Photo-actuating structures inspired by the chemical sensing and signal transmission observed in sun-tracking leaves have recently been proposed by Dicker et al.1 These structures are complex multicomponent material systems, designed to actuate in order 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 photoacid2 or base3, 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.
However, the realisation of such a structure requires the development of new modelling tools and an improved understanding of the repeatability of hydrogel actuation. This work first presents new modelling concepts for predicting hydrogel swelling in systems where the pH change is a variable resulting from the equilibrium interaction of all free and fixed (hydrogel) species in the system. The model developed incorporates a homogenised solution method to predict the final composition of the combined system. Donnan equilibrium4, considering conservation of mass and electro-neutrality within the hydrogel is then calculated to arrive at the final ion composition in both the hydrogel and surrounding solution. This in turn allows for the osmotic pressure and resulting degree of swelling for the actuated hydrogel to be determined. Preliminary findings of interest include a clear optimum for fixed charge concentration for a given hydrogel/solution ratio, and applied change in proton concentration.
This work also examines experimentally the repeatability of reversible hydrogel swelling. In particular, swelling resulting from limited shifts in pH around the hydrogel’s transition pKa is examined. The investigations involve simultaneously performing a titration of the hydrogel (measuring degree of protonation), whilst visually determining the hydrogel’s state of swelling in order to separate the chemical and mechanical contributions to actuation performance. The polyether-based polyurethane and poly(acrylic acid) hydrogel developed by Naficy et al.5and shown to have rapid and repeatable load recovery is used in this work. Experimental results are used to provide both model validation and guidance for model improvements and material/system design.
 Dicker et al. Bioinspir. Biomim. 2014, Accepted.
 Shi et al. J. Am. Chem. Soc.2011, 133, 14699-14703.
 Irie, M. J. Am. Chem. Soc. 1983, 105, 2078-2079.
 Ricka and Tanaka. Macromolecules 1984, 17, 2916-2921.