Newly Developed Super Stretchy, Self-Healing Material Paves Way for Artificial Muscles

An experiment conducted in the laboratory of Stanford University’s chemical engineering professor Zhenan Bao has resulted in the creation of a super-stretchy material. It began when one of the Bao’s team members, Cheng-Hui Li was testing the stretchiness of an elastomer, which he had just synthesized.

A new, extremely stretchable polymer film created by Stanford researchers can repair itself when punctured, a feature that is important in a material that has potential applications in artificial muscle. (Image credit: Bao Research Group)

Generally, elastomers can be stretched to two or three times of their original length and will return to their original size. A standard stress test involves stretching an elastomer to the point of breaking. However, Li, a visiting Chinese scholar, faced a hurdle while testing a one-inch sample of the newly developed material. The clamping machine, which was usually used to measure elasticity, could only stretch a material to about 45 inches. Therefore, to detect the breaking point of their elastomer sample, Li and another teammate stretched the material by hand, pulling it at the ends. They were able to stretch the sample over 100 inches.

Bao was very surprised at this outcome. “I said, ‘How can that be possible? Are you sure?'” she recalled.

The team published its findings in the Nature Chemistry journal.

The researchers also demonstrated the expansion and contraction of the new elastomer as the material twitched upon exposure to an electric field, suggesting its potential to be used as an artificial muscle.

A flexible fishnet

Bao stated that although artificial muscles have applications in robotics and some consumer technology currently, they still have a few downsides compared to a real bicep.

If the materials currently used to create artificial muscle have small defects or holes, then the resulting artificial muscle will not possess as much resilience. It also cannot self-repair if scratched or punctured. However, the unique stretchy material provides the adequate solution to this problem, as it has extraordinary self-healing characteristics as well.

When a polymer is deformed, heat treatment or a solvent is typically applied to restore the properties. In the case of the new material, it can heal itself at room temperature regardless if the damaged portions are aged for days. The researchers discovered that it self-repaired at temperatures as low as -4°F (-20°C), which is about as cold as a commercial walk-in freezer.

The researchers claim that the unique stretching and self-healing ability was possible due to certain crucial improvements to a specific chemical bonding process referred to as crosslinking. The chemical bonding process, which involves linking linear chains of connected molecules in a fishnet pattern, has allowed the polymers to stretch 10 times in the past.

As a first step, the team made specially designed organic molecules to connect to the short polymer strands in their crosslink to produce a sequence of structures called ligands. These ligands coupled to produce longer polymer chains, resulting in spring-like coils with innate stretchiness. The researchers added metal ions to the material as these ions have an affinity for the ligands. When this composite material is strained, the knots relax and allow the ligands to detach. Likewise when relaxed, the affinity between the ligands and the ions causes the fishnet to be stretched. The resulting material is a stretchy, strong, and self-repairing elastomer.

Basically the polymers become linked together like a big net through the metal ions and the ligands. Each metal ion binds to at least two ligands, so if one ligand breaks away on one side, the metal ion may still be connected to a ligand on the other side. And when the stress is released, the ion can readily reconnect with another ligand if it is close enough.

Zhenan Bao, Professor of Chemical Engineering, Stanford University

Advancing artificial muscle and skin

The researchers discovered that the new polymer could be tuned to be stretchier or heal faster by simply altering the quantity of added metal ions. The material sample, which had exceeded the limits of the measuring machine, was constructed by minimizing the ratio of iron atoms to the polymers and organic molecules in the material. The researchers also illustrated that this unique polymer, containing the metal additives, would twitch when exposed to an electric field.

The team will have to tweak the concept to enhance the degree to which the material contracts and expands, and to control it in a more precise manner. Going forward, this observation holds promise for new applications.

This research also can be adapted to Bao’s efforts to develop artificial skin that probably could be used to repair certain sensory faculties of people using prosthetic limbs. In earlier studies, Bao’s team had developed flexible but delicate polymers, studded with pressure sensors to spot the difference between a butterfly landing and a handshake. This new, tough material could be a part of the physical structure of a well-developed artificial skin.

Artificial skin is not just made of one material. We want to create a very complex system.

Franziska Lissel, Postdoctoral Fellow, Stanford University

Before artificial muscle and artificial skin are implemented in real world scenarios, this sturdy, flexible, electronically active polymers could be adapted for many other uses, such as a new generation of wearable electronics, or long-use medical implants that do not need replacement or repair.

This current discovery was possible after two years of collaboration, under the leadership of Bao, involving Cheng-Hui Li, a organo-metallic chemist who designed the metal ligand bonding scheme; polymer chemist Chao Wang, now an assistant professor of chemistry at the University of California, Riverside, who had made previous iterations of self-healing elastomers; and artificial muscle expert Christoph Keplinger, now an assistant professor of mechanical engineering at the University of Colorado, Boulder.

Other contributors to the study titled, “A highly stretchable autonomous self-healing elastomer,” include Jing-Lin Zuo, Lihua Jin, Yang Sun, Peng Zheng, Yi Cao, Christian Linder and Xiao-Zeng You.

The Air Force Office of Scientific Research, Samsung Electronics, and the Major State Basic Research Development Program of China supported the Stanford research.

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