Scientists a Hokkaido University have found a way to create materials that actually get stronger the more you use them. By mimicking the mechanism that allows living muscles to grow and strengthen after exercise, the team led by Jian Ping Gong developed a polymer that breaks down under mechanical stress, then regrows itself into a stronger configuration by feeding off a nutrient bath.

One of the drawbacks of non-living materials is that they have a very finite service life compared to living, organic materials. Materials like steel, plastic, ceramics, and textiles wear out with use at a surprisingly fast rate compared to comparable living things. Metals undergo fatigue, plastics crumble, ceramics crack, and textiles have a sadly short life compared to the skin they cover.

The reason for this is that living tissue can not only regrow itself, it can become stronger the more it's used. That's why a human heart can pump at a rate of about 72 beats per minute, 24 hours a day, 365 days a year, for over a century. It's also why exercise can make skeletal muscles stronger. A workout in the gym that makes a human healthier would just be so much wear and tear to a machine.

Put simply, muscles become stronger because during exercise the fibers are broken down and then replaced by new, stronger fibers that feed off the amino acids supplied by the bloodstream to form proteins. In recent years, scientists and engineers have come up with new, self-healing materials that can repair themselves when damaged or worn out, but what about a material that, like muscle tissue, can become stronger with use?

To achieve this, the Hokkaido team used what is called double-network hydrogels. Like other hydrogels, these are polymers that are 85 percent water by weight, but in this case, the material consist of both a rigid, brittle polymer and a soft, stretchable one. In this way, the finished product is both soft and tough.

Graph comparing the muscle-like hydrogel with other materials Hokkaido University

However, the clever bit is that under laboratory conditions the hydrogel was immersed in a bath of monomers, which are the individual molecular links that make up a polymer. These serve the same function in the muscle-mimicking material as amino acids do in living tissue.

According to the team, when the hydrogel is stretched, some of the brittle polymer chains break, creating a chemical species called "mechanoradicals" at the end of the broken polymer chains. These are very reactive and quickly join up with the floating monomers to form a new, stronger polymer chain.

Under testing, the hydrogel acted much like muscles under strength training. It became 1.5 times stronger, 23 times stiffer, and increased in weight by 86 percent. It was even possible to control the properties of the material by using heat-sensitive monomers and applying high temperatures to make it more water resistant.

Gong says this approach could lead to materials suitable for a variety of applications, such as in flexible exosuits for patients with skeletal injuries that become stronger with use.

The research was published in Science and the video below discusses the new hydrogel material.

Source: Hokkaido University