# 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
[11] improve the IPMC properties by application of multi-walled carbon nanotubes (MWCNTs) and carboxylic multi-walled carbon nanotubes (CMWCNTs). It was shown that the addition of a small number of MWCNTs or CMWCNTs improved the mechanical properties, output force, deformation rate, and displacement of the IPMC. Self-preparation of an ionomer using direct polymerization of different non-fluorinated monomers [14] or by improving the propertiesof existing non-fluorinated polymers (for example by crosslinking) [15; 16; 17; 18] provides great potential for modification of ionomer membrane properties. A significant number of non-fluorinated monomers and polymers allows for obtaining ionomers with very different properties. This gives the possibility of much better and greater control of the final properties of the IPMC. The application of a non-fluorinated membrane allows for more \"friendly\" conditions during electroless metal plating, without problems with the adhesion of the metal layer to the membrane surface. Moreover, in the case of Nafion-based ionomers, we are significantly limited and, as mentioned above, we can only modify the composition of the polymer suspension by introducing various types of modifiers before obtaining IPMC. Compared to the use of ready-made membranes or suspensions of fluorinated polymers, non-fluorinated polymers give much greater possibilities for IPMC modification at the stage of polymer membrane synthesis (through the appropriate selection of the monomers composition), even without introducing other modifiers. [PERSON] et al. present the application of various co-polymers such as commercial pentablock copolymer--Kraton (obtained from such monomers as _tert_-butylstyrene, ethylene, propylene, styrene and styrene sulfonate) [15], poly(vinyl chloride) crosslinked with 4-sulphophthalic acid (SPVC) and modified using phosphotungstic acid (PTA) [16], and sulfonated poly(ether ether ketone) (SPEEK) [18] on the properties of IPMC. In these works, it was shown that the actuators based on SPVC-PTA and Kraton are characterized by similar force and displacement values of 0.1 mN and 10 mm, respectively, while the IMPCs based on SPEEK had better properties, i.e., force 0.3 mN, displacement--20 mm. [PERSON] et al. synthesized the ionomer membrane from acrylate monomers using digital light processing (DLP) [14]. It was shown that by changing the polymerizable mixture's composition it was possible to modify the final properties of the IPMC. In addition, the use of 3D printing allows for obtaining membranes of different shapes, sizes, and thicknesses, which gives more possibilities in designing their shape compared to commercial flat membranes. Additionally, [PERSON] et al. have shown the application of DLP for ionomer preparation [19]. Thanks to the use of 3D printing, it was possible to quickly and easily obtain a membrane of various compositions, which led to the production of ionomers with different mechanical properties and different bending abilities. Due to the fact that the production of ionomers using 3D printing is a fast, cheap, easy-to-use technique, and gives a lot of possibilities for changing the composition of the membrane, it can be expected that in the near future, 3D printing will be intensively developed in terms of its application to obtain ionomers for IPMC actuators. The main route of electroless plating of the metal electrode on the surface of the ionomer is the chemical reduction of metal cations, mostly Pt\({}^{2+}\), Pd\({}^{2+}\), or Ag\({}^{+}\) to its metallic form on the surface of the membrane [12; 18; 20; 21; 22; 23]. Reduction of Pt\({}^{2+}\) and Pd\({}^{2+}\) is very common because the obtained metallic layer is electrochemically stable, does not oxidize,and is easy to fabricate, but the price of the complex solution for plating is very high. Therefore these cations are often replaced by Ag\({}^{+}\), which also gives good plating results and, at the same time, is much cheaper [24; 25; 26; 27].