## 3 Energy Harvesting
### Energy Harvesting Material: t-TENGs  
Triboelectric nanogenators (TENGs) have recently gained attention for generating renewable energy using the triboelectric effect, an electrostatic process where certain materials become charged when they contact and then separate [25; 26; 27]. Introduced by [PERSON] [28] in 2012, TENG technology has grown quickly, with uses in wearable electronics, environmental monitoring, and healthcare devices [28; 29]. Recent research has focused on creating textile-based TENGs (t-TENGs) by integrating TENGs into flexible textiles. These t-TENGs can efficiently capture energy from human motion or other everyday mechanical activities [28; 29].  
The triboelectric effect in TENGs involves the exchange of electrons between materials with different electron affinities (see Figure 2). When these materials meet and then separate, one material gains electrons while the other loses electrons, creating an electric potential [30; 31]. If they are connected to an external circuit, this potential drives electrons to flow, producing electrical energy. TENGs typically operate in four primary modes: vertical contact separation, lateral sliding, single-electrode, and free-standing [32].  
Research shows that TENGs can harvest energy from a wide range of mechanical sources, including human motion, wind, water, and vibrations [33; 34]. For example, [PERSON] et al. [35] found that TENGs can turn ocean wave energy into electricity, suggesting large-scale possibilities for marine renewable energy. Similarly, [PERSON] et al. [36] examined how TENGs capture low-frequency mechanical energy in outdoor environments, noting their durability and adaptability. TENGs have also been miniaturized for use in self-powered sensors, microelectronics, and portable consumer electronics [36]. In contrast, single-phase magnetoelectric (ME) materials often respond weakly, making them less practical for real-world applications [20].  
Textile-based TENGs (t-TENGs) are a major new area of TENG research. They can be used in wearable and portable energy-harvesting fabrics that convert ordinary human motions such as walking or running--into electricity. These fabrics are flexible, lightweight, and comfortable, allowing them to be easily integrated into clothing to power small devices [28; 37; 38]. [PERSON] et al. [39] introduced gel-based TENGs for flexible sensing, addressing the shortcomings of standard energy-harvesting methods in terms of adaptability and performance in changing conditions.  
Most t-TENG designs use woven structures made of conductive and non-conductive fibers, which produce friction during motion and generate electricity [39]. Optimizing how these fibers are arranged can increase the efficiency and power output of t-TENGs, making them more useful for real-world scenarios [40]. TENGs and t-TENGs have applications ranging from powering wearable devices to supporting large-scale environmental monitoring [41; 42]. For instance, [PERSON] et al. [43] created \"bright clothes\" with built-in t-TENG technology to power small health-monitoring sensors, tracking vital signs such as heart rate and body temperature and transmitting data wirelessly. Another study by [PERSON] et al. [44] showcased a TENG system that collects energy from raindrops and vibrations, offering an eco-friendly way to monitor environmental data. Researchers have also integrated TENGs into backpacks so that they generate power from walking or hiking, which is useful for people who spend a lot of time outdoors or work in the field [44]. Finally, [PERSON], [PERSON], and [PERSON] [34] proposed flexible 3D-printed TENG designs that rely on triboelectric and electrostatic induction effects to harvest energy through contact separation, sliding, single-electrode, and free-standing modes. These designs can capture mechanical energy from various movements, making them suitable for wearable electronics, environmental sensors, and self-powered systems.