Whether it is a structural beam supporting an apartment building or a piece of Scotch tape holding together a school science project, we're accustomed to the idea of materials becoming weaker as they age, but a new creation at the University of Chicago (UC) might just flip the script in this regard. Engineers have developed a new type of gel that can actually strengthen over time due to vibrations and stress, by taking inspiration from the way our bones adapt their mass and strength to support different loads through our lifetimes.
“Every other material becomes weaker when vibrated,” says Aaron Esser-Kahn, who led the research team. “This is the first time we have reversed that process, showing that a material can strengthen itself with mechanical vibration.”
While the adaptive nature of human bone provided part of the inspiration for Esser-Kahn and his team, the other part came from what's known as the piezoelectric effect. This refers to devices and materials that can take mechanical stress and vibration and convert that movement into an electrical charge. We've seen how this technology can be integrated into shoes, roads, and laptop keyboards, but the UC team describe this particular application as the first of its kind.
The researchers started with the idea that perhaps a charge produced through the piezoelectric effect could be used to trigger a reaction within a material that boosts its strength. So they got to work experimenting with different chemistries in an effort to produce a gel with the correct properties. After working their way through dozens of mixes the team eventually landed on a winner, consisting of a polymer gel with a mix of what's known as thiol-ene reactors and zinc oxide piezoelectric particles.
This material can be strengthened through by vibration owing to particles that transduce energy and initiate a thiol-ene reaction, which has the effect of creating new cross-links in the material. In testing, the team turned this soft gel into a material with a stiffness close to that of bone interior, 66 times its own strength, just by subjecting it to vibrations. Interestingly, the material strengthened more in areas where it received the greatest stress.
“Just like bone, the material strengthened to the exact amount of power we put into it,” Esser-Kahn said.
The team sees many possibilities for a material of this type, including buildings that grow stronger with age, or medical implants that integrate with the body more effectively. One area of real promise is adhesives, such as those used in aerospace engineering to meld different materials together.
“Adhesives could be hugely influenced by this,” Esser-Kahn said. “Adhesives are almost always the point of failure in materials. This could lead to specialized adhesives that adhere and set much better.”
The research was published in the journal Nature Materials.
Source: University of Chicago
