Thesis title: Development of 3D multilayer-graphene/PDMS foams for piezoresistive sensing for wereable and flexible technologies
This thesis introduces a significant achievement in flexible and wearable electronics by developing a highly responsive piezoresistive sensor made from a 3D Gr/PDMS foam. These innovative sensors play a crucial role in the rapidly advancing areas of flexible electronics, wearable technology, biomedical devices, and smart fabrics, indeed, their primary function is to detect and react to small mechanical changes. The project's success relied on a fabrication procedure that involved many processes specifically designed to create a Gr/PDMS composite material with outstanding mechanical flexibility and improved electrical characteristics. First, Gr is deposited using Chemical Vapor Deposition (CVD) to form a continuous and robust Gr network, starting with a commercial nickel porous structure as the template. Poly(methyl methacrylate) (PMMA) infiltrates the structure to provide mechanical support during subsequent processing steps. Then, the nickel template is etched away, leaving behind a hollow, freestanding Gr foam filled with PMMA. The next phase involves infiltrating the Gr skeleton with PDMS to create a composite material that combines the mechanical flexibility of PDMS with the superior electrical properties of Gr. Finally, the PMMA is removed, resulting in a 3D Gr/PDMS foam with excellent flexibility, high electrical conductivity, and enhanced mechanical strength. Two different morphologies of nickel templates were used to understand the impact of their properties on the response to mechanical deformation. Extensive characterization studies, including Scanning Electron Microscopy (SEM), Energy-dispersive X-ray Spectroscopy (EDS), Raman Spectroscopy, and X-ray Diffraction (XRD), were conducted to monitor and analyze the material properties at each fabrication stage. In-situ SEM measurements during mechanical deformation demonstrated the mechanical robustness of the Gr/PDMS foam at the microscale. The electrical characterization revealed the foam’s ability to translate compressive strain into predictable resistance changes. Tests with different foam thicknesses showed consistent piezoresistive behavior in the 30$\%$ to 70$\%$ deformation range, while the integration of graphene ink into the contact system improved measurement stability. This setup enhanced the sensitivity of the sensors, ensuring reliable and repeatable measurements, particularly under cyclic loading. These results further emphasize the potential of Gr/PDMS foams in applications requiring high sensitivity and mechanical flexibility. Significant effort was devoted to improving the electrical measurement methodology. Different approaches for fabricating electrical contacts were explored, leading to the use of graphene ink with gel properties to enhance adhesion and stiffness compatibility with the Gr foam surface. A custom sample holder, optimized and produced via 3D printing, improved signal reliability by minimizing signals from contact deformations and reducing foam damage. Preliminary results showed increased sensitivity to mechanical compression and temporal stability, with resistance changes influenced by foam morphology. These results highlight how Gr/PDMS foams hold significant promise for advancing flexible electronics and wearable technology.