Thin Is In
Tubing manufacturers face constant pressure from OEMs to push the limits of wall thickness, strength and performance.
With the increasing popularity of minimally invasive surgery and smaller medical devices, OEMs and their end users want higher-performing tubing that is smaller, thinner, stronger and more multi-functional. This can take the form of higher pressure ratings, better torque performance, less kinking, or the incorporation of active components such as antimicrobials.
Medical device companies continue to demand tighter tolerances on the outer diameter (OD) and inner diameter (ID) to improve compatibility with mating components and over molding.
“Concentricity of tubing is also an area that is becoming more critical on the larger tubing designs,” indicated Carl Savage, vice president of sales and marketing for MICRO, a full-service contract manufacturer based in Somerset, N.J. “Design engineers are looking for tubing to be multi-functional in a device design by attempting to combine prior multi-component designs into a single fabricated tube.”
Increased expectations such as these for tubing specifications make it more challenging for tubing manufacturers to meet these rigorous requirements—especially when customers demand tubular products that are smaller in diameter but still have the same physical strength characteristics of larger tubes.
“These two requirements are fundamentally opposite, which stress both our processing and material input capabilities,” indicated James Shobert, chairman and CEO of Polygon Company, a manufacturer in Walkerton, Ind., that makes fiber composite products, primarily tubular, for a variety of precision industrial markets, including medical devices. “We are seeing continued demand for metallic replacement materials that are non-magnetic and also electrically insulating. Most of the applications being sent our way require a much higher degree of engineering and customization, requiring significantly higher resource allocation.”
There also is a “green” movement toward eliminating certain plasticizers and/or other chemicals that pose human health risks. An increasing number of companies are reluctant to use polyvinyl chloride (PVC) and other materials that have harmful leachable components or additives.
“Another key trend is the demand for single-use bioprocess tubing for bioreactors,” said William Shedd, product compliance manager for Eldon James Corp., a Loveland, Colo.-based manufacturer of tubing and tube connectors for bioprocess and medical applications. “This tubing must meet very tight regulatory standards because the processes are often associated with injectables and vaccine production.”
What OEMs Want
Advanced, higher-performing tubing is in high demand for angioplasty catheters, stent delivery catheters, central venous catheters, and other cardiology, peripheral vascular and neurovascular devices.
For example, ablation catheters use smaller diameter (less than 0.100 inches) multi-lumen tubing with up to 10 lumens.
“Taper tubing is being used to offset pre-balloon forming operations,” said Matt Bills, director of extrusion technology for Duke Empirical, a Santa Cruz, Calif.-based contract provider of custom extrusion, braiding, coiling tipping and other specialized processes for custom catheter and device delivery systems.” According to Bills, braided multi-durometer tubing is preferred for catheters that may be threaded through tortuous anatomy and multi-lumen bump tubing commonly is used for peripherally inserted central catheters, commonly known among healthcare providers as PICC lines.
“For the last two years almost every application has required a much higher degree of customization than in the past,” added Shobert. “From a market perspective, we see our customers diligently working on products that provide differentiation. This creates stress for the manufacturing areas because associated volume is lower, but process complexity is higher.”
OEMs are asking for more combinations of flanging, flaring, expansion, forming and stamping on an increasing number of tubing designs, which can require more tooling and increase operational costs.
Karen Werkheiser, sales manager for Fluortek, a provider of precision plastic extrusion and component assembly in Easton, Pa., indicated customer requests are up for specialty profiles and customized heat-shrink. “We have also added services such as tipping, flaring and flanging that were previously done in-house by the customer or outsourced to a different vendor,” she said.
Shedd has observed an increase in requests for multi-lumen and bonded tubing, which offer advantages in identification of tube lines and provides enhanced aesthetics when used in assemblies. Cut-to-length tubing also is gaining traction. “OEMs are looking more toward products delivered at the component level that can be installed directly into assemblies,” he said. “Cut-to-length product is used in both assembly of analytical devices and in peel-open sterile kits for single use medical applications.”
Of course, with these increased demands for higher-performing, more complex designs comes the pressure of improved process validation and cost reduction. Manufacturers must strive to reduce variability and assure lot-to-lot consistency so the overall device fabrication and assembly process remains robust and minimizes challenges during the later manufacturing stages. This calls for more statistical process data, raw material certifications and downstream process information such as sterilization method compatibility.
“One of the ways to control costs is [by] incorporating the sterilization requirements of finished devices early in the process,” said Geary Havran, president of NDH Medical Inc. in St. Petersburg, Fla., a provider of custom extrusion services for the life-sciences industry. “This allows materials to be selected that can accommodate the preferred sterilization method, while maintaining compatibility of the various components and the assembly process steps such as bonding, thermal bonding and printing.”
Thinner and Stronger
The trend toward minimizing the size of devices is continuing to push the limits of the extrusion process and material properties to produce tubing with smaller overall dimensions and thinner walls, without compromising finished device properties such as kink resistance, columnar strength and pressure ratings.
“We are frequently asked what is the thinnest wall we can extrude?” said Bills. “Producing walls as thin as 0.001 inches without the use of an inner core is challenging but we have gone as low as 0.0007 inches. State-of-the-art processing techniques enable us to produce and hold these thin walls to plus or minus 0.0003 inches or better.”
Walls can’t just be super-thin—they also have to be as strong as their thicker-walled counterparts.
“There is a keen desire in the medical-device community to have tubular products that are smaller in diameter but that still have physical strength characteristics similar to the larger tubes,” said Shobert. “These two requirements are fundamentally opposite, which challenges both our processing and material input capabilities.”
One problem with thinner walls is the increased ability of the tubing to kink. “We can alleviate this problem by combining different materials with stainless steel braid to give optimum performance with the thinnest wall,” said Bruce Nichols, president of RiverTech Medical LLC, a manufacturer of polyimide tubing and thin-walled composite tubing in Chattanooga, Tenn. “We can make polyimide tubing down to .0005-inch wall thickness and have produced braided tubing with a wall thickness as low as .002 inches.”
Savage noted that the challenge with thin tubing is maintaining a uniform wall thickness throughout the drawing processes. “A 0.005-inch wall thickness is very common in 5 millimeter types of devices, but recent requests have been surfacing whereby a wall thickness of 0.002 inches on 5 millimeter tubing is being designed into devices,” he said.
Not all applications are ideal for thin tubing, however. Besides kinking, thin tubing also can develop aneurisms under pressure. Therefore some procedures, such as bioprocess applications, are more compatible with thicker-walled tubing because it does a better job of sustaining vacuum pressure.
Extruders need state-of-the-art controllers and screw design, as well as a deep understanding of the extrusion process, for producing high-performance, thin-walled tubing. “This combination allows for some of the most advanced and complicated extrusions to be produced at the highest quality possible,” said Bills.
State-of-the art extrusion equipment now makes it possible to extrude multiple layers and diverse geometric configurations. Profile tubing, a process where a final non-round shape is drawn into the tubing as the last processing operation, is relatively a new development that is being widely used by design engineers.
“Previously, a round tube needed secondary fabrication to obtain a profile shape,” said Savage. “Now it can be done as part of the mill drawing operation.”
Some new equipment that has impressed Shedd is Guill Tool & Engineering Co. Inc.’s “MicroAdjust,” an extrusion head that centers using a single screw on the horizontal and vertical axis.
“This is a simple, low-tech thing, but it can save a lot of time and frustration,” he said. “Movement is more precise compared to a conventional concentricity bolt, which also reduces the risk of over-adjusting.”
For Eldon James, the most significant technological advances affecting the company are in downstream equipment. This includes highly accurate microprocessor-based measurement devices and cutting and winding equipment. New measurement equipment monitors in process deviation and is capable of statistical process data output. This capability greatly increases efficiency by reducing trial-and-error start-up measurements.
“Although it’s expensive, this technology is an effective way to compete with cheap labor in Asia,” Shedd stated. “It also gives us the control over quality needed to satisfy the most technical applications.”
When asked about the return on this capital investment, Shedd indicated it was too early to tell.
“Most of our equipment has been purchased in the last two or three years and large medical tubing projects often take several years to develop,” he said. “The bottom line is that unless a company is producing strictly commodity product, it needs state-of-the-art equipment and facilities to bring customers on board.”
Tubing runs fast, particularly in smaller sizes, so it is important to have downstream winding and cutting equipment that can match the output of the extruder head. Applying microprocessor-controlled automation at this end of the process not only increases efficiency, but contributes to product uniformity and fewer rejects.
“It is difficult to compete with commodity low-tech products made offshore, so addressing the up-market technology sectors makes more sense from a business perspective,” Shedd said. “Newer extruder lines with top-of-the-line downstream gear are capable of operating about three times faster than older equipment. Having that extra capacity can make the difference in being able to accept a large contract.”
Cleaning and sterilizing a product is just as important as manufacturing it—no matter how high-quality the final product is, contamination will render it useless. Tube cleanliness, especially with more complex tubing, always is a major challenge.
“We have invested heavily in custom cleaning systems, in our facilities in both New Jersey and Korea,” MICRO’s Savage said. “We utilize a combination of ultrasonic and electrochemical processes to ensure we send out the cleanest possible tubing products.”
Partnerships with material suppliers and compounders are critical to the development and implementation of new materials. Successful projects typically involve a partnership with the raw material supplier, the extruder and the end product designer. “Often equipment needs to revolve around the continuing improvement of extrusion equipment, such as incorporating new generation controls and drives, enhanced data collection and analysis software and proprietary screw designs among other refinements,” said Havran. “Frequently standard off-the-shelf equipment must be customized or modified to optimize specific manufacturing processes.”
Duke Empirical has been working with a materials manufacturer to develop lubricious materials for use in catheter delivery systems. This helps reduce the co-efficient of friction between the tubing and whatever may be passing through it. “Currently it is challenging to accurately apply a spray-on lubricious coating to the inner diameter of micro-diameter tubes,” said Bills. “It is nearly impossible to do for tubing with micro inner diameters and long lengths. By adding lubricious fillers to the base polymer, we can easily extrude micro-diameter tubes and have the inner diameter and outer diameter lubricious. This saves time and money for OEMs that are looking for a solution to a common problem.”
Several new suppliers and new grades of materials are creating additional options for medical device designers and specification developers. New medical devices are being designed around specific material developments that are enabling new or improved device performance, including lowering infection rates and improved biocompatibility. “The challenges with respect to replacement materials are cost, the ability to use a substitute material without fundamentally changing the device assembly process and overcoming reluctance to change what is viewed as having worked well for many years,” said Havran.
Although they can be highly effective materials, resin blends sometimes also present regulatory concerns regarding traceability and quality. “It is always an advantage if a resin supplier can produce its own custom blend,” said Shedd. “It solves some issues like traceability and blend consistency that can be challenging when blending in-house.”
Generally, pellet blends must be blended, extruded, re-pelletized and then re-extruded into the final medical product, which may require specialized equipment and verified cleaning protocols, which are beyond the scope of some extruders.
“We have seen an increased interest in TPE (thermoplastic elastomer) materials,” said Shedd. “These materials tend to be clean in terms of composition because they do not contain plasticizers, which are generally leachable and negatively bioactive. They can also be produced without animal derivatives (a requirement for some medical applications) and are compatible with multiple sterilization methods, including autoclave.”
Unique fibers such as steel, glass, carbon, Kevlar, Teflon, metallic and a variety of thermoplastic fibers can be built into tube structures to add enhanced physical properties, especially strength.
“We are researching new base fiber materials and introducing nanoparticles into both our fiber architecture and base resin systems,” said Shobert. “Most thermoplastic tubing materials are not continuous fiber in orientation, either filled or short; because our products are fiber-based we have a wider availability of options that can be incorporated into our tubing.”
Polygon Company’s unique thermoset products provide the somewhat lower physical-strength characteristics of metallic tubing such as stainless steel, inherent die-electric properties and significantly higher strength characteristics than that of thermoplastic tubing.
“Because we incorporate filaments directly into the structure of our tubing walls, which we call fiber architecture, we have the capability to vary the architecture to create variable physical strengths,” said Shobert. “This allows us to fundamentally explore the potential of placing separate elements within the wall with completely different performance parameters, opening up a wide variety of options for our customers.”
Materials such as coated or plated fibers are of extreme interest to Polygon. “Advances in higher denier filaments with higher tensile moduli are always on our radar screen,” said Shobert. “An area of concern, however, is that from a volumetric perspective, our medical applications do not represent a very high market opportunity for most fiber producers, who may also carry product liability concerns.”
RiverTech Medical manufactures polyimide tubing and thin-walled composite tubing that combines different materials with stainless steel braid to impart optimum performance with the thinnest walls possible. Composite tubing consists of stainless steel braid and outer coatings of polytetrafluoroethylene (PTFE) composite, Pebax, Tecoflex and other materials.
“We have been experimenting with different coatings and coating thicknesses, along with braid wire size, PIC count, etc., to give the best results possible,” said Nichols. “Manufacturers are also looking for outer coatings on polyimide tubing to be used as a ‘tie layer’ to ‘fuse’ other materials. For example, if a client wants to fuse Pebax onto polyimide (using the polyimide or PTFE composite-lined polyimide tube as a liner), we can coat a thin layer (.00025 inches) of Pebax on to the polyimide to be used as the tie layer.”
Science vs. Art
Most tubing manufacturers agree there is a touch of “art” in theextruding process—where skilled engineers or operators go beyond the pure science of process and rely on instinct or “touch” to find a “sweet spot” where it all comes together to create the smoothest process and purest tubing with the least amount of steps.
Take, for example, the level of precision for a draw down ratio. “Most polymers process at their best given a certain draw down ratio,” said Bills. “Within a given polymer, several factors come into play when selecting the proper draw down: What is the end use for the tube? Is high burst strength needed? How critical is tensile strength and elongation for the tube?”
Every polymer has a draw down window that can be selected to achieve these end results. Some polymers are very sensitive to the draw down and therefore have a very limited window in which they can be drawn down. Others have a very large window and some of the polymers used in catheter applications are made with these polymers.
“For extrusion, science exists in terms of the process (material selection, draw down, temperatures, and speeds forinstance) to achieve proper molecular orientation,” said Bills. “The ‘art’ comes in the form of being able to string the line, dial in the dimensions, and being able to read the process to produce high-quality tubing that meets specifications, and be able to repeat all of that in multiple runs.
One of the biggest challenges for co-extrusion is finding materials that are compatible in terms of bonding, which often includes a fair bit of trial and error and fine- tuning the process. “Eldon James is a smaller company and we’ve had the good fortune of being able to work with materials developers in cooperative trials,” said Shedd. “This has advantages because not only can we can modify the process, but the materials formulator can adjust the formulation based on what they see during processing. Each individual part of the process—temperature, pressure, vacuum, tooling, and speed—is based on science, but combining those parameters to produce the desired end result requires experience. Extrusion is a dynamic rather than a static process so ‘art’ is definitely a factor. Until you nail the process, it is a combination of art and engineering—after that it becomes science.”
The quality of the masterpiece, of course, depends on the skill of the artists—which means acquiring the best talent. For example, Fluortek has hired several new material and process engineers. “With over 30 years of combined experience, they bring to the company an extensive working knowledge of extrusion science and advanced materials,” said Werkheiser.
Since the parameters of tubing extrusion are constantly changing, the U.S. Food and Drug Administration, OEMs and end usersincreasingly are concerned about how process changes or normal variations affect product performance and reliability. Typical variations in raw material properties between batches are getting more scrutiny after initial validation and processing windows often are locked down with no further changes permitted.
“Much of the data on the effects of various process parameters are developed and retained by extrusion companies,” said Havran. “Seldom is comprehensive extrusion processing data available from raw material manufacturers who typicallyreport injection molding results in their published literature. This leads to theconclusion that there is still a lot of ‘art’ in the extrusion process, although many processors now have developed information banks of data which support a more scientific approach to extrusion.”
Mark Crawford is a full-time freelancebusiness and marketing/communications writer based in Madison, Wis. Contact him at firstname.lastname@example.org.