Synthetic homeostatic materials with chemo-mechano-chemical self-regulation

Abstract

A bilayer material comprising catalyst-bearing microstructures embedded in a responsive gel and actuated into and out of a reactant-containing ‘nutrient’ layer continuously interconverts chemical, thermal and mechanical energy and thereby shows autonomous, self-sustained homeostatic behaviour, which regulates the temperature of the system in a narrow range. Taking their cue from the ability of living organisms to maintain control over their local environment through homeostasis, Joanna Aizenberg and colleagues have developed a way to produce synthetic homeostatic materials that autonomously regulate a wide range of parameters at the micrometre scale through a series of chemo-mechanical feedback loops. They describe a bilayer of hydrogel-supported catalyst-bearing microstructures separated from a reactant-containing ''nutrient'' layer. Reconfiguration of the gel in response to a stimulus induces reversible actuation of the microstructures in and out of the nutrient layer, and serves as an on/off switch for chemical reactions — a sort of artificial homeostasis. This design triggers organic, inorganic and biochemical reactions that undergo reversible, repeatable cycles that are synchronized with the motion of the microstructures and the driving external chemical stimulus. The authors suggest that SMARTS (self-regulated mechano-chemical adaptively reconfigurable tunable systems) could be tailored to modulate variables such as light, pH, glucose, pressure and oxygen. Applications might include robotics, biomedical engineering and building materials. Living organisms have unique homeostatic abilities, maintaining tight control of their local environment through interconversions of chemical and mechanical energy and self-regulating feedback loops organized hierarchically across many length scales1,2,3,4,5,6,7. In contrast, most synthetic materials are incapable of continuous self-monitoring and self-regulating behaviour owing to their limited single-directional chemomechanical7,8,9,10,11,12 or mechanochemical13,14 modes. Applying the concept of homeostasis to the design of autonomous materials15 would have substantial impacts in areas ranging from medical implants that help stabilize bodily functions to ‘smart’ materials that regulate energy usage2,16,17. Here we present a versatile strategy for creating self-regulating, self-powered, homeostatic materials capable of precisely tailored chemo-mechano-chemical feedback loops on the nano- or microscale. We design a bilayer system with hydrogel-supported, catalyst-bearing microstructures, which are separated from a reactant-containing ‘nutrient’ layer. Reconfiguration of the gel in response to a stimulus induces the reversible actuation of the microstructures into and out of the nutrient layer, and serves as a highly precise ‘on/off’ switch for chemical reactions. We apply this design to trigger organic, inorganic and biochemical reactions that undergo reversible, repeatable cycles synchronized with the motion of the microstructures and the driving external chemical stimulus. By exploiting a continuous feedback loop between various exothermic catalytic reactions in the nutrient layer and the mechanical action of the temperature-responsive gel, we then create exemplary autonomous, self-sustained homeostatic systems that maintain a user-defined parameter—temperature—in a narrow range. The experimental results are validated using computational modelling that qualitatively captures the essential features of the self-regulating behaviour and provides additional criteria for the optimization of the homeostatic function, subsequently confirmed experimentally. This design is highly customizable owing to the broad choice of chemistries, tunable mechanics and its physical simplicity, and may lead to a variety of applications in autonomous systems with chemo-mechano-chemical transduction at their core.