A microsystem for in vivo wireless monitoring of plastic biliary stents using magnetoelastic sensors

Abstract

With an interest in monitoring the patency of stents that are used to treat strictures in the bile duct, this paper reports the investigation of a wireless sensing system to interrogate a microsensor integrated into the stent. The microsensor is comprised of a 28-μm-thick magnetoelastic foil with 8.25-mm length and 1-mm width. Magnetic biasing is provided by permanent magnets attached to the foil. These elements are incorporated into a customized 3D polymeric package. The system electromagnetically excites the magnetoelastic resonant sensor and measures the resulting signal. Through shifts in resonant frequency and quality factor, the sensor is intended to provide an early indication of sludge accumulation in the stent. This work focuses on challenges associated with sensor miniaturization and placement, wireless range, drive signal feedthrough, and clinical use. A swine specimen in vivo experiment is described. Following endoscopic implantation of the sensor enabled plastic stent into the bile duct, at a range of approximately 17 cm, the signal-to-noise ratio of ~10⁶ was observed with an interrogation time of 336 s. These are the first reported signals from a passive wireless magnetoelastic sensor implanted in a live animal.

Fig. 1. The biliary stent monitoring system.

a Schematic of the magnetoelastic sensor integrated with the biliary stent for detecting occlusion. b Schematic of the interrogation module used for wireless communication with the sensor

Fig. 2. In vivo experiment.

a Interrogation in progress, and the coils are placed like a belt on the swine specimen. b The first reported signals from a magnetoelastic sensor implanted in a swine specimen. c Fluoroscopic image of the stent after implantation in the swine. d Sensor response in vivo compared to the response on the benchtop in air and water

Fig. 3. Extended in vivo experiments.

a An inflated bag issued to mimic the increased waistline of a larger patient. b Sensor response for increased coil diameters mimicking patients with a larger abdomen, interrogation time 336 s. c Sensor response in vivo with misaligned coils and with an axial offset ~5 cm, interrogation time 340 s

Fig. 4. Custom sensor packaging and stent integration.

a Magnetoelastic sensor in a package, inset shows the standalone sensor. b Schematic of the packaged sensor integrated onto the inner wall of the stent. c Stent integrated with the packaged sensor ready for implantation

Fig. 5. Signal integrity, conditioning, and processing.

a Wireless range and feedthrough challenge associated with an interrogation module and time domain decoupling approach. b Schematic of digital signal processing techniques implemented in Labview^TM. c Signal, and d noise before and after all digital signal processing steps