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Journal Club: Yin et al, 2018

Water‐jet outer sheath with braided shape memory polymer tubes for upper gastrointestinal tract screening

Linkun Yin | Shuxin Wang | Siyang Zuo
International Journal of Medical Robotics and Computer Assisted Surgery (IF 1.634)
DOI: 10.1002/rcs.1944

The authors presented a novel class of endoscope based on braided shape-memory polymer (SMP) tubing and water-jet steering, with the intention to create a cheap and disposable gastroscope. The presented prototype features an insertion length of 250mm, largest outer diameter of 16mm, a central cooling water channel, and six water jet nozzles arrayed around the distal tip, as shown below. In operation, the gastroscope is inserted along the usual esophageal pathway; in a larger cavity such as the stomach it can be steered by allowing the force reaction of its six water jets to deflect the tip in the desired direction. Once in position, the internal volume of the endoscope is flooded with cooling water, causing the endoscope shaft to become rigid, allowing the gastroscope to remain in place without having to use the water jet system. Figure 3 of the paper shows an overview of the system’s layout.

The SMP material used in the construction of the braided water tubes can dramatically vary its Young’s modulus with temperature, from ~2450 MPa in its glassy state, to just ~2.3 MPa in its rubber-like state. By comparison, this is like an ABS-like plastic (1400-3100 MPa) turning into something like an unreinforced nitrile rubber (1.8-2.1 MPa). This dramatic change occurs around 35°C, meaning that inside the human body the material tends to the rubber-like state without additional cooling, and the steering jets are fed using water at 40°C. When the system is to become rigid, the internal cooling channel floods the space between the SMP tubing and the outer sheath with water chilled to 5°C, reverting the SMP to its rigid state. Figure 2 of the paper shows a plot of the material’s Young’s modulus vs its temperature.

The authors report a response time of 15 seconds for the SMP tubing to reach 5°C and 10 seconds to reach 10°C, as well as a heating response time of 12 seconds. The maximum reported bending angle was 57° using a water jet pressure of 0.22 bar.

Evaluation

The prototype was evaluated in a hemispherical phantom and compared to an Olympus GIF-2TQ260 gastroscope in five trials; users were asked to identify five red foam targets inside the phantom using the olympus scope and then using the authors’ prototype. The mean time to identify all targets and standard deviation were published by the authors. A gastroenterologist requiring 80s to identify the targets using a conventional endoscope (with an SD of 7s) took 411s (SD 9s) with the authors’ prototype, although after some training (the nature of which is unspecified in the paper) this fell to 282s (SD 13s).