One reason for this trend is that more clinical procedures are being performed in the surgery center or clinician’s office in an outpatient setting, rather than the hospital. An increase in off-site patient treatments also compels some OEMs to simplify product design and packaging to increase ease of use. Of course, medical device manufacturers want these innovative products to be engineered and produced during increasingly short timelines, which calls for the need to iterate tooling quickly.
Meeting all these expectations requires increasing specialization when it comes to extrusion. For example, multi-lumen, co-extrusion, miniature and micro extrusion, over extrusion, tapered extrusion or braiding reinforced extrusion processes, integrated with several other technologies often are required to manufacture a single extruded part. Specialization is trending toward miniaturizing extruded tubes to be less invasive and provide accessibility to small body areas, such as the eye, brain, or a neonate—a big challenge, especially considering how fragile these critical human structures can be.
“Miniaturization calls for the selection of suitable raw materials, keeping the extruded part design simple, having appropriate systems in place to verify quality and post-extrusion processing,” said Ronald Lilly, vice president of sales and marketing for Forefront Medical Technology, a Singapore-based medical device contract manufacturer.
A key factor in selecting raw materials is melt strength—its ability to be drawn out and still hold its form.“It is not unusual to work with drawn-down ratios as high as 40:1 or even higher, depending on complexity of the design,” added Lilly. “Other factors that impact material selection include tight tolerances, increasing demand for new and better performing polymer materials, disposability, and shorter delivery time.”
OEMs and their customers’ clinical needs are driving demand for higher tubing performance in the field. In addition to strict dimensional requirements, more end users want tubing that has increased performance criteria. Multiple-use devices also are being designed to last longer and perform more functions than ever before.
“A good example of a device designed to perform multiple functions is a central venous catheter (CVC),” said Cassie Botti, director of business development for Fluortek Inc., an Easton, Pa.-based manufacturer of custom medical tubing and partially finished medical devices. “CVCs are used to monitor central venous pressure, drug delivery, dialysis and other functions. Many of these catheters now incorporate drug-eluting coatings to help prevent certain types of infections. We are seeing an increasing number of these designs come across our desks.”
The development of higher-pressure balloons, coupled with the push for smaller-diameter balloon catheters, create greater stress on the shaft tube extrusion. The shaft must be strong enough to withstand the higher pressures the balloon can achieve, yet also have a customized weld of the shaft tube to the balloon that is strong enough to handle these stresses.
“It’s a classic ‘weakest-link-in-the-chain’ scenario,” said Mark Geiger, vice president of sales and marketing for Interface Catheter Solutions, a Laguna Niguel, Calif.-based balloon catheter designer and manufacturer. “Device manufacturers like Interface Catheter Solutions work closely with OEMs to blend the desired performance and size specifications with what can be achieved from a materials and welding standpoint.”
New Materials, New Possibilities
Common extrusion materials are polytetrafluoroethylene (PTFE), melt fluoropolymer, polyolefin, thermoplastic elastomer, thermoplastic polyurethane, polyamide, polycarbonate, polyetheretherketone (PEEK), polyetherimide, and others. Thermoplastic polyurethanes are a common material choice for vascular catheters, including polyester and polyether. Polyether block amide (PEBA) is softer and more elastic than polyamide, making it a good material for interventional catheters. Thermoset polyimide, because it is constructed through a layering process, often is selected for high-strength, thin-wall tubing that will navigate thin blood vessels in the head and neck.
Material manufacturers are producing a wider range of bioabsorbable, antimicrobial and other specialty raw materials for advanced extrusion products. These materials and additives have been selected to tolerate high extrusion process temperatures (tolerance for some of these materials also depends on how long they are subjected to the extrusion temperature).
PEEK is becoming more popular for products that require high strength or heat resistance, such as ablation tools, because of its pushability and buckling resistance. Single-lumen PEEK extrusions can be extruded with diameters as small as 0.01 inches and wall thicknesses of 0.002 inches. PEEK also has been combined with titanium for some orthopedic and spinal surgical devices.
Dayville, Conn.-based Putnam Plastics Corporation, a provider of advanced extrusion technologies for the medical device industry, recently announced it extended its PEEK offerings to include large-diameter, thin-walled tubing up to one inch in diameter for endoscopy and laparoscopy procedures.
“Processing temperatures for PEEK are much higher than traditional catheter materials such as urethanes and polyamides,” said Larry Alpert, new product development manager for Putnam.“Companies that do have the temperature capabilities or controls for extruding PEEK tend to focus on small-diameter tubes or large solid shapes. Larger tubes provide new possibilities for emerging medical devices.”
New lubricious materials also are being developed to reduce friction. Lower durometer materials tend to be “sticky” or have a tackiness that requires the application of coatings to improve surface friction. A solution to this problem is the use of PTFE liners to reduce friction on the interior diameter of catheter shafts; however, this is a more expensive process and also limits the sterilization options, durability and resistance to abrasion. It also can be too stiff for some applications that require high flexibility.
An alternative to using PTFE liners as the lubricious internal diameter for catheter tubing is the use of high-density polyethylene (HDPE) as the inner layer of a tri-layer extrusion, which joins the HDPE liner to an outer material such as nylon, with a tie layer that bonds to both the outer material and the liner.
“The use of a tri-layer extrusion, however, can put limitations on how thin the wall can be, considering there are three materials being put together, each with its own wall thickness and tolerance,” said Robert LaDuca, CEO of Duke Extrusion, a Santa Cruz, Calif.-based medical extrusion supplier that specializes in catheter applications. “Duke Extrusion has developed a class of PEBA, nylon, and polyurethane materials that are highly lubricious, readily bondable, capable of forming the thinnest possible walls, certified to USP Class VI standards and are compatible with gamma and e-beam sterilization methods.”
These materials—PebaSlix, NyloSlix-12, and PolySlix—are being used in a variety of catheter products that take advantage of these novel characteristics for meeting challenging design requirements.
Finding the Right Solution
The need for lower-profile devices continues to be a driving factor in the development of tighter tolerances and thinner-walled extrusions. For example, small-bore, braided catheters and balloon tubes are becoming more popular for angiographic, neural and orthopedic needs, as well as neonatal and pediatric size applications, sometimes with multiple segments that are braided and nonbraided. It is critical for the toolmaker to be able to make small bore, thin-wall extrusions, with tips and dies durable enough to withstand the stresses of high-volume production—and still keep costs down.
“Pricing pressures in the market are an opportunity to develop innovations that can still accomplish the clinical need, with reduced cost through design changes,” said LaDuca. “For example, braid-reinforced tubing for high-pressure line applications meets the clinical need for flexibility, kink-resistance and ability to hold high pressure. As a cost-saving alternative, tube-on-tube co-extrusion design can meet the same product requirements by having a thin inner layer made from a high-tensile strength material for withstanding high pressure, combined with an outer layer made from a flexible tough material to provide kink resistance.”
Multifunctional tri-layer tubing also is increasing in popularity. This process involves the simultaneous extrusion of three different layers around the catheter shaft. Tri-layer tubing provides the advantage of reducing the overall cost of catheter manufacture because it provides low friction for the advancement of a guide wire or catheter through the lumen, without compromising strength and stiffness.
“Customers want a lubricious inner layer of the tubing that slides well over a guide wire, with an outer layer that bonds well to the material of the main shaft tube,” said Geiger. “Our tri-tubing with three layers has an outer diameter of only 0.0206 inches.”
Challenging performance requirements that some customers request provide a perfect collaborative opportunity for designing and producing customized extrusions. If the processor understands the intended use of the tubing, it can bring its own experience to the design process and make valuable suggestions that will improve manufacturability and intended end use. For example, the extruder may discuss processing factors that can impact the resultant tubing properties, such as an area draw-down ratio for the tooling that not only produces the required dimensions, but also optimizes elongation or ultimate tensile strength.
“For example,” said LaDuca, “when producing tubing for catheter applications, there are often reflow processes that are affected by the stresses that are put into the material during the extrusion process. By understanding the application and the issues related to further processing of extruded tubing, the extrusion process itself can be optimized to produce the desired properties. As another example, annealing conditions for tubing can remove stresses that may affect how the tubing behaves in its end application.”
A growing request among OEMs is replacing metal marker bands in catheters with more flexible materials that also are radiopaque. Metal marker bands, which easily are detected under an X-ray fluoroscope, are placed at strategic locations along the catheter so surgeons know exactly where the catheter is during the procedure.
“For specialized applications, many in the industry are moving away from gold, platinum, and tungsten marker bands for radiopacity toward urethane-based and polymer based marker bands,” indicated Steven Labrum, vice president of global OEMs for Merit Medical Systems Inc., a South Jordan, Utah-based manufacturer of medical devices. “These materials are much better than hard metal for navigating tortuous anatomy.”
As companies continue to move toward tighter tolerances and more complex co-extrusions, they also are working hard to control the entire supply chain in order to meet and exceed stringent U.S. Food and Drug Administration (FDA) guidelines—including safer and smarter fluid path delivery systems.
“Safer fluid path refers to using optics to inspect the fluid path during the extrusion process to insure foreign matter does not occlude the fluid path, especially on small micro-bore tubing sets,” indicated Bob Donohue, general manager for the global healthcare division of Natvar (a Tekni-Plex Inc. company) based in Clayton, N.C., which produces medical tubing for device manufacturers around the world.
Closed-loop automation, along with optical inspection, is growing increasingly popular—in part because customers are demanding it.
“OEMs need to have partners who are willing to drive quality to the next level and provide things like process failure mode effects analysis plans, robust inspection control plans, and the ability to have rapid response to quality issues, should they arise,” said Donohue.
Extrusion and downstream equipment also have more features and controls available than they did a few years ago. New closed-loop systems provide enhanced extrusion control, which improves the ability to hold tighter tolerances and increased repeatability.
“In the last three to five years the components for closed-loop systems have greatly improved,” said Botti. “These include the use of a three- or four-axis laser instead of a two-axis, improvements in ultrasonic measurements, software for statistical process control and real-time data collection.”
Catheters are on the path to becoming a “commodity item” so OEMs are pushing hard to develop intelligent devices that incorporate electronics, do more, and stand out from the crowd.
“These can include servo systems, chips, micro electrochemical systems, photonics and optics that allow for robotic control of catheter motion and active feedback such as pressure monitoring and real-time diagnostics,” said Botti.
Most of these next-generation catheter systems are still being developed. A few flexible robotic catheter systems have been developed for minimally invasive cardiac procedures. Mechanical components include a biocompatible passive flexible catheter and active servo system that controls the flexible catheter. More complex, 3-D end-point motion is controlled by pitch and yaw movements (enabled through actuation of articulation control tendons) and insertion motion.
Adding More Services
Fluortek has made substantial investments in upgrading its IT systems to provide enhanced interface capability with the manufacturing floor. Multiple systems now can be monitored proactively from onsite and remote locations.
“Unexpected process variations can be analyzed in real time, as opposed to post-run, resulting in a more consistent and higher-quality product,” said Botti. “This can save us up to 20 percent of our time, in most cases. We continue to invest in optimizing our systems, processes and supply chain to allow for increased flexibility, reduced bottlenecking and increased quality.”
Every medical extrusion house attempts to push the limit of how small and how fine a product it can manufacture. Fluortek’s new PTFE Micro-Bore System, for example, extrudes PTFE tubing (not just monofilament) with as small an outside diameter as 0.006 inches—roughly two times the diameter of a human hair.
Natvar recently opened a global tech center in Holland, Ohio. The $4 million facility is equipped with the latest analytical and product-performance testing laboratories and staffed with a team of professionals from a variety of science and engineering disciplines with deep industry experience. There are large custom pilot scale extrusion, co-extrusion, extrusion coating, extrusion lamination and lamination capabilities to support product development. OEMs also go there to work with Natvar engineering teams on new designs and processes.
“Rapid development is essential for meeting the new demands within the industry, along with solid validation plans to meet and exceed the customer expectations,” said Donohue. “The global tech center and our fully validated engineering labs provide faster speed to market. FDA requirements are changing fast for controlling the supply chain and organizations have to stay on top of what’s required to provide true product validation to satisfy these needs.” Forefront Medical Technology has developed new product capabilities of specialty stainless and nitinol wire reinforced flexible tubing for high-performance catheters and other unique delivery systems. Tubing constructions are available with medical grades of silicone, polyurethane or di(2-ethylhexyl)phthalate-free polyvinyl chloride. Standard construction ranges are 5 to 20 millimeters outside diameters and up to 75 centimeter lengths. Custom constructions are available for unique application requirements. The material is ideal for flexible, non-kinking tubing used for airways, drug delivery systems and proprietary applications.
“This new tubing extends product shelf life and also provides tighter tolerances, multiple durometers and specialty designs and a more durable exterior, which improves the user experience,” said Lilly.
Duke Extrusion is responding to the market’s desire for immediately available tubing that can be purchased in discrete lengths in a variety of materials, sizes, configurations and durometers by expanding its stock catheter tubing product offerings, which now include both PEBA and nylon resin. In the near future, Duke will launch a PebaSlix single-lumen product line.
“We understand that our customers are seeking faster iterations for R&D of new products,” said LaDuca. “By providing the opportunity to try materials in their preferred size range and material type quickly, we improve our customer’s ability to custom specify their exact product, with more assurance the tubing design will perform as expected.”
Moving Forward
OEMs must be certain their complete supply chains are in compliance with the FDA, which has been placing more stringent controls on suppliers. This has compelled OEMs to demand more from their extrusion suppliers. Risk management always has been a constant part of developing devices and choosing suppliers, but now it is especially at the forefront of the OEM decision-making process.
“OEMs are demanding that suppliers use medical grade/FDA-compliant materials, and extrusion houses are becoming more heavily involved in the device design process,” said Botti. “Additionally, OEMs are requesting suppliers share in biocompatibility costs. OEMs have shifted to becoming comfortable with proven, next-generation devices and technologies that were developed outside of their companies and have been pre-proven and tested.”
As medical devices continue to push manufacturing limits with smaller and more complex products, extruders will have to work very hard to find creative ways to make these devices without compromising quality, or driving up costs. Concerns at the top of every OEM list are:
- Time, cost, and quality. “Every customer wants product yesterday, at the lowest cost possible, without compromising quality,” said Botti. “This forces tubing manufactures to constantly strive to be leaner and more efficient.”
- Precision. Tighter tolerances require extremely close monitoring of the entire device manufacturing process, including raw material selection, compounding, extrusion and secondary operations.
- Size. Minimally invasive procedures require smaller devices and smaller devices require smaller diameter tubing and thinner walls.
- Repeatability. Validation requirements also have increased, which in turn, demands a higher level of process and/or product repeatability.
- Adaptability. OEMs expect their extruders to be dedicated partners who respond quickly to their design, timeline, and cost needs. Extruders must be able to manufacture extremely small quantities and take products from infancy through third and fourth iterations until they are ready for production.
This also means inventory needs to be managed and monitored closely to assure that a sufficient but not overstocked variety of resins is readily available to accommodate material changes to the designs. “Extrusion houses must have a certain level/ability to produce in house, custom tooling, or a strong relationship with an outside tooling vendor to maintain low lead times during device developmental stages,” she added.
Mark Crawford is a full-time freelance business and marketing/communications writer based in Madison, Wis. He also writes a variety of feature articles for regional and national publications and is the author of five books. Contact him at mark.crawford@charter.net.