3D heterogeneous stiffness reconstruction using MRI and the Virtual Fields Method

The first extension of the virtual fieldsmethod to the reconstruction of heterogeneous stiffnessproperties from 3D bulk full-field displacementdata is presented in this paper. Data are provided byMagnetic Resonance Imaging (MRI). Two main issuesare addressed: 1. the identification of the stiffness ratiobetween two different media in a heterogeneoussolid; 2. the reconstruction of stiffness heterogeneitiesburied in a heterogeneous solid. The approach is basedon a finite element discretization of the equilibriumequations. It is tested on experimental full-field dataobtained on a phantom with the stimulated echo MRItechnique. The phantom is made of a stiff sphericalinclusion buried within a lower modulus material. Preliminaryindependent tests showed that the material ofthe inclusion was four times stiffer than the surroundingmaterial. This ratio value is correctly identified byour approach directly on the phantom with the MRIdata. Moreover, the modulus distribution is promisinglyreconstructed across the whole investigated volume.However, the resulting modulus distribution ishighly variable. This is explained by the fact that the approach relies on a second order differentiation of thedata, which tends to amplify noise. Noise is significantlyreduced by using appropriate filtering algorithms

Gelation behavior of polysaccharide-based interpenetrating polymer network (IPN) hydrogels

We report the preparation and rheological characterization of interpenetrating polymer network (IPN) hydrogels made from alginate and hydrophobically modified ethyl hydroxyl ethyl cellulose (HMEHEC). To our knowledge, there have been no studies of the gelation behavior of IPNs. We found that the rheology of these systems can be easily tuned, with the elastic modulus of the IPN strongly dependent on the relative ratio of HMEHEC to alginate. The sol-gel transition of these systems was found to satisfy the Winter-Chambon criterion for gelation at various crosslinker densities. From the power law relationship of the dynamic moduli (G (') similar to G (aEuro3) similar to omega (n)), the exponent n appears to be dependent on both the crosslinker density and relative amount of two polymers. The value of n was found to be similar to 0.5 for all samples for stoichiometric amounts of crosslinker. The effect of molecular weight of HMEHEC on the gel point and viscoelastic exponent has also been reported. Alginate seems to dominate the kinetics of the process but the effect of high molecular weight HMEHEC on the gel point, especially at lower proportion was also evident