Magnetic biosensors are an important part of biomedical applications of magnetic materials. As the living tissue is basically a “soft matter.” this study addresses the development of ferrogels (FG) with micron sized magnetic particles of magnetite and strontium hexaferrite mimicking the living tissue. The basic composition of the FG comprised the polymeric network of polyacrylamide, synthesized by free radical polymerization of monomeric acrylamide (AAm) in water solution at three levels of concentration (1.1 M, 0.85 M and 0.58 M) to provide the FG with varying elasticity. To improve FG biocompatibility and to prevent the precipitation of the particles, polysaccharide thickeners—guar gum or xanthan gum were used. The content of magnetic particles in FG varied up to 5.2 wt % depending on the FG composition. The mechanical properties of FG and their deformation in a uniform magnetic field were comparatively analyzed. FG filled with strontium hexaferrite particles have larger Young’s modulus value than FG filled with magnetite particles, most likely due to the specific features of the adhesion of the network’s polymeric subchains on the surface of the particles. FG networks with xanthan are stronger and have higher modulus than the FG with guar. FG based on magnetite, contract in a magnetic field 0.42 T, whereas some FG based on strontium hexaferrite swell. Weak FG with the lowest concentration of AAm shows a much stronger response to a field, as the concentration of AAm governs the Young’s modulus of ferrogel. A small magnetic field magnetoimpedance sensor prototype with Co68.6Fe3.9Mo3.0Si12.0B12.5 rapidly quenched amorphous ribbon based element was designed aiming to develop a sensor working with a disposable stripe sensitive element. The proposed protocol allowed measurements of the concentration dependence of magnetic particles in gels using magnetoimpedance responses in the presence of magnetite and strontium hexaferrite ferrogels with xanthan. We have discussed the importance of magnetic history for the detection process and demonstrated the importance of remnant magnetization in the case of the gels with large magnetic particles.

A new kind of ferrogel with entrapped metallic iron nanoparticles causing unusual magnetodeformation is presented. Crosslinked polyacrylamide (PAAm) based ferrogels embedded with iron nanoparticles (MNPs) were synthesized by free radical polymerization in aqueous medium. Spherical iron MNPs with average diameter 66 nm were synthesized by the electrical explosion of wire and modified by interfacial adsorption of linear polyacrylamide (LPAAm). Extended Derjaguin-Landau-Verwey-Overbeek (xDLVO) calculations based on the superposing of van der Waals, electrostatic, steric, and magnetic contributions showed that polymeric encapsulation of nanoparticles by LPAAm is one of the most suitable pathways for preparing stable aqueous dispersions of iron nanoparticles. Microcalorimetry confirmed the presence of strong interfacial adhesion forces between LPAAm chains and the surface of iron nanoparticles. By keeping the same crosslinking density of a polymer network (i.e. 100 : 1, monomer to crosslinker ratio) and varying the initial monomer concentration, an influence of the extent of polymer network reticulation on the mechanical properties and subsequently, magneto-elastic properties was demonstrated. It was found that the upper limit of the shear modulus for the synthesis of a new kind of polyacrylamide based ferrogel to exhibit any usable magnetodeformation under the application of a uniform external magnetic field of 420 mT is ca. 1 kPa. Magnetodeformation of cylindrical ferrogel samples was observed in the form of an overall volume contraction accompanied by a homogeneous decrease in all dimensions. The deformation was found to be maximum (around 10%) for the aspect ratio of 1/1 and it was lower and similar for the samples with 1/2 and 2/1 aspect ratios. Such a type of magnetic response is significantly different from the behavior observed in the existing reports on ferroelastomers.

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Polyacrylamide ferrogels with embedded nickel magnetic nanoparticles were synthesized by radical polymerization in water. The elastic modulus of ferrogels was measured under unidirectional compression in a uniform magnetic field with a strength of 0, 165, 220 and 275 Oe. With an increase in the content of nickel particles in ferrogel from 0 to 5.3% by weight, the elastic modulus in zero magnetic field increased by 60%. The application of magnetic field parallel to the direction of compression resulted in the increase of modulus, and if the field was applied perpendicular to the compression, the modulus tends to decrease. Such trends are in good agreement with the features of magnetostriction of a non-compressible uniformly magnetized sphere in a constant magnetic field.

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