Smart Materials Get SMARTer
Few synthetic materials are able to mimic the human body’s ability to regulate itself—until now. In Nature, a team of engineers from the University of Pittsburgh and Harvard University, including McGowan Institute for Regenerative Medicine affiliated faculty member Anna Balazs, PhD, distinguished professor of chemical engineering and Robert Von der Luft professor, Department of Chemical and Petroleum Engineering, Pitt’s Swanson School of Engineering, 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 Dr. Balazs. “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, PhD, 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 Dr. 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 Dr. 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 Dr. 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.
Illustration: A strategy for building self-regulating nanomaterials relies on an array of tiny nanofibers, akin to little hairs, embedded in a layer of hydrogel. Simulation image courtesy of Ximin He and Lauren Zarzar, Harvard University.
Abstract (Synthetic homeostatic materials with chemo-mechano-chemical self-regulation. Ximin He, Michael Aizenberg, Olga Kuksenok, Lauren D. Zarzar, Ankita Shastri, Anna C. Balazs & Joanna Aizenberg. Nature; 487,214–218 (12 July 2012)).