Development of Conductive Inks Compatible with Direct Ink Writing Technology for the Fabrication of Flexible Temperature Sensors
Creators: Sinno, Jihad
Publisher: name: University of Alberta Library
Year: 2024

<p>Interest in printed electronics has grown in recent years, specifically for the fabrication of flexible electronics devices including sensors. Flexible temperature sensors that can map the temperature profile of a surface over time hold significant promise for various applications, including wearable devices, healthcare monitoring, and environmental sensing. These sensors have experienced a substantial surge in both research and commercial integration, owing to their remarkable attributes of heightened sensitivity, adaptability, precision, and ease of fabrication.<br>
The most common materials used in temperature sensing and mapping are alloys such as Nickle Chromium/ Nickel Aluminum, Copper/ Constantan, or Platinum Rhodium. However, these materials, used singularly, exhibit brittle behaviour when incorporated into flexible electronics. Additionally, complex top-down fabrication processes are required to utilize these and other bulk metals in miniaturized devices. In this work, temperature-sensing inks are being developed for deposition by the additive manufacturing (AM) technique direct ink writing (DIW). DIW is a versatile printing technique that uses viscous ink formulations to create a variety of patterns and designs that can be applied to the fabrication of flexible temperature sensors.<br>
DIW requires the development of compatible inks for the development of flexible temperature sensors. Most commercially available inks are metallic inks, which are expensive and have low stability over time. An example of this is silver (Ag) ink which is prone to oxidize after some time has passed and is non-biodegradable. An alternative to using metallic inks is the development inks that are based on carbonaceous materials such as graphene, carbon nanotubes (CNTs). These inks however are not as prevalent as metallic based inks.<br>
This study aims to explore the use of DIW along with a sustainable carbonaceous conductive ink to fabricate reusable and eco-friendly flexible temperature sensors. The ink incorporated a carbonaceous material, reduced Graphene Oxide (rGO), synthesized using chemical reduction of Graphene Oxide (GO) via a green reductant, L-Ascorbic Acid (L-AA). Poly (lactic acid) (PLA) was used to create a polymer matrix for the rGO. PLA was chosen for its small thermal expansion (68μm mK-1), biocompatibility, and relatively low cost. Inks of varying weight percentages of rGO in PLA were produced ranging from 0-16 wt %, resulting in the fabrication of composites exhibiting electrical percolation behaviour as well as temperature dependent resistivity. Sensors were then fabricated using DIW via a GEEETech printer with varying ink extrusion speeds.<br>
The outcome of this thesis was the development of a flexible temperature sensor using DIW technology via green conductive inks. The produced sensors exhibited a temperature coefficient of resistance (TCR) of 0.007/οC for the 12 wt% ink printed using a 20-guage nozzle tip at 3 different extrusion speeds: 400, 600, 800 ml/hr. This work demonstrates the viability of using rGO/PLA inks for the fabrication of flexible temperature sensors, and an application of this work is demonstrated by the production of a printed array of 9 sensors on a flexible polyimide substrate that can map the temperature of a surface. This holds significant promise for the integration of various functional materials into inks compatible with DIW resulting in further growth in the field of printed electronics.</p>