A new artificial muscle is stronger, more flexible than natural ones

All thanks to changes in manufacturing process.
Ameya Paleja
A flim made with 10 layers of PHDE and 20 actuatorsUCLA

Researchers at the University of California, Los Angeles (UCLA) have developed a new material to build artificial muscles that are stronger and up to 10 times more flexible than naturally occurring muscles, a university press release said

Scientists have been keen to replicate the muscles in the body, which can then be used to make soft robots and new haptic technologies with a sense of touch. There are many soft materials that are known to material scientists that can do the dual job of delivering a mechanical output while also remaining viable under high strain conditions.

A class of materials called dielectric elastomers (DE) can deliver on both flexibility and toughness, and they are not only light in weight but also have high elastic energy density. DEs can be made from natural or synthetic compounds and are polymers that can change size or shape when an electric field is applied. This makes them ideal materials to make actuators i.e. machines that can convert electrical energy to mechanical work. 

What needed improvement then? 

Currently, DEs are manufactured using either acrylic or silicone and while these have been useful, they also come with certain drawbacks. DEs made from acrylic can handle high levels of strain but they require pre-stretching and lack flexibility. On the other hand, silicone DEs can be made easily but fail to handle high strains. 

Working with the non-profit organization, SRI International (formerly known as Stanford Research Institute), the team at UCLA used commercially available chemicals and an ultraviolet (UV) light-based curing process to improve the acrylic-based DE. 

The researchers were able to change the cross-linking in the polymer chains of the material to make the DE softer, flexible, and simpler to scale without losing endurance or strength. The changes in the manufacturing process allowed the researchers to make thin films of the DE, which they call processable, high-performance dielectric elastomer or PHDE. 

How can PHDE be used? 

A PHDE film is as thin as human hair and equally light in weight. Layering these films can help researchers make miniature actuators that can work like muscle tissue and produce enough mechanical energy to power a small robot.

Soft materials have been layered before. However, the method employed to do so involves the use of a liquid resin that needs to be first deposited and then cured. Such a "wet" process can result in an actuator with uneven layers, which results in bad performance. This is why artificial muscles you might have seen in the past are just one layer thick. 

The UCLA researchers worked on this aspect too and implemented a dry process where the PHDE films are laid down in layers using a blade and then UV-cured. The simplified process has even allowed the researchers to manufacture actuators that resemble spider legs that bend and then jump, or even windup and then spin. 

These new actuators can generate many times more force than biological muscles and are between 3-10 times more flexible than their natural counterparts, the press release claimed. In a demonstration, the researchers showed that the actuator could toss a ball that was 20-times its weight. 

"This flexible, versatile and efficient actuator could open the gates for artificial muscles in new generations of robots, or in sensors and wearable tech that can more accurately mimic or even improve humanlike motion and capabilities," said Qibing Pei, a professor of materials science and engineering at the UCLA. 

The research has been published in the journal Science

Abstract

Dielectric elastomers (DEs) can act as deformable capacitors that generate mechanical work in response to an electric field. DEs are often based on commercial acrylic and silicone elastomers. Acrylics require prestretching to achieve high actuation strains and lack processing flexibility. Silicones allow for processability and rapid response but produce much lower strains. In this work, a processable, high-performance dielectric elastomer (PHDE) with a bimodal network structure is synthesized, and its electromechanical properties are tailored by adjusting cross-linkers and hydrogen bonding within the elastomer network. The PHDE exhibits a maximum areal strain of 190% and maintains strains higher than 110% at 2 hertz without prestretching. A dry stacking process with high efficiency, scalability, and yield enables multilayer actuators that maintain the high actuation performance of single-layer films. 

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