**1. Introduction**

As the increasing trend of better and comfortable lifestyle has led to growing demand for both new technologies and materials, advanced smart and intelligent functional materials have been receiving a large attentions in recent years [1]. The smart materials are those controllable with external environments such as electric or magnetic field, mechanical stress, heat, and light [2].

Among these, magnetorheological (MR) materials become one of the most important smart materials in terms of their huge industrial potentials. They are classified as a functional smart material possessing tunable rheological and viscoelastic properties such as yield stress, shear stress, dynamic moduli, and damping property when an external magnetic field is applied [3–7]. Note that while their electrical analogue electrorheological (ER) materials have been also extensively investigated [8], MR materials prevail ER materials regarding both their scientific investigation and applications due to superior performance characteristics of the MR materials [9].

MR materials in general include several different systems, depending on their media employed such as MR fluid (MRF), MR foams, MR gels, and MR elastomer (MRE) [4]. Historically, MRFs, colloidal suspensions that consist of small soft-magnetic particles with extremely small hysteresis, high magnetic permeability and high saturation magnetization suspended in non-magnetic liquids [10] have been widely studied. Dispersed soft-magnetic particles enhance the apparent shear viscosity and yield stress of the MR suspensions [11], depending on both magnetic field strength applied [12] and concentration of micron-sized magnetic particles [13,14]. They have been extensively adopted in various applications such as MR brake, MR valve, MR mount, MR clutch, and MR damper [15]. The versatility of their wide applications is caused by noiseless operation, rapid field responsiveness of MRFs relative insensitivity to small quantities of contaminants or dust, and easy control [16]. On the other hand, despite that MRFs are possessing significant merits with many potential applications, their disadvantages include sealing issues due to the leakage of the medium liquid, contaminating environment, and sedimentation of the particles [17]. Hence these drawbacks limit to the further expansion of its engineering applications.

Contrary to the MRFs, MREs are the hard (particles) and soft (pristine matrix) hybrid composite smart elastomeric materials [18]. Therefore, unlike MRFs, solid-state MREs overcome the disadvantages of MRFs. MRE contains unique mechanical properties in such a way that their viscoelastic properties change under an applied magnetic field. This behavior is very similar to the MRF, but MRE is a more like solid equivalent [19]. In addition, the MRE is generally composed of a material that contains a rubber matrix, minor additives and magnetic carbonyl iron (CI) particles. In such a material, the coalescence of CI particles occurs within an applied magnetic field, which attributes to the hardness, enhanced elastic modulus, and shear modulus [20]. The typical rubber matrix includes silicone rubber (SR) [21–23], polybutadiene rubber [24,25], nitrile rubber [26], and polyurethane (PU) rubber [27,28] and others. Traditionally, elastomers are nonmetallic materials, being used in wide engineering applications such as seals, gaskets, and tires [29]. Correspondingly, the MREs are mainly used in three main categories in past decades, which are sensing devices, actuators and vibration, and noise control [30]. In recent studies, the MRE is also adapted in biomedical engineering field where it is used as a soft material for magnetic-elastic soft robot [31,32]. The tunable viscoelastic properties of MREs have also overpowered the mobility limitation of small-scale robot due to texture or different material in an unstructured environment [31]. Hence, according to these studies, MREs have drawn dramatic interests in various engineering applications.

On the other hand, MREs can be classified into two different groups, which are isotropic and anisotropic MREs, based on mechanisms of magnetically polarized particle configuration in the MREs. The polarized particles are uniformly suspended in an isotropic MRE, so that the MRE shows homogenous physical behaviors in every direction. For an anisotropic MRE, the magnetic particles are aligned along with the input magnetic field direction during manufacturing process, which leads to the perpendicular direction of a flat MRE specimen. In many studies, the shear direction that is perpendicular to the CI particles' alignment was observed when a rheological measurement of anisotropic MRE was conducted [33–35]. Tian and Nakano [36] proposed the fabrication process of anisotropic MRE with 45◦ CI particles' (less than 60 μm depending on their shape) arrangemen<sup>t</sup> in various silicone oil concentrations, which enhanced the storage modulus (G'). In addition, the combination of isotropic/anisotropic MRE has examined to adjust damping capability and zero-field dynamic stiffness of silicone based MRE [37]. According to these studies, MRE has raised grea<sup>t</sup> attention in engineering fields. On the other hand, along with the CI particles, various magnetic particles such as CoFe2O4 (less than 12 nm) [38], Ni (10 μm) [39], FeCO3 (less than 30 nm) [40], and industrial waste nickel zinc ferrite (less than 2 μm) [41] have been also employed. Siti et al. [42] fabricated MREs using CI and nano-sized Ni-Mg cobalt ferrites (35 to 80 nm) based on SR. Even though, both MRF and MRE possess similar magnetic field properties, the main distinction is that their operation period is dependent within two types of materials. The interesting point is that MRE tends to operate in pre-yield regime [43,44], where MRF is typically operated in a flow regime or post-yield shear regime [45,46]. Hence, the magnetic field performance of MRF is mainly obtained from yield stress while MRE is characterized by field-dependent modulus [47].

This article reviews a recent trend on preparation, characteristics, and applications of smart MRE materials. While the microstructures with isotropic or anisotropic MREs are tested by scanning electron microscopy (SEM) and the magnetic property of the magnetic particles used was analyzed by VSM, the mechanical characteristics of MREs such as tensile, Payne, and loss factor have been conducted. In addition, the rheological properties of MREs including dynamic, amplitude sweep, frequency, sweep, and creep test are also illustrated.
