SMARTS: Self-regulating Artificial Material Systems

Living organisms exhibit unique homeostatic abilities, maintaining tight control of their local environment through inter-conversions of chemical and mechanical energy and self-regulating feedback loops organized hierarchically across many length scales. In contrast, most synthetic materials are incapable of undergoing continuous self-monitoring and self-regulating behavior. Applying the concept of homeostasis to the design of autonomous materials would have transformative impacts in areas ranging from medical implants that help stabilize bodily functions to smart materials that regulate energy usage. We have explored a versatile strategy for creating self-regulating, self-powered, homeostatic materials capable of precisely tailored chemo-mechano-chemical feedback loops at the nano/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 in and out of the nutrient layer and serves as a highly precise “on/off” switch for chemical reactions. We apply this design to trigger a variety of processes—fluorescence quenching, catalytic decomposition of hydrogen peroxide, or a complex enzymatic reaction—that undergo reversible, repeatable cycles synchronized with the motion of the microstructures and the driving external chemical stimulus. 

In this manner, we create exemplary internally-regulated, self-sustained homeostatic systems, SMARTS (Self-regulated Mechano-chemical Adaptively Reconfigurable Tunable System), that maintain a user-defined parameter—temperature—by exploiting a continuous feedback loop between various exothermic catalytic reactions in the nutrient layer and the mechanical action of the temperature-responsive gel. The experimental results were validated using computational modeling that qualitatively captured the essential features of the self-regulating behavior and provided additional criteria for the optimization of the homeostatic function, subsequently confirmed experimentally. This design is highly customizable due to the broad choice of chemistries, tunable mechanics, and physical simplicity, thus promising exciting applications in autonomous systems with chemo-mechano-chemical transduction at their core. 

Publications

Shastri A, McGregor LM, Liu Y, Harris V, Nan H, Mujica M, Vasquez Y, Bhattacharya A, Ma Y, Aizenberg M, et al. An aptamer-functionalized chemomechanically modulated biomolecule catch-and-release system. Nat. Chem. [Internet]. 2015;7 (5) :447-454. Full TextAbstract
The efficient extraction of (bio)molecules from fluid mixtures is vital for applications ranging from target characterization in (bio)chemistry to environmental analysis and biomedical diagnostics. Inspired by biological processes that seamlessly synchronize the capture, transport and release of biomolecules, we designed a robust chemomechanical sorting system capable of the concerted catch and release of target biomolecules from a solution mixture. The hybrid system is composed of target-specific, reversible binding sites attached to microscopic fins embedded in a responsive hydrogel that moves the cargo between two chemically distinct environments. To demonstrate the utility of the system, we focus on the effective separation of ​thrombin by synchronizing the pH-dependent binding strength of a ​thrombin-specific aptamer with volume changes of the pH-responsive hydrogel in a biphasic microfluidic regime, and show a non-destructive separation that has a quantitative sorting efficiency, as well as the system's stability and amenability to multiple solution recycling.
Grinthal A, Aizenberg J. Adaptive all the way down: Building responsive materials from hierarchies of chemomechanical feedback. Chem. Soc. Rev. [Internet]. 2013;42 (17) :7072-7085. Publisher's VersionAbstract
A living organism is a bundle of dynamic, integrated adaptive processes: not only does it continuously respond to constant changes in temperature, sunlight, nutrients, and other features of its environment, but it does so by coordinating hierarchies of feedback among cells, tissues, organs, and networks all continuously adapting to each other. At the root of it all is one of the most fundamental adaptive processes: the constant tug of war between chemistry and mechanics that interweaves chemical signals with endless reconfigurations of macromolecules, fibers, meshworks, and membranes. In this tutorial we explore how such chemomechanical feedback – as an inherently dynamic, iterative process connecting size and time scales – can and has been similarly evoked in synthetic materials to produce a fascinating diversity of complex multiscale responsive behaviors. We discuss how chemical kinetics and architecture can be designed to generate stimulus-induced 3D spatiotemporal waves and topographic patterns within a single bulk material, and how feedback between interior dynamics and surface-wide instabilities can further generate higher order buckling and wrinkling patterns. Building on these phenomena, we show how yet higher levels of feedback and spatiotemporal complexity can be programmed into hybrid materials, and how these mechanisms allow hybrid materials to be further integrated into multicompartmental systems capable of hierarchical chemo-mechano-chemical feedback responses. These responses no doubt represent only a small sample of the chemomechanical feedback behaviors waiting to be discovered in synthetic materials, and enable us to envision nearly limitless possibilities for designing multiresponsive, multifunctional, self-adapting materials and systems.

Media Coverage

Catching and releasing tiny molecules, Harvard press release, March 23, 2015. 

Hydrogels: The catalytic curtsey, Nature Materials, July 24, 2012. 

Materials with SMARTS, Chemical & Engineering News, July 16, 2012. 

Nanomaterial duplicates self-regulation of living organisms, IEEE Spectrum, July 13, 2012. 

New SMART materials regulate, respond to their environment, Txchnologist, July 12, 2012. 

Homeostatic hydrogels to help heat the home, Chemistry World, July 12, 2012. 

Smart buildings, Nature's weekly podcast (at 06:49), July 12, 2012. 

Smart materials get SMARTer, Harvard press release, July 11, 2012.