Few synthetic materials are able to mimic the human body's ability to regulate itself--until now. In the July 12 issue of Nature, a team of engineers from the University of Pittsburgh and Harvard University has presented a strategy for building self-regulating microscopic materials, ultimately paving the way toward so-called smart buildings with more energy-saving features and smarter biomedical engineering applications.
"Consider, for example, what happens when a typical hair dryer becomes too hot: The device just shuts off. The hair dryer does not, however, turn itself back on when the system has cooled down. Hence, this is a very passive way of regulating temperature," said Anna Balazs, research team co-investigator and Distinguished Professor of Chemical and Petroleum Engineering in Pitt's Swanson School of Engineering. "Our design is a much more active way of continuously sensing and regulating the temperature. It's another step toward making smart materials that are just as conscious of their internal workings as the human body is of its inner mechanisms."
The Pitt team, which also included Olga Kuksenok, a research professor in Pitt's Swanson School of Engineering, crafted a new multi-scale model for the novel material, created by embedding "posts," or tiny hairs, into a hydrogel.
"This model captured salient features of our experimental work, including the presence of two fluids lying above the embedded posts and the posts' tips (decorated with catalysts), which interact with chemical reagents in the upper fluid and thereby produce heat," said Kuksenok. "Thus, the scale model captured the components and range of complex phenomena occurring within our experimental system."
This model helped the Harvard team optimize the behavior of the system that the Pitt team created, which they later called SMARTS--a Self-regulated Mechano-chemical Adaptively Reconfigurable Tunable System. SMARTS offers a customizable way to trigger chemical reactions on cue and reproduce the type of stable feedback loops found in biological systems. By building SMARTS from the bottom up, the Harvard team was able to integrate the desired features into the material itself. Whether it is a pH level, temperature, or pressure, SMARTS can directly interact with the desired stimulus, presenting a platform that is customizable, reversible, and efficient.
To demonstrate SMARTS, the team selected temperature as the stimulus. With the posts in the upright position, the tips were able to interact with reagents in the upper fluid layer and thereby generate heat, which then caused the temperature-sensitive gel to shrink. When the gel shrank, the posts bent away from the reagents, and the temperature of the system eventually cooled down. This caused the gel to expand and, consequently, caused the posts to assume an upright configuration.
"The reconfiguration of the gel creates an on/off switch of sorts for the system," said Kuksenok. "The system oscillates back and forth between these two states and, in this manner, regulates the overall temperature. While none of the individual components exhibit oscillatory behavior, the combination of these elements leads to an oscillatory system, which maintains the temperature at a constant level."
The researchers anticipate that this technique could be integrated into handheld portable diagnostics, which are playing a growing role in bringing medicine to developing or rural areas.
"Many biomedical analyses require specific temperatures, pH, or other conditions and are hard to do outside a lab, but if a portable device contains homeostatic materials that can autonomously regulate these conditions, it could bring many more sophisticated analyses to many more people," said Balazs.
According to the Pitt researchers, SMARTs is also an ideal "laboratory" to study the fundamental properties of biological and chemical systems, such as how living systems are able to so efficiently convert between chemical and mechanical processes; furthermore, they believe the mechanical motion of the hair-like posts could be put to work or used for propulsion, like cilia in a living organism.
Balazs and Kuksenok collaborated with Harvard University researchers Joanna Aizenberg, the Amy Smith Berylson Professor of Materials Science and the Susan S. and Kenneth L. Wallach Professor in the Radcliffe Institute for Advanced Study; Ximin He, a postdoctoral research associate in the Wyss Institute of Bioinspired Engineering and the School of Engineering and Applied Science; and Michael Aizenberg, senior staff scientist at Wyss. The Pitt professors also collaborated with Harvard postdoctoral research associates Lauren D. Zarzar and Ankita Shastri, both in Joanna Aizenberg's laboratory.
The research team received support from the U.S. Department of Energy and the National Science Foundation.