Electronic Theses and Dissertations

Date

2018

Date of Award

2018

Document Type

Dissertation

Degree Name

Doctor of Philosophy

Department

Electrical & Computer Engineering

Committee Chair

Bashir Morshed

Committee Member

AMY L. DE JONGH CURRY

Committee Member

Eddie Jacobs

Committee Member

Madhusudhanan Balasubramanian

Abstract

Biopotential signals such as electroencephalography (EEG), electrocardiography (ECG or EKG), electrooculogram (EOG), and electromyography (EMG) play vital roles in health and clinical diagnoses, monitoring, and therapy. In addition, these signals are required for many nonclinical applications such as Neurofeedback and Brain-Computer Interface (BCI). The quality of the measurement relies on the electrical and mechanical properties of the electrode. Conventional wet or gel impedimetric electrodes provide an excellent signal due to the conductive fluids or gel, which reduces the skin-contact impedance and maintains contact during movement. However, they operate for a short duration; the quality of the signal degrades due to the fluid or gel drying out. Dry electrodes promise the ability for long duration sensing and avoiding the drawbacks of the wet/gel electrodes. However, dry electrodes suffer from high interfacing impedance. In this work, dry electrodes based on Carbon Nanotube (CNT) are presented. The CNTs were fabricated in a Vertical Pattern (pvCNT) on a circular stainless steel foil substrate with a diameter of 10 mm and thickness of 2 mils. The pattern on the substrate was developed with a custom shadow mask using sputter coating with Al2O3 and iron. Electrically conductive multi-walled CNTs were grown in patterned pillar formation with a square base of 100 m each side, with an inter-pillar spacing of 50, 100, 200 and 500 m, and heights between 1 to 1.5 mm. The impedances of the electrodes were 1.92, 3.11, and 8.15 for 50, 100, and 200 m spacing, respectively. A comparative in vitro study with commercial wet and gel electrodes showed pvCNT electrode has lower interfacial impedance for a long-term period, comparable signal capture quality, and ability to be used for stimulation. Coating the electrodes by a conductive polymer (Polypyrrole or PPy) is used to improve the mechanical properties of the CNTs. The coating procedure involved applying 10 L of PPy after preparing the pvCNT with 70% ethyl alcohol solution and flash drying at 300C. The impedance of the coated version of pvCNT has slightly increased compared to the non-coated pvCNT version and stayed lower than the electrical impedance of the commercial electrodes. Mechanical tests showed the PPy coated pvCNT has stronger adhesion to the Stainless Steel (SS) substrate. The results demonstrate the feasibility of coating pvCNT dry electrodes with PPy for robustness. Furthermore, we explore the use of this dry electrode for wearable ECG sensors. Fully passive sensors, which are zero power and battery-less, can make wearable devices more practical by eliminating the contact wires subsequently decreasing the weight, costly and high-maintenance batteries. We have previously developed a novel technique of Wireless Resistive Analog Passive (WRAP) sensor based on resistive transducers. The scanner transmits an RF signal at an ISM frequency band (e.g. 8.37 MHz) which is amplitude modulated based on the resistive changes by a transducer. The modulated signal is then captured and analyzed on the scanner or downstream on the users smartphone. In this work, we have proposed a novel conjugate coil pair technique for WRAP sensors and demonstrated the capability for differential signal capture, such as electrocardiogram (ECG or EKG). The WRAP ECG sensor uses an N-channel dual-gate MOSFET (depletion mode) to convert the biopotential signal to the correlated resistive variation of RSD (Source to Drain Resistance). The system was able to cancel the common mode signal and only transmitted differential mode signals. The results show that connecting a pair of sensors in this way could allow accurate measurement of a differential biopotential. This work demonstrates voltage sensitivity down to 40 V towards realizing a battery-less, body-worn WRAP ECG sensor for monitoring ECG signals while the signal is collected using pvCNT electrodes.

Comments

Data is provided by the student.

Library Comment

Dissertation or thesis originally submitted to ProQuest

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