# Review of Soft Actuators Controlled with Electrical Stimuli: IPMC, DEAP, and MRE
## 2 Recent Progress in Development and Applications of Soft Actuators
### Magnetorheological Elastomers (MRE)
#### 2.3.1 Actual Trends in the Modification of MREs  
Due to the composition of MREs, we can distinguish two basic directions of their development in order to improve the response performance to the magnetic field. The  
Figure 8: Diagram showing the production process of isotropic and anisotropic MREs (source: [104]).  
first direction is the selection and modification of the polymer matrix, while the second is the selection and modification of the magnetic particles. The selection of the appropriate polymer matrix is an extremely important stage of MRE preparation. The choice of the polymer matrix is primarily related to its mechanical properties, and, thus, to the dynamic properties of MREs. Generally, a softer polymer matrix leads to higher magnetorheological effects. Other aspects of the polymer matrix or resin before their curing are also important. The viscosity of the resin before curing significantly affects the MRE fabrication, for example, lower viscosity facilitates the orientation of magnetic particles in the magnetic field during the preparation of anisotropic MREs or improves the miscibility of the magnetic particles with the resin. Other factors are also important, such as aging resistance, the ability to stably disperse magnetic particles, as well as price and availability. The listed properties of polymer matrices and resins for their preparation can be additionally modified by various types of additives, such as plasticizers [8,105], carbon additives (carbon nanotubes, carbon black, graphite, graphene) [106,107], and others. Each additive enhances the magnetorheological effect in a different way.  
[PERSON] et al. [8] have investigated the influence of silicon oil plasticizers on MRE properties. It has been shown that the increasing content of silicon oil causes a decrease in the zero-field storage modulus and an increasing relative magnetorheological effect for both isotropic and anisotropic MREs.  
Magnetic properties, on the other hand, can be adjusted by introducing compounds with different magnetic properties as well as introducing modified magnetic compounds. One of the directions of investigation presented in the literature is the application of different magnetic fillers themselves, or as modifiers for an MRE containing CIP. [PERSON] et al. [108] and [PERSON] et al. [103] present the application of Fe\({}_{2}\)O\({}_{3}\) while [PERSON] et al. [102] and [PERSON] et al. [101] present the application of Fe\({}_{3}\)O\({}_{4}\) nanoplates as magnetic filler. [PERSON] et al. [94] present the influence of Ni and Co powder additive on the properties of MREs. However, keep in mind, that the additives of magnetic fillers have an influence on the magnetic as well as mechanical properties of MRE, which allows for modification of the properties of the MREs depending on their application.  
A good example of the investigation presenting both the influence of plasticizer-silicon oil and magnetic modifier-CoFe\({}_{2}\)O\({}_{4}\) additives in the MRE (silicone-based + CIP) properties was presented by [PERSON] et al. [105]. It was shown that the introduction of CoFe\({}_{2}\)O\({}_{4}\) increases the magnetic properties, but also the hardness and elasticity modulus, while the introduction of plasticizer gives the opposite effect. An MRE with the best properties was obtained after the introduction of both modifiers--plasticizer and CoFe\({}_{2}\)O\({}_{4}\).  
An interesting course of research that may gain popularity may also be the use of 3D printing [109, 110, 111, 112]. 3D printing allows fabrication of the MRE with sophisticated shapes. It is a very interesting method of MRE fabrication that may gain popularity in this field of research in the near future and it could be expected that the number of papers describing the application of 3D printing will increase.