Multiphysics model development to characterize fundamental mechanism of bio-responsive hydrogels
Goh, Kek Boon
Date of Issue2019-01-16
School of Mechanical and Aerospace Engineering
Environmentally bio-responsive hydrogel is of tremendous interest in plethora of advanced engineering applications, such as hydrogel-based urease-loaded dialysis membranes and hydrogel-based hemoglobin-mediated oxygen carrier. However, literature reviews reveal that the coupled bio-chemo-electro-mechanical responses of these hydrogels remain poorly understood, due to lack of accurate mathematical models to numerically characterize the environmental-induced hydrogel synergistic performances. Therefore, the present research work focuses on the development of multiphysics models to obtain a deeper understanding of such materials, especially when operating at extreme of physiological environmental conditions. The first academic achievement made in this present work is the development of a multiphysics model for describing the coupled bio-chemo-electro-mechanical responses of urease-loaded hydrogel. By coupling the multiphysics interactions occurring in the hydrogel together, the model consists of three governing mass, momentum and energy conservation equations, and also four sets of constitutive relations, namely mass flux, fixed charge equation, and nonlinear mechanical equation. A novel rate of reaction is also incorporated into the model to describe the urease activity as a function of ambient temperature coupled with environmental solution pH, capturing the environmental-sensitive urease ionization and denaturation states. For model validation, the multiphysics model is examined with the comparison between present numerical results and published experimental observations, where good agreements are achieved, especially for temperature-, pH- and urease-induced swelling deformations and urease catalytic activity of the polyelectrolyte hydrogels. The result shows that the urease catalytic activity patterns differ in anionic and cationic urease-loaded hydrogels by increasing the environmental concentration of sodium chloride at a relatively higher environmental concentration of urea, whereas the urease catalytic activity remains almost unchanged when the environmental pH increases above the acid-base dissociation constant pKa of the polyacidic hydrogel. The result also shows that the osmotic pressure response of urease-loaded hydrogel enlarges linearly by increasing physiological urea concentration making it biocompatible for healthcare diagnostic applications. The second academic achievement is the development of another multiphysics model to elucidate the coupled-stimulated responses of hemoglobin-loaded polyelectrolyte hydrogels. A developed constitutive relation is integrated into the model to capture immobile hemoglobin bioactivity as a function of ambient oxygen coupled with environmental pH. After validation against the reported experimental observations, it is taken that the multiphysics model can effectively characterize the hemoglobin saturation with oxygen for (1) neonatal and (2) adult hemoglobins, and also the pH-induced swelling deformation of hemoglobin-loaded polyelectrolyte hydrogels. The result demonstrates that the hydration-induced swelling deformation of the polyampholytic hydrogel changes in a bowl-shaped fashion by increasing the environmental pH value, in which the pH-induced swelling deformation of initially balanced polyampholytic hydrogel changes from a “bowl” to “V”-shaped like pattern with decrease of immobile acidic and basic components ionization strength. In addition, the result demonstrates that the strength increase of both the immobile acidic and basic components in the initially balanced polyampholytic hydrogel causes the hydrogel to exhibit isoelectric point behavior at wider environmental pH range, whereas the initially unbalanced polyampholytic hydrogel collapses at the environmental pH coinciding with dissociation constant of the dominant immobile charge group, if the initial dominant immobile charge group concentration is twice that of the counter one.