Tubing is available with a variety of options and in a range of styles. Selecting the right tubing for a specific application can be both challenging and daunting. This article attempts to clarify the process for device makers by highlighting some of the most commonly indicated choices.
Regardless of the medical device application, certain tubing attributes are desired across the sector including the need for thin walls to maximize working lumen area; high stiffness to allow thin walled devices to be pushable and transmit operating loads; excellent torque response for actuating rotational mechanisms at a distance, as well as for remotely controlling tip direction; and kink resistance. To accomplish these often competing performance characteristics, more sophisticated materials have been developed, new processing methods used to enhance material properties, and experimentation with new combinations of common materials have delivered uncommon results.
Suppliers are currently pioneering the processing of materials that have been around in other industries for years but are now making their way into medical applications. One such process advancement is the braiding of fiberglass filaments, carbon fiber, and less well known materials, such as Spectra, which is stronger and lighter than Kevlar, and Vectran, a melt spun liquid crystalline polymer five times stronger than steel by weight while exhibiting no creep. In addition to those commercially available materials, custom formulated, proprietary mono- and multifilament polymers can be designed and laminated over fluoropolymer liners ultimately being built into catheter tubing, overcoming the design limitations of traditional materials. The result is the creation of novel, market leading products of the future.
Following are several design considerations that arise in the development of catheter tubing and device delivery systems listed along with prospective responses.
Do designers need to consider this? They should. When any tubing is bent around a curved pathway, the lesser curve is subjected to a shorter path length than the greater curve, resulting in compression of the material below the neutral axis or midline and tension above the midline. Prior to buckling failure, also known as kinking, tubing ovalize will occur and can pinch or lock nested components that are meant to slide if sufficient clearance is not designed into the fit between components or if the hoop strength is not increased with reinforcement by braid or coiled material with strength sufficient to keep the lumen from ovalizing.
2. Guidewire Clearance
How much clearance is enough? A proper answer requires understanding of the application. Rebuttal questions would ask how tortuous a pathway is expected and what are the surfaces and the coefficients of friction that will be interacting. Will there be fluid between the guidewire and the lumen to increase lubricity? Typically, 0.002 to 0.005 in. should be sufficient as a nominal condition; however, tolerance stack ups need to be done for worst case scenarios.
3. Fluid Passage
How much clearance area is enough? The answer depends on several factors. What is the flow rate and what is the dynamic viscosity of the fluid that will be used? How much pressure will be needed to create the desired flow rate? A balloon catheter will inflate and deflate at different inflation/deflation times depending on the contrast to saline ratio used. Has the product been designed for the worst case scenario in this example (i.e., 0% contrast dilution)? Will the fluid have a concentration of immiscible components? As pressure is increased beyond atmospheric conditions, solutions with high concentrations can form precipitates that can occlude small luminal areas where that result could be detrimental and unexpected.
How can the friction be lowered inside lumens? Teflon liners are the industry standard for most guiding catheters, but hydrophilic coatings on the I.D.s of lumens can save space, reduce stiffness, and lower costs.
5. Kink Resistance
What is the minimum bend radius before a tubing will kink? Can this be known prior to building the configuration? Experience is really the key to answering this question as there are too many factors involved to create meaningful theoretical predictions. Availability of similar samples to what is being designed is the key to understanding how material selection, wall thickness, and reinforcement configuration result in a tube performance.
6. Braid Pattern
What braid pattern is best for this application? To optimize compressive strength, one would pick a single over single under pattern, which has a more integrated structure than a double over double under pattern. If torque response is the prioritized characteristic then a 45<deg> braid angle is best but that will add significant lateral stiffness and detrimentally effect kink resistance versus a higher angle braid, all other factors being equal.
What is the difference between coil reinforced and braid reinforced? Coils do not transmit torque nearly as well as braid reinforcement; however, coils will be more effective in increasing hoop strength, kink resistance, and flexibility.
8. Wall Thickness
How thin can you go? Between lumens, 0.001 in. is attainable depending on the material chosen. For the outer jacket, enough material should be used to flow into the islands of the braid and yet, be the minimum needed for braid coverage so that the surface remains smooth.
The tubing needs to be stiff but soft. In other words, can it be made so that it is pushable, yet flexible at the same time? It’s all about the transitions. Two ways to get smooth transistions are to use variable durometers on the jacket and liner by staggered overlapping of the durometers like shingling where the liner and jacket durometers are not changing at the same linear distance from the tip of the shaft. A second way is to vary the braid angle so that the tip is at a higher braid angle than the proximal portion of the shaft.
What material is best for my application? When to use polyurethane versus Pebax versus nylon. The hidden truth behind raw material suppliers is that all materials are not equal. Polyurethane is known to have higher gel concentrations and greater lot to lot variability in processing parameters than polyether block amides, such as Pebax. How to deal with lot to lot variation? Let the extrusion house deal with it through raw material sorting and process changes that do not impart physical property changes to the resultant tubing. If processes change too much, shrinkage and other undesirable characteristics (e.g., lower mechanical properties) can result.
What about visualization? Size and shape matter, but it’s really all about the density. The first question is what type of imaging will be used? Fluoroscopic is the most common, but MRI and CT are becoming more widely used due to 3D visualization capabilities, visualization of tissue type differentiation, and image modification. In the more advanced centers worldwide, the ability to visualize anatomic structures in real time 3D using a variety of modalities has been shown to have important clinical benefits. For example, in aneurysm repair, an important feeder vessel may be seen in a 3D reconstructed view that would be hidden or missed in a 2D representation or series of 2D image collections. In aneurysm repair, using embolic materials, such as coils, glue, and particles, the ability to isolate the segment to be treated and rotate it in space can really alter the diagnosis and subsequent treatment prescribed. If a collateral vessel is feeding an aneurysm but is not detected, the consequences would be a type II endoleak, resulting in continued sac enlargement and possibly subsequent need for reintervention (now complicated by a previous treatment), or an equally bad or worse result obtained where an ischemic condition is created with resultant end organ failure.
Some therapies are being developed in conjunction with real time MRI imaging where devices are in the body and being visualized in the imaging area. As there are high magnetic fields being used, devices that have the same performance requirements that have been developed using high tensile stainless steel wire reinforcement are required but can no longer use the slightly magnetic metal. Now that imaging is driving material selection more than ever before, there are many devices that need to be redesigned for MRI compatibility. It’s desirable to work with suppliers that have met the processing challenges that exotic materials bring and to find a supplier who can deliver a consistent product that meets the performance requirements in a short timeline and at efficient cost. One of the best ways to find and evaluate such a supplier is to ask about the availability of samples, and to preview the company’s high technology product offerings that suppliers market at various industry trade shows and journals.
In conclusion, when designing for specialized applications that require new materials, new configurations of materials, or newly developed processes, one should be aware of the advanced technologies that are available and work with suppliers who can provide experienced-based advice to more rapidly convert those technical advances into novel products and clinical applications.
Robert LaDuca is the CEO of Duke Empirical Inc., an outsourcing company that specializes in medical product design, catheter delivery systems, and medical tubing extrusion. LaDuca can be reached at 831-420-1104, ext. 201 or firstname.lastname@example.org.