Development of Printed Devices for Self-Powered Wearable Electronics

Wearable electronic devices encounter significant constraints, particularly in terms of power supply, flexibility, and comfort, requiring novel solutions for widespread adoption. This dissertation aims to address these challenges by developing highly conformable self-powered electronic devices for wearable applications, using printed electronics technologies. It investigates the fabrication of an ultra-thin piezoelectric sensor, piezoelectric nanogenerators, and a triboelectric nanogenerator, specifically designed for biomechanical applications.

First, an ultra-thin piezoelectric sensor was fabricated with a minimum thickness of 7.5 μm. This sensor comprised inkjet-printed interdigitated electrodes (IDEs) using poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) and a blade-coated poly(vinylidene fluoride-co-trifluoroethylene) (P(VDF-TrFE)) piezoelectric layer on a Parylene-C substrate. The ultra-thin piezoelectric sensor’s application for healthcare purposes was demonstrated in pulse wave (PW) measurements. Additionally, the performance of the fully printed IDE-based piezoelectric device was analyzed by comparing its sensing and energy harvesting capabilities with a metal-insulator-metal (MIM)-based piezoelectric device.

Next, the printed piezoelectric nanogenerators (PENGs) were integrated with surface-mounted device (SMD) components, creating a hybrid energy harvesting module. This module consisted of a piezoelectric nanogenerator and a full-wave rectifier, using polyethylene terephthalate (PET) as a substrate and nanoparticle silver (Ag) ink to form the conductive paths. The PENGs were developed using PEDOT:PSS for inkjet-printed electrodes and P(VDF-TrFE) as the active layer.

Lastly, the research focused on enhancing the stretchability of P(VDF-TrFE) through the development of a porous film, using freeze-casting and freeze-drying methods. Comparative analysis of the piezoelectric and triboelectric properties of the porous film versus a solid P(VDF-TrFE) film was conducted. Additionally, the study demonstrated the application of the porous P(VDF-TrFE) film in triboelectric nanogenerator (TENG) systems, specifically by utilizing porous P(VDF-TrFE) and Ecoflex as the triboelectric contact pair. The TENG was fabricated through a combination of doctor blade coating and three-dimensional (3D) printing.

The results indicate that highly conformable piezoelectric devices can be fabricated using printed technologies by minimizing their thickness, and at the same time simplifying the fabrication process. The developed sensors show potential for pulse wave measurements for the evaluation of cardiovascular parameters, and for use in biomechanical energy harvesting applications. However, the power generated for the latter case was relatively low compared to existing devices, with MIM-based devices producing a power density of 7.8 μW/cm3, while IDE-based devices generated 44.0 nW/cm3. Nonetheless, the developed IDE-based and MIM-based devices can be integrated with a full-wave rectifier showing that printed electronics facilitates the development of hybrid systems, which incorporate flexible and traditional electronic components. Furthermore, the research outcomes highlight the importance of engineering materials to customize their properties and structures to enhance their functionalities, such as improving flexibility, stretchability, skinconformability, or charge generation mechanism (i.e., piezo- vs. triboelectricity). Notably, the fabricated TENG, based on the porous P(VDF-TrFE), demonstrated a superior power generation, reaching 9.9 mW/m2 compared to the PENGs.

In conclusion, this thesis significantly contributes to the field of self-powered wearable electronics by providing valuable insights into the development of conformable and highly flexible self-powered printed devices. The research outcomes not only advance the understanding of the working mechanisms of these devices but also pave the way for the widespread adoption of printing technologies as a viable fabrication method. In addition, this study emphasizes the importance of tailoring the material/device structure (e.g., morphology and electrode structure) and geometry (e.g., thickness) to improve the performance of these novel devices.