In vivo wireless readings from a bile duct stent sensor in a pig

New magnetoelastic sensor within a stent transmits information about potential blockages, demonstrating potential use for a wide range of body sizes.

A wireless monitoring system successfully communicated with a sensor embedded into a bile duct stent placed in a pig, providing the first in vivo results of the technology which could improve timely interventions for blockages, according to a University of Michigan study recently published in Nature Microsystems & Nanoengineering.

When the ducts that carry bile—a fluid that helps digestion—become obstructed, a common solution is stent placement within the duct by an endoscopic procedure called endoscopic retrograde cholangiopancreatography. Over time, bacterial “sludge” or stones can block these bile duct stents, causing jaundice, bacterial infections or even life-threatening sepsis if not treated urgently by antibiotics and stent replacement. 

Left: A graphic of the liver, gallbladder, pancreas, and duodenum with the bile duct connecting them. Middle: An arrow points from the bile duct to zoom into the biliary stent positioned inside of the duct with the polymer package and sensor attached to the outside. Right: A schematic of a human with the belt-like detector placed around the waist. The edges of the belt are purple with an associated flow chart that reads: sinewave generator, buffer amplifier, transmit coils. The inside of the belt is labeled blue with an associated flow chart that reads: receive coil, low noise amplified, protection circuit, acquisition & DSP.
The biliary stent monitoring system. A) Schematic demonstrating where the magnetoelastic sensor integrated into the bile duct stent is positioned. B) Schematic of the belt-like detector placed around the waist used for wireless communication with the sensor. Credit: Nambisan et al., 2024.

Currently, medical providers monitor biliary stent blockages indirectly through blood tests. Placing a sensor within the stent will allow direct, real-time measurement of sludge accumulation, allowing for more timely interventions and better patient outcomes.

“This novel stent sensor provides the opportunity to detect impending biliary stent obstructions without waiting for clinical symptoms, blood tests or imaging tests, all of which delay intervention,” said Richard Kwon, a clinical professor of internal medicine and gastroenterology at the University of Michigan Medical School and co-author of the study.

The research team developed a 8 mm long sensor, a little less than half the diameter of a penny, and only 1 mm wide, encapsulated in a 3D printed polymer structure for protection and seamless integration into the stent. 

A ridged, rectangular, plastic piece labeled “polymer package” propped up against the tip of a standard pencil, about the same size. An arrow points to “sensor”: four small, film-like rectangles positioned at the plastic piece’s four corners. An inset shows the film-like sensor on a penny, less than half its diameter.
The magnetoelastic sensor encapsulated in the 3D printed polymer structure. The inset shows the standalone sensor on a penny for scale. Credit: Nambisan et al., 2024.

After the procedure, information is read from the stent by placing a belt-like detector around the waist that emits a magnetic field. The sensor, which is batteryless, operates by changing resonant frequency in response to the mass of any blockage accumulated on the sensor, and wirelessly responds to the applied magnetic field using a property known as magnetoelasticity. The detector design aims to provide comfortable, non-invasive blockage monitoring for patients which would be done at checkups about every three months after stent placement.

The design of the hardware and digital signal processing techniques helped boost the signal-to-noise ratio to be greater than 1,000,000 to 1, overcoming the hurdle of reduced signal imposed by miniaturization and placement within a fluid environment.

“The high value measured for the signal-to-noise ratio at 17 cm interrogation distance indicates that in the future the readout distance can be greatly increased as necessary for humans, accommodating differences in anatomy,” said Yogesh Gianchandani, a professor of electrical engineering and computer science at U-M and senior author of the study. 

In addition to increasing communication range, the design reduces signal feedthrough—ensuring the sensor signaling and receiving ends do not cross over one another. This is done through time domain decoupling, where one end is suspended while the other is working and vice versa.

“Successfully receiving signals from a live animal marks a major advancement in low profile, batteryless magnetoelastic sensor technology, paving the way for new and expanded applications,” said Ramprasad Nambisan, a doctoral graduate of electrical engineering and computer science at U-M and lead author of the study.

To continue developing this technology, the researchers plan to develop a version that works with metal stents. In the longer term, they will further miniaturize the sensor, allowing several sensors each with a different resonant frequency distributed along the stent, allowing for localized detection of sludge accumulation.

Developing more inexpensive electronics for the belt-like detector will facilitate more animal trials and ultimately clinical trials in humans. 

As the technology continues to advance, magnetoelastic sensors have the potential to be used in other places in the body including peripheral vascular stents, long-term coronary stents and ureteral stents.

This research was funded in part by the National Institutes of Health (R01DK102663).

Full citation: “A microsystem for in vivo wireless monitoring of plastic biliary stents using magnetoelastic sensors,” Ramprasad M. Nambisan, Scott R. Green, Richard S. Kwon, Grace H. Elta, and Yogesh B. Gianchandani, Nature Microsystems & Nanoengineering (2024). DOI: 10.1038/s41378-024-00772-8