Scientists make new muscle-like material that gets stronger after a 'workout'

This is an Inside Science story.

Researchers from Japan have come up with a way to encourage materials to grow stronger over time, like the muscles in our body. The new technique could allow engineers to design adaptable and healable materials for a wide range of applications.

When we lift weights in the gym, the mechanical stress causes our muscle fibers to rip and tear, but this damaging action actually allows the fibers to regrow stronger afterwards. In contrast, nonliving materials such as rubber bands or paper clips tend to weaken and deteriorate after repeated use.

Drawing inspiration from muscle tissues, a research group from Hokkaido University in Sapporo, Japan, designed a hydrogel that contains two intertwining networks of polymers -- materials formed by long chains of molecules such as those found in plastics.

The hydrogel threads are submerged and stretched in a solution that contains monomers -- building materials for polymers similar to what amino acids are to muscles. The stretching causes the threads to partially tear, then the monomers in the solution allow them to regrow and become stronger.

The researchers observed the properties of the hydrogel threads after repeatedly stretching them and letting them heal. The threads eventually became 1.5 stronger and 23 times stiffer, and they also displayed a considerably bulkier physique, having beefed up to almost twice the original mass.

The researchers suggest this new fabrication technique could be used to customize specific materials to specific challenges, by starting with different polymers and monomer combinations. It may lead to healable and tunable materials useful in biomedical applications, such as for prosthetics and orthotics.

The paper was published today in the journal Science.

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