Advancements in medical technology and material science have created new engineering challenges for medical devices used in minimally invasive interventional procedures. Medical device companies are making devices that are smaller (less invasive), more complex, and multifunctional, which require more precision and tighter tolerances from their contract manufacturers. For example, in the extrusion market, device companies continue to develop complex new designs for single products that require co-extrusion, multi-lumen, multidurometer segments, or variable coil/braid reinforcements.
As challenging as these expectations are, what is perhaps hardest to design into new products is low cost. The medical industry expects more technology and function, but at a lower price point. Device companies are continuously looking for ways to save money on their new launches; some have even initiated corporation-wide global initiatives with goals to reduce costs over the next several years.
Increasingly complex designs, materials, tooling, and prototyping all drive up costs — yet cost savings must still be engineered into the final product, without compromising quality or function. One way to do this is by improving efficiencies that speed up the manufacturing process (automation, for example) and reduce waste. Another method is to work with a partner skilled in the practice of designing for manufacturability, that is, engineering products and processes that eliminate certain steps, such as finishing and assembly. For example, overmolding, multi-shot molding, and other injection-molding technologies can bond different materials together in a single step, thereby reducing assembly steps. More sophisticated process control and validation, such as robotics or sensors and visual systems, can monitor manufacturing process parameters in real time, observe the ejection of product from molds or extruders, and inspect final products in microscopic detail — all of which reduce production costs.
Yet another cost-control method medical device companies are taking seriously is aligning themselves with vertically-integrated outsource partners, like Teleflex Medical OEM (Kenosha, WI), that can provide expert, single-source, concept-to-completion services that meet all their design, production, and regulatory needs. This greatly facilitates communication and decision-making while improving quality control, waste management, and speed to market. Any catheter provider that wants to be a key part of the supply chain must provide integrated design, engineering, material selection, prototyping, testing, validation, manufacturing, assembly, packaging, and labeling capabilities — and also know the regulatory landscape in detail.
Miniaturization is Huge
Driven by the proliferation of minimally- invasive surgical procedures, devices continue to become smaller — which requires more innovative processes for design and production. It is much more difficult working with the tighter specifications that miniaturized products require. For example, tubing can have interior diameters as small as 0.004 inch ±0.0005 inch with walls as thin as 0.005 inches. Catheter sizes can be as small as 1 Fr – that size is for a braid reinforced catheter, not just a simple extruded tube.
Microcatheters, one of the most common miniaturized products, are ideal for diagnostic and therapeutic neuroradiology or occlusive therapies. Outside diameters are as small as 0.026 inches, interior diameters 0.014 inches, and minimum wall thicknesses are 0.005 inches. Liner and outer jacket materials, braid or coil reinforcement, durometer and stiffness, variable shaft diameter, embedded marker bands, and thin-wall construction can all be customized according to use. Microcatheters are available with flat or round wire and coilreinforced configurations. Flat wire is generally used for thinwalled applications where performance is critical. Coiled shafts are more flexible and kink-resistant, but may not provide the same torque and push capabilities as a braided shaft.
Microcatheters are often used in delicate neurovascular cerebral procedures for the treatment of strokes and aneurysms, so very high precision manufacturing is required. Building a catheter, regardless of size, is often an “inside out” process where the inner liner (most likely polytetrafluoroethylene, or PTFE) is layered with the coil or braid support, followed by bonding with the outer shaft.
The big challenge is that each layer has its own specifications and tolerances, which collectively add up (“tolerance stacking”) to meet the specifications and tolerances determined for the final product — a key consideration during material selection. Manu fac turing and process controls are tightly controlled, and because the final product is essentially microscopic, rigorous inspection and control systems are required to detect the smallest imperfections.
Complex Extrusion Engineering
Medical device companies are designing more complex products with difficult or asymmetrical shapes, including multi-lumen, bump tubing, co-extrusion, and braid/coil reinforcement. These products often require tighter tolerances, thinner walls, greater kink resistance, and special fluoropolymers and thermoplastics with certain characteristics and properties.
Tubing is often reinforced with braided or coiled material to add strength. Teleflex Medical OEM has developed a proprietary manufacturing process that can produce a variable pitch, coiled shaft in a continuous process. Often a shaft doesn’t require the same amount of reinforcement in all areas — being able to change the tightness (pitch) of the wound coil along the length of the catheter, where that degree of strength or stiffness isn’t required, increases functionality, flexibility, and reduces weight. This is ideal for large-diameter delivery systems where catheters often have pushability/flexibility issues because of their size — which may be better managed with a customized, variably pitched coil.
Lubricity is another critical quality. Neurovascular catheters in particular must be highly lubricious on both interior and exterior surfaces. A popular material for catheters is PTFE, a fluoropolymer resin that is highly lubricious, chemically resistant, and presents excellent dielectric insulation properties. PTFE can be extruded with ID as small as 0.005 inches and wall thicknesses as narrow as 0.001 inches. PTFE does, however, require etching to improve its bonding with other materials.
Etching is typically done with a chemical solution that removes fluorine molecules from the exterior of the PTFE tubing, creating a surface with a deficiency of electrons. This surface then becomes more chemically responsive to bonding to a different material. Some drawbacks to etching do exist — the outer surface will often darken in color, exterior surface lubricity is reduced, and additional processes are necessary to stop the etch from affecting the PTFE. It is important to note that the etching process does not impact the inner surface of the PTFE tube.
Etching, however, can be eliminated with a proprietary Teleflex Medical OEM technology that co-extrudes EFEP (perfluoronated ethylene-propylene copolymer) with polyamideor PEBA-type materials in a one-step process. EFEP is a lubricious material that can be used as a liner or outer material. EFEP co-extrusion is faster and cheaper than using FEP (fluorinated ethylene propylene) or PTFE, which must be etched. EFEP co-extrusion also provides outstanding transparency.
EFEP works where conventional fluoropolymers cannot be used. For example, it can be co-extruded as an outer layer or tube liner, or laminated over coil- or braid-reinforced assemblies. EFEP provides the advantages of fluoropolymers (lubricity, chemical resistance, and biocompatibility), plus the advantages of traditional materials like polyamide or PEBA (flexibility, ease of bonding, and over molding). Tubing stiffness can be tailored to specific applications by varying the durometer of the coextruded materials. The most common application for EFEP is multilayer tubing for IV or other fluid-delivery applications. EFEP is a USP Class VI-approved material that can be gamma- or ETO-sterilized.
Medical device companies are increasingly interested in making products from other types of thermoplastic resins, such as PEEK. Material producers continue to engineer new hybrids and blends that have enhanced characteristics such as strength, flexibility, and durometer. This is where knowledge of material science — both for well-established materials and new materials — really comes into play. By quickly determining which material will deliver the best results for the product’s end use and the manufacturing process selected, experienced extruders can save their clients significant development time and reduce production costs (especially with prototyping and molding).
It is, of course, a requirement for interventionalists to see exactly where catheters are going during their often-delicate surgical procedures — in other words, the catheters must be radiopaque (visible on X-ray). This is typically accomplished by using steel marker bands in the catheter tubing — although the steel shows up fine on X-ray, it creates an undesirable stiffness that even the best physicians can find frustrating.
To solve this problem, Teleflex Medical OEM developed a novel multilayer technology that uniformly embeds a tungsten layer in the catheter. Tungsten is radiopaque yet highly flexible in powder form. This allows the catheter to be seen easily under fluoroscopy, without the stiff metal marker bands that can limit flexibility — a perfect option for those procedures that require highly flexible and radiopaque catheters.
The biggest challenge with tungsten encapsulation or other radiopaque fillers is that they must be evenly distributed and not contribute to increased wall thickness or increased outer diameter beyond the agreed specification; the inner profile/inner lumen must also not be narrowed. It is essential that the extrusion process is precisely controlled so the tungsten powder is properly embedded in the correct concentration and dense enough to be visible under X-ray, without being “lumpy.”
Physicians, hospitals, and health-care systems seek out devices that increase patient outcomes and reduce hospital stays, ultimately reducing the cost of health care. This has created intense interest in antimicrobial coatings on medical devices and certain instruments to reduce the possibility of post-surgical infections.
Medical industry suppliers are now producing resins that are “doped” with antimicrobial agents that then become a permanent part of the final plastic product. Silver is a popular antimicrobial agent that is incorporated into medical products today — silver ions are highly effective in killing bacteria and do not damage human tissue. Antimicrobial resins have been engineered that can kill deadly micro-organisms such as MRSA, a dangerous bacteria that is often found in hospitals.
Antimicrobial agents do not typically affect the physical or chemical behavior of their host polymers or thermoset materials during the molding process. Antimicrobial agents are “carried” by another compound that is mixed with the resin prior to extrusion; the agent, carrier, and plastic selected depend on the usage of the product. A key step in the process is calculating the amount of agent needed; the plastic is formulated to release silver ions at a pre-determined rate over the expected life of the product.
Although device companies don’t necessarily know all about these new technologies and advanced materials, they fully expect their key suppliers and design, development, and production partners to be expert advisors on these topics.
The best way for a medical device company to assure performance, quality, speed to market, and the lowest possible costs is to involve a single-source vertically integrated extrusion/ catheter vendor at the very earliest stages of product design and development. This is where the companies exchange information and share their knowledge about the latest technologies, materials, and processes. It’s a little like seeing a physician — the best results happen when the device company opens up candidly to the vendor’s questions about expectations, concerns, product usage, deadlines, markets, costs, or other barriers. Acceptance by the medical device company of this upfront “value engineering” from the outsource partner is what often leads to beneficial design modifications that streamline design, reduce prototyping and extrusion/manufacturing charges, accelerate throughput, improve quality, and speed up time to market.