Hydrogel and Microfluidic Enabling Technologies for Wearable Biomonitoring Devices: Sweat and ECG Sensing.

SHAY, TIMOTHY WILLIAM . Hydrogel and Microfluidic Enabling Technologies for Wearable Biomonitoring Devices: Sweat and ECG Sensing (under the direction of Dr. Orlin D. Velev and Dr. Michael D. Dickey) Wearable devices in healthcare can greatly benefit from the development of new microfluidic sampling methods for sensing biomarkers in sweat. Sweat contains many useful bioindicators that could enable traditional blood based testing techniques in a non-invasive wearable device. Current commercial wearable devices are limited to a combination of motion and heart rate sensors, which provide little insight to the health of the user. Several research groups have shown that sweat sensing patches can be used to analyze the physiological status of the user, but they have the drawbacks that they only work during periods of high sweat rate and cannot manage the flow of fluid to direct it away from their sensors after testing, resulting in higher error over time. This thesis will present new microfluidic technologies that may enable the next generation of wearable biosensors. The goal of our research is to harness the osmotic properties of hydrogels with a paper microfluidic network to enable continual sweat sensing. These technologies will solve issues related to sensing during low sweat rate and fluid management. We first describe hydrogels that are doped with glycerol or sodium chloride to create a high osmotic pressure in order to pump sweat from the body, mimicking the sweat gland itself, and into our device. Flowrate control and sensing were demonstrated for this pumping mechanism in a lab setting. We then investigated evaporation as a means to continually drive fluid flow through a paper microfluidic network. Evaporation off of the back end of a microfluidic device enables long-term operation of a sweat collection device. Flow control and sensing modalities were demonstrated for this platform. We show that even with the accumulation of salt due to evaporation, we can still achieve pumping for durations of up to 10 days. These two technologies, osmotic hydrogel pumping and paper based evaporative pumping, were integrated into a single device to be tested on human subjects. During periods of high sweat rate, the paper microfluidic strip acts as a wicking material to draw sweat from the skin and pass it through a microfluidic channel. During periods of low sweat rate, the osmotic properties of the hydrogel draws fluid from the body, which can then be wicked through the paper microfluidic channel. This microfluidic pumping method ensures the continual sampling of fresh sweat with minimal mixing before sensing has occurred. We present initial results that show the feasibility of this concept on a proof of principle scale. We also introduce possible extensions or modifications of these concepts that may further develop the field of wearable sweat sensing devices. We will also show how this same hydrogel interface can also be used as a soft electrode to perform ECG measurements on the user. The hydrogel can be made conductive through the addition of ions. We can then implement the use of a liquid metal to create our electrodes, resulting in a device comprised entirely of soft materials. The electrical properties of this system were first tested through impedance spectroscopy. Working prototypes were then created and tested on human subjects, showing better performance than commercial electrodes. The technologies introduced in this thesis will have a large impact on the future of wearable non-invasive biosensing devices. We have presented and tested prototype devices on a proof of principle level that perform passive sweat collection and management. Noninvasive continual biochemical sensing is a field of untapped potential and will enable patients to more closely monitor serious medical conditions. These devices will ultimately help patients better monitor their own bodies, which will result in better health for the individual.