Modern medicine has come a long way from the reed catheters of ancient Syria or the metal tubes of Benjamin Franklin’s time. As doctors discover more and more ways to utilize minimally invasive procedures, the demand for lower profile catheters with increasingly greater applications rises. Additionally, more control is required from the doctor’s perspective in order to successfully navigate through the body’s tortuous anatomy. With a catheter-based delivery system this means that not only must the device be smaller, but also exponentially more robust from a performance perspective. Advanced material formulations have paved the way for these added performance characteristics while helping to create a lower cost environment for design engineers to develop new and innovative products.
Medical catheter manufacturing has historically been plagued with competing demands for flexibility, torque response, kink resistance, ultrathin walls, and lubricity for the passage of other devices through the catheter channel. This added lubricity aspect was particularly challenging, as numerous developments have focused on tackling the problem. Yet, as each new idea for ways to increase lumen lubricity was explored, either one of the above listed qualities was sacrificed, or new problems occurred. One major issue with adding lubricity to the inner lumen was an added difficulty during the sterilization process due to material reaction. Attempts to manufacture a single layer material with increased lubricity were fraught with other problems, such as particles not fully dispersing into the resin and chemical leaching, which would contaminate the material surface. Seemingly unavoidable, this search for an extrusion with better lubricity led to the development of a tri-layer extrusion.
Tri-Layer: An Important Step Along the Way
Tri-layer extrusion, by pairing together an inner layer of lubricious material, such as high-density polyethylene (HDPE), and an outer layer of catheter shaft material, either Pebax™ (polyether block amides) or Nylon 12, with a bonding agent, commonly Plexar™ or Orevac™, allows the catheter to utilize the lubricious property of the HDPE while maintaining the flexibility and torque response of the outer layer, without adding resistance for a guidewire passed down the channel. Tri-layer extrusion appeared to be the best solution to the need for catheters with improved lubricity. Coupling the benefits of an outer layer to both maintain kink resistance and torque response, while maintaining the inner layer’s lubricious property, tri-layer extrusion expanded the medical applications possible with catheter systems.
Beyond this, tri-layer extrusion allows the catheter developer to thermally bond materials to the outer layer, whether a medical balloon or an atraumatic distal soft-tip. Most notably, trilayer material is used extensively in Percutaneous Transluminal Coronary Angioplasty catheters, which have a rapid exchange port for the guidewire. The tri-layer material serves as the lumen for rapid exchange of the guidewire and ensures that the guidewire is easily inserted and retracted without any unnecessary friction. Additionally, tri-layer material is also used in Transcatheter Valve Replacements, neurovascular applications, and infusion tubing, among other purposes, which require low friction.
Despite its benefits, tri-layer extrusion has not been the ideal solution for increased inner lumen lubricity. In trilayer extrusion, although the inner layer lubricity is sufficient, there is a high risk of delamination between the layers. This is often a result of improperly extruding the bonding agent (the middle tie-layer). Delamination of the layers could mean that a guidewire gets snagged when passing through the trilayer extrusion, which creates a potential for the inner layer to buckle. This failure to properly extrude the middle tie-layer has to do with the complicated process of extruding three very different materials simultaneously in an extremely small configuration.
The major difficulty is due to the extrusion manufacturer not being able to confidently determine whether or not the tie-layer portion of the tri-layer material is present in a uniform distribution. Furthermore, the extrusion may not consistently yield tubing with three distinct layers, but rather the layers may “bleed” into each other. This becomes very troubling, as it puts to question every catheter utilizing tri-layer extrusion with a potentially significant product failure risk. Should the layers “bleed” together, the inner lubricity becomes undesirable, resulting in added friction upon passing anything through the channel and amounts to a tube without the lubricity required to perform its desired function.
To curb the risk associated with the tendency toward delamination, tri-layer extrusion requires significantly enhanced inspection and very costly inspection equipment. This amplified inspection, combined with the resulting decreased yields, leads to tri-layer extrusion carrying a much higher cost per part for the end catheter manufacturer or, even more troubling yet, a significant risk to the end user.
Single-Layer Formulation Adds Advantages
To mitigate the failure risks as well as to address the price issues presented by delamination of the tri-layer extrusion component, a custom formulated material was developed that eliminates the need for tri-layer tubing completely. This unique material formulation allows the engineer to extrude a single layer that can perform better than tri-layer extrusion on several fronts.
As a single layer tubing, the extrusion process and resulting inspection is much more straightforward, and the risks associated with delamination of layers are eliminated. This greatly reduces the cost to manufacture and inspect while, most importantly, drastically reduces product failure risks. Beyond this, a custom formulation can offer a greater lubricity than the standard HDPE utilized in tri-layer tubing. Also, since it is a single layer extrusion, this material formulation can be extruded in a thin-wall configuration, which is something not readily achieved when fabricating tri-layer extrusion.
This material was formulated so that it has a greater lubricity than HDPE, but unlike HDPE, it has an ideal melt-flow profile. This allows it to be utilized in catheter manufacturing in the same manner as standard catheter materials (Nylon 12 or Pebax™). (See Figure 1)
The formulation was developed using a twin screw compounding process to create a homogenously blended resin, which ensures high performance throughout the entire extruded tube. This custom resin formulation extrudes well into a tube with tight tolerances on both ID and OD. Because it is a custom compound and not a coating, the lubricious properties are realized both on the ID and the OD of the material, which makes it valuable in areas beyond just rapid exchange port extrusions. For example, tubing that might otherwise require a polytetrafluoroethylene (PTFE) or fluorinated ethylene propylene (FEP) liner may find sufficient lubricity utilizing this formulation without the added expense of laminating material over a lubricious fluoropolymer inner layer. (See Figure 2)
Another area in which this material has proven useful has been in tubing that requires a lower shore-hardness durometer (such as 35D and under). These extrusions will function much better utilizing this innovative formulation than standard shore 35D material, since the added lubricity helps reduce the usual “gummy” or “tacky” feeling related to low durometer tubing. The added lubricity also helps to ensure easier passage through the channel, while still maintaining the advantages of a soft, low durometer. Also, this formulation works particularly well with concentric, telescopic tubing for delivery of stents, heart valves, and other devices, as the added lubricity helps ensure smooth passage. (See Figure 3)
In developing this new material, it was important that not only was the lubricity increased, but also that the melt flow properties were maintained. Maintaining these melt flow properties ensures that the resulting product is well suited for standard reflow, thermal bonding, and tip forming operations. This allows for a wider range of applications in catheter manufacturing versus other lubricious material options. For example, due to their thermal properties, materials such as HDPE, FEP, PTFE, etc. are not easy to work with, as they will not behave in the same manner as standard catheter materials such as Nylon-12 or Pebax™, a problem that has been addressed by the new material. It is also bondable with medical grade adhesives, and the extrusions are stable when sterilized by Gamma, ethylene oxide, or E-beam sterilization methods. The custom formulation was developed to work across a full range of durometers, which has ensured unique solutions to a myriad of applications requiring added lubricity. Its formulation has also been applied to base resins of both Nylon 12 and poly urethane, which can open the doors to more novel device developments.
Furthermore, because this formulation is a single layer extrusion, it can be easily processed, without the uncertainty of whether or not an inner tie-layer has been affected. In this way, it can be reflowed for catheter jacketing, and can be bonded to a medical balloon, or reflowed to create custom geometry such a tip forming or flaring, which may all present difficulty when using standard tri-layer extrusions. With the added lubricity afforded by this formulation, catheter engineers are finding new ways to solve low-profile, high performance needs across a wide spectrum of medical applications.
The material, under the trade name PebaSlix™, is USP Class VI tested for biocompatibility. Utilizing the base resins of Nylon 12 or polyurethane, it is referred to by the trade names NyloSlix™ and PolySlix™, respectively.
This article was written by Lorie Lodico IV, Engineering Sales, Duke Empirical, Inc., Santa Cruz, CA. For more information, Click Here .