# Strategic Development of Piezoelectric Nanogenerator and Biomedical Applications
## 3 Strategies
### Hybrid Structures  
The coupling of two or more nanogenerators together is referred to as a hybrid structure. Superior performance is a promising feature of hybrid structures, and the output of such structures is the sum of more than one effect, which improves the performance. An HNG is a type of nanogenerator that can harvest energy from various sources. Numerous combinations have been investigated, for example, piezo-tribo and piezo-pyro hybrids. In this system, the same nanogenerator simultaneously exploits triboelectric and piezoelectric effects. The ability to harness energy from different sources simultaneously improves the efficiency of the device. In 2014, [PERSON] et al. [22] reported a novel arc-shaped nanogenerator based on piezoelectric and triboelectric mechanisms. They developed a piezo-tribo-nanogenerator based on BTO/PDMS. The BTO layer acted as the piezo-electric layer in the assembly, which was placed on an ITO/polyethylene terephthalate (PET) insulation layer. The PDMS layer maintained the optimal separation between the top and bottom surfaces. Al foil was utilized as the bottom electrode. The peak-to-peak voltage achieved using this hybrid device was 22 V, with a current density of 1.13 \(\upmu\)A. [PERSON] et al. [120] investigated and compared six different piezo-tribo combinations based on the number of terminals connected to the materials. Furthermore, [PERSON] et al. compared numerical simulations and experimental measurements of piezo-tribo-nanogenerators in different cases. Their numerical studies led them to conclude that the coupling of piezoelectric and triboelectric effects can be controlled by manipulating the structure, the polarization direction, and the electrode connection. Moreover, the possibility of a zero output due to the negative coupling between the piezoelectric and triboelectric effect could not be ruled out. Next, they concluded that the output could be modulated by realizing the piezoelectric-triboelectric nanogenerator (PE-TENG) in successive and simultaneous coupling modes. The enhancing or weakening mode could be determined based only on  
Figure 9: Fabrication of a 3D-printed kirigami PENG. (**a**) Schematic diagram of a 3D-printed PENG with kirigami structure. (**b**) Output voltage of the fabricated device. (**c**) Current density with 20 wt\(\%\) of BaTiO3 NP/P(VDF-TrFE) at 60 N at 5 Hz on a 1 cm\({}^{2}\) area [118]. Adapted with permission from [PERSON] (2020). Copyright © 2020 Elsevier. the structure, polarization direction, and electrode connection, regardless of the coupling mode. The output could be modulated in the simultaneous coupling mode by using an appropriate design that allowed the stress inside the piezoelectric material to change with the gap between the two connected surfaces in a specific relationship. This approach has the advantage of achieving the desired output. Furthermore, they concluded that each structure could generate a multilevel electric output using different electrode connections. In recent years, several approaches have been reported; [PERSON] et al. [121] designed an HNG via the integration of organic and inorganic nanostructured materials. They fabricated a device wherein the bottom part operated as a piezoelectric component. The piezoelectric element was fabricated by growing ZnO nanostructures on a chromium-deposited shim substrate using a hydrothermal method. Subsequently, poly(methyl methacrylate) (PMMA) was used to cover the ZnO nanostructure and Al was deposited on it, which formed the piezoelectric part. The triboelectric part consisted of a polytetrafluoroethylene (PTFE) layer, and an Au layer was deposited on it. Reactive ion etching was employed to form nanostructures on the PTFE surface. A copper layer was deposited on the nanostructured PTFE layer to form the electrode. Using double-sided copper tape, an arc-shaped PET was affixed to the PTFE substrate Finally, both the piezoelectric and triboelectric parts were assembled. The HNG produced 186 V of output voltage, with a current density of 10.02 uA/cm\({}^{2}\).