Living organisms exhibit unique homeostatic abilities, maintaining tight control of their local environment through inter-conversions of chemical and mechanical energy and selfregulating feedback loops organized hierarchically across many length scales 1-7 . In contrast, most synthetic materials are incapable of undergoing continuous self-monitoring and self-regulating behavior due to their limited single-directional chemo-mechanical 7-12 or mechano-chemical 13, 14 modes. Applying the concept of homeostasis to the design of autonomous materials 15 would have transformative impacts in areas ranging from medical implants that help stabilize bodily functions to smart materials that regulate energy usage 2, 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 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 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 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 chemomechano-chemical transduction at their core.The survival of organisms relies on homeostatic functions such as the maintenance of stable body temperature, blood pressure, pH, and sugar levels 1,3,[5][6][7] . This remarkable self-

We develop a computational model to capture the complex, three-dimensional behavior of chemoresponsive polymer gels undergoing the Belousov-Zhabotinsky reaction. The model combines components of the finite difference and finite element techniques and is an extension of the two-dimensional gel lattice spring model recently developed by two of us [V. V. Yashin and A. C. Balazs, J. Chem. Phys. 126, 124707 (2007)]. Using this model, we undertake the first three-dimensional (3D) computational studies of the dynamical behavior of chemoresponsive BZ gels. For sufficiently large sample sizes and a finite range of reaction parameters, we observe regular and nonregular oscillations in both the size and shape of the sample that are coupled to the chemical oscillations. Additionally, we determine the critical values of these reaction parameters at the transition points between the different types of observed behavior. We also show that the dynamics of the chemoresponsive gels drastically depends on the boundary conditions at the surface of the sample. This 3D computational model could provide an effective tool for designing gel-based, responsive systems.

With newly developed computational approaches, we design a nanocomposite that enables self-regeneration of the gel matrix when a significant portion of the material is severed. The cut instigates the dynamic cascade of cooperative events leading to the regrowth. Specifically, functionalized nanorods localize at the new interface and initiate atom transfer radical polymerization with monomers and cross-linkers in the outer solution. The reaction propagates to form a new cross-linked gel, which can be tuned to resemble the uncut material.

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We undertake the first computational study to determine the effect of light on polymer gels undergoing the Belousov-Zhabotinsky (BZ) reaction. The BZ gels are unique materials because they can undergo rhythmic mechanical oscillations in the absence of external stimuli. The BZ reaction, however, is photosensitive. Via simulations, we demonstrate that the interplay between the chemoresponsive gels and the photosensitive reaction can cause millimeter sized BZ gels to exhibit autonomous, directed motion or reorientation away from 4 the light. In effect, we show that these synthetic BZ "worms" display a fundamental biomimetic behavior: movement away from an adverse environmental condition, which in the context of the BZ reaction is the presence of light.

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A remarkable feature of certain biological species is their ability to dramatically alter their shape in response to environmental cues. Here, a computational model for photoresponsive polymer gels that contain spirobenzopyran (SP) chromophores is developed, and it is shown that these materials can undergo 3D biomimetic shape changes under non-uniform illumination. The SP moieties are hydrophilic in the dark, but become hydrophobic under illumination with blue light. Hence, with the incorporation of these chromophores into gels in aqueous solutions, light can be harnessed to control the gel's swelling or shrinking and, thereby, dynamically alter the gel's shape. The model is fi rst validated by determining the effects of uniform illumination on the temperature-induced volume phase transitions in these gels, and show good agreement between these results and available experimental data. These gels can also be patterned remotely and reversibly by illuminating the samples through photomasks and, thus, molded into a variety of shapes with feature sizes on the submillimeter length scale. Furthermore, by repeatedly rastering the light source over the sample, the system can be driven to exhibit another biomimetic behavior: sustained, directed motion. The introduction of a temperature gradient provides a means of further controlling this autonomous movement. The results point to a robust method for controllably reconfi guring the morphology of soft materials and driving the self-organization of multiple reconfi gurable pieces into complex architectures.

A single biological cilium can sense minute chemical variations and transmit this information to neighboring cilia to produce a global response to the local change. Herein, we undertake the first computational study of self-oscillating, artificial cilia and show that this system can ''communicate'' to undergo a biomimetic, collective response to small-scale chemical changes. The cilia are formed from chemo-responsive gels undergoing the oscillatory Belousov-Zhabotinsky (BZ) reaction. The activator for the reaction, u, is generated within these BZ cilia and diffuses between the neighboring gels. We find that the spatial arrangement of the BZ cilia affects the local distribution of u, which in turn affects the dynamic behavior of the system. Consequently, two closely spaced cilia bend away from each other and the chemo-mechanical traveling waves within the gels propagate top down. By increasing the inter-cilia spacing, we dramatically alter the behavior of the system and uncover a distinctive form of chemotaxis: the tethered gels bend towards higher concentrations of u and hence, towards each other. This chemotaxis is particularly pronounced in an array of five cilia, where we observe a ''bunching'' of the cilia towards the highest concentration in u, accompanied by the synchronization of the chemomechanical waves. We also show that the cilial oscillations can be controlled remotely and noninvasively by light. By selectively illuminating certain cilia, we could ''play'' the array like a keyboard, causing a rhythmic variation in the heights of the gels. These attributes could be exploited in a range of microfluidic applications, where the controllable communication among the BZ cilia and selfoscillating surface topology can be harnessed to transport microscopic objects within the devices.