# Research Progress on Conducting Polymer-Based Biomedical Applications
## 3 Biomedical Applications
### Actuators for Artificial Muscles  
The volume change of the CPs due to the electrochemomechanical reactions pave a way towards their utilization as actuators for artificial muscle applications [129]. The volumetric expansion of a CP in electrolyte is controlled by tuning the applied voltage. It is notable that the oxidized state is stiffer than the reduced state due to the ionic crosslink at the polaron site and delocalization of \(\pi\)-electron [130]. The size of anions used for the process determines whether the electrochemomechanical reaction occurs via anion drive or cation drive, as illustrated in Figure 6. Small anions, such as \(\text{ClO}_{4}-\) and bis-(trifluoromethylomenthesulfonyl)imide (TSFI\(-\)), come out of the bulk CPs during reduction, and thereby the reduced state is shrunk, which is called anion drive (Figure 6a). On the other hand, when large anions, such as dodecylbenzenesulfonic (DBS\(-\)) acid, are used, long alkyl chains of the anions cause entanglement and immobilization in the network. The reduction of the CPs then occurs via the cation drive, where cations inserted from the electrolyte solution result in a swollen polymer network by the volume of additional cations (Figure 6b) [131].  
In actuators, a linear or angular movement can be derived from the volumetric change of the CPs. One of the most widely used configurations is a bilayer structure composed of a CP film grown on a metallic electrode with a nonconductive film. The device exhibits macroscopic bending behaviors shown in Figure 7 due to swelling or shrinking induced by a mechanical stress gradient across their interface resulting from electrochemical reactions. Specifically, the anion-driven device bends towards the CP-convex form upon oxidation via swelling of the CP. On the other hand, the cation drive results in bending the device in a CP-concave manner, as they shrink by oxidation. It is yet unavoidable to use a metallic counter electrode in a bilayer actuator. However, it often results in deterioration of the actuating film caused by corrosion of the metal electrode exposed to a harsh environment (i.e., a sudden change in pH and chemical) [129].  
Figure 6: Electrochemomechanical reactions using small anions result in the shrinking of CPs upon reduction by anion drive (**a**), whereas large anions cause CPs to swell when reduced by cation drive (**b**). Reprinted with permission from [131]. Copyright 2014 Springer.  
In an effort to exclude the metallic counter electrode, a trilayer actuator was developed in which two CP films were attached to double-sided tape [132]. When the trilayer structure is immersed in an electrolyte (Figure 8), two opposite volume changes are produced as one of the CP films undergoes anion-driven swelling, whereas the other film experiences cation-driven shrinking [129]. The main advantage of the CPs, as actuators for artificial muscles, is their low operating voltage (as low as 2-10 V) [133]. In addition, they provide higher strains in a more cost-effective way than carbon nanotubes [134]. However, the electromechanical coupling, the conversion efficiency from electrical to mechanical energy, is not as high as that in carbon nanotubes. Therefore, the application of the CPs in a large-scale actuator has been hampered [135; 136].