# Research Progress on Conducting Polymer-Based Biomedical Applications
## 3 Biomedical Applications  
### Biosensors  
A biosensor is an analytical device that generates electrical signals according to biological responses from an analyte. As the biological species is monitored by a sensing element in a biosensor, measurable electrical signals proportional to the concentration of the analyte are generated. As shown in Figure 2, a biosensor is mainly composed of a bioreceptor, transducer, and electronic module [55]. Various sensing elements, such as antibodies, cells, enzymes, and tissues, can be  
Figure 1: p-doping of heterocyclic polymers (X = S, N, O): a polaron and a bipolaron are subsequently introduced upon oxidation. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Polymer** & **Chemical Structure** & **Applications** & **References** \\ \hline Polypyrrole (PPy) & \(\big{\{}\)\(\big{\}}\)\(\big{\{}\)\(\big{\}}\) & Fuel cells, corrosion protection, computer displays, microsurgical tools, biosensors & [33; 46; 47; 48; 49] \\ \hline Polyaniline (PANI) & \(\big{\{}\)\(\big{\}}\)\(\big{\)\(\big{\}}\)\(\big{\)\(\big{\}}\)\  
used as bioreceptors that interact with specific analytes. The extent of the interaction caught on the bioreceptors is transformed by the transducer into an electrochemical, optical, or thermal signal [56; 57; 58]. Once the transduced signals are delivered to the electronics, they are often magnified by an amplifier. Subsequently, a signal processor displays the amplified signals in a user-friendly way. Biosensors can detect various signals such as current, potential, change in conductivity, and light absorbance [59]. Biosensors have evolved through three generations depending on the level of integration of a bioreceptor and a transducer as illustrated in Figure 3[60]. The first generation involves the bioreceptor immobilized in a dialysis membrane, which is attached to the surface of the transducer. In the second generation, the bioreceptor is either adsorbed or covalently connected to the transducer without the need for the membrane. The third generation includes the bioreceptor directly bound to the transducer and the electronics, allowing their complete integration as a biosensor unit. In biosensors, the CPs are used as transducers owing to their great electrical conductivity as well as their great sensitivity and versatility of available monomer types. Furthermore, biomaterials for a receptor can be easily integrated with the CPs, thereby enabling the facile assembly of biosensors. Furthermore, the CPs are cost effective and can be adapted to a variety of fabrication techniques to form biosensors. PPy, PT, polycarbonate (PCz), poly(o-phenylenediamine) (POPD), PA, and polyfuran (PFu) are commonly used as biosensors. The CPs have been exploited in diverse applications for biosensors including medical purposes and environmental monitoring. Extensively studied blood glucose biosensors are a good example of the medical applications [61; 62; 63]. They use enzymes to oxidize glucose in a number of steps and the concentration of glucose is detected by a transducer composed of the CPs. PPy, PANI and polyaminobenzoic acid (PAB) are typical CPs used in glucose biosensors [64; 65]. Cholesterol  
Figure 3: Three generations of biosensors. Figure 2: A schematic of a biosensor. Adapted with permission from [55]. Copyright 2011 Intech. biosensors monitor the level of cholesterol in blood, which is considered an important measure in clinical diagnostics. PPy can be used as a transducer in cholesterol biosensors to immobilize enzyme cholesterol oxidase by physical adsorption. In cholesterol biosensors, the oxidation of cholesterol in PPy films results in amperometric responses [66]. Biosensors are also capable of monitoring environmental hazards. In gas sensors for environmental monitoring, the CPs exhibit a fast change in electrical conductivity resulting from the reduction of the polymers when organic vapor or moisture is detected [67; 68; 69]. PANI films have been reported to be effective in ammonia gas sensors, which is beneficial for food-packaging applications [70; 71].