UC San Diego

Flexible and wearable electronics have become ubiquitous since the introduction of the first epidermal electronics back in 2011. Since then, many sensors various active materials have been developed to monitor various human electrical signals such as through electrocardiography (ECG), electrocorticography (ECoG), electroencephalography (EEG), and electromyography (EMG). These sensors usually typically are built in either thin (standard CMOS processes) or thick film technology (printing techniques). In thin film technology, gold and platinum often are chosen as the material of choice due to its ease in processing. Similarly, in thick film technology (printing), there is a larger selection of materials available, such as carbon black, carbon nanotube, and silver/silver chloride, to name a few.

While typical metals such as gold and platinum suffice for recording of most human electrical signals (higher frequency range), it is not ideal for the recording of the stomach electrical activity via electrogastrography (EGG), which is mostly at the low frequency region (~0.05 Hz). At low frequency (near DC), those metallic materials are polarizable in nature, thus very susceptible to DC noise. Further, the high electrode-skin impedance due to low ion mobility will form a barrier for the biopotential waveform to cross. In this dissertation, I explore ways to marry techniques from thin and thick film technology, resulting in an adhesive-integrated flexible and wearable sensor array with very thin form factor and able to leverage Ag/AgCl as its active electrode's material.

In Chapter 1, I successfully explore this novel technique to yield an electrode array for the recording of electrogastrography (EGG) in healthy human subjects. The electrode array is successful in capturing the electrophysiological signal at the frequency domain associated with the stomach activity.

In Chapter 2, I conduct an electrochemical performance characterization study of the flexible electrode across multiple diameters, materials, and frequency ranges. The data are then fitted with a custom circuit model that describes the electrode/electrolyte behavior. This chapter will pave a foundation for future systematic design of electrode arrays by optimizing their properties for various biopotential recording applications.

In Chapter 3, I successfully explore the recording of cervical neuronal activities associated with parasympathetic and sympathetic nervous system using the custom flexible electrode array that attaches to the location on the neck where vagus nerve is approximately located under it. This work successfully presents a non-invasive neuronal recording technique that has not been explored in human subjects (previous work was only done on a rat model).

In Chapter 4, I successfully design a multi-electrode array that is capable of recording electromyography (EMG) activity from the human bladder muscles. This work hopefully will pioneer the breakthrough in non-invasive urodynamics studies to address the drawbacks of current urodynamics approaches. The drawbacks include invasive monitoring via an indwelling catheter, discomfort or pain to the patient, a risk of urinary tract infection (UTI), and a risk of artifact due to filling of bladder at supra-physiologic rates.

While my approach of constructing flexible and wearable multi-electrode array has been successful in monitoring several human electrophysiological signals in healthy subjects, further validations through larger clinical trials are needed to prove adoption and clinical usefulness among larger patient population. Regardless, this body of work represents a critical step towards clinical non-invasive measures of human physiology.