Originally Published MPMN January/February 2009

Whereas OEMs repeatedly voice their desire for tighter tolerances, thinner walls, and ever-shorter lead times, a new set of demands for medical tubing is becoming increasingly more audible. With the increased use of minimally invasive surgery (MIS) and the emerging field of natural orifice transluminal endoscopic surgery (NOTES), the need for minuscule, multifunctional, problem-solving tubes that offer design flexibility is on the rise.

"It used to be that you could just have a homogeneous tube--a tube that was either going to carry fluid or be a path for a guidewire," observes Tim Lynch, operations manager for MicroLumen Inc. (Tampa, FL; www.microlumen.com). "But today, the devices are getting more and more complex. They're trying to do more and more in smaller devices, and therefore there's really not enough room to have tubes that function only as a tube--the real estate in there is critical and tubes have to have multiple features and multiple purposes." Aiding in living up to this expectation of multitasking are multiple materials and geometries in the form of composite tubing.

Offering Reinforcement

Providing functional flexibility in composite tubing is the combination not just of several materials, but geometries as well. Jim Shobert, chairman of Polygon Co. (Walkerton, IN; www.polygoncompany.com) likens the design of composite tubing to that of a bridge. When building a bridge, there is first laid out a latticework of metal over which concrete is then poured. The metal latticework serves to reinforce the concrete, increasing the structure's strength.

Like bridges, composite tubing often features coiling or braiding to reinforce the tube structure, especially in applications requiring thin walls. Potential advantages of such a construction can include better tensile and burst strength, as well as kink reduction in flexible tubes.

"I always use that [bridge] example because it is very analogous to composites," Shobert says. "You take a fiber structure and then put the polymer around it and you can vary the material and the mechanics of that structured tube based on the fiber geometry. Not only can you change the geometry, but you can also change the material constituent to change physical characteristics." Depending on the materials and geometries employed, a composite tube can offer rigidity, chemical resistance, precise tolerances, and a high strength-to-weight ratio.

Giving Devices the Slip

The materials employed in the tube ultimately dictate its properties and suitability for particular applications. For example, Teflon (PTFE), renowned in the cookware circuit for its nonstick properties, is also a key component in many companies' medical-grade composite tubing. Frequently used as a liner in composite tubing, PTFE provides its patented nonstick and lubricious surface to facilitate device delivery. "Teflon is so slippery that the product being pushed in the middle [of the tube]--whether it's an angioplasty balloon or stent-type product--can move more freely with that lubricious surface," says Michael Badera, president, Precision Extrusion Inc. (Glens Falls, NY; www.precisionextrusion.com).

Precision Extrusion, a specialty high-end extrusion house, is venturing into composite tubing for the first time in response to customer demand. A natural outgrowth from its efforts to expand its value-added capabilities, the company has developed a PTFE-lined braid-reinforced catheter. Contributing lubricity for device delivery or guidance, PTFE lines the catheter shafts. The shafts can be braided using round, single- or double-ended, flat, or ribbon wire with different tensile strengths. Variable stiffness is achieved along the catheter shaft from the proximal to the distal end using nylon, polyurethane, or Pebax jacket materials. Precision Extrusion's catheter also features clear and radiopaque distal tips. "The properties are variable depending on the type of wire that's used, the braid pattern, and the hardness of the materials that go into the outer jacket," Badera explains.

This ability to vary properties and hardness throughout a tube is among the main advantages of composite tubing, according to Lynch of MicroLumen. He points out that composite tubing allows both for stiffness at the proximal end of a catheter, where the surgeon may be working, and flexibility within the body at the distal end. Plus, the addition of other elements can further enhance the composite product. For example, platinum marker bands enable the tube to be seen during fluoroscopy.

MicroLumen supplies a variety of tubing products, including braid- and coil-reinforced composite tubing engineered with PTFE or polyimide liners. Claiming to offer the thinnest PTFE liners in the industry at roughly 0.0005 in., MicroLumen provides Teflon liners in a range of different dimensions. Etching the liners on a silver-plated copper mandrel eliminates the need for additional labor required to stretch the liners over mandrels, Lynch says of MicroLumen's process. He adds that this offering also helps to keep manufacturing costs down by enabling processes to be performed continuously.

Shedding the Sheath

As is the case with enhancing tube lubricity through the incorporation of PTFE liners, composite tubing can serve as a problem solver in various applications. For example, Polygon supplies its PolyMed composite tubing line as an option for manufacturers that want to move away from metal tubes without sacrificing strength or performance.

Engineered to replace stainless steel, aluminum, or thermoplastics, PolyMed composite tubing features circumferentially wound or unidirectional geometries made from continuous electrical-grade fiberglass encapsulated within a thermoset resin matrix. Along with providing rigidity and electrical insulation, this construction boasts the strength of metal without the need for a secondary protective sheath, according to Polygon.

"If you use a stainless-steel or metal tube in a laparoscopic instrument where you're doing any sort of cauterization, you have to be careful of capacitance coupling," warns Shobert. "Typically, you have to put a plastic sheath over the metal so that [capacitance coupling] doesn't happen. With our materials, it's all electrically insulated, so the capacitance coupling issues cease to be a problem."

Available in both single- and multilumen configurations, PolyMed is also corrosion resistant, sterilizable, and radiolucent. It is offered with inner diameters as small as 0.042 in. and with wall thicknesses as thin as 0.010 in.

Looking into the Future

Owing to its design flexibility and ability to be tailored to suit an application's needs, composite tubing appears to be concurrently on the rise with MIS procedures. MIS applications dictate the need for tubing that features such unique characteristics as variable stiffness and the benefits of combined material properties working together. And while composite tubing is helping to meet these demands, it's not doing so without challenges.

"For the entry sites where someone might have formerly used a 7, 8, or 9 French device, people are now going in with 4, 5, and 6 French devices, forcing us to do more and more, less invasively to the patient," Lynch says. That's what is pushing everything to be more specific. "We have to accomplish more in the same or less space than we used to," he adds.

Another challenge is that composite tubing has been overlooked by many engineers over the years for applications for which it would have been ideal, according to Shobert. "Our biggest issue is education of the engineers that this is the material of choice," he muses. "Engineers come out of college typically educated in metals and thermoplastics. Engineers have not in the past been trained in the nuances of composite materials." Lynch concurs, adding that a concerted educational effort by tubing suppliers is necessary so that engineers are informed about the options available to them as well as the capabilities of suppliers.

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