# Strategic Development of Piezoelectric Nanogenerator and Biomedical Applications
## 3 Strategies
### Doping and Compositing
An Sn and Ca-co-doped BTO (0.82 Ba(Ti\({}_{0.89}\)Sn\({}_{0.11}\)) O\({}_{3}\)-0.18(Ba\({}_{0.7}\)Ca\({}_{0.3}\)) TiO\({}_{3}\)) material, synthesized via the freeze-drying method, generated an open-circuit output voltage of 39 V and a short-circuit current of 2.9 \(\upmu\)A, which were greater than those of Zr-doped BTO-based PENGs [95]. Thus, doping can remarkably improve piezoelectric properties, with unique mechanisms and outstanding performance. Figure 6: The output voltage of ZnO-based PENG with different dopants [5; 76; 86; 87; 88; 89; 91]. (III) Polymers  
Many investigations have been carried out on polymers such as PVDF, P(VDF-HFP), etc. Doping is capable of improving the \(\beta\)-phase of the polymer, which improves the overall performance, as demonstrated in the following reports. [PERSON] et al. [96] investigated the effect of Eu\({}^{+}\) doping on P(VDF-HFP)-based PENGs. In their work, they observed that Eu\({}^{+}\) doping resulted in an improvement in the \(\beta\)-phase of more than 99%. In addition, they observed improved dielectric properties, which occurred due to the interaction between the O-H moieties of Eu\({}^{+}\) and electronegative CF\({}_{2}\) group of P(VDF-HFP). The fabricated device was able to deliver an open circuit voltage of 5 V with an open circuit current of 0.3 uA. [PERSON] et al. [97] fabricated PVDF/graphene-composite-based PENGs and investigated the effect of Ce\({}^{3+}\) doping. The device was fabricated via the electrospinning method and had an excellent piezo-active \(\beta\)-phase without a post-pooling process. The fabricated device delivered an output voltage of 11 V, with a current density of 6 nA/cm\({}^{2}\). Apart from metallic dopants, carbon materials have also been utilized as dopants, improving the d\({}_{33}\) value of the piezoelectric host and thereby improving the performance of the fabricated device. Carbon materials such as carbon nanotubes (CNTs) [98], graphene [99], and reduced graphene oxide (rGO) [100] act as conducting phases that improve the conduction network, which eventually elevates the piezoelectric performance. In addition, carbon materials reinforce mechanical properties [99]. [PERSON] et al. [101] fabricated a PENG based on a BTO/PDMS composite using the easily modified solid-state method. The output performance was enhanced by doping acetylene black C with BTO; the d\({}_{33}\) value increased from 5.4 pC/N to 15.3 pC/N. The PENG based on BTO/PDMS/C recorded an output voltage of 7.43 V when periodically beaten by the vibrator, and a 143% increment was observed compared with undoped BTO. [PERSON] et al. [93] fabricated a graphene-doped BTO/PVDF composite-based PENG using an electrospinning method. In their work, they compared the performance of PVDF, PVDF/BTO, and PVDF/BTO/graphene to determine the role of graphene doping. The PENG of PVDF with 0.15 wt% graphene and 15 wt% BTO generated an output voltage of 11 V. Recently [PERSON] et al. [100] fabricated a PENG based on rGO-doped BTO and investigated the effect of rGO doping on PVDF/BTO by comparing the output performance of rGO-doped PVDF/BTO and undoped PVDF/BTO. The undoped PVDF/BTO generated an open-circuit voltage of 4.65 V and short-circuit current of 1.82 uA, whereas the rGO-doped PVDF/BTO provided a circuit voltage of 16.9 V and short-circuit current of 3.53 uA. When the doping concentration exceeds a certain value, the composite turns into a conductor, and the piezoelectricity disappears. Every piezoelectric material has drawbacks, such as low output, current loss, low flexibility, and high cost. The best way to overcome these problems is to combine different species with unique properties to achieve a trade-off. Typically, materials with flexibility and improved performance have been fabricated by compositing organic or inorganic polymers with different entities. Thin-film composites for PENGs are susceptible to the conversion of minimal mechanical action into electrical energy. Some examples are mentioned in Table 2.