# Research Progress on Conducting Polymer-Based Biomedical Applications
## 4 Current Stage and Challenges  
To date, significant progress has been achieved in exploiting the CPs in the field of biomedical engineering owing to their electrical conductivity and biocompatibility. Nevertheless, some drawbacks still limit the utilization of these polymer materials in biomedical engineering. Biodegradability is one of the critical issues of typical CPs that requires improvement, especially in tissue engineering, where scaffolds are expected to eventually decompose. A scaffold is an artificial support device implanted temporarily in order to boost the differentiation and proliferation of cells until new tissues are formed. Therefore, the CPs used as scaffolds must degrade after they achieve their role [105]. In an effort to ensure biodegradability of a scaffold, the CPs have been blended with other degradable polymers. For example, PLA, a common biodegradable polymer, was mixed with conductive PPy nanoparticles to provide both conductivity and degradability [41]. Figure 9: Release of anionic drugs occurs by anion driven shrinking when a CP is oxidized (**a**) whereas cationic drugs are released during reduction of a CP by cation drive (**b**). [PERSON] et al. reported on the fabrication of stable freestanding nanomembranes by combining a PT derivative that has carboxylate substituents in the 3-position of the heterocyclic ring and a biodegradable polyester [138]. The nanomembranes were fabricated by spin-coating mixtures of a PT derivative, poly(3-thiophene methyl acetate) (P3 TMA) and polybutylene succinate (PBS) for use as bioactive platforms for tissue engineering [139]. Although it is possible to provide the CPs with biodegradability, there is a tradeoff between the electrical conductivity and biodegradability, as the overall structure is modified in such a way that the conductive properties are hindered. Hydrophobicity of the CPs is also an obstacle that must be overcome in order to apply the CPs to scaffolds and biosensors. Despite their excellent mechanical properties and biocompatibility, their lack of hydrophilicity hampers the attachment of cells onto their surfaces [140; 141]. The surface properties can be modified via functionalization. For example, [PERSON] et al. proposed a two-step method, as shown in Figure 10, for the functionalization and surface modification of PLA to promote hydrophilicity as well as to introduce conductive aniline oligomers onto the surface of PLA. It resulted in improved cell adhesion and controllable cell behaviors such as spreading, proliferation, and differentiation in a later stage. Specifically, carboxyl acid and anhydride functionalities were introduced onto a PLA film using photo-crosslinking. Subsequently, aniline tetramers were coupled with the functionalized PLA films. The surface modification enhanced both hydrophilicity and electroactivity at the surface [142]. Actuators based on the CPs are electrically controllable with low voltage and biocompatible with liquid electrolytes such as bodily fluids, which makes the CPs attractive materials for the application. However, the mobility of charge carriers in the CPs needs improvement to provide faster responses in the actuators. By blending the CPs with nanofibers and nanotubes, the charge-carrier mobility can increase due to their porous structures, but making the blended materials biocompatible is another issue [143]. Finally, for drug delivery, there is a limit to the amount of therapeutic substances that can be delivered using the CPs. Increasing porosity and surface area is one way to increase the drug-loading capacity. Films of template-grown CP can enhance the porosity and increase the surface area by forming nanostructures. According to [PERSON] et al., PPy scaffolds created with the help of a 3-dimensional colloidal crystal template made of monodisperse poly(methyl methacrylate) (PMMA) exhibited a high surface area [144]. Nanofibers can also be used as a template to fabricate the CPs with a high porosity and surface area. [PERSON] et al.