Modeling of effect of initial fixed charge density on smart hydrogel response to ionic strength of environmental solution

Abstract

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.

Advances in smart materials: Stimuli‐responsive hydrogel thin films

“…There are many various models that simulate volume transitions, some of which have been recently reviewed . Recently, work by Li et al and Lai et al investigated the swelling dynamics of hydrogels by the multieffect‐coupling ionic‐strength‐stimulus model (MECis), which integrates the Nernst–Planck equation for mobile ion concentrations and the Poisson equation for the electric potential associated with the fixed charges to describe the mechanical displacement of a deformable network and the phase field variable. The MECis model is designed to study hydrogels stimulated by changes in ionic strength in two dimensions.…”

Effect of the viscosity on the electrical response of a hydrosolution

“…A version of this model has been applied recently to model the effect of initial fixed charge density on smart hydrosolution response to ionic strength of environmental solution. 2 In its original version, this model is known as Poisson-Nernst-Planck (PNP) model. The imposed boundary conditions on the ionic current densities depend on the nature of the electrodes.…”

Effect of tensile load on the actuation performance of pH‐sensitive hydrogels

“…where v s is the volume of a solvent molecule, N s is the number of solvent molecules per unit volume of the dry network and v s N s is the volume of solvent molecules divided by the volume of the dry network. In eqs (11a-c) and (12), k x , k y , k z and N s are allowed to vary independently as long as all equations are satisfied. The elastic free energy term in eqs (9) and (10) can be determined by a Gaussian-chain model: 31,32…”

“…In eq (15), v is a measure of enthalpy of mixing and k x k 2 y represents the volume swelling ratio of hydrogel. From eq (12), it is obvious that 1=k x k 2 y is the dry network volume fraction and ð121=k x k 2 y Þ is the solvent volume fraction. Hence, eq (15) correlates the mixing osmotic pressure to the hydrogel swelling ratio through the extension ratios of the deformed network, regardless of how the network has been deformed.…”

“…Piezoelectric materials, electroactive polymers, shape memory alloys, dielectric elastomers, and stimuli‐responsive hydrogels are examples of such materials. Hydrogels are known to respond to a variety of external stimuli, including pH, temperature, chemical composition of the solvent, ionic strength, and electric field . Mechanical properties of hydrogels (especially their stiffness) resemble those of biological tissue, while the stroke generated is one of the highest among actuator materials.…”