Actuation contraction/expansion/bending

So why do some ionic EAPs contract The movement of the ions of the electrolyte comes into play, but to exploit contraction, the placement of the electrodes is also very important. If contraction, rather than bending is desired, the negative electrode is placed external to the EAP, while the positive electrode remains internal (embedded) in the EAP (Fig. 4.5). In the case of the contractile EAP composed of PMA, the ions along the main polymer chains are weak acetic acid groups. The electrolytic solution typically used to swell the gel and provide for good electroactivity is a dilute sodium chloride solution, such as isotonic saline solution. When the electric input is applied to the PMA EAP based actuator with this electrode configuration, the EAP of the actuator contracts quickly and substantially. In selected EAP formulations, when the polarity is reversed, expansion occurs, and contraction and expansion can be cycled repeatedly in selected PMA based EAP materials and actuators [3, 5]. 

Besides the intrinsic conductive polymers, some deformable polymers, such as shape-memory polymers, are usually activated by heating. After incorporating with conductive fillers, such as carbon nanomaterials, they can be simulated by the electricity through Joule heating (Liu et al., 2009 Hu and Chen, 2010 Koerner et al., 2004). This kind of electro thermally active polymer composites can produce expansion/contraction and bending behaviors upon with the electricity. Moreover, these actuators can work durably... 

PPy layers that forms the outside layers on either side of a polymer electrol5de layer in the middle. The pol5mier electrolyte layer could be solid state electrolyte or electrol5de saturated media (i.e., porous PVDF film). Once a voltage is applied, one PPy layer performs as cathode and the other performs as the anode. Since oxidation and reduction occur separately at both sides, expansion and contraction on each side will cause bending actuation (Fig 4.9). 

Actuators that generate movements and forces, such as bending, expansion and contraction driven by stimulation of electrical, chemical, thermal and optical energies, are different from rotating machines such as electric motors and internal combustion engines. There are many sorts of soft actuators made of polymers [1-3], gels [4] and nanotubes [5]. Particularly, biomimetic actuators are interesting because of the application to artificial muscles that will be demanded for medical equipment, robotics and replacement of human muscle in the future. 

Without using ions, electric field or heat, electrochemically synthesized PPy films may exhibit fast and intensive bending in ambient air on the basis of a reversible van der Waals adsorption of polar, organic or water molecules in the vapor state [139-141]. As shown in Table 8.13, the chemomechanical response of the actuator strongly depended on the adsorbate [139]. It was reported that PPy film exhibited contraction when an electric field was applied in ambient air because of the contraction caused by desorption of water vapor and thermal expansion of polymer chains [142]. 

For pH-responsive electrochemical systems, on the other hand, the electrical field indirectly influences the polymer conformation to induce a reversible volume transition. The electricity is used to electrolyze water, generating hydrogen ions at the anode and hydroxyl ions at the cathode. This produces a pH gradient across the solution, which can be used to influence the conformation of pH-responsive polymers. Strategic placement of the polymer in the solution can cause expansion, contraction, or even bending (when placed directly between the electrodes) of a polymer actuator. Switching the direction of the current creates the opposite effect. This second method of actuation... 

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