# Research Progress on Conducting Polymer-Based Biomedical Applications
## 1 Introduction  
Biomaterials generally indicate natural and synthetic substances that interact compatibly with biological systems [1]. The field of biomedical applications is defined by biosensors, tissue engineering, artificial muscles, drug delivery, etc., according to the component of the living organism with which the biomaterials interact for the purpose of medical diagnosis and treatment. Biomedical engineering involves both biology and engineering in an effort to apply engineering fundamentals and materials to medicine and healthcare. The current level of biomedical engineering has been reached primarily by diversifying biomaterials based on specific uses. While, biomaterials were generally acquired from the natural environment before the 20 th century, a new variety of materials--such as ceramics, metals, carbons, synthetic polymers, alloys and composites have been extensively utilized as biomaterials since the early 20 th century. These novel materials exhibit many advantages such as enhanced mechanical properties, chemical and biological performance, high reproducibility, and improved functionality [2; 3; 4; 5; 6; 7]. Furthermore, their properties could be tuned to a target application by controlling the precursormaterials and synthesis parameters. In particular, synthetic polymers have been considered promising biomaterials because they are compatible with biological environments; biocompatibility is one of the key aspects required in biomedical applications [8]. In addition to biocompatibility, polymeric materials possess some advantages, including chemical stability, good processability, and low production cost, allowing them to be widely used in various fields of engineering [9; 10]. However, there are limits in certain biomedical applications where the semiconducting or metallic properties of biomaterials are essential, as conventional polymers are insulators. As a new class of polymers, namely, conducting polymers (CPs) emerged, the limitations began to collapse. CPs are polymers with \(\pi\)-conjugated backbones that show electrical conductivity when given charges by redox reactions. CPs have attracted an immense amount of attention since the initial discovery of metallic polyacetylene in the late 1970s, as it was revealed that CPs can exhibit electrical conductivity in a broad range via a doping process using a relevant dopant [11]. They not only have electrical and optical characteristics similar to those of metals and semiconductors, but also exhibit the advantages of conventional polymers mentioned above. Furthermore, the level and duration of electrical stimulation can be externally controlled with CPs, which is beneficial for biomedical applications [12]. Therefore, CPs have various abilities that are useful for medical diagnoses and treatments of damaged body parts, including (i) ease of functionalization with bioactive molecules, (ii) alteration of a cellular response to electrical or optical or physical properties, (iii) charge transfer from an electrode to ions in living tissue, (iv) dimensional changes upon oxidation and reduction, and (v) entrapment and the release of drugs and biomolecules [13; 14; 15]. Additionally, their biocompatibility in vivo as well as in vitro is another great merit for their utilization in biomedical engineering [16; 17; 18; 19]. Biocompatibility refers to the ability of a biomaterial to function in medical treatment by deriving a beneficial cellular or tissue response from a living host organism while not inciting any undesirable effect in the recipient of the therapy [20]. It is a crucial characteristic of a biomaterial that it is determined by a variety of factors including: physical, chemical, biological, medical, and design elements [21]. Since biomaterials must coexist with the tissue of the host; biocompatible materials need to possess long-term storage capability, and resistance to corrosion and chemical components of biological fluids. In addition, they should not cause any response that is inflammatory, toxic, carcinogenic, or allergic [22].