Materials that get stronger in response to mechanical stress have been developed by polymer scientists from Hokkaido University. The strategy devised by Jian Ping Gong and her team uses double-network hydrogels to emulate the building process of skeletal muscles.
Double-network hydrogels are a soft, yet durable material formed of about 85 wt% water and two types of polymer networks: one rigid and brittle, and the other soft and stretchable. The researchers placed a double-network hydrogel inside a solution containing molecules (monomers), which can be joined to form larger compounds (polymers). This solution simulates the role of circulating blood carrying amino acids to skeletal muscles.
Applying tensile force to the hydrogel causes some of its rigid and brittle polymer chains to break, leading to the generation of a chemical species called mechanoradicals at the ends of the broken polymer chains. These mechanoradicals can trigger the joining up of the monomer absorbed into the hydrogel from the surrounding solution into a polymer network, strengthening the material.
With successive stretching, more breaking down and building up occurs, similar to what happens with skeletal muscles undergoing strength training. Through this process, the hydrogel’s strength and stiffness improved 1.5 and 23 times, respectively, and the weight of the polymers increased by 86%. Further, the team was able to tailor the material’s response to mechanical force by using a specific monomer that altered the gel’s reaction to heat; heated at high temperatures, the gel’s surface became more water-resistant.
The researchers say their work could help with the development of self-growing gel materials for applications as flexible exosuits for patients with skeletal injuries; these suits would potentially become stronger and more functional the more they are used.