Extruded Tubing: Shrinking Dimensions, Greater Complexity

Demands are increasing for complex extruded tubing with shrinking wall thicknesses and multiple lumens. In addition, medical device OEMs are demanding that contract manufacturers demonstrate the ability to produce coextrusions and microbore multilumen designs using a range of biocompatible materials, from PEI and PEEK to PVC and DEHP-free polymers. Addressing these and other issues is Rudi Gall, managing director of Raumedic Inc. (Leesburg, VA).

MPMN: What key extrusion capabilities should the medical device OEM require of an outsourcing partner?

Gall: Tubing and catheters used in medical devices are growing more sophisticated as OEMs seek tubing products that deliver ever-increasing performance levels. The trend toward more-complex tubing configurations is associated with the growing prevalence of minimally invasive surgery. As minimally invasive surgical techniques become more common, medical device manufacturers are clamoring for tubes and catheters that are smaller and more complex. Thus, they should select extrusion service providers with the understanding that single-lumen microbore tubes will not suffice for future medical device applications.

While today's extruded tubing must maintain the same microdiameter sizes as previous-generation tubes, it must also feature multiple lumens with guidewire access capability, fluid-transfer channels, and inflation ports. Lumens that provide steering functionality are also in demand. OEMs should partner with contract manufacturers that demonstrate the ability to work with properly dimensioned drawings that will result in tubes with well-balanced outer diameters, wall thicknesses, and interlumen interstices. Based on those capabilities, contract manufacturers should be able to produce tubing with up to 13 lumens

To achieve improved functionality, medical device OEMs should also be interested in establishing relations with contract manufacturers that exhibit demonstrated coextrusion capability. Coextrusions, which combine multiple polymer materials, are used to increase a tube's pressure resistance up to 1200 psi and higher. Often, such tubes feature an inert polyethylene inner layer and a bondable PVC outer layer. Coextrusions are appropriate if the surface friction of the tubing's inner layer is a concern, a phenomenon that occurs when catheter tubing acts as a guide catheter. In such cases, the inner layer of the tubing can be designed to include a nylon layer. For example, in the case of epidural catheters used to perform regional anesthesia, a small catheter tube is inserted via a tiny metal cannula into the nerve channel in the spine. While such catheters must have very small dimensions and be flexible enough to spare spinal cord nerves, they must also offer kink resistance to negotiate the tight radii between the spinal vertebrae. In addition, such tubing must be highly transparent, enabling anesthesiologists to ensure proper fluid transfer.

To meet the requirements of such coextrusions, the contract manufacturer must be able to handle two-layer tubing designs that can consist of such materials as a polyamide inner layer and a polyurethane outer layer. These materials achieve a firm, permanent bond during the extrusion process. While the polyamide layer is responsible for the mechanical strength of the catheter--in particular, its kink resistance--the polyurethane outer layer can be equipped with printed length markers to enable the surgeon to guide the catheter properly. Visual markers can also be achieved by embedding x-ray contrast stripes in the catheter wall, which can measure as little as 0.002 in.

Medical device OEMs may also require that a prospective contract manufacturing partner be able to produce microbore multilumen catheters, which are used in microdialysis applications to perform quantitative analyses of blood solutes. In such applications, blood components are diffused out of the blood using a thin membrane. Diffusion of the solutes is accomplished by means of osmotic pressure until a concentration equilibrium is achieved between the blood and the rinsing solution. This procedure requires the use of an oval-shaped dual-lumen microbore catheter consisting of a polyamide and a polyurethane layer. While the catheter itself might have a width of 0.03 in. and a height of 0.02 in., the diameter of the lumens might measure 0.01 in. Using a secondary operation, a small window segment is cut into the catheter tip, and a microdialysis fiber is inserted. Such a design enables continuous measurement without the need to extract large quantities of blood from the patient.
 
In short, future medical applications will demand a combination of extrusion and associated technologies, including microextrusions, coextrusions, multilumen designs, metal or monofilament braiding, and wire-inserts. The medical device OEM should therefore choose a contract extrusion partner that can offer such services and is backed by an experienced process, application, and quality engineering team. Moreover, because the equipment and tooling technologies used for producing catheter designs can be challenging, outsourcing partners with an in-house tool and die shop offer added benefit.

MPMN: What types of materials must a medical extruder be able to process?

Gall: The medical device OEM should demand of a prospective partner that it have experience with a range of medical-grade materials that meet biocompatibility, temperature, and durability requirements. For example, such invasive tubing as epidural catheters come into contact with blood, nerves, and bone. Vascular catheters, on the other hand, come into contact with blood and the inner walls of veins and arteries. Thus, contract manufacturers must be able to extrude medical tubing made from biocompatible materials suitable for such environments. Such materials should also be sterilizable.  

In principle, the range of polymers suitable for use in extrusion applications is unlimited. However, of particular interest are those thermoplastics that have an established track record in medical applications, such as thermoplastic polyurethanes, polyamides, polyolefins, and thermoplastic elastomers. Other medical-grade high-temperature thermoplastics such as PEI and PEEK serve as alternatives to metallic materials because of their superior mechanical properties.

Another suitable material is plasticized PVC. However, while PVC is still widely used in the medical device industry, alternative materials without such plasticizers as DEHP are becoming prevalent. Outsourcing partners should therefore be able to offer a range of different materials and compounding capabilities. They should also be able to assist and support OEMs seeking to migrate from plasticizer-containing materials or seeking to perform drug-adsorption studies with alternative materials.

Regulatory documentation and ISO 10993 and USP Class VI test certificates should accompany medical device materials. Some outsourcing firms even have in-house compounding capabilities, giving them additional knowledge of material parameters and processing behaviors. Such companies also maintain materials engineers on their staffs that can address such issues as 'blooming effects,' which cause amide wax to migrate to the surface of some grades of polyurethane. Cosmetically undesirable, this effect also degrades bond strength in secondary solvent bonding processes.

MPMN: What new challenges are involved in fabricating extruded tubing?

Gall: It is possible to coextrude very thin multiple polymer layers, but miniaturization remains challenging. Special microextrusion lines allow manufacturers to produce multilayer tubing for different applications using up to three different polymer materials. Employing such equipment, tubing with an inner diameter of approximately 0.004 in. and a wall thickness of approximately 0.002 in. can be achieved. Such extrusion lines produce extremely small throughputs and consume material at a rate of as little as 50 g per hour.

While the trend toward miniaturization seems to be unstoppable and small dimensions can be achieved, the real challenge will be creating ever-thinner tubing walls with ever-tighter tolerances, placing tubing designs at risk of collapsing. Such miniature designs also raise repeatability and reproducibility issues. As tubing dimensions shrink, another challenge is the incorporation of electronics into medical devices, which demands that manufacturers be able to embed copper wires in tubing walls to enable data transfer. Finally, the need to guide catheters through tiny blood vessels places increasing demands on contract extruders to develop metal- or filament-reinforced tubing.