New polymer could potentially lead to development of artificial muscle

(credit: Bao Research Group)
Credit: Bao Research Group.

Whether it’s a small cut, a sprained ankle, or a pulled muscle, the human body will heal. An artificial muscle, on the other hand, would not experience the same phenomenon. Electrically-responsive polymers used to generate artificial muscles for things such as haptic systems (which revolve around the science of touch) and experimental robots come to the end of their useful lives once they experience any form of mechanical damage. But what if this wasn’t the case?

A human muscle can endure particularly high levels of strain, due to its elasticity. However, to maintain this over a lifetime, the muscle must be able to heal itself. Artificial muscles aim to mimic these characteristics as closely as possible, but there aren’t many materials that currently tick the right boxes.

Scientists from Stanford University have created a new polymer—called Fe-Hpdca-PDMS—that’s extremely stretchy and self-healing, and therefore shows potential for use as an artificial muscle. The team, led by materials scientist Zhenan Bao, is making significant progress toward developing a more resilient material that could be used as artificial muscles and may potentially lead to breakthroughs in the fields of robotics and prosthetics. The polymer, which has been categorised as an elastomer (i.e., a rubber-like polymer) can stretch to about hundred times its length; a 2.5-centimeter sheet can be stretched out to a length of 2.5 meters. Usually, elastomers stretch to two or three times their original length and spring back to original size.

The material also has remarkable self-healing properties – it can fuse back together if punctured. Usually, damaged polymers require a solvent or heat treatment for their properties to be restored, but Bao’s material demonstrates the ability to mend itself at room temperature (and even lower temperatures) regardless of how old the damaged pieces are.

The material is described in depth by Bao’s group in the journal Nature Chemistry. The team’s findings include the fundamental discovery that the polymer’s characteristics can be adjusted quite simply, suggesting that this adaptability could enable researchers to tailor the material as required. For example, if a stretchier or faster healing material was desired, they could achieve this by changing the quantity or type of metal ions included in the material, rather than having to create a new material all together.

The internal structure of the material resembles a ‘fishnet pattern’ consisting of linear chains of linked molecules. The process of generating the pattern involves pre-designed organic molecules, which attach to the short polymer strands in their crosslink. This creates a series of structures called ligands (i.e., ions or a molecule attached to a metal atom by a form of covalent bonding) which in turn are combined together to form longer polymer chains. These chains become string-like coils with the required stretchiness.

On a microscale, the composition of these polymer chains consist of oxygen, carbon, silicon, and nitrogen atoms mixed with an iron salt. Chemical bonds are formed between the iron and the nitrogen and oxygen atoms, joining the polymer chains to one another in crosslinks.

Next come the metal ions, which have a chemical affinity for the ligands. This affinity arises because the iron molecules are attracted to the nitrogen and oxygen molecules in the ligands. When this combined material is strained, the knots loosen and allow the ligands to separate. When relaxed, the affinity between the metal ions and the ligands pulls the fishnet taut.  “Basically, the polymers become linked together like a big net through the metal ions and the ligands,” Professor Zhenan Bao told the Economic Times.

These crosslinks allow the polymer chains to move together without sliding away altogether, which in turn enables the material to stretch. When the crosslinks are stretched, broken, or rearranged, the material changes its shape accordingly. The researches have stated that, on the microscale, it is in fact the iron-ligand bonds which can readily break and re-form, while the iron centres remain attached to the ligands through stronger interactions within the overall structure. Even when cut in half, the elastomer can join back together, provided that the edges are kept within close proximity. This actually occurs because the iron molecules on one edge of the cut material become attracted to the nitrogen and oxygen molecules on the other edge and, in turn, fuse back together. After undergoing this self-healing process, the material continues to retain 90% of its elasticity and strength.

This instantaneous healing is highly beneficial when it comes to making progress in the field of artificial muscle development. Bao states, “Artificial muscles are typically very sensitive to defects and pinholes”. This generally poses a large problem, due to the fact that these defects “really affect their actuation performance”. The presence of a self-healing characteristic in artificial muscles could potentially avoid such defect-induced problems and, as a result, lead to higher durability.

When it comes to comparing this newly developed self-healing polymer to other already existing self-healing polymers, there are a number of distinctions to be made. For example, some other self healing materials will only reform under stringent conditions. Those healed due to weak hydrogen bonds are sensitive to water vapor in the air, whereas Fe-Hpdca-PDMS can sit out in open air for days without any damage. Other designs have to be heated or compressed to trigger the reforming reactions, whereas the Fe-Hpdca-PDMS heals at room temperature without any extra nudging. When it comes to artificial muscles, scientists have so far been using materials such as conducting polymers, ionically conducting polymers, and carbon nanotubes, as well as elastomers of different types. One of these groups consists of electric electroactive polymers (EAPs), which show particularly promising characteristics. Their ability to emulate the operation of biological muscles with high fracture toughness, large actuation strain, and inherent vibration damping make them a good candidate for artificial muscle development. This material does not, however, show any self-healing properties.

There are downfalls for Fe-Hpdca-PDMS, too. Artificial muscles need to respond pretty effectively to electric fields in order to be compatible for use for application in prosthetics and robotics. A biological muscle will usually change its length byup to 40%. However, when an electric field is applied to the material developed by Bao, the change is only 2% – a significant drawback for a muscle.

Nonetheless, there are further opportunities for the polymer to be adjusted and modified in order to make it suitable for use as an artificial muscle. Indeed, Bao says that artificial muscles are just one possible application for these sorts of materials. The research believe the material’s ability to restore a high dielectric strength after recovery from mechanical damage presents further opportunities when it comes to the preparation of highly stretchable functional materials. While more work is needed to control the material more precisely, the researchers have said that their work opens the door to other promising applications, such as artificial skin. They’re now applying this simultaneous break-and-repair chemistry to other kinds of electronic polymers.

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