Medical Tubing Offers More (and Less) to Device Makers

Originally Published MDDI January 2002

TUBING

Medical Tubing Offers More (and Less) to Device Makers

Modern tubing boasts more properties with less bulk than ever before. But the improvements will cost you.

William Leventon

When it comes to medical tubing, more is good. And so is less. Today, tube manufacturers offer more properties, more material options, and more precise process control than ever before. At the same time, there's a lot less to their products than there used to be. Some new tubes feature diameters that measure in the thousandths of an inch, with walls thinner than a human hair.

Though small, specialty tubing can cost many times more than conventional high-volume tubes, according to Mark Saab, president of Advanced Polymers Inc. (Salem, NH). Saab's firm works mainly with medical companies willing to pay for super-small tubing. "By far, the biggest trend in the device industry is downgauging," says Saab, whose company has made heat-shrinkable tubing with diameters as small as 0.008 in. and walls just 0.0001 in. thick.

These diminutive tubes are used to make catheters that can be inserted into the cardiovascular system. The smaller the catheter, the farther it can travel in a network of arteries of diminishing size. Supersmall catheters also cause less trauma when inserted into the body, says Bob St. John, manager of medical products for New England Electric Wire Corp. (Lisbon, NH).

WANTED: LARGER ID THAN OD

While reducing the outside diameter (OD) of their products, tubing manufacturers want to maintain as large an inside diameter (ID) as possible. Larger IDs give doctors more room to insert tools or deliver drugs into the body. "Everyone wants an ID that's bigger than the OD," jokes Mike Badera, president of Precision Extrusion Inc. (Glens Falls, NY).

Pressed to reduce ODs and increase IDs, Badera's company and others have produced tube walls that are astonishingly thin. Precision Extrusion makes tubing with walls about a quarter as thick as a human hair. And some of Advanced Polymers' tubes have such thin walls that they can take the place of coatings. "They're actually thinner than some coatings," Saab says of the tubes, which are used for encapsulation and electrical insulation.

In the production of ultra-thin-wall tubing, manufacturers can have trouble just getting material to flow through the small orifices of the machinery. To minimize flow problems, Putnam Plastics Corp. (Dayville, CT) relies on special tooling designs and materials that are more conducive to thin-wall extrusion. Such materials include Pebax and various nylons and urethanes, according to Bob Poirier, the company's director of sales and marketing.

Advanced Polymers also makes certain modifications to commercially available extrusion equipment to produce its special tubing. For example, the company designs a special extruder screw for each of its tubing materials. "We tweak the screws to give us the most uniform output we can get for each material," Saab explains.

Gear or melt pumps can also help in the extrusion of very small tubes. "If they're designed and used properly, melt pumps can give you very stable output and much better yields off the extrusion line," Saab notes.

For consistency, manufacturers must maintain precise control of the equipment that pulls tubing through the extrusion system. "If the pulling rate varies by much more than fractions of a percent, your size will be all over the place," Badera says.

New England Electric Wire controls pulling speed with programmable logic controllers and feedback loops. The company also relies on laser micrometers to accurately measure tube diameters to five decimal places.

Customers have asked Advanced Polymers to hold tubing tolerances as tight as ±0.00025 in. When the extrusion process cannot guarantee such tight tolerances, the company adds secondary inspection and sorting operations.

If necessary, a laser gauge measures every piece coming off the line. "If a piece is out of tolerance, we throw it away—and suffer the yield loss," Saab says. "It may be that to hold the desired tolerance, the process only runs at 20% yield, so 80% [of the output] is thrown away. We can do that if the customer is willing to pay for it."

ONE PRODUCT, MANY PROPERTIES

Tubing customers are a demanding lot. Not satisfied with precisely made, supersmall tubes, they also want products that combine several different properties. Take a tube that will be used as a catheter. Customers want a soft, flexible tip so the catheter can follow the contours of the arterial system. But they also want a stiff back so doctors can push the catheter into position inside the body.

The tube manufacturers must often combine two or more different materials to produce a single tube with a range of properties. To make the catheter tube described above, Saab says, the manufacturer could marry two different materials during extrusion: an elastomeric tip and a back piece made of a stiffer material. Or the manufacturer could use a soft material that runs the length of the catheter and then stiffen the back by adding another material layer to it.

In cases like these, Putnam Plastics uses a process to make tubes in which a transition is made from a stiff polymer to a soft one. The process, called total intermittent extrusion, produces soft-tipped, stiff-backed tubes in a single extrusion. Thus, the tubes are one integral part rather than separate pieces bonded together, Poirier says.

One of the most common methods for adding stiffness to a tube is to reinforce it with a braided structure. Braiding also lets designers increase the tube's burst strength while thinning the wall at the same time. Tubing manufacturers concerned with maintaining minimal tubing diameters often braid with flat wire. "If you use flat wire, you get thinner walls than you would with round wire," Poirier notes.

Equally important, braiding helps transmit torque from one end of the tube to the other. "If the doctor is twisting the proximal end of a catheter to make the tip go in a certain direction, he needs torque transmission in the shaft," Badera explains. "The wire reinforcing helps with that transmission."

Though New England Electric Wire has made tubing with reinforced walls as thin as 0.007 in., the company continues to explore new options. For example, St. John and his colleagues are evaluating braiding metals with very high tensile strengths. "We use a lot of stainless steel in the 300,000-psi range, but we need to look further up than that," he says.

Wire-reinforced walls can do more than strengthen a tube. New England Electric Wire makes tubes that are part of ultrasound catheter systems. Integral wires in the tubes carry heat energy to help shrink the prostate gland. "Instead of taking a plain tube and stuffing a cable through it, we provide cable that's extruded right into the wall of the tubing," St. John says.

In addition to traditional metal reinforcement, tubing makers are considering nonmetal reinforcing options, such as polyester and Kevlar. By using "threads instead of wires," as Badera puts it, companies may be able to reduce reinforced wall thickness and make tubing even more flexible.

MULTILAYER TUBES

Tubing reinforcement often involves many layers of different materials. Each layer serves a different purpose. For example, a lubricious inner layer can make it easy for a physician to slide guidewires through the tube, says Poirier, whose company produces lubricious layers using Teflon and high-density polyethylene.

The inner layer can also serve as a barrier between the outer material and a substance traveling through the tube. When a tube has an inert polyethylene inner layer, for instance, "you can inject a drug through the middle of a tube and there won't be any reaction between the polyethylene and the drug," Badera says. With concerns about chemical reactions eliminated, the tubing maker can select an outer material strictly on the basis of flexibility, stiffness, bondability, or other necessary exterior design characteristics.

Multilayer tubes are sometimes made in a series of steps. "It's an extrusion process combined with an assembly process," Saab says. For example, a manufacturer might extrude a tube, braid over it with wire or polymer, and extrude an outer jacket over the braiding. The tube might then be drawn through a hot die to fuse the different layers together.

In the method used by Advanced Polymers, a piece of heat-shrinkable tubing is placed over the entire assembly, which is then placed in an oven. As heat is applied, the outer tube acts like a mold, squeezing the different materials together. After the materials have bonded, the assembly is removed from the oven and the heat-shrinkable tubing is peeled off. "So we've taken a structure that was initially made of three or four different materials and literally fused it together," Saab explains.

Although multilayer tubes offer valuable combinations of properties, they're "astronomically expensive" compared with conventional tubes, according to Saab. Explaining that a conventional tube might cost from 5 to 50 cents per foot, he says that multilayer tubes can cost from $2 to $15 a foot.

MATERIAL ADVANCEMENTS

Rather than combining materials with different properties, tubing manufacturers may soon be able to take advantage of advanced materials that have been formulated to provide several properties. For example, a single material might offer the flexibility of a urethane and the lubricity of a fluoropolymer or PTFE. Such materials are commercially available now, St. John notes, but are not ready for medical device applications. "You can't put them in people yet because there are biocompatibility issues," he says.

Other new materials are ready for prime time. One of these, called Topas, offers "crystal-clear transparency," Badera says. "If you're injecting something through the tubing or passing a wire through it, you can see what's happening." Rigid, kink-resistant Topas can also be pigmented, Badera adds.

Another new material type softens at different temperatures. According to Badera, a catheter made of such a material is rigid when the doctor inserts it into the patient. Once inside the body, however, the material softens so the catheter won't cause trauma as it is moved through the blood vessels.

At Kent Elastomer Products Inc. (Kent, OH), the concern is not trauma but latex allergy. Because of this well-known allergy, "we're always looking for latex alternatives," says Cindy Harry, Kent's director of sales and marketing.

In some of Kent's tubing, chloroprene and various flexible TPEs take the place of natural rubber. "Right now, we can provide properties that are similar to those of latex, but we give up properties in other areas," Harry notes. For example, the company might have to give up shape memory to get better tear resistance or sealability.

As a result, Kent has given its material suppliers a goal: Come up with a formulation that provides all the properties of latex at a comparable or lower price. Although Kent's suppliers have produced some good materials, they have yet to hit the cost target. With latex raw materials running 50 cents a pound and TPE at $2 a pound, Harry notes, the cost challenge is "quite a hurdle to get over."

CONCLUSION

Medical tubing is far from the simple, mundane product it appears to be. Today's specialty tubes are often complex structures offering many different combinations of properties. Tubes have also shrunk dramatically in recent years, to the point where a human hair is thick compared with some tube walls. Manufacturing such products poses considerable challenges, requiring special extrusion equipment, precise process control, and painstaking inspections. The process may become a bit easier soon, thanks to innovative materials that offer new combinations of properties. But specialty tubing will probably always be difficult to make—and expensive to buy.

William Leventon is a New Jersey–based freelance writer who frequently covers the medical device and diagnostic industry.

Photo courtesy of Roni Ramos.

Copyright ©2002 Medical Device & Diagnostic Industry