The Chemo-Electro-Mechanical Behavior of the pH Sensitive Hydrogels in Transient Condition

Hydrogels are 3-D network polymeric materials that exhibit a phase-transition due to a change in their environment so that the response causes the hydrogel to swell or shrink. The changes in the solvent pH cause the hydrogel to swell or shrink, which is exploited for designing a drug delivery system. The time taken for drug uptake or delivery is an important aspect in these applications. The development of such systems require thorough understanding of the kinetic behavior of hydrogel swelling. We report the results of our research effort to model the time-dependent behavior using the finite element analysis. The hydrogel response is modeled using three partial differential equations in which two are nonlinear and third equation uses nonlinear material constants. Solving all three nonlinear equations using the finite element method for transient condition is a significant challenge. Our model is implemented using the generic finite element software COMSOL and the method can be used with any software having similar capabilities. We discuss the simulation of phase-transitions in transient conditions emphasizing the response to change in the solvent pH. The simulation was carried out on hydrogels with diameters from 200 μm to 500 μm while varying the solvent pH from 3 to 6. The results are compared with the published experimental data. A major achievement of this work is that the transient analysis is carried out in 2D, while the most reported literature are confined to one-dimensional analysis.