(H-285) Intra-Catheter String Array Wrapping to Regulate Circumferential Bending

Cardiovascular devices

Multi-layered tubular devices are increasingly utilized across the spectrum of medical technologies. Examples span from endotracheal tubes, to endoscopy tubes, gastrointestinal drains, and endovascular catheters. Many of these tubular devices utilize embedded biomaterials that are interwoven into the tubular wall biomaterials. These interwoven biomaterials are composed of an array of plastics or metals, and can be placed in various different orientations such as longitudinal, circular, braided, or spiral configurations [1]. A limitation of current technology is the ability to precisely distribute these interwoven components in multi-layered tubular medical devices to help regulate the circumferential distribution of bending, maneuverability, and advancement of devices to facilitate a desired medical therapy [2]. For multi-layered devices, it is important to consider the surface contact between the materials and their impact on its mechanical characteristics [3]. The Multiflex catheter outer lumen encases an array of steel wires; when a vacuum is induced between the multiple layers, the inner and outer tubes cinch against the wire array and the catheter becomes rigid (Figure 1). Using a multifaceted testing approach, we evaluated different expanded polytetrafluoroethylene (ePTFE) wrapping configurations on their ability to secure the string array in the multi-layer, which is a key feature of the catheter that influences variable flexural rigidity (Figure 2). We hypothesized that further interweaving biomaterials in the multi-layer and along the catheter’s unique string array can further impact the catheter’s ability to transition from flexible to rigid states, and improve the bending distribution of the catheter along its longitudinal axis.

Three-point bend testing was conducted to determine the flexural rigidity along the length of each catheter in their flexible and rigid state. Marks were placed in 5 centimeter increments starting from the distal end of the catheter to determine the load cell location during testing. This allowed for a regulated recorded force to be applied and produce a force-displacement assessment along each catheter’s length.

Shape-hold testing mimics the catheter angle as it passes an acute turn in the vasculature, such as the aortic arch or the abdominal aortic bifurcation, but it excludes outer barriers to allow for catheter deformation. Based on previous testing results, a line was identified to serve as the axis for angular deflection measurement. Located on the right side of the apparatus, this zone is where the catheter position changes the most because of the steep increase in curvature at the barrier outlet. The catheter was conformed to the U-shaped apparatus while it is in its flexible state, and then actuated to become rigid. This served as the initial state for the angle of deflection. When an endovascular wire was fully passed through the catheter endoluminal length it served as the final state for the angle of deflection (Figure 3). Five guidewires of increasing flexural rigidity were passed through each of the catheters. The difference in angular deflection of the states was measured using imageJ and plotted relative to wire stiffness.

Catheters 1 and 2 recorded the highest flexural rigidity maxima and minima. Conversely, Catheter 3 recorded the lowest flexural rigidity range in both states (Figure 4), demonstrating a relationship between the wrapping style and effectiveness of each state.

Catheters 1 and 3 showed similar results despite their difference in wrapping style. Catheter 2 shifted up on the graph because of its ability to resist deflection and maintain its bent shape (Figure 5). This correlates with the three-point bend test results, but there are other factors impacting flexural rigidity. We were able to derive an equation that provided a baseline for our observed shape-holding results. When plotting this equation with our results, we observed two distinct plateaus in the data collected for each prototype when wires with a flexural rigidity value of 10 N cm2 or less were passed through , representing two different mechanisms that impacted their shape holding ability.

The mechanisms that cause these plateaus may be explained by a model similar to the three-point bend test; The force needed to deform the string array is determined by the space between two points of wrap contact, with an increase in wrap spacing decreasing the force. Once the strings overcome friction from the ePTFE wrapping and slip, they begin to converge on the plane coinciding with the bend to be in their lowest energy state. This grouping of strings improves the catheter’s shape holding ability, inducing the first plateau. The stiffer wires then provide a stronger force on the string array, creating bulges localized in the wrap spacing that press against the outer lumen. This behavior attributes to the second plateau and is surpassed once stiffer wires are introduced. With this, we may begin to characterize the relationship between wrap geometry, catheter design, and biomaterial selection.

The ability of the closer, tighter wrapping used in catheter 2 demonstrates its value in improving the bending distribution, and more uniform minimum and maximum bending stiffness. By standardizing wrap configurations in relation to our derived equations and biomaterial compatibility, interwoven biomaterials can be specialized to facilitate catheter-mediated medical therapies.

The author thanks Dr. Mohamed Zayed and Dr. Guy Genin for guidance in this project, and collaborators within the Engineering and Medical school for their companionship and support. This work was funded by the Cardiovascular Research Innovation in Surgery & Engineering Fellowship.

[1] Kokkinidis, D.G. and Armstrong, E.J., 2020. Current developments in endovascular therapy of peripheral vascular disease. Journal of thoracic disease, 12(4), p.1681.

[2] Chandrasekaran V et al. JMBBM 113 (2020). OI: 10.7936/nwn0-9a57.

[3] Hartquist CM et al. J. Biomech 119 (2021). DOI: 10.1016/j.jmbbm.2021.104459.