# Applications of Nanomaterials in RFID Wireless Sensor Components
## 4 Nanomaterials for RFID-Based Sensing System
### Polymer  
Polymers are favored materials for the main substrate of sensor tags. Commonly used polymers include polyethylene terephthalalate (PET), polyethylene naphthalate (PEN), polyethylene (PES), polyimide (PI), polycarbonate (PC), polydimethylsiloxane (PDMS), and more [129]. For example, [PERSON] et al. fabricated a RFID sensor based on the cost-effective and mechanically and thermally superior PET substrate [96]. The PET surface was activated by oxygen plasma treatment, and the formed hydroxyl and carboxyl groups facilitated easy drop casting. This resulted in a RFID sensor based on PET that maintained mechanical stability even when bent and preserved integrity in environmental conditions (Figure 4a). Polymers not only serve as substrates but also provide an environment where other elements such as sensing materials, antennas, and IC chips can operate optimally. For instance, [PERSON] et al. investigated the impact of the substrate's dielectric constant on the sensitivity of sensing materials for sensor performance optimization (Figure 4b) [97]. A polymer substrate with a high dielectric constant is ineffective in low humidity conditions,  
Figure 4: RFID-based sensing-tag substrate formed using various polymers. (**a**) Ammonia gas sensor based on PET substrate [96]. (**b**) Nafion 117 humidity sensor performance depending on the polymer-based substrates of various dielectric constants [97]. (**c**) Swelling and shrinkage of PDMS substrate and preservation of silver ink printed loop structure depending on vapor exposure [98]. which reduce sensor sensitivity. Conversely, in high humidity environments, a polymer substrate with a high dielectric constant proves to be more effective. Table 2 shows the frequency shift and signal amplitude variation values of humidity sensors composed of substrates with different dielectric constants. The dielectric constant of the substrate influences the sensor's sensitivity by varying its mechanism under different conditions. Furthermore, the substrate's dielectric constant affects factors such as impedance matching systems, antenna bandwidth, and overall readout efficiency of a RFID system [130]. Therefore, careful consideration is required in the selection of polymers due to their properties, such as dielectric constant, that influence the sensitivity and communication performance of the RFID sensor. When designing tags, the influence of the sensing process on the polymer substrate must be taken into account. For example, [PERSON] et al. observed the state of PDMS based on exposure to solvent vapors. PDMS serves as the substrate for vapor sensors, and by printing an ink solution of silver, a conductive loop is formed (Figure 4c) [98]. During gas detection, PDMS expands mechanically and forms cracks in the printed silver loop in the presence of solvent vapors. However, upon solvent removal, PDMS contracts, and the silver loop recovers, demonstrating reversible characteristics. Thus, PDMS proves to be a suitable substrate for gas detection environments. Therefore, while numerous polymers exist for sensing-tag substrates, a careful selection of polymers considering their interaction with other elements is essential for withstanding harsh sensing environments. Conducting polymers can exhibit various properties due to the conjugation structure of their chains. In particular, the increased density of charge carriers in the polymer chain structure not only enhances electrical conductivity but also allows for high sensitivity to specific compounds by utilizing diverse monomer chemical structures and additional dopants. Commonly used conductive polymers in RFID sensor systems include polyaniline (PANI), polypropylene (PPy), poly(3,4-ethylenedioxythiophene)-polystyrene sulfonate (PEDOT:PSS), polythiophene, and poly(p-phenylene vinylene) [131]. The high electrical conductivity of conductive polymers enables the fabrication of antennas for sensing tags. [PERSON] et al. designed a compact bandwidth antenna for UHF RFID using PEDOT:PSS (Figure 5a) [101].