We analyze the effect of nanoparticle concentration on the physical properties of magnetic hydrogels consisting of polymer networks of human fibrin biopolymer with embedded magnetic particles, swollen by a water-based solution. We prepared these magnetic hydrogels by polymerization of mixtures consisting mainly of human plasma and magnetic nanoparticles with OHfunctionalization. Microscopic observations revealed that magnetic hydrogels presented some cluster-like knots that were connected by several fibrin threads. By contrast, nonmagnetic hydrogels presented a homogeneous net-like structure with only individual connections between pairs of fibers. The rheological analysis demonstrated that the rigidity modulus, as well as the viscoelastic moduli, increased quadratically with nanoparticle content following a square-like function. Furthermore, we found that time for gel point was shorter in the presence of magnetic nanoparticles. Thus, we can conclude that nanoparticles favor the cross-linking process, serving as nucleation sites for the attachment of the fibrin polymer. Attraction between the positive groups of the fibrinogen, from which the fibrin is polymerized, and the negative OHgroups of the magnetic particle surface qualitatively justifies the positive role of the nanoparticles on the enhancement of the mechanical properties of the magnetic hydrogels. Indeed, we developed a theoretical model that semiquantitatively explains the experimental results by assuming the indirect attraction of the fibrinogen through the attached nanoparticles. Due to this attraction the monomers condense into nuclei of dense phase and by the end of the polymerization process the nuclei (knots) of the dense phase cross-link the fibrin threads, which enhances the mechanical properties.

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The effect of chain-shaped aggregates on the dynamic properties of magnetic liquids is studied under the assumption that the chains can be modeled as straight and rigid. On the basis of microscopical analysis the macroscopical expression for average stress tensor is obtained. The effective viscosity and viscoelastic characteristics are estimated.

This paper is devoted to the steady-state rheological properties of two new kinds of ferrofluids. One of these was constituted by CoNi nanospheres of 24 nm in diameter, whereas the other by CoNi nanofibers of 56 nm in length and 6.6 nm in width. These ferrofluids were subjected to shear rate ramps under the presence of magnetic fields of different intensity, and the corresponding shear stress values were measured. From the obtained rheograms (shear stress vs shear rate curves) the values of both the static and the dynamic yield stresses were obtained as a function of the magnetic field. The magnetoviscous effect was also obtained as a function of both the shear rate and the magnetic field. The experimental results demonstrate that upon magnetic field application these new ferrofluids develop yield stresses and magnetoviscous effects much greater than those of conventional ferrofluids, based on nanospheres of approximately 10 nm in diameter. Besides some expected differences, such as the stronger magnetorheological effect in the case of ferrofluids based on nanofibers, some intriguing differences are found between the rheological behaviors of nanofiber ferrofluids and nanosphere ferrofluid. First, upon field application the rheograms of nanofiber ferrofluids present N-shaped dependence of the shear stress on the shear rate. The decreasing part of the rheograms takes place at low shear rate. These regions of negative differential viscosity, and therefore, unstable flow is not observed in the case of nanosphere ferrofluids. The second intriguing difference concerns the curvature of the yield stress vs magnetic field curves. This curvature is negative in the case of nanosphere ferrofluid, giving rise to saturation of the yield stress at medium field, as expected. However, in the case of nanofiber ferrofluid this curvature is positive, which means a faster increase of the yield stress with the magnetic field the higher the magnitude of the latter. These interesting differences may be due to the existence of strong interparticle solid friction in the case of nanofiber ferrofluids. Finally, theoretical models for the static yield stress of the ferrofluids were developed. These models consider that upon field application the ferrofluid nanoparticles are condensed in drops of dense phase. These drops tend to be aligned along the field direction, opposing the flow of the ferrofluids and being responsible for the static quasielastic deformation and the yield-stress phenomena. By considering the existence of interparticle dry friction only in the case of nanofiber ferrofluids, the developed models predicted quite well not only the magnitude of the static yield stress but also the differences in curvature of the yield stress vs magnetic field curves.

Magnetopolymer materials (ferrogels, magnetic hydrogels, magnetic elastomers) are new kind of smart materials whose physical and mechanical properties can be controlled by an external magnetic field. We present an overview of experimental, theoretical and computer studies of magentomechanical properties and behavior of these systems.

Magnetic hydrogels are becoming increasingly demanded for technical and biomedical applications, especially for tissue engineering purposes. Among them, alginate-based magnetic hydrogels emerge as one of the preferred formulations, due to the abundance, low cost and biocompatibility of alginate polymers. However, their relatively slow gelation kinetics provokes strong particle settling, resulting in nonhomogeneous magnetic hydrogels. Here we study magnetic hydrogels prepared by a novel two-step protocol that allows obtaining macroscopically homogeneous systems, consisting of magnetic microparticles embedded within the alginate network. We describe a comprehensive characterization (morphology, microstructure and mechanical properties under shear stresses) of the resulting magnetic hydrogels. We 2 pay special attention to the effects of particle volume fraction (up to 0.33) and strength of the magnetic field on the viscoelastic properties of the magnetic hydrogels. Our results indicate that magnetic hydrogels are strongly strengthened against shear stresses as magnetic particle concentration and applied field intensity increase. Finally, we report an adaptation of the two-step protocol for the injection of the magnetic hydrogels that might be adequate for implementation in vivo. Interestingly, injected magnetic hydrogels present similar morphology and mechanical properties to noninjected hydrogels. To conclude, we report magnetic alginate hydrogels with adequate homogeneity and injectability character. These characteristics, together with the broad range of their mechanical properties, make them perfect candidates for cutting-edge technology.

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We present results of an experimental and theoretical study of the magnetic properties of soft magnetic gels consisting of micron-sized magnetizable particles embedded in a polymer matrix. Experiments demonstrate hysteretic dependences of composite magnetization on an applied magnetic field and non-monotonic, with maximum, dependence of the sample susceptibilities on the field. We propose a theoretical approach which describes the main physical features of these experimental results.