Abstract
The rapid advancements in flexible electronics have revolutionized the field of wearable devices, enabling comfortable and seamless integration into our daily lives. However, the limited battery life of these devices remains a significant challenge. Batteries impose constraints on device design, size, weight, and usage time, hindering the full potential of flexible electronics. Energy harvesters have emerged as promising alternatives to batteries, harnessing ambient energy sources to power wearable devices. Various types of energy harvesters, including solar cells, thermoelectric generators, piezoelectric devices, and triboelectric nanogenerators, offer the potential to extend the operational lifespan of wearables by converting mechanical, thermal, or light energy into electrical power. This PhD dissertation discusses the current advancements in flexible electronics, highlights the limitations imposed by batteries, and explores the advantages of energy harvesters as replacements for batteries in wearable devices, opening doors to self powered and longer-lasting wearables. In the realm of energy harvesters, triboelectric nanogenerators (TENGs) have gained significant attention due to their unique advantages over other energy harvesting technologies. TENGs offer high energy conversion efficiency, versatility in mechanical inputs, and the ability to function not only as energy harvesters but also as mechanical sensors. Unlike solar cells that rely on sunlight or thermoelectric generators that require temperature gradients, TENGs can generate electricity from a wide range of mechanical stimuli, including human motion, vibrations, or even wind flow. Moreover, TENGs can be seamlessly integrated into wearable devices, providing self-powered capabilities and eliminating the need for external batteries. To enhance the device output, increasing the surface area is crucial. However, the currently available solutions for increasing the surface area of TENGs are not cost-effective, posing a challenge in practical implementation. Additionally, TENGs do have limitations in harsh environments, such as high humidity or extreme temperatures, which can affect their performance and durability. Overcoming these limitations is an ongoing area of research, with the aim of expanding the applicability of TENGs in diverse wearable and sensor applications. This PhD thesis focuses on gaining a deeper understanding of triboelectric nanogenerators (TENGs) and proposing novel solutions in terms of both structural and material aspects. The initial research effort involved the development of a surface structure for TENGs called the FS-TENG (Flexible, biocompatible, and ridged silicone elastomers based robust sandwich-type triboelectric nanogenerator) with the maximum power output density of 390 mW/m2 . Later on the device was improved in terms of electrical output and named as SER-TENG (Surface textured double layer triboelectric nanogenerator for autonomous and ultra-sensitive biomedical sensing), which featured ridged structures. Through experimentation, it was determined that v an optimal ridge size of 1 mm resulted in the highest output power, reaching a maximum of 490 mW/m2 . Furthermore, these devices were utilized in the creation of mechanical sensors for biomedical applications such as monitoring breath rate, pulse rate, and gait. In the subsequent phase, the analysis focused on investigating the impact of ridge shape on the sensing capabilities of the SER-TENG. Three different ridge shapes were examined, namely pointed, ŕat, and curved. The findings revealed that the flat ridged surface exhibited a superior ability to sense high loads beyond 50 N, while the curved-ridge surface demonstrated a balanced response encompassing both low (from 5 N) to high forces (70 N), establishing a favorable middle ground. The research culminated in the creation of an innovative composite material that combined the tribological properties of silk with the stretchability of a hydrogel. To achieve this, glycerol, known for its low freezing point and high melting point, was incorporated and converted into composite hydrogel. The resulting material was successfully fabricated and exhibited an impressive stretchability of up to 150 %. To assess its stability in harsh environments, the material underwent qualitative inspection. Remarkably, even after being subjected to a freezing temperature of -18 degrees Celsius for a duration of 24 hours, the silk-glycerol based hydrogel samples remained completely flexible without any signs of damage or stiffness. Since this PhD research is focused on energy harvesters based on triboelectric nanogenerator. This PhD study exploited the understanding the basics of triboelectric nanogenerators (TENG) and its applications in smart wearables. The detailed explanation of this topic is discussed in chapter 2. Chapter 3 covers the materials and methods for the fabrication and the characterization of the proposed TENG devices. Later on, chapter 4 discusses in brief about all the results that have been published as well as the detailed explanation of unpublished work is also mentioned in this section. The remaining chapters covers the published data with chapters 5 and 6 discusses about the novel structured triboelectric nanogenerators known as FS-TENG and SER-TENG while the last chapter 7 covers the details of the material modification based triboelectric nanogenerators.