When it comes to medical tubing for catheters, IV products, drug-delivery devices, and other applications, it's not just what's on the inside that counts. In fact, a material's properties are often equally as important to the outer diameter (OD) of the tube as they are to the inner diameter (ID) in terms of achieving superlative performance. Consisting of configurations that feature different materials for the inside and outside layers of the tube, coextruded medical tubing offers the advantage of IDs and ODs optimized with the functional characteristics best suited for a given medical device application. As a result, multilayer medical tubing can be produced to accommodate the increasingly complex needs of the industry by ultimately offering the best of both worlds.

Multiple Layers, Multiple Functions

Because it offers the ability to produce a tube with its ID and OD optimized for their respective environments and functions, coextrusion offers advantages to manifold medical device applications. Chief among these applications, however, are guide catheters through which a physician must pass guidewires or probes to deliver stents or other medical devices to the proper destination in the body for treatment.

A lubricious ID is imperative for such medical tubes in order to facilitate device delivery, according to Don Centell, vice president of engineering, thermoplastics division, at medical extrusion specialist Vesta Inc. (Corona, CA). However, the tube may only require a thin-wall material with lubricious properties for contact purposes. Pairing that thin-wall, lubricious ID material with an implant-grade OD material that offers kink resistance to navigate the tortuous anatomy of the body can yield an ideal tube design for delivery devices, Centell notes.

Coextrusion is also the processing method of choice for various drug-delivery applications. The use of light-sensitive therapies, for example, may require multilayer tubing to prevent exposure of the drug to light, which can result in reduced efficacy. To meet this need, the extrusion branch of Raumedic Inc. (Leesburg, VA) offers the Rausorb line of tubing, designed to absorb light while retaining transparency for clinician reference. Rausorb tubes feature a drug-compatible ID material coupled with an OD material that has been mixed with absorbing substances that are individually adapted to the activation range of the therapy, according to the company. This outer jacket layer of the tube serves to filter wavelength ranges from 200 to 450 nm.

In a diabetes device, on the other hand, a polyethylene ID material may be selected because of its compatibility with the drug, while an implant-grade polyurethane may prove to be most suitable for the OD of the tube, according to Richard DiIorio, technical sales manager, northeast, for Raumedic. Material selection for an IV or drug-delivery device, he adds, is often limited by compatibility issues between the properties of the required therapy and those of certain materials. But a drug-friendly material may also present problems during assembly of the finished device.

"Essentially, you have a tube that is transmitting a drug; however, you often have to also connect that tube to various connectors, luers, and fittings," DiIorio explains. These types of applications, he notes, often require solvent bonding of the medical tubing to some sort of connector. But a material that can be solvent bonded is not necessarily compatible with the designated drug and vice versa. "In this case, you may want to perform a coextrusion of a material on the ID that works well with the drug and, on the OD where the parts of the tubing and the connector are going to mate and touch each other, you may want a different material that bonds well," DiIorio says.

In addition, coextrusion can offer the benefit of reducing the number of operations required to produce a particular medical tubing application. To create braided high-pressure tubing, for example, companies may typically employ a three-step process that entails extruding a tube, braiding over it, and then extruding over the braided structure, DiIorio explains. Because it requires three separate operations, this approach can be expensive and cumbersome. By coextruding two materials, however, companies can achieve strong, high-pressure tubing using only one process. Although the up-front tubing cost for coextruding high-pressure medical tubing would be more than with the multistep method, companies would likely see a dramatic reduction in overall tubing costs, DiIorio comments.

Also among the many benefits of coextrusion is that it enables the inclusion of colored or radiopaque stripes in a medical tube configuration. While colored stripes can help with catheter orientation, radiopaque stripes incorporated within the wall of the tube are often useful for viewing catheters under fluoroscopy. "The stripe itself is usually a compound--often the same as the tube's base material--but it might be loaded with 30% barium sulfate or 40% bismuth subcarbonate," Centell says. "It serves as an aid for physicians to understand where they're at in the human anatomy. A lot of times you'll see that in pain-management devices."

The Tie that Binds
Coextrusion of compatible materials offers a means of creating multilayer tubing that accommodates the different functional requirements of the ID and the OD for a particular application. But sometimes the two desired materials just aren't compatible with each other. As an example, Centell cites the case of a tubing application for which a customer selects high-density polyethylene (PE) for the inner layer of the tube and wishes to pair it with nylon or soft-durometer Pebax. "We can extrude [these incompatible materials] together, but it would be very easy to separate them back apart, which is not something you want to happen in a clinical setting," he says.

To overcome this barrier, companies may have to introduce a third, middle layer to the tube design, aptly named a tie layer. Compatible with both the ID and OD materials, the tie layer serves solely as a bonding mechanism for the incompatible materials.

"In the medical industry, [designers] want the characteristics of the nylon or Pebax for the OD and the high-density PE for its lubricity, but the tie layer becomes a necessary evil," Centell notes. "Unlike other industries that may use three-, four-, or five-layer products, those are serious considerations when it comes to medical applications." Because designers typically want to minimize the impact of the additional layer on the overall size of the tube, the tie layer is frequently applied as thinly as possible--measuring in many cases approximately 0.0004-in. thick, Centell adds. Alternatively, thinner ODs may have to compensate for the incorporation of a tie layer.

Increasing the complexity of a tube by including a tie layer also presents new considerations that must be taken into account for the final product, according to DiIorio of Raumedic. "Whenever you coextrude tubing, within those layers, you need to have concentricity," he states. "So, you need to have some sort of uniform shape between the various layers; the rings of the [different layers of the final tube] must be consistent, and that's a very difficult thing to master."

Miniaturization Meets Multilayer Tubing
Necessitating the use of a process dubbed trilayer extrusion, the incorporation of a tie layer in a medical tube design has become increasingly common in recent years, especially in conjunction with the rise of minimally invasive procedures, Centell notes. He adds that Vesta currently runs trilayer extrusions roughly 10 or 12 times per month compared with two or three times per month just a few years ago.

This rise in activity can be attributed to the fact that miniaturization has become paramount in the medical device industry as physicians attempt to access areas that were previously inaccessible or difficult to treat, including the brain and peripheral arteries. Facilitating such progress, coextrusion, and trilayer extrusion in particular, is enabling the development of these tight-tolerance, small-diameter delivery devices. "Not only does [coextrusion] provide the desired functional characteristics to the ID and the OD, but now that you've got these three layers--though they may be really thin--structurally the tubing is actually a bit stronger," Centell says. "It can handle greater burst strengths, yet it still has the ability to have a low profile and navigate these tortuous areas of the anatomy."

Coextrusion has also offered a solution to problems that have arisen as tubes have shrunk in size. Formerly, hydrophilic coatings were often applied to the ID of a tube to enhance lubricity, for example. But as tube diameters have diminished, this approach has become increasingly difficult, according to Centell. Coextrusion, he states, has provided a relatively simple technique for achieving a lubricious ID that also eliminates the secondary operation and cost associated with applying a hydrophilic coating.

But as tube size continues to shrink while complexity grows, new challenges continue to emerge. "We're seeing the limits being pushed," DiIorio of Raumedic comments. "Customers are demanding very tiny, precision tubing. But now they're demanding this in even smaller sizes in combination with multilumen or multimaterial tube configurations." Among the most significant barriers to achieving these tiny, complex tubes, Centell adds, are the limitations imposed by extrusion tooling. To resolve this issue, Vesta is working with crosshead manufacturers to improve their designs in order to meet customer demands.

"We see some very interesting concepts and questions arise from theoretical designs," DiIorio says. "They're not all possible within physical capability, but they're pushing limits, and it challenges us to be better and make better product."