# Research Progress on Conducting Polymer-Based Biomedical Applications
## 1 Introduction
The biocompatibility of a material is measured through various tests concerning the interaction of the material with physiological fluids and cells, and stimulation of an immune response, both in vitro and in vivo [21]. In vitro studies are conducted to roughly assess a possibility of a material of interest harming relevant cell types. Assays composed of MTT (3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolzolium bromide) or its variants are commonly used to test cellular metabolic activity since high rates of MTT reduction is expected from rapidly dividing cells [23]. In addition, DNA synthesis, cell proliferation, cell membrane integrity, and toxicity based on cell death or tissue injury are also measures tested by in vitro studies [24; 25; 26]. There may be effects of a biomaterial not only from direct contact with cells but also from indirect exposure since it can generate diffusible components including residual solvents, monomers and breakdown products [23]. On top of the in vitro tests, in vivo studies are also essential to fully assess the biocompatibility of a material, as the cell-based assays used in vitro do not concern the rest of the human body. A material that is not directly cytotoxic can cause a destructive reaction. For example, [PERSON] at al. discovered that a UV-cross-linkable chitosan depressed cell viability in a moderate degree in cell culture but caused vigorous peritoneal adhesions in the peritoneal cavity [27]. The evaluation of biocompatibility can vary depending on the applications of biomaterials. In drug delivery, a material that does not cause any tissue injury may still damage a recipient of the treatment from drug delivery, as well as unpredicted complications including intravascular coagulation and embolism [28; 26; 29]. Therefore, it is important to consider the effect of drug delivery systems on an entire human body to eliminate any potential risk. The biocompatibility in tissue engineering refers to the ability of a scaffold to perform as a substrate to promote cell growth. In this context, biocompatiblematerials for tissue engineering should be able to support appropriate cellular activity such as the facilitation of molecular and mechanical signaling systems for tissue regeneration [30]. The biocompatibility of conducting polymers is affected by their polymer compositions including chemical structures, functional groups, morphologies, and synthetic processes [31]. Diverse conducting polymers such as polypyrrole (PPy) and polyaniline (PANI) have exhibited a great cellular response and support of the growth of various cell types, which is an essential aspect of biocompatibility in biomedical applications [32; 33; 34; 35; 36]. Furthermore, the biocompatibility of conducting polymers can be readily improved by introducing biocompatible molecules, segments and side chains to the polymers [37]. An in vivo study by [PERSON] et al. exhibited great biocompatibility of chemically synthesized PPy particles in mice over six weeks of treatment without any negative effects on cell viability and proliferation [18]. PANI has also been reported to show great biocompatibility in terms of dermal irritation and sensitization by [PERSON] et al. [38]. In spite of the attractive features of a CP mentioned above, a few challenges that hamper their widespread adoption in biomedical applications still remain. This article seeks to review recent research efforts and progress in the use of CPs for biomedical applications. In particular, this review focuses on the motivation and function of the CPs in the research fields of biosensors, tissue engineering, artificial muscles, and drug delivery. Furthermore, the challenges that the CPs are currently facing in the research fields above will be discussed regarding the charge carrier mobility, biodegradability, hydrophilicity, and loading capacity of biomolecules. Finally, a summary of representative approaches that could resolve the challenges is presented.