Highly Stretchable Elastomer May be Candidate for Artificial Muscles

Science magazine highlighted a study in Nature Chemistry with the headline "Artificial muscle can heal itself". While the study describes an amazing elastomer, there are only minimal muscle applications.

Researchers from Stanford University developed a new silicone rubber elastomer, based on a poly(dimethylsiloxane) (PDMS)-derivative. The material can be stretched to 45 times its original length, a characteristic that the team attributed to the constant breaking and reforming of metal-ligand coordination bonds throughout stretching. The material can recover to its original length in one hour, with almost full recovery of its original stress-strain behaviour. The team cut the material in two, and then rejoined the ends. After 48 hours at room temperature, the material exhibited 90% healing efficiency. Furthermore, this healing ability was unaffected by surface ageing – that is, if the cut ends were left apart and exposed to air for 24 hours and then rejoined, the material saw no difference in healing efficiency compared to when the cut ends were immediately rejoined. This is in contrast with materials that rely on hydrogen bonding, where immediate re-contact is necessary for efficient healing. These characteristics are impressive, and appear to address two major issues of metal-ligand self-healing to heal strongly without an external stimulus.

Now onto the artificial muscle part. Given the dieletric strength of the material, the team tested its reactivity to an externally-applied electric field. The material could be seen to deform when cycled between 0 and 11 kV. However, deformations were minimal and the team still needs to characterize the contractile force, which is also likely minimal. Furthermore, how the material would behave in vivo as an artificial muscle implant is unclear: is it toxic? does it behave similarly in biological fluids? how would it be powered? Outside the body, perhaps the material could be applied as an actuator for prosthetic devices.

Calling this material artificial muscle is a…stretch. Nevertheless, here the researchers have created a good approach to preparing highly stretchable materials. The applications for self-healing materials are numerous, including anti-corrosion coatings and anti-crack structures. While a typical limitation is the difficulty in automatically joining the cut ends after damage, we could foresee this material contracting under an electric field to pull its damaged ends together and heal.

Science: Artificial muscle can heal itself…

Study in Nature Chemistry: A highly stretchable autonomous self-healing elastomer…

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