Printed organic electronics: towards sensing and biosensing

This work is aimed at impacting on the sensing and monitoring of bio-signals and biomolecules relevant to assess the state of health of people possibly improving the state of the art where the available instruments generally require complex analytical methods commonly implemented by dedicated equipment in a hospital setting. Large research efforts are in progress in research labs to develop and qualify friendly usable devices that could better implement homecare and personalized monitoring aiming at intervening as early as possible to maintain the best health possible for each person. Within this ambitious and quite challenging framework, this PhD project is contributing by developing biosensors that show quite relevant features in terms of flexibility, stretchability, conformability and biocompatibility. Features that are quite essential for devices that should be easily and efficiently interfaced with the human body. Our devices belong to the well-known family of Electrochemical Organic Transistors (OECT) and were implemented by an innovative manufacturing protocol, i.e. the Aerosol Jet Printing (AJP) technique, on flexible/stretchable biocompatible substrates. We chose AJP technique since we believe and hence demonstrate that could be applied to a wide range of printable inks allowing the fabrication of in-line all-printed devices with high definition features. We hence have first of all studied the printing parameters for all the needed components including contacts, active channel on different substrates, optimizing them to achieve the best definition and functionality of the materials. For the active channel we studied poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS), that could be considered the benchmark polymer in Organic Bioelectronics. We fabricated OECTs using two different flexible substrates: polyimide (Kapton) and a fibroin film. We demonstrate that we can achieve state of the art performance of our printed devices compared to similar ones in terms of architecture and materials used. For the most advanced bio-electronic sensing device, we selected fibroin film as a substrate because not only shows better biocompatibility than a Kapton foil but we could also achieve a functionality of great relevance: optimized fibroin films give a particularly good skin self-adhesion, improving also signal transmission without the need of the commonly used gels, features that Kapton doesn’t offer without the addition of gels and other Our OECTs printed on fibroin have hence been directly applied on the human skin for ECG signal monitoring in a standard clinical configuration. This represents the most important achievement: the development of a fully-printed OECT on a flexible fibroin substrate that could monitor of ECG signals both in vivo and real time giving signals amplitude even better with respect to commercial electrodes with comparable time resolutions. These results are a very good basis for the future planned activities aimed at developing devices with an increased quality response for directly monitoring bio signals generated by neuronal activity. We have also explored skin-attachable optical sensors. To this aim, we have studied fibroin photoelastic properties that are fundamental considering the continuous mechanical stimulation induced by the human body during movement. We made experiments on the two major fibroin phases (Silk I and Silk II) by applying longitudinal strains on a silk fibroin whispering gallery mode (WGM) cavity fabricated on an optical fiber taper. In synthesis the major relevant results shown in this thesis, concern hence materials preparation, inks development, printing optimized processes, all printed device fabrication, that have been used to develop and qualify OECTs on fibroin as very promising all printed devices for biomedical applications. We also explored fibroin itself as a base material for optical sensors, achieving the feasibility towards developing wearable optical sensor for biomedicine and rehabilitation. We show the feasibility of fully biocompatible, cheap, easy to handle and scalable systems that could contribute to the development of wearable and all-day user electrophysiology and optical sensors. We are working to further improve the devices by increasing their sensitivity and time response mainly working on the gain and noise suppression. The idea is to possibly extend the fields of applications to neural signals, intrinsically faster and less intense than ECG, to electrophysiology, and to wearable optical sensors