# Strategic Development of Piezoelectric Nanogenerator and Biomedical Applications
## 3 Strategies
### 3D Printing  
3D printing is an additive manufacturing technique; it is well known for its ability to fabricate complex structures and it provides a simple bottom-up approach. 3D printing can be used to fabricate every part of a nanogenerator. There are various types of 3D printing, such as fused deposition modeling (FDA), SLS, stereolithography, and DIW, of which SLS and DIW are the most commonly used for the fabrication of PENGs. [PERSON] et al. [21] proposed a unique PVDF/graphene composite using the SLS method. They fabricated PVDF/graphene with enhanced piezoelectric properties. The fabricated device provided an open circuit voltage of 16.97 V and a short circuit current of 274 nA. Furthermore, the composite was able to produce sufficient energy to light 10 (LED) bulbs. [PERSON] et al. [116] reported a PENG based on a PVDF/BTO/CNT composite (Figure 8b), with an improved d\({}_{33}\) value and piezoelectric performance. A microporous wood-like structure was fabricated via SLS and supercritical carbon dioxide foaming (Figure 8a). The composite powder was prepared using a series of techniques including melt extrusion, granulation, low-temperature grinding, drying, and screening. The CNTs improved upon the polarization efficiency of the PVDF/BTO composite and the d\({}_{33}\) value was increased from 0.7 to 2.6 pC/N. The recorded output voltage was 19.3 V, with an output current of 415 nA.  
In the case of DIW, a syringe is filled with the material ink, and appropriate pressure is used to print the material on the substrate. Thus, it is necessary to produce a printing ink with an appropriate viscosity, as the viscosity of the ink strongly affects the fabrication of the device. The ink experiences a shear strain while passing through the needle, which reduces the viscosity of the ink. After the release of the shear strain, the viscosity suddenly increases, which stops the printed ink from spreading. Thus, to produce high-resolution printing, the shear thickness must be maintained [117]. [PERSON] et al. [118] noted that 3D printing is unsuitable for stretchable devices; it reduces effective contact, which hinders the overall performance of the fabricated device. To overcome this obstacle, they fabricated a PENG using the kirigami technique. They demonstrated the fabrication of a flexible and stretchable PENG, while maintaining the ease of fabrication (Figure 9a). A device based on BTO/P(VDF-TrFE) was fabricated with an Ag flake electrode, and a T-joint-cut kirigami structure was demonstrated (Figure 9). The fabricated device could harvest energy through stretching as well as pressing; the device could withstand being stretched by more than 300% without reducing its output performance. This device produced an open-circuit voltage of 6 V with a short-circuit current of 2 \(\upmu\)A/cm\({}^{2}\) (Figure 9b,c), and the maximum power density achieved was 1.4 \(\upmu\)/cm\({}^{2}\). Another easy printing method used in the fabrication is screen-printing; [PERSON] et al. [119] demonstrated the fabrication of a PENG using screen-printing of the surface-modified BTO/PVDF nanocomposite. Surface modification was performed using a triethoxy(octyl)silane (TOs) coating; consequently, the TOs coating prevented the agglomeration of BTO nanoparticles, reduced defects, and improved the output performance. A device with well-dispersed BTO nanoparticles exhibited an output performance of 20 V after 7500 cycles. A power density of 15.6 \(\upmu\)W/cm\({}^{2}\) was achieved, which was 150% higher than that of an ordinary BTO/PVDF-based PENG.  
Figure 8: (a) Schematic diagram of the preparation of porous foam using Sc-Co\({}_{2}\). (b) Actual photograph of printed device. (c) SEM image of pore morphology [116]. Reprinted with permission from [PERSON]. (2021). Copyright © 2021 American Chemical Society.