Researchers at the Hokkaido University have developed a method to produce hydrogel and materials that strengthen in response to mechanical stress – mimicking growth of the skeletal muscle.
The new strategy is promising for long-lasting materials that can adjust and become stronger depending on the surrounding conditions, according to the researchers, whose findings are detailed in paper published in the journal Science.
For the strategy, the team drew inspiration from process that strengthens human skeletal muscles. Due to strength training at the gym, for instance, muscle fibers break down, influencing new, stronger fibers formation. For this phenomenon, the muscles must be delivered with amino acids, the building blocks of proteins, which combines together for the formation of new muscle fibers.
The team of researchers led by Jian Ping Gong from the Hokkaido University created a strategy using ‘double-network hydrogels’ that mirrors the building up of skeletal muscles. These hydrogels are soft but tough materials made of around 85% of water weight and two different types of polymer networks – one soft and stretchable and other rigid and brittle.
When the double-network hydrogel is placed inside a solution containing ‘monomers’, a type of molecules which can join together to form larger compounds or polymers, it mimics a role of the blood circulation which carries amino acids to the skeletal muscles.
The team then applied tensile force to the hydrogel which caused few of its rigid and brittle polymer chains to split, leading to fabrication of a chemical species known as ‘mechanoradicals’ at the tails of the broken polymer chains.
According to the researchers, these mechanoradicals can induce the monomers – absorbed into the hydrogel from the surrounding solution – to join together and form a polymer network, thereby strengthening the material.
Successive stretching or application of tensile force causes more breaking down and building up, similar to that of skeletal muscles receiving strength training. Through this process, strength of the hydrogel improved 1.5 times and stiffness 23 times, while the polymers’ weight surged by 86%.
In addition, the researchers were able to adjust the material’s response to stretching or mechanical force by employing a particular monomer that changed the hydrogel’s reaction to heat; when heated at high temperatures, surface of the hydrogel become more resistant to water.
The new development could help fabricate self-growing materials for applications such as flexible exosuits for individuals suffering from skeletal injuries; the more these suits are used, they would potentially become more functional and stronger, Prof. Gong said.