# Strategic Development of Piezoelectric Nanogenerator and Biomedical Applications
## 1 Introduction  
Advances in technology have enabled researchers to miniaturize electronic components and develop portable devices, such as mobile phones, laptops, and smartwatches. The miniaturization of electronic devices is inevitable in the development of modern technologies to ease operations in almost all sectors. Sectors such as the bio-implants sector rely heavily on miniaturized devices, such as pressure sensors, pacemakers, and nerve stimulators [1; 2; 3]. These bio-implant devices require external power for their proper functioning. Utilizing batteries to power these microdevices is not a practical approach as their size is considerably small compared with the currently developed batteries. The development of self-powered systems that can harvest energy from various sources from their surroundings to power these micro-implants is a significant breakthrough in addressing this limitation [4]. Thus, battery problems, such as their limited cycle life and large size and the need for frequent maintenance, can be resolved using self-powered systems. Nanogenerators are well-known examples of self-powered systems that can harvest different forms of energy [5]. Research on nanogenerators has evolved exponentially over the past few decades. Depending on the form of energy conversion, nanogenerators can be classified as piezoelectric nanogenerators (PENGs), triboelectric nanogenerators (TENGs), or pyroelectric nanogenerators (PYNGs) [6; 7; 8]. PENGs are promising candidates owing to their high efficiency, low cost, facile fabrication techniques, and wide range of applications. PENGs work on the basis of the piezoelectric effect, also referred to as the direct piezoelectric effect, which is a unique property of a material that generates an electricfield when subjected to mechanical stress. Piezoelectric materials are non-centrosymmetric materials that experience electric polarization under external mechanical stress. For example, ZnO has Zn\({}^{2+}\) cations and O\({}^{2-}\) anions stacked layer by layer in a crystal structure; in an undisturbed state, the charge centers of the cation and anion coincide with each other. When external stress is applied to the crystal, structural deformation occurs and the charge centers are displaced. The separation of charge centers generates polarization, and when an external load is connected to the crystal, the electrons begin flowing from the external circuit to achieve a new equilibrium. Thus, mechanical energy is converted to electrical energy [9]. This phenomenon is reversible; when an electrical field is applied to the piezoelectric material, a change in dimension can be observed owing to the generation of internal stress [10]. Based on their structure, PENGs can be classified into three types: vertical-nanowire integrated nanogenerators (VINGs), lateral-nanowire integrated nanogenerators (LINGs), and nanocomposite electrical nanogenerators (NEGs). In 2006, for the first time, [PERSON] et al. [11] fabricated a VING utilizing vertically aligned ZnO nanorods and reported an efficiency of 17%-30%. In 2007, [PERSON] et al. [12] developed a fully operational VING device comprising ZnO nanowires underneath a zigzag metal electrode to harvest the energy from ultrasonic waves. In 2008, [PERSON] et al. [13] introduced a single-ZnO nanowire generator placed horizontally on a polyamide film, a flexible substrate, thereby generating potential by means of lateral bending. This model was further modified by adding multiple ZnO nanowires with a lateral alignment, referred to as LING. Subsequently, various approaches have been reported to fabricate LING. For example, [PERSON] et al. [14] demonstrated a unique fabrication technique, wherein vertically aligned ZnO nanorods, grown via physical vapor deposition, were transferred to another substrate to create horizontal nanorod alignment and generated a high output voltage (2.03 V). In 2010, [PERSON] et al. [15] introduced NEG, wherein PENG was developed as a nanocomposite, comprising ZnO nanorods in an epoxy matrix.