# Strategic Development of Piezoelectric Nanogenerator and Biomedical Applications
## 3 Strategies
### Effect of Morphology
(**a**) Crystalline PZT ribbons are synthesized on an MgO host substrate, which is subsequently etched, and the ribbons are transferred and printed onto flexible PDMS rubber. (**b**) Photograph of a piece of PDMS with PZT ribbons covering the top surface. (**c**) Schematic of a specimen indicating 31-mode piezoelectric bending and measurement. The PZT layer; top and bottom contact electrodes and substrate are indicated. (**d**) Oscillating pressure (left axis) and induced dielectric displacement (right axis) [66]. Adapted with permission from [PERSON] et al. (2010). Copyright © 2010 American Chemical Society. Several lead-free perovskite materials, such as BTO, BFO, KNN, and BNT, are available. BTO is a popular perovskite lead-free material used for the fabrication of PENGs. BTO can be synthesized to form various morphologies, such as nanotubes, nanowires, and nanopillars [69]. The morphologies of the fabricated materials strongly influence their piezoelectric properties, such as energy conversion and efficiency. Reportedly, a nano-generator based on BTO nanotubes composited with PDMS was used to form a transparent device (shown in Figure 5A(a)). the BaTiO\({}_{3}\) nanotubes were synthesized via the hydrothermal method. In Figure 5A(b,c) one can observe the randomly oriented BaTiO\({}_{3}\) nanotubes in the PDMS matrix. The fabricated device generated an output voltage of 5.5 V and a short-circuit current of 350 nA [70]. This device was used to drive a commercial light emitting diode (LCD) display. Compared with nanotubes, the BTO nanowire/PDMS composite-based nanogenerator exhibited slightly enhanced performance. A device based on BTO nanowires/PDMS nanocomposite delivered an output voltage of approximately 7.0 V, with a current of 360 nA [71]. [PERSON] et al. [72] fabricated a PENG based on the BTO micropillar morphology. A P(VDF-TrFE)/BTO micropillar composite was synthesized via the sol-gel method, and it was polarized under a high electric field and ultraviolet (UV) light. The P(VDF-TrFE)/micropillar composite-based PENG exhibited an enhanced output voltage of 13.2 V with a current density of 0.33 \(\upmu\)A/cm\({}^{2}\). In addition to nanorods, nanowires, and nanopillars, the unique morphology of BTO nanofibers was also investigated. The BTO-powder-incorporated PDMS matrix demonstrated an output current of 1 \(\upmu\)A and an open-circuit voltage of approximately 12 V [73]. Other lead-free perovskite materials, such as BFO, KNN, and NBT, have also been utilized in different morphologies. [PERSON] et al. [16] introduced an NBT-based lightweight nanogenerator. In their work, NBT with a one-dimensional (1D) morphology was synthesized via a solid-state reaction. The piezoelectric composite (polycaprolactone/NBT nanoparticles) was prepared using the casting technique. The composite film was sliced. Subsequently, a gold (Au) layer was sputtered and copper wires were attached to the film using silver (Ag) paste. Then, a Kapton layer was attached to both sides of the composite film. The final operational PENG device provided an output voltage and current of 22 V and 140 nA, respectively, under an applied compressive force of 6 N. KNN can be formed in different morphologies. [PERSON] et al. [74] fabricated a PENG based on KNN nanorods with a high aspect ratio (8.5). A hydrothermal method was used to synthesize the KNN nanorods, and a KNN nanorod/PVDF fibrous web was prepared via electrospinning (Figure 5B(a)). Aluminum (Al) tape was affixed to the fibers, which acted as the top and bottom electrodes (Figure 5B(b)). The fabricated device provided an open-circuit voltage of 23.24 V and a short-circuit current of 9 \(\upmu\)A. The KNN nanotubes exhibited a superior output current compared with the KNN nanorods [75].