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

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Abstract

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.

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Figure 1: General design of SMARTS.
Figure 2: Oscillations in exemplary chemical reactions triggered by pH changes.
Figure 3: Homeostasis in SMARTS via self-regulated chemo-thermo-mechanical feedback loops.
Figure 4: Computer simulations of the self-sustained thermal regulation.

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Acknowledgements

We thank P. Kim for assistance with the gel formulation, M. Khan for microstructure fabrication, R. S. Friedlander for assistance with confocal imaging, M. Kolle and A. Ehrlicher for technical assistance, and A. Grinthal for help with manuscript preparation. The work was supported by the US DOE under award DE-SC0005247 (experiment) and by the US NSF under award CMMI-1124839 (computational modelling).

Author information

M.A. and J.A. planned the project and supervised the research. X.H. and M.A. designed and conducted the experiments and data analysis. X.H., L.D.Z. and A.S. conducted the characterization. X.H. and L.D.Z. carried out microfluidic device design. A.S. carried out hydrogel deposition optimization. O.K. and A.C.B. developed the model and numerical code and carried out the computational simulations. All authors wrote the manuscript.

Correspondence to Joanna Aizenberg.

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The authors declare no competing financial interests.

Supplementary information

Supplementary Information

This file contains Supplementary Text and Data 1-5, Supplementary Figures 1-10 and additional references. (PDF 1813 kb)

Supplementary Movie 1

This movie shows pulsed O2 gas generation controlled by the actuation of microfins within a microfluidic channel. Alternating flow of aqueous solution of pH 3 (yellow) and pH 6 (purple) was used to control the actuation of the microstructures out of and into the H2O2 layer and thus the cyclic bubble generation. The colour arises from bromophenol blue indicator. Movie played at a realtime speed. (MOV 8431 kb)

Supplementary Movie 2

This movie shows a self-regulated chemo-mechanical feedback system showing self-contained autonomous oscillation and the corresponding temperature regulation during 6 h. The movie presents three cycles at the beginning, two cycles 3 h later, and two cycles 6 hours later. Movie played at 60x speed. (MOV 11769 kb)

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He, X., Aizenberg, M., Kuksenok, O. et al. Synthetic homeostatic materials with chemo-mechano-chemical self-regulation. Nature 487, 214–218 (2012) doi:10.1038/nature11223

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