# Research Progress on Conducting Polymer-Based Biomedical Applications
## 3 Biomedical Applications
### Biosensors
In a humidity sensor, films of CPs, such as poly(3,4-ethylenedioxythiophene)/polystyrene sulfonate (PEDOT/PSS) and poly(anilinesulfonic acid) have been shown to be capable of detecting a change in a wide range of humidity [72; 73]. The main challenge of biosensors is to combine biologically reactive elements such as antibodies, cells, and enzymes with the CPs in order to function as an electrical signal generator. The CPs are great matrices for the immobilization of the bioreceptors due to their biocompatibility, sensitivity, and durability. In addition, they are flexible to endure the embedment of the biomolecules [74; 75]. Such immobilization can be done by physical adsorption, chemical adsorption, covalent bonding, crosslinking, or entrapment [76; 77; 78; 79; 80; 81]. Physical adsorption takes advantage of physical interactions such as Van der Waals and electrostatic forces between bioreceptors and CPs. The anionic bioreceptors are attached to the cationic CPs through electrostatic forces. It is worth noting that physical adsorption does not require the functionalization of monomers. However, the leaching of the bioreceptors may occur over time due to the weak forces of physical adsorption. In this context, chemical adsorption that involves covalent binding between bioreceptors and the CPs is introduced. Despite its simplicity, the precise control of biomolecule orientation on the surface film remains a challenge. Such random orientations may block active sites on the surface of the CPs. In this regard, a targeted coupling strategy where crosslinking agents and functional groups, such as -NH2, -COOH, and -SH, are selectively introduced onto biomolecules. Such targeted coupling strategies enable the precise control of the biomolecule's orientation. However, it is difficult for the biomolecules to maintain enzyme activity due to the harsh treatment by toxic chemicals involved in the crosslinking process [82; 83; 84; 85]. Entrapment techniques where monomers, dopants, and biomolecules are mixed together in a solution are usually performed in electrochemical polymerization. In this strategy, polymerization of the CPs and immobilization of the biomolecules occur simultaneously. However, it is not cost effective because a high concentration of biomolecules is required during the process. The procedure of entrapment causes sensing elements to become less accessible to the analytes. Furthermore, the quaternary structure of proteins is damaged by the hydrophobic CPs, and therefore their biological activity as sensing elements decreases [86; 87]. Despite the advantages of low cost and easy production, the CPs in biosensors have limitations--in that the CPs are more vulnerable to surrounding chemical conditions including oxygen, acids, and bases. In addition, the charge carrier mobility is not satisfactory compared to other conducting materials such as carbon nanotubes and silicon nanowires [88]. [PERSON] et al. suggested coaxial nanowire composites composed of PPy and carbon nanotubes in an effort to exploit complimentary attributes, such as the flexibility and light weight of the CPs and the high electrical conductivity of carbon nanotubes [89]. Specifically, PPy was electrodeposited onto an array of carbon nanotubes, resulting in individually coated nanotubes with a great electroactive surface area [90]. Glucose oxidase was immobilized by the oxidation of pyrrole to form a glucose biosensor. In Figure 4a,b, the scanning electron micrographs of the carbon nanotubes are compared before and after the electrodeposition of PPy. The layer of evenly coated PPy, thinner than 100 nm, was found in the array of carbon nanotubes with a spacing between 100 nm and 400 nm. Figure 4c illustrates the electrical current according to the concentration of glucose. The response increases linearly with the detected amount of glucose up to 20 mM, which is higher than the requirement for practical glucose biosensors, typically 15 mM.