Synthesis, Characterization, and Applications of Solution-processed Nanomaterials: From Thin-film Transistors to Flexible “Smart Bandages”
Technical Report Identifier: EECS-2016-30
May 1, 2016
Abstract: In parallel with the continued scaling of traditional CMOS devices, another paradigm of electronics has taken shape: flexible electronic systems. Flexible displays, electronic textiles, bio-inspired sensors, and wearable or implantable medical devices are just a few applications that benefit from large-area form factors and mechanical flexibility, both of which are challenging to achieve with conventional wafer-based electronics. Advances in thin-film materials and devices over the past several decades have helped to drive the development of flexible electronics. Printing solution-based electronic materials is a particularly desirable path towards flexible devices. Because it is a purely additive process, printing results in lower overall process complexity, eliminates etching steps, and reduces material usage. Additive processing also enables the integration of various functional materials onto the same substrate, even when the materials or processing technologies would otherwise be incompatible. Printed electronics are compatible with low-cost roll-to-roll manufacturing techniques, offering a significant cost advantage over traditional microelectronic fabrication. Solution-based electronic materials utilizing metal oxides (In2O3, ZnO, SnO2, etc.) have been the focus of intense research efforts to enable high-performance printed electronics because of their high field-effect mobility in amorphous or disordered states. In this work, indium oxide nanocrystal inks are demonstrated as a promising pathway towards highperformance, air-stable, solution-processed transistors. Thin-film transistors (TFTs) that utilize indium oxide nanocrystals as the channel material were developed, and the impact of materials synthesis and device fabrication on the TFT performance was explored in the context of printed electronic devices. To highlight the merits of flexible electronics based on solution-processed nanomaterials and to demonstrate how these materials will enable innovation, one specific application was demonstrated. Biomonitoring devices benefit from lightweight form factors that can make conformal contact with the body, and are thus a particularly interesting proof-of-concept application. Here, a “smart bandage” prototype was designed to detect and monitor tissue wounds in vivo. A flexible, electronic device was developed that non-invasively maps pressure-induced tissue damage, even when such damage cannot be visually observed. Employing impedance spectroscopy across flexible electrode arrays in vivo on a rat model, it was observed that the frequency spectra of impedance measurements were correlated in a robust way with the state of the underlying tissue across multiple animals and wound types. Tissue damage detected using the impedance sensor is represented visually as a wound map, identifying regions at risk of developing a pressure ulcer and thus enabling intervention. These results demonstrate the feasibility of an automated, non-invasive “smart bandage” for early monitoring and diagnosis of pressure ulcers, improving patient care and outcomes.