Application of Magnetoelastic Thick Films for Wireless, In Vivo Monitoring of Pressure at Abdominal Aortic Aneurysm

According to the National Center for Health Statistics, cardiovascular diseases remain the number one killer in the United States. Among the various types of cardiovascular diseases, the aortic aneurysm is ranked number nine. The abdominal aortic aneurysm (AAA), in particular, is an abnormal, localized dilation of the abdominal aorta wall caused by weakened or diseased aorta walls. One of the treatments for this disease is using an endovascular surgery at which an endovascular graft is delivered to the aneurysm site through the femoral arteries. The deployment of the endovascular graft will exclude blood flow to the aneurysm, thus preventing further expansion of the aneurysm sac. Although this technique is preferred over open surgeries due to its minimal invasiveness, an event known as the endoleak, where the endovascular graft fails to retain the blood and leads to leakage to the aneurysm sac, may occur. Here, we are developing a novel pressure monitoring system to remotely and continuously measure the pressure in the aneurysm sac. The main component of the system is a pressure-sensitive material, made of a magnetoelastic, magnetically soft film attached or coated on the endovascular graft. When under an AC magnetic field (excitation field), the magnetoelastic film generates a secondary magnetic field. Due to its magnetoelastic property, the amplitude of the secondary field varies with applied stresses, allowing remote pressure monitoring. To eliminate noises from the excitation field, the generated secondary field is measured at twice the excitation frequency to obtain the 2nd-order harmonic field, which is used for tracking the pressure variations. A scaled-up prototype of the pressure monitoring system was constructed and examined to demonstrate the feasibility of this technology. A commercial magnetoelastic thick film, Metglas 2826MB from Metglas, Inc., was attached on a polycarbonate substrate and covered by a thin polycarbonate protective layer. The substrate was then embedded in a plastic tube with flowing liquid to represent the condition of an aorta. Liquid pressure in the tube was altered during the experiment by restricting or relaxing the flow channel. In this study, a $10mm×40mm$ file (Film A) and a $5mm×40mm$ film (Film B) were fabricated and tested. The amplitude of the 2nd-order harmonic field produced by the films was inversely proportional to the fluid pressure. It was also shown that films with different sizes exhibited different signal sensitivity with the smaller film (Film B) exhibited greater sensitivity. This experiment indicates that feasibility of the pressure monitoring technology.