Biliary strictures, narrowings of the bile ducts, are treated with endoscopic placement of plastic biliary stents. However, these stents frequently become occluded with sludge (30–75% within 12 months), leading to serious complications like cholangitis and sepsis. Current methods for detecting stent occlusion rely on indirect indicators like elevated liver enzymes or imaging, which are often late and imprecise. Early detection is crucial to prevent complications. This research proposes a novel wireless monitoring system using a magnetoelastic sensor integrated into the stent. The sensor, based on changes in resonant frequency and quality factor due to sludge accumulation, provides a direct measure of stent patency. Magnetoelastic materials offer a suitable transduction mechanism, which has been used in other applications, but needs to be adapted to the in-vivo environment. This paper focuses on the challenges associated with miniaturizing the sensor, ensuring wireless communication, minimizing signal feedthrough, and designing a clinically viable system.

Existing methods for detecting biliary stent occlusion are indirect and often reveal problems only after significant blockage has occurred. Elevated liver enzymes and imaging techniques like ultrasound or magnetic resonance imaging (MRI) detect downstream effects of the occlusion rather than the blockage itself. These methods are not always timely, and thus do not prevent the need for emergency intervention and hospitalization. The current study builds upon prior work by the same team, demonstrating the concept of monitoring viscous mass accumulation with a wireless magnetoelastic sensor. Previous benchtop experiments showed a correlation between mass accumulation and changes in the sensor's resonant frequency and quality factor. This study aims to translate these promising results into a practical, in vivo system.

The developed microsystem consists of a magnetoelastic sensor (28 µm thick, 8.25 mm long, 1 mm wide) with attached permanent magnets for biasing. The sensor is packaged within a 3D-printed polymer structure to protect it during endoscopic retrograde cholangiopancreatography (ERCP) deployment. The interrogation subsystem uses electromagnetic excitation and detection via transmit and receive coils. Time domain decoupling minimizes drive signal feedthrough. The system's design addresses challenges related to sensor miniaturization, placement within the stent for compatibility with ERCP instruments, wireless range, signal strength, signal-to-noise ratio (SNR), and clinical usability. The in vivo experiment involved implanting the sensor-enabled stent into the bile duct of a swine model. The transmit and receive coils were placed around the animal, and the sensor response was measured. Additional experiments investigated the effect of coil size and alignment on signal quality. Benchtop characterization assessed the impact of sensor orientation and position relative to the coils on signal strength. The animal test protocol was approved by the University of Michigan IACUC.

The in vivo experiment yielded a signal-to-noise ratio (SNR) of approximately 10⁷ at a range of ~17 cm with an interrogation time of 336 seconds. This represents the first successful wireless interrogation of a magnetoelastic sensor implanted in a live animal. Comparison of in vivo and benchtop data showed a decrease in the quality factor and resonant frequency in vivo, attributable to the higher viscosity of bile compared to water. Experiments simulating larger patient sizes and coil misalignment demonstrated the system's robustness, with only minor changes in resonant frequency and SNR despite increased coil size and axial offset. Benchtop tests demonstrated sensitivity to sensor orientation and positioning, indicating the importance of careful coil placement during interrogation. Prior unsuccessful attempts highlighted the importance of robust packaging and a sensitive interrogation system for successful in-vivo deployment.

The successful in vivo demonstration of the wireless magnetoelastic sensing system opens pathways for real-time, outpatient monitoring of biliary stent occlusion. The high SNR at a considerable wireless range suggests potential for further sensor miniaturization and the creation of sensor arrays for localized detection of sludge accumulation. Future work includes developing a more cost-effective and portable interrogation system suitable for broader animal trials and eventual human clinical trials. The technology's adaptability to other stent types (urethral, arterial) and potentially even metal stents holds promise for replacing invasive diagnostic procedures. The ability to map the progression of stent occlusion could greatly improve clinical research efficiency and our understanding of this condition.

This study successfully demonstrated the first in vivo wireless interrogation of a magnetoelastic sensor integrated into a biliary stent. The high signal-to-noise ratio and reasonable range achieved, even with variations in coil size and positioning, pave the way for developing a clinically viable system to improve the management of biliary stent occlusion. Future research focuses on miniaturization, cost reduction, and clinical translation. Ultimately, this technology could lead to more timely interventions, reduced complications, lower costs, and better patient outcomes.

The study's in vivo experiments were conducted on a limited number of swine, which may not fully represent the variability observed in human anatomy. The current interrogation system uses research-grade hardware and requires further development to create a clinically practical and cost-effective device. The long interrogation time of 336s might be considered a limitation that needs to be reduced for clinical use. The study did not directly assess the accuracy of the sensor in detecting early stages of sludge accumulation. Future studies should address these limitations.