Elastic Electronics: Creating Flexible Materials with Conductive Properties

Catherine Zheng | catherine.zheng@yale.edu January 29, 2020

Elastic Electronics: Creating Flexible Materials with Conductive Properties

Image courtesy of Lawrence Wang

As robotics has blossomed in the past decade, some robots have been perfected and implemented into daily life or put to use in factories for manufacturing goods. However, this is just the tip of the potential of modern robotics.

Through implementing more human-like properties as well as developing artificial intelligence, researchers are continuously working to make robots better and enabling them to act on their own. Significant progress has already been made in making robots closer to resembling humans, allowing them to be more easily integrated into society.

One step that researchers have taken to give robots more human-like properties is through developing soft robots. As opposed to conventional robots, soft robots have a soft structure usually consisting of one complete system that moves by hydraulics or pneumatics. Soft robots are superior to conventional link-and-joint robots in many ways.  First, they are softer and safer around humans than the hard metal frames of conventional robots, which can be rather dangerous. In addition, the manufacturing of soft robots is often more efficient and cheaper than that of conventional robots, which have many individual pieces that need to be crafted separately. Soft robots can be produced all at once.

In order to create a soft robot, the material used must be soft or malleable, as the name suggests.  Previously, soft materials like elastomers (rubber compounds), gels, and fluids were used to create various soft electronics. Liquid crystal elastomers (LCEs), polymers with crosslinked liquid crystals dispersed throughout, have been especially promising for soft electronics. Specifically, they allow for more elasticity than traditional elastomers while maintaining conductivity. When these polymers are exposed to heat, they display a shape-memory effect so that they contract back to their original shape when heated. When the polymer is cooled, it expands whereas heating causes it to contract. With this material, the shape-memory effect can also be achieved through a photochemical transition, meaning that when it is exposed to light, the shape of the elastomer is also affected. 

LCEs are a special type of polymer that, in one state, are made up of molecular units forming an almost crystal structure with the molecules organized into layers. When heated, those molecules become disordered, so the material goes from behaving more like a crystal to a fluid. To make this an elastomer, meaning that it behaves more like a rubber, polymer chains are added to those molecular units. Thus, even when the material is heated and the molecules are distorted, the polymer chains still connect them together and the material still stays intact and rubbery.

A downside of most materials like LCEs and rubbers, however, is that they are not electrically responsive, magnetically responsive, and/or photoresponsive. This means that they are not responsive to stimulation by electricity, magnetism, or light—limiting their uses. In addition, they are mostly thermally insulating. “Once you heat it up, it takes a long time for the material to cool down and return to its original shape,” explained Carmel Majidi, a professor from Carnegie Mellon University. The other issue is that, because LCEs are not electrically conductive, they cannot be heated through electric current flow. To raise their temperature, the researchers had to use a heat gun or a hairdryer instead of using a direct source of electricity.  In order to make LCEs more conductive and responsive, some tried adding metal filler particles. However, these particles made the polymers stiff and brittle, detracting from the elasticity that makes LCEs desirable as a soft material.

In order to resolve these issues, researchers from the University of Texas at Dallas and Carnegie Mellon University—including Majidi—created a material that incorporates a liquid metal alloy made of gallium and indium into LCEs—giving the polymer conductivity without sacrificing elasticity. Gallium and Indium on their own are solid at room temperature, but when mixed in just the right ratio, you end up with an liquid alloy that is an ideal elastomer. “By adding these little microscopic droplets of the liquid metal to any kind of polymer, plastic or rubber, we can tune their thermal and electrical properties,” Majidi explained.

With these elements incorporated into the polymer, this research combines the best of both worlds—the thermal and electrical properties of metals and the elasticity, shape memory, and mechanical properties of these LCEs and soft polymers.

The end goal is to create a material that can “think” on its own, in that it can move and change shape based on its environment rather than direct simulation. The electrical conductivity of this material allows it to respond to external electrical stimuli in an active way. “For example, we could kind of create this composite so that when you punctured it or if you damage it and tore it, the material would basically sense that damage, and then cause electrical signals and current within the material to flow in a certain way, so that the material would suddenly contract or undergo some type of change shape change in response to that mechanical damage,” Majidi said.

Thus, this liquid metal elastomer would be ideal for soft robots since it stretches, bends, moves, and responds to stimuli as a result of the liquid metal microdroplets incorporated into the polymer network. Some think this elastomer could be used as a rubber skin for robots replicating the capabilities of nervous tissue or muscle and allowing them to better sense their environments.

One additional benefit of these polymers or elastomers is that they all work well together. Since their chemistry is very compatible, it is easy to mix and match materials with different uses for a common purpose. Therefore, all of these polymers that researchers have been developing can eventually be combined, each serving a different purpose.

The excitement of soft robots is starting to spread into industry. A startup company called Arieca,  co-founded by Majidi, produces and sells rubber composites filled with little drops of metal alloy. “It’s nice to explore commercial translation of academic research so not just do stuff that is interesting scientifically, but also has commercial, real-world, industrial applications,” said Majidi.

This technology lends itself well to a wide variety of applications. Polymers and materials science are a hot topic of research in the soft robotics field, especially with regards to creating new “robot skins.”  In the future, researchers are hoping to create materials that are more responsive and perform even better, as well as looking into additional applications of this elastomer in prosthetic devices or wearable electronics.

SOURCES

Ford, M. J., Ambulo, C. P., Kent, T. A., Markvicka, E. J., Pan, C., Malen, J., … Majidi, C. (2019). A multifunctional shape-morphing elastomer with liquid metal inclusions. Proceedings of the National Academy of Sciences, 116(43), 21438–21444. doi: 10.1073/pnas.1911021116

Interview with Dr. Carmel Majidi

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