Abstract

Wrist-wearable bioimpedance systems have potential applications in continuous, noninvasive health monitoring, including monitoring of tissue hydration and glucose-related physiological changes. Such systems can benefit from miniaturized dry electrode arrays that are compatible with flexible substrates. In this dissertation, miniaturized carbon electrode arrays were fabricated using carbon-nanotube-templated microfabrication (CNT-M) and used for broadband bioimpedance measurements on the wrist. To support reproducible CNT-M fabrication, a metrology study was also performed to develop a spectroscopic-ellipsometry-based method for measuring the ultrathin iron catalyst films used for carbon nanotube (CNT) growth. Because iron catalyst thickness strongly affects CNT growth, a metrology method for ultrathin iron catalyst films was developed using spectroscopic ellipsometry. Multisample analysis, interference enhancement, and a consecutive-layer measurement approach were used to characterize thermally evaporated iron films with target thicknesses from 1 to 14 nm. The partially oxidized iron films were represented with an optical model, from which the iron thickness before oxidation—termed the equivalent iron thickness—was estimated. The method was highly sensitive to equivalent iron thickness and produced repeatable measurements with a variation of approximately ±0.015 nm. Variations measured across different locations on a sample were consistent with uncertainty propagated from the thickness of the underlying alumina layer. The resulting method provides a rapid, nondestructive means of measuring ultrathin iron catalyst films. Second, a process was demonstrated for fabricating miniaturized carbon electrode arrays using CNT-M and integrating the arrays with flexible printed circuit boards. CNT forests were grown from patterned iron catalyst films and infiltrated with carbon to form solid electrode structures 300–600 µm tall. Cross-sectional scanning electron microscopy showed relatively complete carbon infiltration. The electrode arrays were aligned and transferred to contact pads on flexible printed circuit boards using an anisotropic conductive film adhesive, which formed both the mechanical attachment and the electrical connection. The alignment error was approximately 100 µm, and the connection resistance from individual electrodes to their corresponding connector contacts was typically below 10 Ω. Resistance measured between adjacent electrode connections exceeded 40 MΩ. Five completed arrays were used in multiple test sessions with human subjects without observed functional degradation. Finally, the completed dry carbon electrode arrays were used for broadband bioimpedance measurements across the human wrist over the radial artery. A vector network analyzer performed repeated frequency sweeps from 300 kHz to 200 MHz during 45-min measurements on two human subjects. Two electrode sizes were compared under a controlled applied pressure of 3.0 kPa. The larger electrodes produced lower impedance magnitudes, although the reduction was less than proportional to the increase in electrode area. The measured impedance changed rapidly after electrode application and gradually stabilized over time, consistent with acclimation at the electrode–skin interface. Acclimation-related changes were generally largest below 1 MHz and much smaller above approximately 10 MHz. These results show that time-dependent impedance changes associated with dry-electrode acclimation are reduced at higher frequencies. Together, these studies establish methods for ultrathin iron catalyst metrology, fabrication of miniaturized carbon CNT-M electrode arrays, and broadband characterization of reusable dry electrodes on the wrist, supporting the development of future wrist-wearable bioimpedance systems for continuous, noninvasive health monitoring.

Degree

PhD

College and Department

Computational, Mathematical, and Physical Sciences; Physics and Astronomy

Rights

https://lib.byu.edu/about/copyright/

Date Submitted

2026-08-14

Document Type

Dissertation

Keywords

Bioimpedance, wearable sensors, microfabrication, carbon nanotubes, thin films, spectroscopic ellipsometry

Language

english

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