Inkjet-Printed Aqueous Graphene Suspensions on Flexible Substrates for Multimodal Wearable Continuous Health Monitoring

Inkjet-Printed Aqueous Graphene Suspensions on Flexible Substrates for Multimodal Wearable Continuous Health Monitoring

ABSTRACT: Flexible and gel-free dry electrophysiological electrodes offer excellent alternatives to the benchmark with better skin conformality and dispensing with the hydrogel electrolyte layer, where the integrity of recorded signals highly depends on the resilience of this Inkjet-printed graphene layer. However, the implicit complex fabrication methods and resource-intensive nature of existing dry electrodes, designed for single use, necessitate additional technological innovations. This paper presents an original integration of inkjet-printing, graphene, and poly(ethylene terephthalate) substrates to create flexible, inexpensive, and reusable dry polyethylene terephthalate electrodes for monitoring human physiological biopotential signals, including ECG, EEG, electrooculography, and electromyography. The electrodes exhibited high electrical conductivity and maintained their performance even after one hundred cyclic bends. Additionally, they demonstrated robust signal collection even ninety days postfabrication, with reusability and strong graphene-substrate bonding verified through Scotch tape experiments. Assessing against gel-based commercial electrodes, a ninety-nine point three four percent correlation ratio was obtained from the recorded ECG signals. Similarly, a correlation ratio of ninety-five point one four plus or minus four point seven five percent was achieved with ten diverse participants. The capability of these electrodes to effectively record neural signals in practical applications was demonstrated in real-time detection of sleep and drowsiness in car drivers. These findings highlight the potential of inkjet-printed graphene electrodes in advancing wearable health monitoring technologies.

One. INTRODUCTION

In the past decade, flexible electronic devices have arisen for various applications including energy storage and harvesting (e.g., supercapacitors, triboelectric nanogenerators), lighting and display applications (e.g., organic light-emitting diodes), health monitoring, diagnosis and preventive care (e.g., skin patches, tattoo electronics). Thanks to their miniature form-factor, suitability for pick-and-place integration with surface mount low-power CMOS, massive scalability and adaptability for roll-to-roll manufacturing, flexible hybrid electronics has emerged as a disruptive technology. Especially, in the personalized wellness domain, where continuous monitoring and acquisition of data from daily activities is needed, low-power wearable systems that can conform to the body even in dynamic conditions are critical. To that end, various substrate-functional layer combinations have been investigated to realize flexible health monitoring devices by utilizing existing fabrication techniques typically with certain modifications and integration strategies.

However, a significant manufacturing bottleneck remains: balancing the high throughput of these systems with economic cost-effectiveness and environmental sustainability, particularly concerning material waste and the use of nonrecyclable substrates.

Metals such as silver and copper, oftentimes coming in paste or ink formulation with suitable viscosity, are the most widely used functional materials in flexible electronics due to their high conductivity. While these materials have been used as standard, two-dimensional layered materials, especially graphene and its derivatives like graphene oxide and reduced graphene oxide, offer the unique combination of biocompatibility and environmental sustainability needed for "single-use" or "disposable" skin-interfacing sensors. The appeal of such materials derives both from the sustainability perspective due to their cost-effective solution-based preparation methods, scalability, environmental friendliness and suitability for the circular economy; and also from the functionality perspective owing to physiochemical properties including mechanical flexibility and stretchability, large surface area for functionalization and drug loading,

biocompatibility, and abundant oxygen-containing groups for electrochemical sensing, to name a few.

Despite these advantages, the transition from graphene "source material" to a functional device is governed by the compatibility between the material's formulation and the chosen fabrication process. While techniques such as screen printing, aerosol jet, and gravure printing have been utilized, they often face challenges regarding material waste, high setup costs, or the need for physical masks. Inkjet printing distinguishes itself from the above-mentioned methods as a fully digital, additive, and mask-less technique, offering high-resolution patterning with nearly zero material waste-a critical factor for scalable, sustainable manufacturing.

Building on these advantages, inkjet-printed graphene can be effectively utilized in monitoring various electrophysiological signals through flexible dry electrode forms. Key examples of such biosignals are electrocardiography, electromyography, electroencephalography, and electrooculography, which provide excellent assessment tools in the diagnosis and treatment of cardiovascular diseases, rehabilitation, neurological disorders, and human-machine interface applications, respectively. To enable high-quality acquisition of electrophysiological signals, electrodes interfacing with skin must possess specific properties. (One) Low skin-electrode impedance is crucial for collecting signals with a high signal-to-noise ratio. The importance of this property is evident in EEG signal recording, in which high skin-electrode impedance can result in complete signal loss at the skin and electrode interface before reaching the electronic circuitry. (Two) High electrical conductivity is an interdependent property with electrode impedance that grants a reliable signal transfer from the skin to electronic devices. (Three) Electrode adaptation to skin deformations-achieved through the use of flexible materials-reduces motion artifacts associated with natural body movements and skin deformations, ultimately enhancing comfort and wearability. (Four) The biocompatibility of the materials that come into direct contact with the skin is pivotal for minimizing the risk of skin irritations and trauma.

Despite significant research into the fabrication and performance evaluation of dry electrodes, past efforts have usually failed to address an ongoing paradigm in the development of biomedical devices which demands: (One) scalable fabrication techniques that can cater to a large population while keeping the final product costs low; (Two) the use of recyclable materials to mitigate electronic waste; and (Three) minimum dependence on natural resources. The current reliance on complex fabrication processes (e.g., cleanroom-based processes), nonrecoverable materials (e.g., poly(dimethylsiloxane) and thermoplastic polyurethane), and finite natural elements (e.g., copper, silver and gold) for products designed for a single-use presents challenges that require careful and innovative solutions. This is where additive, inkjet printing of two-dimensional materials like graphene enter the scene, however with critical technical challenges in manufacturability and device integration which have been largely unmet thus far. Essentially, the success of inkjet printing approach depends fundamentally on the rheological properties of the graphene ink. Unlike other printing methods, inkjet printing requires a meticulous balance of viscosity and surface tension to ensure stable drop-on-demand ejection and eventually successful print formation.

To address this issue, herein we detail, to the best of our knowledge, the first systematic report on flexible inkjet-printed dry graphene electrodes on recyclable and affordable polyethylene terephthalate substrates, achieved by fine-tuning and optimizing the inkjet-printing process. We demonstrate the interplay between ink rheology, substrate surface properties, inkjet printing waveform and jetting profile, to report the optimized conditions for successful printing of dry, flexible graphene electrodes on PET, gathered as a culmination of extensive process development trials. To demonstrate the functionality of these flexible dry electrodes, we employ them in capturing a wide range of human electrophysiological signals, including ECG, EMG, EEG, and EOG, when interfaced with the skin. The performance of the printed graphene electrodes demonstrated a high correlation ratio with simultaneously recorded ECG signals from commercial silver-silver-chloride electrodes, with a similar high correlation ratio observed between printed silver nanoparticle electrodes and silver-silver-chloride electrodes. Additionally, the generalizability of the printed electrodes in recording electrophysiological signals in a larger population, composed of participants with varying ages, genders, and body mass indices, showed statistical results similar to those of clinical-grade electrodes. The electrodes also exhibited high conductivity, excellent resilience to cyclic bending when evaluated by recording ECG, and consistent bonding between the printed materials and substrates, as assessed by scotch tape experiments. Finally, to demonstrate the applicability of the reported electrodes in real-life wearable applications, the printed graphene electrodes showed high accuracy in recording neural signals for detecting drowsiness in car drivers, thereby contributing to accident prevention.