# Magneto-Rheological Elastomer Composites. A Review
## 1 Introduction  
Magneto-rheological elastomer (MRE) composites belong to the category of smart materials, showing a fast (milliseconds) and reversible transformation, including a controllable stiffness and frequency-dependent viscoelastic behavior, when subjected to a magnetic field [1]. Such composites generally show variations in magneto-rheological (MR) effect for applying in magnetic flux densities of the order of magnitude of 0.8 T [2]. The conventional use of MR fluids creates the settling of particles within the matrix as the function of time. To solve the issue of sedimentation of particles, MR elastomer emerges as an alternative polymer for the MRE composite [3]. The properties could be influenced by the function of temperature. Figure 1 is a schematic representation about the role of temperature for the transformation of MR fluids to elastomer [4]. The MR effect in the composite opens a considerable property making them suitable for the application in mechanical systems where the active control of vibrations or transmission of torque are required. Such as in the areas of tunable vibration absorbers, shear type dampers, composite dampers, isolators, seismic vibration dampers, brakes, control valves, clutches, and artificial joints [5; 6]. Furthermore, application fields using the magnetic control are those related with the thermal energy transfer, sound propagation, and isothermal magnetic advection, until you get to the biomedical applications [7; 8]. Similarly, magnetic nanocomposites find important applications in the areas of remediation, catalysts, and microwave absorbers in response to external stimulus [9]. In addition, other than elastomeric polymers, current research also focuseson biocompatible and thermoresponsive polymers in the fabrication of magnetic field responsive composites [10]. Conventional MRE composites consist of two-phase matters obtained by dispersing magnetizable micron-sized solid particles in a liquid matrix, up to 30 vol %. Due to their considerable saturation magnetization (\(\mu_{0}\)M\({}_{\mathrm{s}}\) = 2.1 T), iron and carbonyl iron particles are commonly used to this purpose, whilst silicone oils are employed as carrier liquids. Additives are necessary to inhibit sedimentation and aggregation of filler particles within the matrix, in addition, they provide lubricating properties [11]. Silicone rubber, one of the most common elastomers, is used a as matrix consisting of two-phase liquids with base/binder in the 1:1 ratio. Generally, in the composites fabrication after the adding of the magnetic particles into the matrix, a curing is carried out either in the absence (isotropic MREs) or in the presence (anisotropic MREs) of an external magnetic field. The efficiency of the MRE composite, oxidative and chemical stability, and durability, strictly depends on various factors, among those, particles aggregation plays a key role, considering that the interface particles-matrix is determinant in the durability of the composite [12; 13]. Thus, in designing MRE composites the main purpose is to obtain the highest MR effect, by inducing magnetization of the particles used as filler [14]. In fact, when the magnetic field is not present, it is possible to observe a random distribution of the particles within the matrix; thus, leading to an isotropic composite. Differently, the presence of a magnetic field, during the preparation, leads to the particles magnetization and attracting one another along the field lines, with the formation of an anisotropic aggregate that spans the system. The so obtained composite shows high yield stress and a large shear rate-dependent apparent viscosity, with increase in viscoelasticity due to the applied magnetic field. The magnetically induced shape change, because of the MRE based magnetostrictive sensors, is very effective with relatively soft materials.