Wireless Magnetoelastic Monitoring of Biliary Stents.

Abstract

This thesis describes a wireless magnetoelastic system for monitoring the mass accumulation and viscosity changes that accompany the progressive occlusion of biliary stents. The system comprises four components: a biliary stent, a magnetoelastic sensor, a biasing magnet, and an external interrogation module. Design considerations for each component are highlighted and predictive analytical and finite element tools are developed. Two system generations are detailed. The first-generation system utilizes a 37.5 mm x 2 mm ribbon sensor and neodymium magnets to bias the sensor. The second-generation system comprises a patterned 7.5 mm x 29 mm sensor and a strontium-ferrite-polydimethylsiloxane magnetized layer that conform to the stent topology and curvature. Both generations utilize photochemical machining for batch-patterned stents. A resonant frequency change of 2.8% and 6.5% for each respective generation is observed as the viscosity is varied from 1-10 cP. Resonant frequency response to mass loading is similar for both generations, showing a 40% decrease after applying a mass load of 2.5x the mass of the sensor. Advanced sensor designs are also detailed, including a sensor that can evaluate the sludge distribution along the stent length with a resolution of 5 mm, a sensor with varying feature density resulting in a sensitivity increase of 2x compared to other designs, and a hybrid-ribbon sensor that has a full-scale range of at least 10x the mass of the sensor. Finally, an in situ experiment is conducted with a system implanted into a porcine carcass, demonstrating a wireless range of 5 cm. This work has investigated the application of wireless magnetoelastic resonant sensors to monitoring of biliary stents. Methods for interpreting the sensor response to viscous and viscoelastic loads have been demonstrated. In addition, the magnetoelastic sensor performance has been pushed to new limits in terms of full-scale range and mechanical robustness. Advances include development and preliminary experimental verification of modeling tools for resonant magnetoelastic sensors, tailoring of magnetoelastic sensor geometry for enhanced functionality, and demonstration of photochemical machining as a viable process for batch-patterned sensors and stents. Importantly, this work illustrates the long-term potential for passive, wireless magnetoelastic systems for monitoring progressive occlusion of stents.