# Tuning the Cell and Biological Tissue Environment through Magneto-Active Materials
## 4 Fundamentals of Smart Polymeric Materials: Focus on Magneto-Active Elastomers and Hydrogels
### Magnetorheological Elastomers (MREs)  
Magnetorheological elastomers (MREs) are smart polymeric materials that change mechanical or physical properties when an external magnetic field (EMF) is applied. They are the combination of a polymeric network with nano- and micron-sized magnetic particles. The magnetic interaction of the particles in the presence of a magnetic field is transmitted as forces to the polymeric matrix. These forces acting on the polymeric network result in either deformation or a change in the mechanical properties of the material [118]. The first one is commonly known as magnetostriction, and it consists of the deformation of the polymeric network induced by an external magnetic field. Although magnetostriction is a universal phenomenon for almost all ferromagnetic materials, it is more noticeable in MREs due to their viscous and elastic properties, mainly their softness. The second important feature of MREs, and maybe the more intriguing, is referred to as magneto-rheological (MR) effect. This phenomenon is explained as the change of material stiffness in the presence of a magnetic field. An increase in the shear and tensile moduli under an EMF have both been observed for MREs [119]. Unlike magnetostriction, the magneto-rheological effect is rarely observed in conventional ferromagnetic materials, making magneto-active materials a unique kind of material [120]. Due to the composite nature of these kinds of material, attention must be set on the two components that form them: the matrix material, which is a polymer that gives the MREs their basic mechanical properties and shape; the magnetic particles, which are of micron- or nano-size and give the MREs their responsiveness to external magnetic fields. The most used polymeric materials for the fabrication of MREs are silicone rubbers, due mainly to their availability as resins, in liquid state, which facilitates the homogeneous distribution of the magnetic particles and the easy suspension of the particles during the synthesis process. Furthermore, silicone-based polymers are vulcanised fast at an elevated temperature as well as at room temperature, and are non-flammable, non-toxic, slightly dissipative and temperature-sensitive, and, lastly, highly deformable, which allows for higher magnetostriction and MR effect. However, some issues related to their poor mechanical properties persist. These limitations mainly affect the use of silicone rubbers in most load-bushing applications due to its low strength and shorter fatigue life. However, for some biomedical applications that do not require high-strength materials, silicone-based rubbers are still the most preferred material [121]. To overcome the limitations due to the low strength of silicone rubbers, many investigators have turned their attention to the use of thermosets and thermoplastics polymers due to the wide range of mechanical properties that can be achieved by simply modifying the type of concentration of the constituent elements.Another alternative to the use of silicone-based rubbers for the fabrication of MREs is natural rubber, which generally have