Computational investigation of magnetic hydrogels and elastomers
Date of Issue2019-04-26
School of Mechanical and Aerospace Engineering
Recently magnetic hydrogels and elastomers attract considerable attention due to their unique characteristics, such as fast response, biocompatibility, and remote actuation. As such, they are used for wide-range applications, such as chemical absorbent, drug delivery, and microfluidics. However, the fundamental mechanism of the magnetic hydrogels and elastomers responsive to the externally imposed magnetic field still remain unclear, due to the lack of efficient models to quantitatively predict their responsive performance with desired accuracy. Therefore, this work aims to develop novel multiphysics models with capability of accurately predicting and characterizing the primary mechanism of the magnetic hydrogels and elastomers in response to the external magnetic field and/or coupled with other stimuli. The first achievement of the present research work is the development of a multiphysics model for simulation of the responsive behavior of the magnetic-sensitive hydrogel, with the effects of magneto-chemo-mechanical coupled fields, which is termed the multi-effect-coupling magnetic-stimulus (MECm) model. In this model, the magnetic susceptibility for magnetization of the general magnetic hydrogel is defined as a function of finite deformation, instead of a constant for an ideal magnetic hydrogel. The constitutive equations, formulated by the second law of thermodynamics, accounts for the effects of the chemical potential, the externally applied magnetic field, and the finite deformation. In particular, a novel free energy density is proposed with consideration of magnetic effect associated with finite deformation, instead of volume fraction. After examination with published experimental data, it is confirmed that the MECm model can well capture the responsive behavior of the magnetic hydrogel, including the deformation and its instability and hysteresis under a uniform or nonuniform magnetic field. The parameter studies for optimized design of the magnetic hydrogel are then carried out for influences of various material properties and magnetic parameters, including shear modulus, magnetic intensity, and volume fraction of the magnetic particles, on the behavior of the magnetic hydrogel in the equilibrium state, for a deeper insight into the fundamental mechanism of the magnetic hydrogels. The second achievement is the extension of the MECm model for characterization of the transient fluid-structure interaction of the deformable magnetic hydrogel with surrounding fluid flow, through the fully coupled arbitrary Lagrangian-Eulerian (ALE) algorithm. The extended model is validated by comparing with both the published finite difference result and experimental data, where good agreements between them are achieved. It is verified that the present model enables to predict the transient performance of the magnetic hydrogel placed in moving fluid. Furthermore, two magnetic hydrogel-based devices are designed and optimized by the extended model. The first is the magnetic hydrogel-based microfluidic system for replicating various physiological and pathological conditions in the human body, by which the desired flow patterns can be generated in real time due to the fast-response deformation of the magnetic hydrogel. The second device is the magnetic hydrogel-based drug targeting system for delivering the movable and deformable drug-loaded hydrogel to the specific site by variation of the inlet fluid flow velocity, the hydrogel size and position, the maximum magnetic field strength, and magnet position. The third achievement is the development of a multi-effect-coupling magnetic-pH-stimuli (MECmpH) model for optimization of cell physiological microenvironment within the magnetic-pH-sensitive hydrogel-based scaffold. In particular, the positions of seeding cells and the concentration of potassium (K+) within the scaffold are optimized by the MECmpH model, based on (i) the threshold of mechanical force for mechanotransduction effect at cellular-size level, and (ii) the common biological requirement for the cell growth. In the MECmpH model, the four responsive mechanisms of the magnetic hydrogel are characterized, including hydrogel magnetization, ionic polarization, diffusions of the solvent and ions, and nonlinear finite deformation, when subjected to various biophysical and biochemical stimuli, such as the magnetic field due to current intensity and the solution pH. After validation of the MECmpH model with experimental data in open literature, it is found that the higher pH level and current intensity, and the shorter hydrogel-magnet distance contribute to the larger deformation of the magnetic scaffold and thus the stronger mechanical force on cells. Moreover, the cell seeding positions within magnetic scaffold are optimized for better cell culture through the controllable current intensity. Furthermore, the physiological concentration of K+ within the scaffold is also optimized through initial fixed charge density within the scaffold. The last achievement is the development of a magneto-mechanical model for magnetic elastomer microactuator, where the transition is investigated between the uniform and nonuniform magnetic fields and the field-induced nonlinear deformation, due to reversible variation of the geometric size of the magnetic elastomer from infinite to finite one. In this study, both the elastomer and its surrounding medium are covered in the computational domain, where the Maxwell stress over the elastomer-surrounding interface is considered as an additional mechanical boundary, and the nonlinear magnetization included in a proposed free energy density. After validation of the present model with both the published theoretical and experimental works, it is concluded that increasing the geometric size of the magnetic elastomer leads to a more uniform magnetic field, and thus the elastomer undergoes the transition from the nonuniform to uniform magnetic fields, if its size increases to a critical point.