The impact of high energy crosslinking on the network structure of gelatin hydrogels was investigated in comparison to physically entangled gels by small-angle X-ray scattering (SAXS). Physically entangled gelatin of increasing concentration exhibited a nearly constant correlation length of several nanometers. These gels had scattering behavior close to that of polymer coils swollen in a good solvent, as evidenced by the Porod exponent of 1.8. The mass fractal dimension decreased towards 1, indicating increased formation of semiflexible gelatin triple helices and rod-like structures as a function of the gelatin concentration. In contrast, electron irradiation lead to a decrease in the correlation length at doses above 20 kGy. Covalent crosslinking induced by electron irradiation lead to increased branching and formation of globular structures, as observed by a steady increase of both the Porod exponent and mass fractal dimension. Furthermore, the network mesh size systematically decreased from approximately 45 nm to under 20 nm with both additional physical and chemical crosslinking. These mesh sizes as obtained by SAXS were used to estimate the network shear modulus using several polymer models and were compared to macroscopic rheology measurements. Finally, SEM images of freeze-dried samples revealed changes in the microstructure of the irradiated hydrogels. Overall, fundamental differences in the network structures stemming from the crosslinking method were observed across a wide range of length scales.
As emerging responsive materials, ferrogels have demonstrated significant potential for applications in areas of engineering to regenerative medicine. Promising techniques to study the behavior of magnetic nanoparticles (MNPs) in such matrices include magnetic particle spectroscopy (MPS) and magnetorelaxometry (MRX). This work investigated the magnetic response of gelatin-based ferrogels with increasing temperatures, before and after high energy crosslinking. The particle response was characterized by the nonlinear magnetization using MPS and quasistatic magnetization measurements as well as MRX to discriminate between Néel and Brownian relaxation mechanisms. The effective magnetic response of MNPs in gelatin was suppressed, indicating that the magnetization of the ferrogels was strongly influenced by competing dipole-dipole interactions. Significant changes in the magnetic behavior were observed across the gelatin sol-gel transition, as influenced by the matrix viscosity. These relaxation processes were modeled by Fourier transformation of the Langevin function, combined with a Debye term for the nonlinear magnetic response, for single core MNPs embedded in matrices of changing viscosities. Using high energy electron irradiation as a crosslinking method, modified ferrogels exhibited thermal stability on a range of timescales. However, MRX relaxation times revealed a slight softening around the gelatin sol-gel transition felt by the smallest particles, demonstrating a high sensitivity to observe local changes in the viscoelasticity. Overall, MPS and MRX functioned as non-contact methods to observe changes in the nanorheology around the native sol-gel transition and in crosslinked ferrogels, as well as provided an understanding of how MNPs were integrated into and influenced by the surrounding matrix.
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