# Research Progress on Conducting Polymer-Based Biomedical Applications
## 3 Biomedical Applications
### Scaffolds for Tissue Engineering
These hydrogels exhibit rubber-like flexibility, tunability, and biocompatibility, which are essential requirements for the use in scaffolds [116; 117; 118; 119]. An electroconductive hydrogel made of poly(ethylene dioxythiophene) and poly(acrylic acid) (PAA) has been developed by free radical polymerization since they display distinguished electroactivity, mechanical properties, and swelling [120]. Despite their excellent electrical conductivity, physiological pH of living tissue (7.4) diminishes electrical conductivity of electroconductive hydrogels, as the charges on the CP backbone return to the neutral states [121]. Also, hydration in physiological media allows physically trapped CPs to escape the network, resulting in a further decrease in electrical conductivity [122]. Single component CP hydrogels fabricated from CPs as a main matrix can be a solution to the limitations of electroconductive hydrogels. However, it is challenging to form such hydrogels since conjugated polymers lack flexibility, water solubility, and functional side chains [123]. Therefore, only a few studies have been reported on the single component hydrogels. Nevertheless, there are potential candidates for these hydrogels, and polythiophenes are one example owing to a variety of water-soluble polythiophenes that have been developed [124]. [PERSON] et al. demonstrated the fabrication of single component hydrogels composed of poly (3-thiophene acetic acid) (PTAA), which is based on polythiophenes [125]. 1,1\({}^{\prime}\)-carbonyldiimidazole (CDI) was utilized as a crosslinking agent to form the hydrogels since it is soluble in water and washable from the network. The crosslinking density was determined by the ratio of CDI to the monomer of PTAA. In Figure 5a, scanning electron micrographs of the hydrogels with different crosslinking densities are shown. All of the hydrogels exhibited porous networks with varying wall structures and pore size distributions. A well-established morphology of C2C12 myoblast cells 72 h after the incubation on the surface of the hydrogels is shown in Figure 5b,c, which indicates that the hydrogels assisted the adhesion and proliferation of the cells. Depending on the crosslinking density, the swelling ratio and porosity as well as mechanical properties could be tuned. Excellent swelling ratio up to \(\sim\)850% was observed when the crosslinking density was low. The compressive modulus was also affected by the crosslinking density and measured to be 17.88 kPa with a high crosslinking density, which is comparable to muscle tissue. Furthermore, great electrical conductivity of \(\sim\)10\({}^{-2}\) S cm\({}^{-1}\) was obtained in physiological conditions. Figure 5: Scanning electron micrographs of the poly (3-thiophene acetic acid) (PTAA) hydrogels with different crosslinking densities: low (left), middle (middle), and high (right) (**a**). Morphology of C2C12 myoblast cells at 72 h of post-seeding observed by SEM (**b**) and by a fluorescence assay (**c**). Adapted with permission from [125]. Copyright 2012 John Wiley and Sons. Tissue engineering usually concerns three approaches: (1) utilization of biomolecules to help host cells regenerate tissue, (2) introduction of repair cells directly to the defective area, and (3) growth of cells on a scaffold in bioengineered atmospheres that would help the regeneration of tissue [126]. Tissue scaffolds should mimic the chemical, mechanical, and topographical properties of the ECM composed of proteins and polysaccharides [127]. Therefore, a controllable geometry, high degree of porosity with appropriate pore size, great surface-to-volume ratio, and a high degree of pore interconnection are required for tissue scaffolds [128]. Nanofabrication technologies that are able to synthesize nanostructures satisfying such factors include electrospinningning, chemical etching, and photolithography. Electrospinning is commonly used to generate ECM-like nanofibrous scaffolds. PPy, PANI, and PT have been proven to be compatible with cells and tissues in in vivo and in vitro systems [126].