Customized Electrodes for Printed Biosensors and Wearable Electronics

Abstract

The development of technology is drastically shifting the foundation of the medical field, including the way patients are diagnosed and treated. Until present day, the typical way healthcare providers would diagnose and treat patients involved regular trips to doctors’ offices for frequent monitoring, care, and supervision. However, this method created a level of unsatisfaction and inconvenience for the patient as it significantly interrupted their daily life. The advancements of electronics in combination with the coronavirus pandemic sparked a change with how clinicians cared for their patients. Telehealth was introduced and point-of-care tests (POCTs) and wearable electronics became increasingly popular as ways to reduce medical costs and clinical visits. Clinicians had the ability to review, diagnose, and treat the patient’s health status from afar, ultimately leading to increased patient treatment and overall care. Despite the advancements offered by point-of-care tests and wearable electronics, further development is still necessary to improve their performance and ease-of-use while decreasing the cost to realize solutions that can have lasting impact on patient care.Printing technologies offer a potential solution to the problems associated with POCTs and wearable electronics as they allow for the low-cost fabrication of customizable electronics utilizing a variety of inks on a wide range of substrates. In addition, printed electronics can readily be fabricated at low-throughput to create customized prototypes or at high-throughput for mass production, both at low-cost because of the variety of printing techniques available. In addition, printing can be completed at high resolutions allowing for device miniaturization, decreasing the amount of biofluid needed for testing in POCTs to increase the level of comfortability for the patient. What’s more, printing can also provide a path to creating conformal, robust, and breathable electronics for wearable applications by using inks that have demonstrated high strain capabilities (i.e., flexibility). Overall, the use of printing for realizing medical electronics has endless possibilities for revolutionizing the industry and overcoming the limitations currently presented within medical device fabrication. The work contained in this dissertation describes scientific discoveries and innovations related to two types of electronics for biomedical applications that can be fabricated by different printing techniques. The first is a fully printed and low-cost prothrombin time/international normalized ratio (PT/INR) biosensor that was coupled with a 3D printed handheld device to create a simple, fast, and robust measurement of human clot time within a fully integrated point- of-care measurement system. After creating the biosensor, it was improved upon by incorporating simultaneous testing with a single fingerstick of blood to ensure reliability between tests. In addition, the role of morphology and electrode configuration on the printed PT/INR biosensor was investigated and it was discovered that there was an unexpected dependence on the morphology of the electrodes. Three distinct morphologies were studied: aerosol jet printed silver nanoparticles (AgNPs), aerosol jet printed silver nanowires (AgNWs), and evaporated silver (Ag). In general, AgNPs exhibited the best sensor performance, due to relatively low conductance and high porosity. Overall, the printed impedimetric PT sensor functionalization showed promise for leading to a system that overcomes the challenges of commercial PT/INR coagulometers. The second body of work in this dissertation involves the creation of a new multi- component electrode with innovative materials for monitoring electrophysiological signals and applying functional electrical stimulation (FES). Wearable electrodes were fabricated using a mixed ionic-electronic conductor (MIEC) as a skin-interfacing encapsulation layer with a liquid metal network (LMN) of eutectic Gallium Indium (EGaIn) as the highly conductive material for sensing. Bilayer electrodes consisting of a layer of liquid metal (LM) ink with a thin layer of MIEC spin coated on top as well as composite electrodes that consisted of a mixture of a ratio of LM particles and MIEC were fabricated each with varying amounts of MIEC and studied. The electrode-skin impedance of each electrode was tested on a newly improved skin phantom, consisting of a PVA cryogel dermis and a PDMS epidermis, that mimicked the electrical properties of skin. It was discovered that the MIEC acts as a replacement to the electrolytic gel that is present in wet electrodes allowing the electrode to become conformal with the skin. However, too much MIEC within the electrode results in the LM layer becoming smothered and not being able to penetrate through the MIEC while not enough MIEC does not allow the electrode to fully conform to the skin. Both scenarios resulted in high electrode-skin impedances and reduced signal quality leading to the determination that a mid-range of MIEC within the electrodes with spin speeds of 2000 RPM and ratios around 1:5 of particles to MIEC were superior. In addition, the effect of particles size within the composite electrodes on performance were studied to discover that electrodes with an larger average particle size exhibited the lowest electrode-skin impedances due to their ability to rupture more easily allowing for more connectivity of LM throughout the electrode, ultimately, increasing its conductivity. Finally, the electrodes were also validated for biomedical applications when they were used to monitor electromyography (EMG) and apply FES and resulted in high-fidelity signals with low signal-to-noise values that had a similar trend in performance to the electrode- skin impedance results. Overall, the findings and advancements herein demonstrate the impact printing fabrication methods can have on medical devices, allowing for the creation of low-cost and reliable POCTs and wearable electronics. These advancements include the development of a fully printed, reliable, and low-cost PT/INR biosensor and multi-component MIEC/LM electrodes for electrophysiological monitoring and FES applications. Combined, these contributions lay the groundwork for the future of printed electronics for biomedical applications and demonstrate how they can significantly increase overall patient care and comfort.