Design, Modeling and Fabrication of Flexible Sensors for IoE Applications using Emerging Technologies

Abstract

Flexible and stretchable electronics is one of the most promising fields for diverse scientific and industrial areas such as electronic skin, wearables devices or biosensing. In this way, there is a big emerging effort focused on the synthesis of flexible conductive materials as an alternative to the conventional rigid siliconbased technology to satisfy the requirements of this kind of applications (flexibility, lightness, transparency, etc.). Since Andre Geim and Konstantin Novoselov were awarded with the Nobel Prize in Physics in 2010 for their groundbreaking experiments regarding the twodimensional material graphene, it has attracted the interest of many researchers due to its unique electrochemical, mechanical and optical properties. However, although it is a perfect candidate to be used in flexible and stretchable electronics, these expectations have not been yet materialized into end-user applications since its current synthesis methods remain costly and unscalable. For these reasons, the interest of some research activities around graphene is shifting to the graphene-derived materials which, even though they do not present a pristine monolayer structure, capitalize part of the unique graphene’s properties and are paired with synthesis processes suitable for a mass-production of samples. In this context, the main objective of this thesis is the study of promising graphene-derived materials, such as graphene oxide (GO), reduced-graphene oxide (rGO) and laser-induced graphene (LIG), for their use in flexible electronics devices. In particular, the work carried out during this doctoral thesis includes from the synthesis and study of these materials to the exploitation of their properties to develop end-user devices and applications. Regarding the fabrication processes, this thesis is focused on a scalable method based on laser assisted photothermal processes to obtain conductive graphenederived patterns on flexible substrates, specifically, laser-induced graphene and laser-reduced graphene oxide (LrGO). In addition to the laser-synthesis of these nanomaterials, this work also addresses other fabrication techniques for the mass-production of flexible electronics over large area substrates, such screen and inkjet printing. Apart from the description of these fabrication techniques and the characterization of the different materials synthesized, this thesis presents different kinds of sensors and devices based on these materials, including temperature and humidity sensors, heaters, supercapacitors and electrocardiogram (ECG) electrodes. Furthermore, some of these devices have been studied in final applications. Thus, the LrGO-based temperature sensors were integrated in an IoT sensing platform with Bluetooth Low Energy capability, whereas the ECG electrodes were tested in combination with a commercial wearable and custom processing techniques in order to be used for the ubiquitous and long-term monitoring of the heart rate. Furthermore, this work also reports a pioneering research on the fabrication of laser-lithographed graphene oxide memristors. Finally, the use of memristors for the implementation of memcapacitor and meminductor emulators has also been addressed. The eleven articles published in indexed journals and the contribution to a conference of international relevance reflect the success of results achieved during this thesis.