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.

The effect of hydration on the biomechanical properties of fibrin and fibrin-agarose (FA) tissue-like hydrogels is reported. Native hydrogels with approximately 99.5% of water content and hydrogels with water content reduced until 90% and 80% by means of plastic compression (nanostructuration) were generated. The biomechanical properties of the hydrogels were investigated by tensile, compressive, and shear tests. Experimental results indicate that nanostructuration enhances the biomechanical properties of the hydrogels. This improvement is due to the partial draining of the water that fills the porous network of fibers that the plastic compression generates, which produces a denser material, as confirmed by scanning electron microscopy. Results also indicate that the characteristic compressive and shear parameters increase with agarose concentration, very likely due to the high water holding capacity of agarose, which reduces the compressibility and gives consistency to the hydrogels. However, results of tensile tests indicate a weakening of the hydrogels as agarose concentration increases, which evidences the anisotropic nature of these biomaterials. Interestingly, we found that by adjusting the water and agarose contents it is possible to tune the biomechanical properties of FA hydrogels for a broad range, within which the properties of many native tissues fall.

The high magneto-viscous response of magnetorheological fluids (MRFs) comes from the large size (≈1 μm) of the magnetic particles dispersed in the carrier liquid. Unfortunately, in the absence of a magnetic field, this large size constitutes the origin of some problems facing the technological applications of MRFs. These problems are (i) the instability of the suspensions caused by the fast settling of the high density magnetic particles used, and (ii) the poor redispersibility due to an irreversible aggregation. In this work, we used an electromagnetic induction method to study the stability of MRFs containing micron-sized iron particles dispersed in ferrofluids composed by oleate-covered magnetite nanoparticles dispersed in kerosene. Interestingly, we demonstrated that the sedimentation rate in iron/ferrofluid suspensions can be significantly lower than in iron/kerosene MRFs.

In this work, the magnetorheological properties of suspensions of micron-sized iron particles dispersed in magnetite ferrofluids were studied. With this aim, the flow properties of the suspensions in the steady-state regime were investigated using a commercial magnetorheometer with a parallel-plate measuring cell. The effect of both magnetite and iron concentration on the magnitude of the yield stress was studied for a broad range of magnetic fields. In addition, the experimental values of the yield stress were compared with the predictions from the chain model. With this purpose the values of the yield stress were obtained by means of finite element calculations. Interestingly, it was found that the experimental yield stress increases with the concentration of magnetite nanoparticles in the ferrofluid. Unfortunately, this behaviour is not obtained from calculations based on the chain model, which predict just the opposite trend.