lift weights well above Hall’s record, they lack the versatility that enables human beings to combine strength with dexterity. Now, following milestones such as autonomous movement, the ability to climb stairs and open doors, and even the development of synthetic skin, artificial muscles have become the latest robotic frontier.   

Until now, two robotic technologies were used to replicate the capabilities of muscles. On the one hand are pneumatic actuators that can generate movement by injecting liquid or air into soft pouches. This system can produce a substantial amount of pressure, but it is quite bulky and potentially dangerous because of its lack of precision.   

On the other hand, there are actuators based on dielectric elastomers. These apply a voltage to a pair of electrodes resulting in the expansion or contraction of the material (elastomer) between them. Elastomers can be composed of polyurethane, acrylates, silicones or liquid crystal. Each of these materials shows a different response to electricity, with a different degree of sensitivity. This newer system is more flexible but less reliable, and wear and tear becomes an issue for the material subjected to the electric current.

Towards a more humanlike musculature

What if we could create a robotic technology able to hold an egg without breaking it while also lifting heavy weights, like humans can? In other words, bring together the power of pneumatic actuators and the precision and lightness of the dielectric elastomer-based technology. Such was the reasoning of the physicist Christoph Keplinger and his research team at the University of Colorado when they developed several prototypes able to complete tasks similar to those carried out by human muscles.

Their system makes use of several pouches full of a liquid akin to that found in a pneumatic actuator. However, instead of contracting or expanding them by reducing or increasing the amount of liquid they hold, they chose to apply a voltage similarly to dielectric elastomer actuators. As a proof of concept, they managed to lift a raspberry without bruising it. One of the biggest advantages of this technology is its self-healing quality, which results from the use of a liquid instead of a solid elastomer, preventing wear from continued use.   

The researchers at the University of Colorado have called it HASEL (Hydraulically Amplified Self-healing Electrostatic Actuator). The manufacturing techniques required to produce it are comparatively simple and cheap, which means that, if the subsequent tests are successful, the next generation of robots could well sport human-like muscles.

One of HASEL’s most interesting applications will be within the domain of human prosthetics, such as legs or arms, allowing users to handle fragile objects without damaging them just like when using a natural limb. It could also prove helpful in the handling of delicate foods in industrial processes.

Source: Science, National Geographic