# Strategic Development of Piezoelectric Nanogenerator and Biomedical Applications
## 5 Summary and Outlook  
PENGs have attracted significant attention owing to the outstanding achievements made in relation to these devices in recent years, and focus is now being placed on the fabrication of high-performance PENGs in energy conversion devices. In addition, regarding flexible and wearable electronics, a focus is being placed on more flexible, reliable, and bio-safe piezoelectric devices based on organic materials and piezoelectric polymers, such as PVDF and P(VD-TrFE). Thus, PENGs must provide a high output voltage and current with a long and uninterrupted operating life, which can be achieved by customizing piezoelectric materials using strategies such as morphological tuning, doping, developing composites, and synthesis techniques. The overall growth of high-performance PENGs was catalyzed by newly introduced approaches such as 3D printing and fabrication, as well as different device strategies such as an electrode, surface, and interfacial modifications. Novel materials, such as g-C\({}_{3}\)N\({}_{4}\) and BNN, which have superior piezoelectric properties compared with previously reported materials, are also being explored. A new feature of nanogenerators is HNG, which harvests energy from multiple sources simultaneously, and such a hybrid structure drastically improves the output performance.  
Overall, the future applications of PENGs are based on the following aspects. (1) PENGs have been recognized as potential candidates for biomedical applications. A PENG can be used as a power source for microdevices or as a self-powered bio-implant. The growth of recent bio-implants has slowed due to the limitations of their power sources. Batteries have major drawbacks, such as the use of bio-hazard materials and their high fabrication and maintenance costs. By contrast, PENGs can be bio-safe, involve low-cost fabrication processes, and do not require frequent maintenance. They are smaller than existing batteries and are thus perfect candidates for biomedical applications. (2) To catalyze the growth of PENGs, finding new approaches in order to improve their existing performance is a necessary step. The output performance of the device can be improved by combining different strategies during fabrication. Specific strategies can be exploited; for example, [PERSON] et al. [113] combined doping and interfacial modification. (3) New materials, such as g-C3N4 and BNN, exhibit a high piezoelectric response compared to existing piezoelectric materials because of their highly crystalline nature. These materials can be synthesized using simple methods, such as solid-state reactions. (4) The fabrication of hybrid structures is a potential approach to improving performance. Hybrid-structure nanogenerators can harvest multiple energy sources simultaneously, which results in considerable improvements in the output voltage and current.  
Writing--original draft preparation and curation, O.Y.P. Review and editing, O.Y.P., S.B.P., S.L.P. and R.S.R. reviewed the article. Supervision N.L.T., S.B.P. and S.L. All authors have read and agreed to the published version of the manuscript.  
This work was supported by a National Research Foundation of Korea (NRF) grant funded by the Korean government (2021R1A2C1011248).  
NLT is thankful to DST-SERB for providing the financial assistance through Teachers Associateship for Research Excellence (TARE) scheme (TAR/2021/000307). The financial assistance through DST-PURSE Phase-II (2018-2023) and UGC-DSA Phase-II (2018-2023) is highly acknowledged. SBP is thankful to Council of Scientific and Industrial Research for providing CSIR-SRA fellowship (Pool no. 9081-A) under Pool scheme.  
The authors declare that they have no known competing financial interests.