Surprising shrinkage of expanding gels under an external load

Abstract

H ydrogels are fascinating and useful in that they can show large volume changes in response to various stimuli, such as temperature or chemical environment 1 . Here we report the peculiar observation that chemically crosslinked hydrogels that normally expand owing to a change in electrolyte pH can be made to shrink in certain circumstances. Specifically, these hydrogels contract when tested at a constant compressive force and subjected to a pH change that causes expansion in the absence of the applied load. When tested under tension, the gels always expand. Although the effects of external stress on the swelling of gels is known 2-4 , the concomitant change in gel mechanical properties during pH switching was found to be a more dominant effect in our studies. However, existing mechanical models 5,6 used to predict dimensional changes in actuator materials could not explain both the tensile and compression results. In addition, we show that the friction between metal plates of the apparatus and the gel is a key factor in explaining the contractile actuation under compressive loads. The observations reported in this paper are important for the successful design and use of hydrogel actuators in devices such as valves for microfluidics.Volume changes occur in polyelectrolyte hydrogels when the thermodynamic balance between expansion forces (electrostatic repulsion and osmotic pressure of counterions) and contractile forces (stretching of elastically active network chains) is shifted by changes in the chemical environment. Hydrogel 'artificial muscles' 7 can generate huge dimensional changes in response to changes in temperature 8 , pH 9 , solvent 10 and electric fields 11 . Polymer conformation changes are also accompanied by large changes in equilibrium water content (EWC) of the gel, which can conveniently be measured gravimetrically. Many examples of changes in EWC with changes in pH are available in the literature, for example in the gels based on chitosan 12 .Various applications for actuating gels have been proposed, with the microscopic devices seeming most promising owing to their reasonable response times 13 . In practical applications, the influence of the external stresses on the operation of actuators cannot be neglected. Changes in mechanical properties such as

On the Motive Power of Chemical Transformations in Open Systems

“…2a that for small swelling ratios the elastic modulus of the gel changes with the concentration of acetone. This is a general phenomenon also observed in other types of gels in acid environments-tensile tests have shown [9] that the Young's modulus of the gel under uniaxial tension may increase from 1.0 to 1.2 MPa when the PH of the surrounding solution is changed from 7 to 2. The effect of a changing modulus is schematically represented in Fig.…”

Hydrogels for Soft Machines

“…[20] The chemical changes causing the volume change can both cause volume and stiffness changes, such that the results of combining pH changes and applied force can be unexpected. [123] This coupling between swelling thermodynamics and mechanical stress leads to a number of other peculiar phenomena, such as negative Poisson's ratios, [124][125][126] and to strange responses to bending and other complex loads.…”

Continuum Models of Stimuli-responsive Gels

“…Such free swelling, however, rarely occurs in practice. Inhomogeneous but equilibrium states occur, for example, when the network itself is modulated [38][39][40] , or the network is subject to a load or constraint [41,42] . The swelling of a gel is sometimes modeled by prescribing a constant volumetric strain.…”