Smart Materials have SMARTS

In the July 12 issue of Nature, a Harvard-led research team showcased a strategy for building self-thermoregulating nanomaterials that can be tailored to maintain a determined temperature, pressure, or any other measure by meeting the environmental changes with a compensatory chemical feedback response. 

This group of materials is called Self-regulated Mechano-chemical Adaptively Reconfigurable Tunable System, or SMARTS for short. As the name suggests, this group of innovative materials has offers a customizable way to automatically turn on and turn off chemical reactions in a way that mimics how biological systems naturally adjust to the dynamics of their surroundings. 

This development greatly benefits medical implants because it allow for more intelligent and efficient medical implants. In terms of its structure, SMARTS looks like a microscopic toothbrush with tiny fibers capable of either standing up or lying down, which results in creating or breaking contact with the later of the material that contains chemical 'nutrients'. Joanna Aizenberg, a lead author of the research, explains the structure of SMARTS as something similar to the hair on a person's arms. “When it is cold out, 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 works in a similar way.”

At first glance, SMARTS may not seem like a big deal; there already exist glasses that dim and brighten depending on the intensity of light and piezocrystals that can convert vibrations into electrical signals, but one major downfall of these two examples is that they response to one specific stimuli and cannot self-regulate. 

A demonstration of the material can be seen in the video below. 

In the video, the stimuli used is temperature and a hydrogel is embedded with an array of tiny nanofibers, which cause the hydrogel to either expand or contract in response to the temperature changes. 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. 

Figure 1: Process of a homeostatic material maintaing constant temperature.

Aizenberg likens the process to homeostasis, saying, "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. 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." 

Through additional refinement, the technique could eventually be incorporated into the material of medical implants to help stabilize bodily functions. Some examples include sensing and adjusting glucose or carbon dioxide levels in the blood.