Kirill Keller, David Grafinger and Francesco Greco
Publication Date: August 12, 2021
https://doi.org/10.3389/fmats.2021.688133

Abstract

As printed electronics is evolving toward applications in biosensing and wearables, the need for novel routes to fabricate flat, lightweight, stretchable conductors is increasing in importance but still represents a challenge, limiting the actual adoption of ultrathin wearable devices in real scenarios. A suitable strategy for creating soft yet robust and stretchable interconnections in the aforementioned technological applications is to use print-related techniques to pattern conductors on top of elastomer substrates. In this study, some thin elastomeric sheets—two forms of medical grade thermoplastic polyurethanes and a medical grade silicone—are considered as suitable substrates. Their mechanical, surface, and moisture barrier properties—relevant for their application in soft and wearable electronics—are first investigated. Various approaches are tested to pattern conductors, based on screen printing of 1) conducting polymer [poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS)] or 2) stretchable Ag ink and 3) laser scribing of laser-induced graphene (LIG). The electromechanical properties of these materials are investigated by means of tensile testing and concurrent electrical measurements up to a maximum strain of 100%. Performance of the different stretchable conductors is compared and rationalized, evidencing the differences in onset and propagation of failure. LIG conductors embedded into MPU have shown the best compromise in terms of electromechanical performance for the envisioned application. LIG/MPU showed full recovery of initial resistance after multiple stretching up to 30% strain and recovery of functionality even after 100% stretch. These have been then used in a proof-of-concept application as connectors for a wearable tattoo biosensor, providing a stable and lightweight connection for external wiring.

Laura M. Ferrari, Usein Ismailov, Francesco Greco, Esma Ismailova
Publication Date: July 10, 2021
https://onlinelibrary.wiley.com/doi/10.1002/admi.202100352

Abstract

Tattoo electronics is one of the emerging technologies in skin compliant biosensing. The growing interest in their large application in health monitoring raises several interrogations on how these sensors interface with the skin. In this paper, the bioimpedance at the interface of the skin and ultra-conformable tattoo electrodes made of conducting polymers are focused on. The electrochemical characteristics of these electrodes differ from traditional gelled Ag/AgCl electrodes. The modeling of equivalent circuits in different skin-electrode configurations proposes the explanation of the biopotentials transduction mechanism. The strong agreement between the circuit model and experimental values reveals the capacitive coupling of conducting polymer tattoo electrodes where circuit’s values reflect the electrodes’ and skin physical characteristics. Additional studies underline an enhanced signal stability in inter/intra-subject evaluations using dry tattoos beneficial for broad long-term recordings. This study provides a comprehensive explanation of the skin/tattoo electrode interface model. The understanding of this interface is essential when designing next generation wearable biomonitoring devices using imperceptible interfaces.

Federico Maria Vivaldi, Alexander Dallinger, Andrea Bonini, Noemi Poma, Lorenzo Sembranti, Denise Biagini, Pietro Salvo, Francesco Greco*, and Fabio Di Francesco*
Publication Date: June 24, 2021
https://pubs.acs.org/doi/10.1021/acsami.1c05614

Abstract

Notwithstanding its relatively recent discovery, graphene has gone through many evolution steps and inspired a multitude of applications in many fields, from electronics to life science. The recent advancements in graphene production and patterning, and the inclusion of two-dimensional (2D) graphenic materials in three-dimensional (3D) superstructures, further extended the number of potential applications. In this Review, we focus on laser-induced graphene (LIG), an intriguing 3D porous graphenic material produced by direct laser scribing of carbonaceous precursors, and on its applications in chemical sensors and biosensors. LIG can be shaped in different 3D forms with a high surface-to-volume ratio, which is a valuable characteristic for sensors that typically rely on phenomena occurring at surfaces and interfaces. Herein, an overview of LIG, including synthesis from various precursors, structure, and characteristic properties, is first provided. The discussion focuses especially on transport and surface properties, and on how these can be controlled by tuning the laser processing. Progresses and trends in LIG-based chemical sensors are then reviewed, discussing the various transduction mechanisms and different LIG functionalization procedures for chemical sensing. A comparative evaluation of sensors performance is then provided. Finally, sensors for glucose detection are reviewed in more detail, since they represent the vast majority of LIG-based chemical sensors.

Bernhard Burtscher, Günther Leising, Francesco Greco
Publication Date: June 10, 2021
https://doi.org/10.1021/acsaelm.1c00249

Abstract

Generation of ultrathin, transferable, and imperceptible electronic devices [e.g., organic photodiode (OPD)] for multiple applications, such as personalized health monitors and wearables, is emerging due to the continuous development of materials and manufacturing processes. For such devices, the choice of a suitable substrate is of utmost importance. A water decal transfer from a temporary tattoo paper is adopted here as a substrate for ultrathin and conformable organic components because of easy and reliable transfer of a ≈600 nm robust and transparent polymer nanofilm of ethyl cellulose. Strategies for the fabrication of a transferable OPD on a temporary tattoo are investigated. A device with an overall thickness <1 μm and its performance after transfer are demonstrated. Then, efforts are put into fabricating an OPD by inkjet printing with a water-soluble active layer consisting of polythiophene and fullerene derivatives to aid cost- and material-efficient, large-scale production possibilities. Additionally, a second semitransparent electrode made of printed aluminum-doped zinc oxide and silver nanowires is used to allow usage from both sides to enhance the application potential. Both OPD examples presented here need improvement of the device performance but permitted us to highlight the versatility and application potential of temporary tattoos for transferable components. Target surfaces for the final application after transfer include artificial (flat and smooth, e.g., glass, or even complex and rough, e.g., concrete, paper, and so forth) as well as natural ones.

Featured in Journal Front COVER of June 2021 Issue