# Review of Soft Actuators Controlled with Electrical Stimuli: IPMC, DEAP, and MRE
## 2 Recent Progress in Development and Applications of Soft Actuators
### Ionic Polymer Metal Composite (IPMC)
#### 2.1.1 Recent Trends in the Modification of IPMC  
The ion-exchange membrane is an important part of the IMPC that influences the final properties of the actuator. Apart from the appropriate mechanical properties (Young modulus, elasticity, and strength) and a good ability for ion transport, the ionomer should be resistant to chemical corrosion and high temperature, which is related to \"hard\" conditions of electroless plating methods. Therefore, the most common ionomers used are prepared from perfluorinated membranes, mainly Nafion. Thanks to fluoric atoms introduced to polymer structure, these membranes provide great chemical and thermal stability, and resistance to chemical and electrochemical oxidation, which allows for avoiding degradation during the electroless metal plating and IPMC working. However, fluorinated polymers present very pure adhesive properties that cause a lot of trouble with the adhesion of metal electrodes to their surface. Therefore, the IPMC manufacturing process requires additional membrane surface preparation prior to electroless metal plating. The surface of the membrane is sanded with sandpaper (improvement of mechanical adhesion) and the membrane is boiled in a hydrochloric acid solution. In the case of non-fluorinated membranes, there is no need to prepare the surface for the application of the metallic layer. This simplifies the preparation of the IPMC. Another problem is related to the condition of the IPMC operation. In most cases, ionomers are swelled by the water, which provides conditions for ion diffusion in the membrane. The ion's mobility in the water environment is high thus, the response to an electric signal is fast, but during the IPMC working, water can easily evaporate, causing high limitation in ion diffusion and dynamic response to the electric signal decrease. Another problem is the electrochemical stability window of water, which is equal ca 1.23 V (in a neutral environment). Above this value, water decomposes, which causes a progressive decrease in IPMC properties. Therefore, researchers are focusing on various paths of modification and improvement of the IMPCs to solve the described problems. Several important routes can be distinguished: (i) introduction of various modifiers to the Nafion suspension and subsequent preparation of the membrane, (ii) use of non-fluorinated ionomers (self-synthesized or commercially available), (iii) modification of the composition and properties of the metallic electrode. Modification of Ionomers Based on Fluorine Polymers  
Nafion is the most popular ionomer used in IPMC formulations. It can be purchased as a flat membrane or as a polymer suspension in water. The use of Nafion suspension offers great opportunities for IPMC modification, as it allows the suspension to be mixed with various types of modifiers [9; 10; 11; 12; 13]. Therefore a lot of scientists focus on this topic. [PERSON] et al. [12] have shown that the introduction of poly(ethylene oxide) (PEO) in proper concentration into the Nafion structure increases the water uptake, which has a significant influence on the IPMC properties. Increasing the concentration of PEO in the membrane causes a decrease in membrane stiffens, an increase in displacement (for PEO concentration up to 20%), and a decrease in blocking force. Other used modifiers are carbon nanotubes  
Figure 2: Reaction of IPMC Polymer to an applied potential. (CNTs) [10; 11; 12]. In that case, CNTs are often applied as membrane and electrode modifiers. [PERSON] et al. [12] have applied CNTs for modification of the Nafion membrane and surface of electrodes (electroless plating of Pt on membrane surface followed by electrophoretic deposition of CNTs). A high increase in the electrical conductivity of electrodes and IPMC performance was observed. Similar work was presented by [PERSON] et al. [10], who used CNTs mixed with Nafion, prior to electroless plating, as an additional layer on the membrane surface. [PERSON] et al.