Many warm-blooded animals and many living organisms regulate their internal systems via homeostasis. Using efficient interconversions of chemical and mechanical energy through self-regulating feedback loops, they can keep tight control over their bodies’ conditions. Dr. Tracy He lectures on the concept of applying the concept of homeostasis to artificial systems using autonomous, “smart” materials. Chemical energy of catalytic reactions and the mechanical energy of an environmentally responsive hydrogel interact to create self-powered feedback loops on the nano- or micro- scale.
On Friday, April 19, Dr. Tracy He from the Wyss Institute for Biologically Inspired Engineering at Harvard University spoke about the concept of man-made self-regulation as a part of the Jones Seminar Series. She outlined the essential theory as well as practical techniques for implementing her “SMARTS,” or Self-regulated Mechanochemical Adaptively Reconfigurable Tunable Systems (1).
Scientists have long pursued creating systems that respond to and sense environmental stimuli and chemical signals like warm-blooded animals do. Given the global problems in energy and health-care, these sorts of systems could be adapted to address a wide spectrum of needs.
Dr. He discussed the biological models after which her systems are based: warm-blooded animals and many living organisms maintain homeostasis. Through efficient interconversions of chemical and mechanical energy through self-regulating feedback loops, they control internal conditions such as temperature or pressure (1, 2). The SMARTS systems that she and her team develop are based on a precisely tailored chemo-mechano-chemical feedback loops based on two components analogous to the skeleton and muscles of a biological system. When combined, chemical energy is converted to mechanical energy and then funneled back into chemical energy in a self-powered feedback loop (1).
The “skeleton” structure is chosen for its high aspect ratio microstructures (e.g. a structure that is much taller than it is wide). Within this constraint, a variety of materials, such as elastic polymers and rigid semiconductors, and skeleton shapes, such as pillars, blades, fins, and hexagonal cells, are available for use in tailoring the SMART system to its particular function (1). The “muscle” structure involves stimuli-responsive hydrogels composed of cross-linked polymer networks that swell or shrink in aqueous environments in response to stimuli. They provide a means to translate environmental stimuli to a mechanical response from the “skeleton” structure (1).
Pouring the hydrogel around the selected structure combines the two into the beginnings of a system. When the hydrogel is stimulated and shrinks, it creates tension in the structure (i.e. pillars) that will cause a mechanical shift, such a bending over from an erect position (1).
Various techniques are used to create the chemo-mechanical feedback loops. Often, two layers of hydrogels are combined to form a bilayer with particular substrates in each layer. In addition, researchers coat the tips of pillars with a thin film of gel containing catalysts, dyes, or other helpful substances that will react or facilitate reactions with the substrates in a particular layer of the hydrogels (1).
For example, for a particular experiment replicating temperature homeostasis, the skeletal structure tips were coated with a reagent that would react to create heat in the top layer of the hydrogel bilayer. However, the hydrogel in the bottom layer is stimulated to shrink when the temperature exceeds a certain value. When it shrinks, the pillars bend and remove the tips from contact with the top layer of hydrogel, stopping heat generation (1). And thus, basic temperature homeostasis is achieved.
Continuing with the same example, as the system cools down, the hydrogel will swell again, allowing the pillars to become erect and beginning the process again in a periodic feedback loop. This system regulated itself for six hours with no intervention from the researchers and displayed self-oscillation of temperature (1).
The possibilities afforded by SMARTS could substantially impact the design of such materials. Batteries could be engineered to regulate their own energy output. In the interest of energy efficiency, buildings could regulate their own temperatures without additional energy input. Biological systems such as our own could be helped out by artificial systems when bodily self-regulation goes wrong (1, 2).
Of course, challenges remain in the limitations of the lifetimes of these systems. Though the above example discussed a six-hour span of self-oscillating temperature, researchers would have to design systems usable for years in order to achieve practical applications. Dr. He concluded with guarded optimism for the outlook of the systems, anticipating further innovation and collaboration (1).
1) T. He. “Warm-Blooded Plastics: Bio-Inspired Solutions to Smart Materials Systems.” Thayer School of Engineering, Dartmouth College. Hanover, 19 April 2013.
2) Jones Seminar: Warm-Blooded Plastics: Bio-Inspired Solutions to Smart Materials Systems (19 April 2013). Available at http://engineering.dartmouth.edu/events/warm-blooded-plastics-bio-inspired-solutions-to-smart-materials-systems/ (20 April 2013).