Wireless magnetoelastic monitoring of biliary stents

This paper presents a system for wirelessly monitoring the accumulation of sludge in a biliary stent. Two generations of the system are detailed. The first-generation system utilizes a 2 x 37.5-mm ribbon sensor with a mass of 18 mg, along with 0.8-mm-thick x 1.6-ram-diameter neodymium magnets to bias the sensor. Both components are integrated with a 4-mm-diameter stainless steel stent. The second-generation system comprises a sensor and a magnetic layer [consisting of strontium ferrite particles suspended in polydimethylsiloxane (PDMS)] that conform to the meshed topology and tubular curvature of a 5-mm-diameter Elgiloy stent. The second-generation sensors have an active area of 7.5 x 29 mm and a mass of 9.1 mg. The sensors in both generations are fabricated from 28-[micro]m-thick foils of magneto-elastic 2826MB Metglas, an amorphous Ni-Fe alloy. Analytical and finite-element models that predict sensor response in the dynamic biological environment are presented. The response of each system to viscosity changes that precede and accompany biliary sludge accumulation is tested, with resonant frequency changes of 2.8% and 6.5% over a 10-cP range for each respective generation. Sludge accumulation is simulated with successive coatings of either paraffin or an acrylate terpolymer. Resonant frequency response to this mass loading effect is similar for both generations of the system, showing a 40% decrease after applying a mass load of 2.5 x the mass of the sensor. [2008-(1154] Index Terms--Elgiloy, magnetoelasticity, Metglas, microsensors, photochemical machining, resonant sensors.