Ferrogels with well-dispersed single-domain magnetic nanoparticles (NPs) were obtained by the infusion of iron salts in physically cross-linked poly(vinyl alcohol) (PVA) hydrogels followed by coprecipitation. Freeze−thaw (F−T) cycling was used as a cryogenic technique to form mechanically strong and highly swellable hydrogels. The networked structure of the final material was used as a constrained environment for the precipitation of iron oxide NPs and formation of the magnetic gel. A homogeneous, single-domain ensemble of more than 15 wt % iron oxide NPs (in only one cycle of absorption) could be obtained through this easy technique. Moreover, the capacity of these magnetic ferrogels to absorb high amounts of ethanol/ water solutions allows impregnation of these materials with ibuprofen and subsequent release of the drug at physiological pH. The biocompatibility of the components and the use of the nontoxic PVA cross-linking strategy (F−T cycling) make these materials promising for drug-delivery applications.
■ INTRODUCTIONThe rational combination of nanostructures with polymers has proved to be a powerful tool for the development of new stimulus-responsive multifunctional materials. 1 Among polymeric matrixes used in nanocomposites, hydrogels are especially attractive and the ideal choice for potential applications in the biomedical, environmental, and pharmaceutical fields. 2 Hydrogels are networks constituted by crosslinked hydrophilic polymer chains with the capacity of absorbing large amounts of water and biological fluids. 3 In the past several years, the interest and attention paid to hydrogels have progressively changed from large-scale products toward more sophisticated applications requiring improved performance. 2b,c,4 This is the case, for example, for stimulusresponsive hydrogels 4b,c and biomimetic materials, 4a with multiple and promising applications in actuation, in vitro diagnostics, tissue engineering, and so on. Ferrogels are a class of stimulus-responsive materials composed of magnetic nanoparticles (NPs) embedded in a gel matrix. 5 The synergic combination of magnetic NPs with the soft, hydrophilic, and wet matrix of hydrogels enables the creation of materials with unique and advanced properties useful for the design of smart drug-delivery systems, actuators, and sensors. For example, the presence of NPs enables the building of materials that can be deformed 5b,c,6 or heated 7 by the action of a magnetic field. In the presence of magnetic forces, because of the elastic character of ferrogels, particle movement couples to gel movement, producing significant deformation and/or changes in porosity under the action of quite moderate magnetic fields. 5b,c These mechanical actions would be responsible for changes in drug release and bursting, on−off behavior observed for some ferrogel systems. 6,8 On the other hand, the action of an alternating magnetic field produces remote heating of the system through the hyperthermal effect that occurs by Neél (inner fluctuation of the magnetic m...