Flexible printed electronics for touch and strain feedback

Abstract

This era of technological revolution demands more innovation in making the electronics and devices used in everyday human lives more and more personal and interactive. The colossal interest and research in the development of flexible and multi-functional sensors are a result of this demand. However, it is still a significant challenge to fabricate such flexible wearable devices using simple and cost-effective approaches that can be utilized in mass production or directly integrated into textile manufacturing without introducing additional complicated steps that require high-end laboratory-grade technology. This thesis addresses the issue regarding the need for a mass-producible process for developing wearable and flexible sensors by utilizing ancient technology like stencil printing that is fast, simple, and cost-effective. In this work, a graphene/carbon black complex-based conductive ink was developed, which utilizes the filler behavior of carbon black particles among graphene sheets to enhance the electrical performance of the ink. The polymer-assisted binding using polycarbonate enabled the ink to be easily printable with desired patterns with excellent adhesion and stability. First, a functional tattoo capable of sensing different pressure levels was developed, which works by utilizing the change in capacitance of a two-dimensional printed capacitor with the change of pressure. An interdigitated comb geometry was used to fabricate the capacitive sensor that increases the interactive area between the electrodes, resulting in enhanced performance of the two-dimensional capacitor. The capacitive touch sensor could differentiate among multiple pressure levels at different time ranges with fast and consistent performance. Furthermore, a stretchable textile-based strain sensor was developed with the ink on a cotton fabric substrate that can detect human motion and bending by measuring the change in resistance using the same stencil printing methodology. The sensor was capable of detecting different levels of bending and also was able to differentiate between fast and slow bending motion. Furthermore, the textile-based strain sensor could withstand multiple washing cycles and could exhibit similar performance trends for strain sensing.