Timothy Shay

PhD, Chemical Engineering, 2017
North Carolina State University

M.S., Chemical Engineering, 2014
North Carolina State University

B.S., Chemical and Biological Engineering, 2012
University of Wisconsin – Madison

Research Focus: Novel Human-Device Interfaces: Hydrogel EKG Electrodes and Microfluidic Sweat Monitoring

Growing and aging populations have emphasized the need for low cost medical diagnostic tools. Wearable biosensors could reduce hospital and clinic visits while lowering costs associated with staffing and equipment. We use hydrogels as a novel biomimetic body interface for early prototypes of wearable health monitoring devices. The focus of the research are new electrocardiogram (EKG) and sweat capture devices.

To obtain a clean heart’s electrical signal from the body for an EKG, a low resistance electrode with a low impedance device-skin interface is needed. A four point probe conductivity test was initially performed on hydrogel pieces to determine their bulk resistance. It was shown that the inclusion of ionizable groups on the hydrogel molecular backbone was able to lower the resistance by two to three orders of magnitude. These electrolytic hydrogels were then interfaced with a copper wire encased in PDMS to create an EKG electrode, which was able to provide a clean EKG on a human subject. Future work entails optimizing the impedance of the hydrogel-skin interface and investigating whether a liquid metal such as eutectic liquid gallium (EGaIn) could be used to create a truly flexible electrode.

Sweat collection is another parameter of interest because it allows sampling of many analytes such as cortisol, glucose and various other ions that provide a non-invasive measure of the body’s overall health. The hydrogel patches can be tuned to create osmotic pressure gradients while in contact with the body to promote fluid intake for sweat capture. To understand the hydrogel’s ability to work as a sweat capture interface, both the diffusion through the gel and intake of fluid from the body was characterized. Diffusion of acidic solutions, meant to mimic sweat, was measured via a pH color change indicator. The diffusion penetration profiles were successfully modeled with both Matlab and COMSOL. The ability of the hydrogel in a superporous form to draw water from skin was demonstrated on a peach model. Water intake rates were measured and correlated to both water content of the hydrogel and peach. The two hydrogel interface ideas were then combined to create a pH color change sensor that detected the acidity of a peach. Future sweat capture work includes combining both the intake and evaporation from hydrogels with microfluidic networks to create a pseudo-osmotic capillary pump that can deliver continuously sweat samples to embedded sensors. Work will also be performed to create body hydration levels based off of sweat intake in the hydrogels.