Development of polyvinylidenefluoride based sensors for wearable electronics and health monitoring

In the era of digital transformation, the integration of flexible, wearable, and self-powered sensor systems into smart electronics has become a priority for human-machine interfaces and real-time biomechanical monitoring applications. This thesis study comprehensively addresses the design, synthesis, characterization, optimization, and applicability of graphene-based nanocomposites for piezoelectric (PENG), triboelectric (TENG), and iontronic capacitive sensors to develop mechanical energy harvesting and self-powered sensing systems. In the first stage of the thesis, graphene nanoribbons (GNR) were synthesized by oxidatively opening multi-walled carbon nanotubes using the intercalation method. Additionally, nitrogen-doped graphene (NG) was produced using a thermal annealing process with urea from graphene oxide; the material's surface potential and electronic properties were significantly improved due to its high content of pyrrolic and pyridine-type nitrogen. After characterizing the synthesized nanostructures, they were incorporated into polyvinylidene fluoride (PVDF), poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP), and chitosan matrices at different ratios and combinations to obtain flexible and durable nanocomposite films with a high electroactive phase ratio. A PVDF film containing 1% GNR by weight, with approximately 83% β-phase content, exhibited high piezoelectric performance, demonstrating an open-circuit voltage of 26 V and a power density of 16.52 μW/cm². It was determined that the PENG prepared using the PVDF-GNR film can effectively convert biomechanical energy into electrical energy. For a triboelectric nano-generator system, a multi-layered hybrid structure was developed, consisting of a flexible thermoplastic polyurethane nanofiber substrate with a tribo-positive layer of chitosan/NG and a tribo-negative layer of PVDF-HFP/GNR/ionic liquid composite. The use of heteroatom-doped nanofillers has improved surface roughness, charge transfer efficiency, and triboelectric output performance; a TENG device based on a chitosan film with 0.05% NG content achieved an open-circuit voltage of 112 V and a short-circuit current of 6.42 μA. The device was able to distinguish between walking, running, jumping, finger bending, and striking motions and accurately detect electrical signals specific to different breathing patterns. Additionally, the use of the PVDF-HFP/GNR/ionic liquid composite as a piezoelectric iontronic sensor has been investigated. The developed hybrid iontronic sensor demonstrated high sensitivity (1.64 kPa−1), fast response/recovery time (98 ms/103 ms), low pressure detection capability (1.29 Pa), and high mechanical durability (1000 cycles). The iontronic sensor has been able to monitor biomechanical and physiological movements such as joint bending, pulse, and swallowing in real time with high accuracy, proving itself a strong candidate for electronic skin applications. In conclusion, through this thesis study, flexible, low-cost, high-performance energy harvesting and sensing devices were developed by designing nanostructures, employing surface engineering, and utilizing heteroatom doping methods. The synergistic interactions between piezoelectric polymer matrices and GNR and N-doped graphene nanoscale materials have contributed to the development of durable and highly efficient devices for next-generation wearable electronics incorporating triboelectric and iontronic systems.