Stimulus-Responsive Hydrogel Research


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GEL Papers

Chemically-induced swelling of hydrogels

John Dolbow, Eliot Fried, and Huidi Ji
Journal of the Mechanics and Physics of Solids, 52, pp. 51-54, (2004)

Abstract

We present a theory for the chemically-induced volume transitions of hydrogels. Consistent with experimental observations, we account for a sharp interface separating swelled and collapsed phases of the underlying polymer network. The polymer chains are treated as a solute with an associated diffusion potential and their concentration is assumed to be discontinuous across the interface. In addition to the standard bulk and interfacial equations imposing force balance and solute balance, the theory involves an ancillary interfacial equation imposing configurational force balance. Motivated by experimental observations, we specialize the theory to the situation where the time scale associated with the interface motion is slow compared to those associated with diffusion in the bulk phases. We present a hybrid eXtended-Finite-Element/Level-Set Method (XFE/LSM) for obtaining approximate solutions to the equations arising under this specialization. As an application, we consider the swelling of a spherical specimen whose boundary is traction-free and is in contact with a reservoir of uniform chemical potential. Our numerical results exhibit good qualitative comparison with experimental observations and predict characteristic swelling times that are proportional to the square of the specimen radius. Our results also suggest several possible synthetic pathways that might be pursued as a means to engineer hydrogels with optimal response times.

This paper is available in pdf form (38 pages, ~0.5 MB)


A Numerical Strategy for Investigating the Kinetic Response of Stimulus-Responsive Hydrogels

John Dolbow, Eliot Fried, and Huidi Ji
Computer Methods in Applied Mechanics and Engineering, in press, (2005)

Abstract

We present a strategy for obtaining numerical solutions to a system of nonlinear, coupled evolution equations describing volume transitions in stimulus-responsive hydrogels (SRHs). The theory underlying our sharp-interface model of phase transitions in SRHs is provided along with the assumptions leading to a specialized formulation that is the starting point for the numerical method. The discrete formulation is then developed with a hybrid eXtended Finite-Element/ Level-Set Method (XFE/LSM). Domain integral methodologies are used consistently to extract interfacial quantities such as the mechanical driving traction, the jump in normal component of the solute flux, and requisite geometric information. Several benchmark studies are provided to demonstrate the accuracy and robustness of the numerical strategy. We then investigate various features of SRH kinetics including the regimes of unstable and stable phase transitions, surface pattern formation, and bulk phase separation.

This paper is available in pdf form (35 pages, ~7.1 MB)



Kinetics of Thermally-Induced Swelling of Hydrogels

Huidi Ji, Hashem Mourad, Eliot Fried, and John Dolbow
International Journal of Solids and Structures , in press, (2005)

Abstract

We present a continuum model for thermally-induced volume transitions in stimulus-responsive hydrogels (SRHs). The framework considers the transition as driven through the motion of a sharp interface separating swollen and collapsed phases of the underlying polymer network. In addition to bulk and interfacial force and energy balances, our model considers an interfacial normal configurational force balance. To account for the large volume changes exhibited by SRHs during actuation, the governing equations are developed in the setting of finite-strain kinematics. The numerical approximation to the coupled thermomechanical equations are obtained with an extended finite element/level-set method. The solution strategy involves a non-standard operator split and a simplified version of the level-set update. A number of representative problems are considered to investigate the model and compare its predictions to experimental observations. In particular, we consider the thermally-induced swelling of spherical and cylindrical specimens. The stability of the interface evolution is also examined.

This paper is available in pdf form (32 pages, ~2.3 MB)


This material is based upon work supported by the National Science Foundation and the Department of Energy.