Hydrogels are 3-D network polymeric materials that exhibit a large volume phase-transition due to a of change
in their environment so that the response causes the hydrogel to swell or shrink. Since hydrogels have been
found to be useful for chemical sensing and delivery, there is a growing interest in their use for medicine. This
,requires a thorough understanding of the hydrogels characteristic response to pH. The hydrogel response can be
explained by various physical equations which are often challenging to solve. We discuss the simulation of such
phase-transitions in steady-state conditions emphasizing the response to solvent pH and other environmental
stimuli. We demonstrate a method for simulating pH response of hydrogels and describe numerical model
and its implementation in detail. Though a few models have been developed for simulation of these hydrogel
characteristics, these have been based on custom programs implemented in individual laboratories and often
not generally accessible. Hence, our modeling effort is implemented using the generic finite element software
COMSOL and the method can be used with any software having similar capabilities. The effect of buffer
solution concentration, fixed charge density, the solution pH on the swelling characteristics are studied. Results
are compared with published experimental data.
In this study, we compare the characteristics of ferrogels prepared using γ-Fe<sub>2</sub>O<sub>3</sub> and Fe<sub>3</sub>O<sub>4</sub> nanoparticles. The
magnetic nanoparticles with ~ 20 nm diameter were distributed in N-isopropylacrylamide (NIPAM) gel prepared using
N,N'-methylenebisacrylamide (BIS), ammonium persulfate (APS) and N,N,N',N'-tetramethylethylenediamine
(TEMED). Particle distribution and agglomeration characteristics of the prepared ferrogels were investigated using ultra
small angle x-ray scattering (USAXS) and transmission electron microscopy (TEM). The ferrogel samples prepared
using Fe<sub>3</sub>O<sub>4</sub> and γ-Fe<sub>2</sub>O<sub>3</sub> particles have similar particle distribution. The ferrogels, prepared with γ-Fe<sub>2</sub>O<sub>3</sub> nanoparticles,
however, demonstrate significantly different agglomeration characteristics compared to the ferrogels prepared using
Fe<sub>3</sub>O<sub>4</sub>. In both systems, the agglomerated particles appear to be spherical, with few of those indicating chain like
structures. Based on the particle concentration and sizes, the DC SQUID magnetometry data of these samples showed
the magnetic moments range between 0.9 to 2.5 emu/g. Details of our results and analysis are presented.
This paper presents the results of our work in building a finite element based numerical model, to study and understand
the actuation characteristics of gels especially in response to the changes in pH, temperature and electrical potential.
Steady state behavior of the gel is considered initially within an overall model based on 'equilibrium mechanical
condition' with buffer and fixed ion concentrations as the most affecting parametric variables. The finite element
analysis was carried out using commercially available multi-physics software, 'COMSOL'. This simulation used the
variables fixed charge density, pH (2-12), buffer solution ionic concentration (2- 10 mM) and electric potential (0 to 2
V). The variation in gel deformation characteristics with respect to temperature and applied electrostatic potential are
presented. The typical dimensions of the actuator considered are 1 mm × 3 mm. The gel deformation or displacement
with varying length to width ratio and applied potential is also described. A detailed analysis of these results is
discussed in this paper.