Modelling and Analysis of pH Responsive Hydrogels for the Development of Biomimetic Photo-Actuating Structures
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Modelling and Analysis of pH Responsive Hydrogels for the Development of Biomimetic Photo-Actuating Structures Michael PM Dicker1, Ian P Bond1, Jonathan M Rossiter2, Charl FJ Faul3 and Paul M Weaver1 1
Advanced Composites Centre for Innovation and Science (ACCIS), University of Bristol,
Clifton, United Kingdom. 2
Department of Engineering Mathematics, University of Bristol, Bristol, United Kingdom.
3
School of Chemistry, University of Bristol, Bristol, United Kingdom.
ABSTRACT Photo-actuating structures inspired by the chemical sensing and signal transmission observed in sun-tracking leaves have recently been proposed by Dicker et al. The proposed light tracking structures are complex, multicomponent material systems, principally composed of a reversible photoacid or base, combined with a pH responsive hydrogel actuator. New modelling and characterization approaches for pH responsive hydrogels are presented in order to facilitate the development of the proposed structures. The model employs Donnan equilibrium for the prediction of hydrogel swelling in systems where the pH change is a variable resulting from the equilibrium interaction of all free and fixed (hydrogel) species. The model allows for the fast analysis of a variety of combinations of material parameters, allowing for the design space for the proposed photo-actuating structures to be quickly established. In addition, experimental examination of the swelling of a polyether-based polyurethane and poly(acrylic acid) interpenetrating network hydrogel is presented. The experiment involves simultaneously performing a titration of the hydrogel, and undertaking digital image correlation (DIC) to determine the hydrogel’s state of swelling. DIC allows for the recording of the hydrogel’s state of swelling with previously unattained levels of resolution. Experimental results provide both model material properties, and a means for model validation. INTRODUCTION Photo-actuating structures inspired by the chemical sensing and signal transmission observed in sun-tracking leaves [1,2] have recently been proposed by Dicker et al. [3,4] (Figure 1). These structures are designed to actuate such that a desired orientation be maintained between the structure and a light source. Within the structure sensing occurs from the light exposure of a reversible photoacid [5] or base [6], whilst actuation results from the swelling of a pH responsive hydrogel. The two components are separated, with a control signal being sent from one location to the other by the diffusion of ions, and the resulting pH change. By carefully designing the system geometry such that light exposure is a function of the device’s orientation (through angled shades), and by linking through solution appropriate sensing and actuating elements, it is anticipated that precise and stable control can be imparted to the structure. It is believed that such a structure will share the properties of robustness and efficiency often found in Nature, and find application in the tracking of photovoltaic panels for increa
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