System Level Challenges for Microfabricated Magnetoelastic Transducers in Implantable Biomedical Applications

Abstract

Magnetoelastic devices have potential utility as sensors and actuators in implanted biomedical applications because of their inherent passive wireless nature which avoids the need for batteries and simplifies the implanted module. This work focuses on addressing system level challenges in two applications, one requiring an actuator and one a sensor. The first application is mitigation of glaucoma valve encapsulation; challenges addressed in this application include the integration of an actuator onto the valve and especially the development of an acoustomagnetic interrogation module for verifying actuator functionality. The second application is biliary stent monitoring; challenges include protection of the sensor during and after deployment, and development of an interrogation module with vastly increased wireless range. An Ahmed glaucoma valve (AGV) is used in the treatment of glaucoma to drain fluid from the eye and decrease the elevated intraocular pressure. However, adhesion of cells and encapsulation of the AGV often lead to its failure. The magnetoelastic system described in this work consists of a magnetoelastic actuator integrated onto the AGV that can agitate the nearby liquid and potentially mitigate encapsulation, and an interrogation module to excite the actuator. The interrogation module addresses the signal feedthrough and clinical utility challenges by implementing physical domain decoupling and tailored signal processing. The in situ experiments with the system resulted in the first recorded acoustic signatures from a magnetoelastic sensor in an implanted environment; the measured signal-to-noise ratio was 3-6. In vivo experiments performed on live rabbits along with the in situ results indicate that the magnetoelastic system does not adversely affect the health of the animal and can feasibly provide sufficient wireless range and actuation amplitudes after implantation. A magnetoelastic system for biliary stent monitoring consists of a magnetoelastic sensor integrated into the stent, and an interrogation module to communicate with the sensor. The change in the resonant response of the sensor by mass loading and viscosity shifts due to sludge accumulation can diagnose early occlusion and allow timely intervention. A self-biased sensor is designed and fabricated, decreasing the footprint considerably while preserving its resonant frequency. In vitro experiments mimicking sludge accumulation were performed, and the ratio of the shift in resonant frequency to the quality factor was found to be well-correlated with occlusion. The receiver operator characteristic of this parameter indicated an accuracy of 97.84% for a detection threshold of 50% decrease in flow rate through the stent due to occlusion. To protect the sensor during and after endoscopic deployment, a Nitinol reinforced polymer hybrid package is designed and used. In vitro tests utilizing bile-resistant bacterial cultures were conducted to evaluate the effect of the packaged sensor itself on the stent occlusion dynamics. Non-inferiority statistical tests performed on the results indicate that the instrumented stent is not inferior to the normal stent in terms of the occlusion time (p-value < 0.05). The implementation of the interrogation module addresses challenges, including wireless range, signal feedthrough, and clinical utility, and these are solved in part by time domain decoupling and signal processing techniques. The complete magnetoelastic system was successfully demonstrated in vivo, with the sensor-integrated stent implanted in the bile duct of a live pig at a wireless range of ≈17.15 cm with a signal to noise ratio ≈106. These are the first reported signals from a stent-integrated magnetoelastic sensor implanted in a live animal.