Computational modelling and investigation of mechanical and electrochemical response of photo-sensitive hydrogels

• Main Author: Chen, Xiao
• Other Authors: Lam Khin Yong
• Format: Thesis-Doctor of Philosophy
• Language: English
• Published: Nanyang Technological University 2021
• Subjects: Engineering::Mechanical engineering
• Online Access: https://hdl.handle.net/10356/145822
• Institution: Nanyang Technological University

In the past years, photo-sensitive hydrogels have been extensively applied in various engineering fields, such as actuators, biosensors and electrolytes, due to their distinctive advantages, including spatial and temporal control with great ease and convenience, and wireless manipulation. However, the elementary mechanism remains unclear, regarding their mechanical and electrochemical response behaviors, especially when subjected to multiple coupled stimuli. This leads to a limitation to the exploration of their further applications. Currently, most published works were experimental-based trial-and-error, and the understanding of response mechanism might be still poor. Therefore, it is worthwhile to develop theoretical models with proper accuracy to account for the primary response mechanism of photo-sensitive hydrogels subjected to the external photo stimulus as well as other coupled stimuli. In this thesis, two academic achievements are obtained regarding the modelling of photo-sensitive hydrogels and they are briefed as follows. The first academic achievement of the present study is the development of a multi-effect-coupling photo-stimuli (MECp) model to characterize the behavior of photo-sensitive hydrogels in response to light-thermo-pH-salt coupled stimuli. The MECp model consists of several parts: (i) the formulation of chemical reaction kinetics, (ii) the Poisson-Nernst-Planck equation accounting for the interaction between diffusion and electrostatic fields, and (iii) the nonlinear mechanical equation for the conservation of momentum. In particular, the presented hydrogel is regarded as a reactor, where a ternary reaction system is assumed regarding photochromic groups in terms of local pH. Based on this assumption, the ordinary differential equations are then solved analytically using the Laplace transform for the quantification of each fixed chemical species. Furthermore, the ionic strength is considered in the formulation of the electrochemical potential gradient to characterize the effect of salt on the photo-stimulated response. In this study, the large mechanical deformation is expressed in the form of tensor, and the constitutive relation is derived by means of thermodynamic inequality. The validation of MECp model is performed by comparison with experimental results in the literature. The parameter studies by the MECp model are then conducted to study how photo-sensitive hydrogels respond mechanically and electrochemically to several external stimuli, including light intensity, temperature, and surrounding pH. The second academic achievement is the modification of MECp model to characterize the metal-ion-complexation response of photo-sensitive hydrogels. In order to simplify the chemical reaction system, it is assumed that the rates of non-metal-complexation reactions remain constant, and thus the analytical method could be employed for the analysis of coupled chemical reactions. It is confirmed by the experimental observation that the theoretical model could predict the response of photo-sensitive hydrogels subjected to metal ions. Through numerical investigation, it is then demonstrated that the metal-ion-complexation response could be tuned by multiple factors, including fixed acid groups, light intensity, buffer pH, temperature, buffer salt concentration, and polymeric material.