A chemo-electro-mechanical model is presented for analysis of the influence of the initial fixed charge density on performance of ionic-strength-sensitive hydrogel, termed the multi-effect-coupling ionic-strength-stimulus (MECis) model. It is composed of Nernst-Planck convection-diffusion equations which describe the chemical field, Poisson equation which provides the electrical potential associated with the fixed charge equation based on the Langmiur adsorption theory, and the mechanical equation which demonstrates the deformable behavior of the polymeric network of the ionic-strength-sensitive hydrogel. In the MECis model, the ionic strength of the bathing solution is considered as a key stimulus and incorporated into the Nernst-Planck equations and fixed charge formulation via activity coefficient and apparent dissociation constant, which influence the convectiondiffusion characteristics and binding reaction. The initial fixed charge density is taken as an important chemical parameter for the indication of the amount of the ionizable monomer groups within the ionic-strength-sensitive hydrogel, since it reflects the swelling ability of the hydrogel. The initial fixed charge density also influences the binding ability of the mobile ions to the polymeric network that determines the driving force for the swelling, which is reflected by the fixed charge equation in the MECis model. The present simulation results are compared with the experiments for examination of the MECis model, and it is concluded that the MECis model can predict well the influence of the initial fixed charge density on the swelling behavior quantitatively and provide a good simulation platform for designing and optimization of the hydrogel-based BioMEMS. The parameter study is then conducted for analysis of the influence of the initial fixed charge density on the responsive characteristics of the ionic-strength-sensitive hydrogel including the ionic transport, electrical potential, and displacement.

A multiphysics model is presented in this paper for analysis of the influence of various equilibrium constants on the smart hydrogel responsive to the ionic strength of environmental solution, and termed the multieffect-coupling ionic-strength stimulus (MECis) model. The model is characterized by a set of partial differential governing equations by consideration of the mass and momentum conservations of the system and coupled chemical, electrical, and mechanical multienergy domains. The Nernst-Planck equations are derived by the mass conservation of the ionic species in both the interstitial fluid of the hydrogel and the surrounding solution. The binding reaction between the fixed charge groups of the hydrogel and the mobile ions in the solution is described by the fixed charge equation, which is based on the Langmuir monolayer theory. As an important effect for the binding reaction, the equilibrium constant is incorporated into the fixed charge equation. The kinetics of the hydrogel swelling/deswelling is illustrated by the mechanical equation, based on the law of momentum conservation for the solid polymeric networks matrix within the hydrogel. The MECis model is examined by comparison of the numerical simulations and experiments from open literature. The analysis of the influence of different equilibrium constants on the responsive characteristics of the ionic-strength-sensitive hydrogel is carried out with detailed discussion.

a b s t r a c tIn this paper, a chemo-electro-mechanical model is presented which considers the characteristics of the three phases of the ionic-strength-sensitive hydrogel, including the solid polymeric network matrix, interstitial fluid and ionic species. It is termed the multi-effect-coupling ionic-strength-stimulus (MECis) model and composed of Poisson-Nernst-Planck system for diffusion of the chemical ionic species in the interior hydrogel and external solution, associated with the fixed charge equation for simulating the interaction between the fixed charges and mobile ions, and the mechanical equilibrium equation to characterize the deformation behavior of the solid polymeric network matrix. The simulation results of the MECis model are examined by comparing with the experimental data published in open literature. It is demonstrated that the present MECis model could simulate well the responsive behavior of the ionicstrength-sensitive hydrogel quantitatively. The parameter study is conducted by the MECis model for analysis of the influence of the Young's modulus on the characteristics of the smart hydrogel, and it is concluded that the present model can be employed as a good platform for design and optimization of the smart hydrogel-based BioMEMS.

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A multiphysics model is developed in this paper for simulation of the volume transition mechanism of the smart hydrogel in response to the changes in the ionic strength of bathing solution as an important measure of the ionic concentration of that solution, which is termed the multi-effect-coupling ionic-strength-stimulus (MECis) model. In the present works, the ionic strength is treated as a main stimulus and incorporated into both the ionic convection-diffusion system in the Nernst-Planck flux and the fixed charge density equation characterized by Langmuir isotherm theory. Due to the diffusion and convection, the osmotic pressure is produced by the difference in the ionic concentration between the interior hydrogel and exterior solution, which drives the swelling of the smart hydrogel. The deformation of the ionic-strength-sensitive hydrogel is described by the momentum conservation law, in which the osmotic pressure is a main driving source. Apart from osmotic pressure, however, the repulsive force between the fixed charges is also considered in the mechanical equilibrium equation as another driving source for the swelling of the hydrogel. The simulation is conducted for one-dimensional steady-state problem, and then compared with the experimental data and other theories from open literature. The comparisons demonstrate that the MECis model can simulate well the swelling behavior of the ionic-strength-sensitive hydrogel qualitatively and quantitatively. Probably it is able to predict the responsive characteristics of the bathing solution including the distribution of diffusive ionic concentrations and electrical potential.