Khin Yong Lam

Modeling of multiphase smart hydrogels responding to pH and electric voltage coupled stimuli

A model, entitled the multi-effect-coupling pH-electric-stimuli (MECpHe) model, is presented and analyzed for the response of smart hydrogels to changes in the coupled stimuli of an external electric field and the solution pH. It considers finite deformations, the electric potential and distribution of fixed charge density in the hydrogel and surrounding solvent. The MECpHe model is validated with previously published experimental measurements and good agreement is shown. A steady-state study is carried out for various pH values and applied electric voltages to ascertain the impact of these on the deformation of the hydrogel and distribution of ionic species, electric potential, and fixed charge density, both inside the hydrogel as well as in the surrounding solvent.

Modeling and simulation of microfluid effects on deformation behavior of a red blood cell in a capillary

A modified SIMPER algorithm is developed for analysis of microfluid effects on the motion and deformation of a red blood cell (RBC) in a capillary. With consideration of very small Reynolds number in microfluidics, this algorithm not only speeds up the convergence of the momentum equations by combining the advantages of the SIMPLEC and SIMPLER algorithms together, but also satisfies the continuity equation with higher accuracy by integrating a fine adjustment technique. In order to validate the modified SIMPLER algorithm, the behavior of RBC in a capillary is simulated at different velocities. When the mean RBC velocity is 0.1mm/s, the RBC exhibits a characteristic parachute shape in the steady state, which agrees well with the numerical results previously reported. Apart from that, a quantitative validation with the experimental data is performed by examining the relationship between the mean velocity and deformation index of the RBC, showing an excellent agreement. The effects of crucial parameters are investigated systematically on the motion and deformation of the RBC, including the RBC radius, elastic modulus and bending stiffness of RBC membrane, initial velocity of suspending fluid, as well as the density and viscosity ratios of the suspending fluid to RBC. The simulation results demonstrate that all of the parameters have influences on the RBC behavior by changing the interaction between the RBC and suspending fluid.

Modeling the effect of environmental solution pH on the mechanical characteristics of glucose-sensitive hydrogels

Many environmental conditions can influence the mechanical characteristics of the glucose-sensitive hydrogels. In this paper, a multi-effect-coupling glucose-stimulus (MECglu) model is developed to study the influence of environmental solution pH on the swelling behavior of soft smart hydrogels responding to change in surrounding blood glucose concentration. In order to characterize the chemo-electro-mechanical behaviors of the hydrogels, the model is composed of the Nernst-Planck type of diffusion-reaction partial differential equations for mobile species with consideration of the enzyme reaction catalyzed by the glucose oxidase and the catalase, the Poisson equation for electric potential, and the nonlinear equilibrium equation for mechanical large deformation of the glucose-sensitive hydrogel. In the MECglu model, the formulation of the fixed charge groups bound onto the corsslinked polymeric network is associated with the change of the ambient solution pH. Using these nonlinear coupled partial differential equations, we demonstrate that the computational mechanical deformation by the MECglu model consists well with the experimental observations published in the range of practical physiological glucose concentration from 0 to 16.5 mM (300 mg/ml). The simulations are also carried out for analysis of the influences of physiological pH on the distributive profiles of reacting and diffusive species concentrations and the electric potential as well as the mechanical deformation of the glucose-sensitive hydrogels. The simulations by the model can efficiently support the design and optimization of the insulin delivery system based on the glucose-sensitive hydrogels with the immobilized glucose oxidase and catalase.

Multiphysics Modeling of Electrochemomechanically Smart Microgels Responsive to Coupled pH/Electric Stimuli

A multiphysics model is developed to simulate the responsive behavior of smart pH-/electric-sensitive hydrogels when immersed into pH buffer solution and subjected to an externally applied electric field, which is termed the MECpHe model. Comparison with experimental data shows the MECpHe model to be accurate and stable. The influence of the externally applied electric voltage is discussed with respect to the distribution of diffusive ionic species and the displacement of the hydrogel strip. The influences of initial charge density and ionic strength on the swelling ratio and the bending deformation of the microgel strip are studied.

Computational Analysis of Dynamic Interaction of Two Red Blood Cells in a Capillary

The dynamic interaction of two red blood cells (RBCs) in a capillary is investigated computationally by the two-fluid model, including their deformable motion and interaction. For characterization of the deformation, the RBC membrane is treated as a curved two-dimensional shell with finite thickness by the shell model, and allowed to undergo the stretching strain and bending deformation. Moreover, a Morse potential is adopted to model the intercellular interaction for the aggregation behavior, which is characterized as the weak attraction at far distance and strong repulsion at near distance. For validation of the present technique, the dynamic interaction of two RBCs in static blood plasma is simulated firstly, where the RBCs aggregate slowly until a balanced configuration is achieved between the deformation and aggregation forces. The balanced configuration is in good agreement with the results reported previously. Three important effects on the dynamic behavior of RBCs are then analyzed, and they are the initial RBC shape, RBC deformability, and the intercellular interaction strength. It is found that the RBC is less deformed into a well-known parachute shape when the initial RBC shape is larger. Similarly, if the elastic shear modulus and bending stiffness of RBC membrane increase, the RBC resistance to deformation becomes higher, such that the RBC is less deformed. The simulation results also demonstrate that the RBC deformability strongly depends on the intercellular interaction strength. The RBCs deform more easily as the intercellular interaction strength increases.

Coupled chemo-electro-mechanical simulation for smart hydrogels that are responsive to an external electric field

By reformulating the fixed charge density with finite deformation, a previously developed model for pH-sensitive hydrogels is further refined in this paper to simulate electric-sensitive hydrogels when immersed in a bath solution subject to an externally applied electric field; it is termed the refined multi-effect-coupling electric-stimulus (rMECe) model. This model consists of coupled nonlinear partial differential governing equations with the effects of chemo-electro-mechanical multi-energy domains. The reformulated fixed charge density incorporates the effect of the applied voltage. The rMECe model is validated by numerical comparison of the simulation results with those of published experiments. One-dimensional steady-state simulations are carried out for the equilibrium behavior of an electric-sensitive hydrogel strip. Then detailed discussions are given of the influence of the externally applied voltage on the responses of the hydrogel strip.

Numerical Design of Microfluidic—Microelectric Hybrid Chip for the Separation of Biological Cells

A miniature microfluidic-microelectric hybrid chip is numerically designed for separation of biological cells, where the characteristic length of the chip is close to the cell radius. A mathematical model is developed to characterize the motion and deformation of a biological cell in the hydrodynamic and nonuniform electric coupled fields, in which the mechanical and dielectric behaviors of the cell are taken into consideration. Subsequently, the model is validated by comparing with the experimental results published previously. By taking a red blood cell (RBC) as the sample of biological cell, the chip structure is numerically designed from the viewpoints of the electrode width, fluid flow velocity, and electric potential, respectively. Using the designed microfluidic-microelectric hybrid chip, the effects of the shape and initial position of the RBC on the separation ability are then analyzed. After that, the separation of the RBCs with the different permittivities or conductivities using the designed chip is simulated, and the deformation behaviors of the RBCs are discussed as well. At the high frequency, the permittivities of the RBCs play a dominant role in the separation of the RBCs, which causes the RBCs moving toward or away from the electrode array. However, the conductivity of the RBC plays a significant role at the low frequency. With suitable suspending fluid therefore, the separation of cells with different permittivities or conductivities can be achieved using the microfluidic-microelectric hybrid chip designed by the present work.

Qualitative and quantitative analysis of dynamic deformation of a cell in nonuniform alternating electric field

The present paper is dedicated to numerical modeling of the deformation of a cell in a nonuniform ac electric field. A two-fluid model is presented to characterize the flow mechanism of a cell suspending in a medium, in which the mechanical force of cell membranes and dielectrophoretic forces due to the nonuniform ac field are incorporated. The dielectrophoretic forces are then validated by comparing them with the dipole moment approximation. The simulation results demonstrate that the cell deformation depends on the frequency of the electric field. At low frequency, the cell is repelled away from the electrodes, and tilts clockwise. The lower part of the cell deforms thinner, whereas the upper part becomes fatter. At intermediate frequency, the cell moves almost along the centerline of the microchannel, and deforms slightly. The cell orientation oscillates with the average value of zero. At high frequency, however, the cell is attracted toward the electrodes, and tilts counterclockwise. The lower part of...

Modeling and analysis of pH-electric-stimuli-responsive hydrogels

A multiphysics model is presented in this paper, called the multi-effect-coupling pH-electric-stimuli (MECpHe) model, for analysis of smart hydrogels responding to the stimuli of solution pH coupled with electric voltage, when the hydrogels are placed into pH buffer solution subject to an externally applied electric field. The MECpHe model considers the chemo-electro-mechanical multiphysics effects and formulates the fixed charge density with the coupled effect of buffer solution pH and electric voltage. The model is formulated mathematically by a set of nonlinear partial differential governing equations for predicting the displacement and the swelling ratio, as well as the average curvature of hydrogel strip, and for simulating the distributive profiles of diffusive ionic species concentrations and the electric potential, as well as the fixed charge density. The model is validated by the comparison of the present computation results with experimental data extracted from open literature. The steady-state simulation of the smart hydrogels is then conducted for the pH-electric-coupled stimuli. The influences of the externally applied electric voltage are discussed in detail on the distributions of diffusive ionic species concentrations, the displacement, the swelling ratio and the average curvature of the hydrogel strip.

Development of a multiphysics model to characterize the responsive behavior of urea-sensitive hydrogel as biosensor

A remarkable feature of biomaterials is their ability to deform in response to certain external bio-stimuli. Here, a novel biochemo-electro-mechanical model is developed for the numerical characterization of the urea-sensitive hydrogel in response to the external stimulus of urea. The urea sensitivity of the hydrogel is usually characterized by the states of ionization and denaturation of the immobilized urease, as such the model includes the effect of the fixed charge groups and temperature coupled with pH on the activity of the urease. Therefore, a novel rate of reaction equation is proposed to characterize the hydrolysis of urea that accounts for both the ionization and denaturation states of the urease subject to the environmental conditions. After examination with the published experimental data, it is thus confirmed that the model can characterize well the responsive behavior of the urea-sensitive hydrogel subject to the urea stimulus, including the distribution patterns of the electrical potential and pH of the hydrogel. The results point to an innovative means for generating electrical power via the enzyme-induced pH and electrical potential gradients, when the hydrogel comes in contact with the urea-rich solution, such as human urine.