The trends driving custom extruded medical tubing are diverse. There is a growing need for increasingly smaller sizes, in both rigid and flexible formats. Long-term durability, biocompatibility, and multiple-surface requirements are also posing challenges to device makers. In addition, neurovascular applications are becoming more prevalent as that segment grows. Some devices require small-diameter tubing with thin walls to meet these challenges. Others require constructed tubing, such as a thermoplastic material that is applied in a braided fashion over the virgin extruded tubing.

Combined, these demands are stimulating the alchemy of material science and spurring the development of new tubing designs in both extrusion and pultrusion technologies. Every area is growing, and materials, methods, and designs are changing on almost a daily basis.

Thermoplastics

“The general challenges that we are all facing these days are smaller tubing, thinner walls, softer materials, and tighter tolerances,” says Apur Lathiya, COO of ExtruMed LLC (Placentia, CA). “When you put all those together, it becomes a very difficult extrusion. So we focus on working on our process itself, whether it is standardizing our tooling heads, working on temperature controllers, or upgrading our basic equipment with different types of screws and barrels.” ExtruMed extrudes custom thermoplastics, with a focus on lead tubing for active electronic applications, implantable cardioverter-defibrillators (ICDs), and neurostimulation devices that require small-diameter, tight-tolerance tubing.

ICDs, pacemakers, and neurostimulation devices are all products in fast-growing markets. They require not only expertise in extrusion but also heavy scrutiny of each product, up to 100% inspection. “We look for cosmetic and dimensional failures,” Lathiya says. “Much of this is done under a microscope to weed out the smallest defect. Many of these devices are meant to stay in the body forever.”

Staying up to date on new materials is also critical to remaining competitive. ExtruMed has been recently working with a polyurethane-silicone copolymer. Lathiya believes this is the next generation of material for active electrode applications. “What makes it so attractive is the biocompatibility of the silicone combined with the mechanical properties of the polyurethane.”

PEEK Processing

Polyetheretherketone, or PEEK, began moving into the implantable device area circa 2000. Because it's an extremely high-temperature compound, a lot of variables contribute to the material flow. With extrusion, understanding a material's flow performance is critical.

Upchurch Medical, a unit of the IDEX Medical Group (Oak Harbor, WA), extrudes and molds PEEK and other engineering plastics for surgical devices. Materials are extruded and molded, and also assembled, in an ISO 13485–certified Class 100,000 cleanroom. “We do a variety of extrusions. It's generally fine, small tubing, both flexible and rigid,” says Ron Lowell, director of marketing for IDEX. “We get down to 0.0015 in. on IDs and walls. We do multilumen, tapered and oval shapes, profiles, and various custom shapes.”

Exotic shapes are a growing area. Lowell states that, increasingly, customers are asking for difficult shapes. Even a straight, round tube is difficult when it needs extremely tight tolerances. An OEM asking for a tube that measures 0.0015 in. in diameter is asking for a tube with the diameter of a human hair.

Large diameters can also prove problematic. Because of the very high temperatures required to melt them, they tend to shrink more than other materials as they cool. This shrinkage can distort the end configuration. The only way to compensate is to thoroughly understand the shrinkage parameters and plan for them. Shapes for high-temperature materials can be common for stainless-steel tubing, for example, but can be difficult to create using PEEK.

One key area in which PEEK can replace stainless steel is in devices used for minimally invasive surgery. With thoroscopy, for example, it is critical to be able to see the procedure—and sight requires light. Metals create artifacts, or glares and distortions, in the image, meaning a surgeon cannot see. PEEK is very strong, as is stainless steel, but does not cause artifacts. Therefore, many devices that have historically been made with stainless steel are moving to plastic.

Silicone Innovations

Paul Mazelin of Specialty Silicone Fabricators (Paso Robles, CA) says that as a contract manufacturer of silicone components, the popular request in silicone is for multimaterial and multicolor extrusions. “A good example is a catheter tube in which an OEM may want a more flexible distal end but a more rigid proximal end,” Mazelin says. “The hardness of the material is measured by the durometer. We can create a higher durometer at the proximal end and a lower durometer at the distal end to make the tube more flexible, and add approved pigment to one extrusion to show where the change takes place.”

Accomplishing this requires feeding the tooling with two barrels of material, each with different levels of reinforcing fillers, and switching back and forth between the barrels.

Another common request is for printing or marking on tubing. “The most common application is to show depth marking on catheter tubing to indicate how far the doctor has inserted that catheter into the patient,” Mazelin says.

Aside from the durometer switch, Specialty Silicone has also developed a trademarked technology called Geo-Trans, which performs geometrical transitional extrusions.

“We can change the cross section while we are extruding,” Mazelin explains. “It's all based on tooling that enables us to go from a single-lumen to a dual-lumen configuration or bifurcate, if needed. We bifurcate when the customer wants tubing that is together inside the body that has to plug into different pieces of diagnostic equipment outside the body.” Bifurcation, he says, allows the device manufacturer to avoid a secondary molding step to join the pieces together. It reduces cost and increases the reliability and subsequent efficacy of the product.

Secondary Applications

According to Mark Saab, president of Advanced Polymers (Salem, NH), secondary applications have their own trends. “First, companies are asking for more-complex tubes,” he says. “These might be tubes that are braided or have multiple layers or materials.” Such products do not only require an extrusion process—they need one or more secondary processes in addition to extrusion. Advanced Polymers makes custom extrusion tubing and produces heat-shrink tubing and medical balloons.

For example, complex tubing used to reinforce a tube may have a braid over the primary extrusion. A braiding operation might consist of plastic fibers or metal wires over the tubing. After the braiding is done, there may be a second extrusion operation over that entire assembly to encapsulate that braiding.

“You get into multiple extrusion processes,” Saab explains. “You might be extruding the same tube two or three times.” Typically, he says, these reinforced tubes are used in catheters that require reinforcement or certain high-torque characteristics.

Imagine an application that allows a doctor to rotate the back of the catheter and have the front of the catheter respond with a 1:1 ratio. “A normal long plastic tube won't do that because it's soft and flexible,” says Saab. “Turn the back, and the tip doesn't turn.” But a guiding catheter or a diagnostic catheter must be able to be steered and manipulated into position. It has to be torqueable, and 1:1 is the ideal ratio. “Those types of catheters require built-up shafts,” notes Saab. “They are not just a single extrusion. They need the stiffness and torque in the back end and softness and flexibility on the other end.”

Saab says that the biggest challenge is in extruding tubes for making ultra-thin-walled, small-diameter, heat-shrink tubing.

“We are extruding tubes that are then expanded in the heat-shrink, or balloon, process. When those tubes are expanded, their walls are stretched and get very, very thin. Many final products are only a quarter of a thousandth of an inch thick. So any defect in the extrusion can cause a hole or defect in the final product.”

These shrink tubes are used in extremely critical applications. Very small, very thin, flexible balloon-type catheters, with diameters as small as 0.008 in. and walls as thin as 0.001 in., have high burst pressures and mechanical properties. They are used in angioplasty, stent delivery, and cardiac catheterization procedures.

“The materials used in balloons for angioplasty are typically PET, a family of nylons and polyurethanes,” Saab says. “It's a relatively limited number of materials, but a lot of blending goes on that results in custom formulations to fine-tune certain mechanical properties of those balloons.”

Aside from braiding and overextruding, more OEMs are starting to ask for challenging additions such as tipping, flaring, necking, and access ports. They are also looking for processes such as machining, bonding, annealing, and thermoforming. Lowell of IDEX Medical says that many of these secondary operations are not always easy to do. “Sometimes very-thin-wall tubing is made into an exotic shape, and then it needs to be machined down or to have another shape inside of it. These processes can be very hard to do while still maintaining the required tolerances,” he says.

Pultrusion

Another process is making inroads into the tubing business. “We use a chemical reaction in which we take a number of liquid chemicals and mix them into a resin,” says Jim Shobert. Shobert is the chairman of Polygon Co. (Walkerton, IN), a composites company that makes fiber-reinforced composites for medical applications. “The liquid material is introduced into a die at the same time the filaments are being pulled through the die. It hardens as it goes through and creates a tube.” Like extrusion, the process is continuous. It uses the same type of caterpillar pullers seen on extruders, but the forces are extremely high. Most extrusion puller forces range from 1000–2000 psi, but pultrusion forces would be about 3000–6000 psi for the same surface area. “We are pulling full filaments through a die at much higher force, but in principle, it is the same,” says Shobert. “It's a thermal-set matrix material rather than a thermoplastic. Once the reaction takes place, you can't apply heat and remelt the material, whereas
with thermoplastics, the reapplication of heat will soften and melt them again.”

Many of these products are suction and irrigation tubing, which are rigid. This tubing is meant to compete with stainless steel and titanium. In laparoscopic surgery, a metal tube must have a thermoplastic shrink tube over it to give it electrical resistance. “In surgery, when the doctor energizes the cutting or cauterization tool, any conductive capacitance on that tube will make it burst through the body,” Shobert explains. Pultruded tubing is inherently insulated, meaning it does not need a secondary sheath over it. In addition, it doesn't distort under x-rays or cameras. “It can even be used in an MRI environment,” he says.

Shobert notes that like extrusion manufacturers, they have also been working with multilumen tubing. “Over the past few years, we've been doing multicavity lumens with simpler designs, with all the features in one cross-sectional area,” he says.

“Multilumen is a very sophisticated die. We got started with it when a client wanted a new tube for saphenous vein removal for open-heart surgery,” Shobert explains. “They were using a multipiece design, and we worked with them to develop a material that had multiple lumens where it both harvested and removed the vein.”

New Horizons for New Materials

As demands for material properties increase, companies are looking to create new materials to address the challenges. The Polymer Technology Group Inc. (Berkeley, CA) develops plastic compounds, and designs and builds devices and tubing. Much of the tubing made by Polymer Technology goes into pacemaker leads and neurostimulation leads.

One of the company's key developments is a patented technology for modifying the surface of a polymer. Instead of applying a coating onto the surface of the tubing, they build the desired surface chemistry into the material itself.

The composition of the pellets defines the surface chemistry of the polymer, Robert Ward, president and CEO says. “So we modify polymers. Polymers have high-molecular-weight, long-chain molecules. We terminate the ends of these long chains with end groups that we know will come to the surface of anything we make from that polymer.” Because the targeted end group is such a small part of the entire molecule chain, the overall chemistry of the polymer is hardly changed. However, the end groups are very mobile, because they're only chemically bonded to the polymer at one point.

The surface modifier for the polymer is chemically coupled to the polymer, making it a part of the polymer. It is, therefore, not prone to be lost by wearing off, if abrasion occurs, or sloughing off into the bloodstream or into the tissue. Because it is built-in, the surface modification occurs spontaneously shortly after the tube is extruded. The molecules of the end group move to the surface, find each other, and create a single layer at the surface.

“On a molecular scale, it gives the surface a totally different chemical composition,” Ward says. “We can choose end groups by need. For example, we can choose those that create more biostability. We can pick other end groups that make the surface more slippery, making it easier to slide the tube into a blood vessel or move against tissue. We can pick end groups that make the reaction of the surface with blood more favorable so that blood clots are less likely to form.” Other applications include placing drugs on the end of the chain that interfere with the clotting mechanism and placing antimicrobials on the end groups that aid healing and ward off potential infection.

Ward believes that this will be the next generation of pacemaker-lead and neurostimulation-lead material. It addresses the whole market for implantable tubing. Defibrillator leads are getting smaller, and therefore the tubing used to insulate the electrical conductors must be thinner. Moreover, it has to get stronger and be stable in the body at the same time. “It's beyond what silicone can do, as far as strength in very thin tubing. It needs the biostability of standard polyurethane,” Ward says.

Adding the Art to the Science

Saab says, “Unlike a lot of other processes for which you can buy a machine from a manufacturer, start that machine up, and make product, extrusion is different, and a lot of it is art. You can buy the best equipment in the world, but if you don't have somebody who understands the process and how to make the right adjustments and measurements, it's just not going to happen.”

And the smaller the tubes, the bigger the problems. One challenge is that tubes that have ODs of 0.008 in. or smaller are sometimes so small that most of the commercially available measuring equipment either has difficulty seeing them or can't see them at all. Consequently, a lot of the in-line gauging and measuring equipment is difficult to apply to many extremely small tubes.

Saab says that the first line of control is adjusting and modifying inspection equipment in-house until it does suit the need. The next step is to implement a lot of manual in-process checking, which involves cross sectioning a piece of tubing, examining it under a microscope, and then making measurements off-line. Most tubes are flexible. They are hard to cut. They are hard to measure. They are hard to handle. There is nothing easy about the process. It requires a lot of operator skill in addition to hands-on handling.

“We purchase about half of our major extrusion equipment. We custom build the other half in-house, or we buy custom equipment and then modify it to our in-house specifications,” Saab says. “All of the tooling—the part that shapes the tube—is built in-house and we control that. There are a lot of proprietary trade secrets in this industry regarding tooling and process parameters. Equipment is key. But, you can only get so far with commercially available equipment. It still has to be customized. It is an art.”

Real-time in-line inspection and statistical process control data acquisition are practiced at IDEX Medical to regulate both the ID and OD of products during processing. If a dimension is approaching the specification limit, the system self-controls so that upstream, a modification is made to temperature, pressure or pull strength—whatever is needed to put the tubing in process back into the nominal margin.

Accomplishing real-time control requires a combination of advanced equipment and in-house engineering and program development. “There is excellent equipment available on the market, but it needs to be adjusted,” Lowell says. “You have to do the experimentation so your equipment knows how to talk to itself and also knows what to do. Some of these materials, such as PEEK, are very high temperature, which means that they are difficult to manage.”

Experts also believe that versatility in designing tooling for both single and multilumen thin-wall tubing is what drives quality extrusion.

The Bottom Line Is Service

Whether strictly a contract extrusion facility or a multifaceted facility, one goal is to add value to the finished product with expert technical service. The customer should be able to approach the extrusion provider with a concept or a goal that can be tested for feasibility of design, selection of the correct material, and for cost-effective manufacturability.

“A technical department should have the expertise and experience to work with a customer's engineers to not only develop the design of the tubing, but to ensure that it is manufacturable in a cost-effective way,” Lathiya says. “Sometimes we get requests for products that are either not manufacturable or that can only be manufactured with an exorbitant price and a very low yield. We need to understand the application and work with their engineers.”

Lowell agrees. “First, we will consult with customers and go back to basics. We want to make sure they are selecting the right material for the application. They may not be able to do it with the material they selected, so we might suggest a more cost-effective material or one that has characteristics that will help get them closer to their requirements. Sometimes a slight design change will help.” If there is any doubt about the feasibility of a project, extrusion facilities typically offer an R&D quote. The process requires an investment and some risk on both sides to figure out the challenges. If it goes well, the supplier can then provide a quote for a finished product. If it doesn't, most extruders can usually suggest changes that can make the application work for the end-user, with a concentrated mix of art and science.

Joyce Laird is a freelance writer based in Arleta, CA.

Copyright ©2007 Medical Device & Diagnostic Industry