8: Review of Magnetic Gels as Smart Materials

Chapter 8 reviews magnetic gels. Zrinyi and co-workers were the first to develop magnetically active gels, the responses of which could be vastly accelerated by an imposed magnetic field. This chapter is a compact review of magnetic gels based on related research and development performed by Zrinyi and co-workers. Note that magnetic gels are considered a member of the smart materials family and in ways are similar to soft silicon rubber magnetic composites used in our daily life as soft magnetic stickers. However, magnetic gels are softer and more stretchable and maneuverable in the magnetic field and can sustain soft actuation at the micro and nano levels. A prelude to the development of ferrogels was a classic paper on ferrohydrodynamics by Rosenzweig published in 1985. Ferrogels are chemically cross-linked polymer networks swollen by a colloidal ferrofluid. A colloidal ferrofluid, or a magnetic fluid, is a colloidal dispersion of monodomain magnetic particles. Typically the monodomain magnetic particles have typical sizes of about 10–15 nm, and they are superparamagnetic, in which magnetization can randomly flip direction under the influence of temperature. Magnetic gels or ferrogels belong to the general family of magnetostrictive materials, which produce strain when exposed to a magnetic field. One may also embed magnetic coils within these materials to be able to also electrically control the deformation of ferrogels. Magnetic gels belong to the family of hydrogels, polymeric gels and general polyelectrolyte gels. They are highly swollen molecular networks that are cross-linked and create a hydrophilic solid.