## 3 Energy Harvesting
### Energy Harvesting Material: Cellulosic  
Cellulosic materials, primarily obtained from plant biomass, consist of cellulose-a renewable, biodegradable, and abundant biopolymer. As sustainability concerns increase, cellulose has gained significant attention in fields such as energy storage, electronics, and sensors thanks to its environmentally friendly properties, low cost, and wide  
Figure 3: Power generated from piezoelectric material. availability [50]. These qualities also make cellulosic materials ideal for green energy applications, where they serve as a low-carbon alternative to synthetic polymers often used in electronic devices [49; 50]. Recent advances have focused on leveraging the natural attributes of cellulose for energy harvesting, most notably through cellulose-based nano-generators that convert mechanical energy (for example, from human movement) into electrical energy [51; 52]. Cellulose stands out for its high mechanical flexibility, robust chemical stability, and ability to undergo surface modification, all of which can enhance its triboelectric properties [51; 52]. Cellulose-based triboelectric nanogenerators (TENGs) generate electricity through friction between two materials, tapping into the triboelectric effect that creates surface charges upon contact and separation. Combined with cellulose's biodegradability, this approach offers a sustainable path toward energy-harvesting solutions in public areas such as pedestrian walkways and transportation hubs [53]. Triboelectricity in cellulosic materials can be boosted by functionalities their surfaces to increase charge density [54; 55]. Beyond triboelectricity alone, cellulose can be combined with piezoelectric and electrostatic components to form hybrid nanogenerators that capture a broader range of mechanical motions (compression, tension, and bending), further improving power output [53; 56]. For instance, [PERSON] et al. [57] demonstrated that chemically modifying cellulose surfaces can enhance electron transfer with a paired triboelectric material, doubling energy output. [PERSON] et al. [58] introduced a cellulose-based TENG made with micro- and nano-structured cellulose fibers, boosting the effective surface area. The increased number of contact points improved charge density and thereby boosted power generation-enough to power low-energy devices such as LEDs or small sensors. Hybrid nanogenerators incorporating cellulosic materials alongside other harvesting methods (piezoelectric or electromagnetic) provide even more ways to capture mechanical energy [52]. [PERSON] et al. [53] combined a cellulose-based triboelectric layer with a piezoelectric polymer to create a hybrid nanogenerator, achieving a continuous power output from human motion. This design led to a 40% increase in power compared with using cellulose-based TENGs on their own. In a similar vein, [PERSON] et al. [58] explored blending cellulose with carbon-based nanomaterials such as graphene oxide to improve electrical conductivity and surface triboelectric properties, thus enhancing the overall efficiency of cellulose-based nanogenerators. These hybrid systems have shown promise in powering wearable devices and small IoT systems in busy public spaces such as malls and airports. Because cellulose is flexible, it is a strong candidate for wearable applications [59]. Researchers have integrated cellulose-based nanogenerators into clothing, allowing the garments to convert everyday human motions, such as walking or hand movements, into usable electricity. [PERSON], [PERSON], and [PERSON] [38], as well as [PERSON] et al. [60], designed wearable TENGs embedded in textiles made of cellulose fibers, capable of storing energy in a small battery or supercapacitor. This stored energy can power low-demand sensors for health monitoring or fitness tracking. Cellulose also offers comfort and biocompatibility, as it is non-irritating to the skin and can be treated with antimicrobial agents to prevent odor [59; 60]. [PERSON] et al.