Bio-inspired 'smart' materials can self-regulate

Self-powering smart materials that are able to mimic the human body's ability to regulate itself have been developed by engineers in the US.

The advance, made by a team from the University of Pittsburgh and Harvard University, could pave the way for more intelligent and efficient medical implants, as well as so-called smart buildings with more energy saving features.

The researchers also expect that that the materials, called SMARTS (Self-regulated Mechano-chemical Adaptively Reconfigurable Tunable System), could have applications in areas such as robotics and computing.

Structurally, the materials resemble a microscopic toothbrush, with bristles that can stand up or lie down, making and breaking contact with a layer containing chemical 'nutrients'. "Think about how goosebumps form on your skin," explained Joanna Aizenberg, a professor of Materials Science at the Harvard School of Engineering and Applied Sciences. "When it's cold, tiny muscles at the base of each hair on your arm cause the hairs to stand up in an insulating layer. As your skin warms up, the muscles contract and the hairs lie back down to keep you from overheating. SMARTS work in a similar way."

By building dynamic feedback loops into SMARTS from the bottom up, the team was able to integrate the desired regulatory features into the material itself. "Whether it is the pH level, temperature, wetness, pressure, or something else, SMARTS can be designed to directly sense and modulate the desired stimulus using no external power or complex machinery, giving us a conceptually new robust platform that is customisable, reversible, and remarkably precise," said Prof Aizenberg.

To demonstrate SMARTS, the team chose temperature as the stimulus and embedded an array of tiny nanofibers, akin to little hairs, in a layer of hydrogel. The hydrogel, similar to a muscle, can either swell or contract in response to changes in the temperature. When the temperature drops, the gel swells, and the hairs stand upright and make contact with the 'nutrient' layer; when it warms up, the gel contracts, and the hairs lie down. The key aspect is that molecular catalysts placed on the tips of the nanofibers can trigger heat generating chemical reactions in the 'nutrient' layer.

"The bilayer system effectively creates a self-regulated on and off switch controlled by the motion of the hairs, turning the reaction on and generating heat when it is cold," Aizenberg noted. "Once the temperature has achieved a pre-determined level, the hydrogel contracts, causing the hairs to lie down, interrupting further generation of heat. When it cools again below the set-point the cycle restarts autonomously. It's homeostasis, right down at the materials level."

The researchers anticipate that with further refinement the technique could be integrated into materials for medical implants to help stabilise bodily functions, perhaps sensing and adjusting the level of glucose or carbon dioxide in the blood.

"In principle, you can turn anything—heat, light, mechanical pressure—into a chemical signal within the gel," Aizenberg concluded. "Likewise, the reactions triggered by the moving hairs can produce many different types of compensatory responses. By matching signals and responses, we can, in principle, create a wide variety of self-regulating feedback loops."

Laura Hopperton

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