The evolution of technology continues to miniaturize electronic gadgets from the time of the invention of semiconductor technology [1]. The electronic gadgets used daily are mostly battery-dependent for their power requirements ^[2][3][2,3]. The drawback of batteries comes under consideration of their limited lifetime, and they becomes environmental hazards after their expiry ^[4][5][6][4,5,6]. Furthermore, the inclusion of DC circuitry in a device makes it heavy and expensive ^[7][8][7,8]. The research on renewable energy technology continues to give promising alternative to the use of conventional energy-based electronic equipment ^[9][10][11][9,10,11]. Among the alternative energy sources, the solar cell possesses some advantages over others for alternative energy generation, and a lot of research is investigating this solution ^[12][13][14][12,13,14]. However, its main drawback is its dependency on sunlight. Due to clouds and rainy seasons, the yield of the solar cell degrades ^[15][16][17][15,16,17]. On the other hand, a lot of mechanical energy gets lost due to our daily activities, such as walking, running, vibrations, body movements, or cars passing, and by nature’s activities, such as wind flow and tides ^[18][19][18,19]. To convert these mechanical energies to electrical energy, a lot of conversion techniques have been established by researchers, such as electrostatic, piezoelectric, electrochemical, and magnetostrictive methods ^{[20][21][22][23][24]}[20,21,22,23,24]. To convert mechanical energy, the piezoelectric effect has shown a great contribution, and a lot of research has been carried out and continues to be on it ^[25][26][25,26]. Currently, for scavenging mechanical energy, the triboelectric nanogenerator [TENG] has attracted the attention of the scientific community and plenty of research has continued into different applications for the technology, which was invented a decade ago by Z. L. Wang ^{[27][28][29][30][31][32][33][34][35]}[27,28,29,30,31,32,33,34,35]. The main advantage of this energy scavenger is the choosing of diverse materials, since almost every material takes part in triboelectrification in contact with other materials, and they become positively and negatively charged ^{[36][37][38][39][40][41]}[36,37,38,39,40,41]. The working mechanism of a triboelectric nanogenerator for the mechanical energy scavenging and its conversion to electrical energy is based on contact-electrification and the induction effect ^[27][28][27,28]. Until now, several reports have been published on the self-powered sensing and detection applications of TENG ^{[27][42][43][44][45][46]}[27,42,43,44,45,46]. As there is no use of a battery for the power source and each material can show tribo-properties when using bio-friendly materials, people have developed a lot of self-powered bio-medical sensors using triboelectric nanogenerators ^{[47][48][49][50]}[47,48,49,50]. Wearable electronics for bio-medical sensing applications using triboelectric nanogenerator is a promising field in futuristic gadgets for the healthcare sector ^{[51][52][53][54][55][56][57][58]}[51,52,53,54,55,56,57,58]. There are two categories of TENG-based sensors or detectors used for healthcare monitoring; one is wearable devices, and another is implantable devices. Wearable TENGs are attached to different parts of the human body to obtain a source of mechanical energy from various kind of movements, such as stretching, squeezing, running and, simultaneously, they give the corresponding physiological signals ^{[42][44][52][53]}[42,44,52,53]. On the other hand, bio-friendly and biocompatible materials are used to develop implantable devices to implant in human organ, such as the stomach or muscle to obtain the corresponding physiological signal of the body organs ^[49][59][60][49,59,60]. As TENG converts mechanical to electrical energy and human body movements can be a mechanical energy source, by using TENG fitted to human body we can obtain electrical energy and, simultaneously, the device can act in self-powered sensing applications. The biggest advantage of using TENG-based sensing systems for bio-medical application is that there is no use of an external power source; additionally, they are lightweight, have simple features, and are low-cost. However, there are some challenges, such as sensitivity for very weak forces, and stability of performance over various ambient conditions.

Triboelectrification is a natural phenomenon observed in our daily life, and it occurs when two different materials come in contact with each other. Initially, it was considered as one of the adverse phenomena to the industries, until the first useful implementation of the triboelectric effect was developed by Van de Graaff through the famous Van de Graaff generator [61]. Later on, a triboelectric series was established by Alpha lab in 2009, whereby they have shown the triboelectrification of different materials and created a table with the polarity of them ^[62][63][64][62,63,64]. On the basis of electron affinity and, hence, the ability of attraction or repulsion of electrons by a material while it is brought under contact to other materials, it is classified as a positive and negative tribo-material; human hair, skin, and nylon are positive, while cellulose, PVC, and Teflon are negative tribo-materials [62]. It will be a highly efficient triboelectric nanogenerator if two materials are chosen from the top opposing ends of the triboelectric series because, after friction, the produced surface charge density will be higher. There are several factors affecting the output performance of a TENG other than surface charge density. The influencing factors, such as frequency and force, and the environmental factors, such as humidity, temperature, and presence of gaseous molecules, are also the considerable parameters which decide the output. In 2015, Wang, et. al established a formula of the figure of merits for TENG, which is summarized in ^[35][64][35,64]. It is shown that the performance figure of merit (FOM_P) for TENG consists of a material FOM (FOM_M) and structural FOM (FOM_S). The FOM_P is considered as a standard equation to determine the performance of TENG. The most important material-dependent parameter is surface charge density, although there are some external factors which restrict the ability to generate surface charge [64]. The fundamental principle of TENG was first established by Z. L. Wang to scavenge mechanical energy, and his group showed for the first time the utilization of triboelectric effects for industry use ^[27][65][27,65]. The working principle of TENG is a coupling effect of contact electrification and electrostatic induction ^[27][66][27,66]. Here, when a material comes in contact with another material, the surface charge transfer takes place between the materials. Depending on the transfer of electrons from one material to another, one material becomes positively charged and another becomes negatively charged and, accordingly, a triboelectric series is established [64]. Due to the electron induction phenomena, an opposite kind of surface charge is induced on the electrodes attached to the materials and, hence, an electric potential develops in-between the two electrodes attached to the materials ^[27][64][27,64]. This developed electric potential in-between the two electrodes changes its polarity with the contact separation process and delivers alternating current at the output load. The two tribo-layers are separated by a gap, and this can be considered as a parallel plate capacitor ^[67][68][67,68]. The fundamentals of the presented TENG are based on Maxwell’s displacement current [66]. There are four fundamental modes of operation of TENG, as follows.

2.1. Vertical Contact-Separation (C-S) Mode

This mode is the widely applied mode for developing TENG configurations, as it is easy to fabricate and straightforward, as shown in Figure 1a. The working principle is as follows: when two different materials with attached electrodes comes into contact with each other under the presence of external force, they produce a surface charge depending on their electron affinity. When the two charged surface starts to separate, an electrical potential develops, which induces the opposite kind of charges on the surface of the electrodes. This induction of charges comes from the transfer of electrons from one electrode to the other electrode. During the approach, the potential difference tries to minimize and the direction of flow of the electrons becomes opposite to the case of the separation. Hence, in this way, we obtain alternating current at the output ^[38][69][70][38,69,70].

2.2. Lateral Sliding (LS) Mode

As the name suggests, it generates electricity by sliding the top dielectric layer over the bottom layer, as shown in Figure 1b. The top layer is positive, and bottom later is triboelectrically negative, as per the triboelectric series. When the two layers are in full contact with zero relative displacement, they possess equal and opposite charge density and, thus, no net potential develops at the electrodes. While the top layer slides outward with respect to the bottom layer, a large number of charges becomes unpaired, and that is why a potential develops between the two electrodes and, thus, electrons flow from one electrode to other electrode to counter the potential difference. The flow of current continues until the top tribo-layer slides completely with respect to bottom layer. Again, when the top layer tends to slide inward, the direction of flow of electrons becomes opposite to the previous case. Furthermore, the flow of electrons continues until the electrostatic equilibrium is established between the materials and, hence, the electrodes. In this fashion, the alternating current we obtain at the output is generated ^[38][71][38,71].

2.3. Single Electrode (SE) Mode

Although vertical C-S mode is widely used and more effective friction takes place in the lateral slide mode, these are not suitable for developing a miniaturized device because of the large device size and relatively complex circuitry. Here, a single electrode mode comes into play to take the part of the role for a small-sized device. Here, only one electrode is needed, and the ground acts as another electrode, so the effective flow of electrons for electricity generation takes place between the electrode and the ground, as shown in Figure 1c. It is the simplest working mode of TENG which operates in both vertical C-S mode and lateral sliding mode [71].

2.4. Freestanding Triboelectric (FST) Layer Mode

When a triboelectric layer is movable, such as by sliding or rotating, this mode comes into play with greater efficiency than the single electrode mode. There is no need to attach an electrode and connect wires to the moving triboelectric object. In this mode, two symmetrical electrodes are placed at the same plane and parallel to each other with a small gap, as shown in Figure 1d. The size of the dielectric tribo-layer is the same as the electrode. The movement of the free tribo-layer in-between the electrodes develops potential between them and, consequently, to balance this, the potential electrons flow from one electrode to another. With the change in direction of the moving dielectric layer, the flow of electrons changes its direction and produce alternating electric output ^[72][73][74][72,73,74]. Of the self-powered systems for human health monitoring based on TENG, there are two available ways which have been developed by researchers, namely an implantable device and a wearable electronic system. The implantable TENGs are environmentally friendly and biodegradable, and are implanted into different parts of the human body, such as the muscles, heart, stomach, etc., to harvest electrical energy from their stretching, vibrations, or dynamicity; this helps to diagnose health conditions by monitoring and analyzing the electrical signals coming from the self-powered TENG ^[74][75][76][74,75,76]. On the other hand, wearable TENGs are attached to human body parts, specifically different joints, the chest, and the throat, to scavenge electrical signals from the movements and to obtain different physiological electrical signals.