Mark Crawford, Contributing Writer04.06.16
Driven by rapid improvements in medical device design and complexity, and the strong demand for less-invasive surgical procedures, the extrusion market continues to be a robust part of the medical device industry. OEMs want miniaturized devices with more functionality—for example, smaller and thinner, but stronger and more maneuverable. Demands are high for smaller outside diameters, larger inside diameters, kink resistance, coatings, and tighter tolerances. Designs include multiple lumens, lumens with variable geometries, and co-extruded materials. Ultra-thin tubing walls are frequently requested—for example, diameters as small as 0.004 inches (about the thickness of a human hair) with tolerances in the ±0.00015-0.00020 inch range for demanding applications.
Meeting these requirements pushes the limits of extrusion technology and material performance. Extruders must be experts at their game. To be a “go to” provider, they must understand and apply the latest science, have expert knowledge of material behavior across a range of production settings, and be able to adapt and finesse their equipment (or design new proprietary in-house equipment) to meet customer design and performance specifications—all in a way that can be verified and meets regulatory demands.
As tolerances continue to approach zero, medical device manufacturers (MDM) rely more on higher-performance polymer materials and sophisticated extrusion methods. These include multi-lumen extrusion, co-extrusion, jacketing, tapered/bump extrusions, and braid-reinforced extrusions, as well as the integration of some of these technologies together. Accurate and relevant data capture is also essential to verify the accuracy and repeatability of the extrusion process and the quality of the extruded product. The need for increasingly specific performance criteria for MDMs also raises the demand for better lot-to-lot consistency.
All these expectations exert constant pressure on component suppliers to maintain the best knowledge of materials and extrusion technology and equipment. They must find ways to use specialized tooling and production processes to create more complex and multifunction tubing—for example, hybrid tubing, which incorporates conductors in the walls of tubing.
“Imaging, electro-surgery, heating, measuring, and other functionalities can all be incorporated into a smaller cross section when bundled into a multi-lumen or hybrid tube configuration,” said Tucker Hill, manufacturing manager for New England Catheter, a Lisbon, N.H.-based provider of custom braid or spiral reinforced tubing, lubricious lined catheter shafts, single/multi-lumen tubing, and hybrid configurations for the medical electronics and device industries.
In contrast to the intense focus on smaller components, there is a growing need for large diameter shafts—up to 35F—that can maintain dimensional stability and concentricity. “In addition to large diameters, customers want shafts that combine engineered elements as thin walls, braid and coil configurations, marker bands, and tri-layer tips,” said Jim McCormack, manager of global marketing communications for Gurnee, Ill.-based Teleflex Medical OEM, a provider of design, development, and manufacturing services for the medical device industry.
Pushing the Limits
“Customers are constantly asking us to make things smaller,” said Chris Mazelin, marketing manager for Specialty Silicone Fabricators, a Tustin, Calif.-based contract manufacturer that specializes in silicone fabrication, assembly packaging, sterilization, and drug-eluting combination products. “For example, customers want lumens of tubing with a diameter of 0.004 inches. This is in response to the increased demand for miniaturized, less-invasive surgical techniques, which is pushing the technological limits of tubing manufacturing.”
“It is not uncommon for us to manufacture extrusions with lumens less than 0.012 inches in diameter, reinforced tubing walls less than 0.005 inches in thickness, or miniature coaxial cables less than 0.01 inches in diameter,” added Hill.
It is not just size—tubing designs such as multi-durometer tubing, multilayer tubing, and material blends/fillers test extruder capabilities on a daily basis. New extrusion equipment continues to show improved digital control. Servo-driven pullers and extruders with pressure control provide greatly improved line speed and output control; traversing spool pay-offs and take-ups, as well as the use of ultrasonic dancers, provide gentler material handling.
“New England Catheter has used these advances to develop in-house pressure control, etching, pre-heating, straightening, guiding, and other equipment made specifically for products and processes,” said Hill.
Extruder manufacturers serving the medical industry have also developed downsized extruders capable of extruding small medical tubes with tight tolerances. These machines are sometimes just smaller versions of larger extruders, which lead to higher efficiencies when extruding small tubing sizes.
“These micro-extruders, with screw diameters as small as 0.25 inches, are very useful in addressing issues of the polymer’s residence time in the extruder,” said Dwain Tarmey, project manager for VistaMed Ltd., an Ireland-based Freudenberg Medical company that develops and manufactures complex extrusions and innovative catheter systems for the medical device market. “Polymers will degrade once melted, so it is important to pump the polymer through the extruder efficiently, maintaining consistent back pressure that is slow enough to have manageable haul-off speeds, but fast enough not to degrade the resin.”
If needed, extruders also invent their own innovative equipment to deliver the functionality and performance that OEMs want in new products—for example, equipment that can change the shape of the extrusion, or the materials being used, during the extrusion process. Specialty Silicone Fabricators has also developed the ability to add pharmaceutical compounds to an extruded tube after the tube is made. The current approach involves adding the pharmaceutical to the melt, which can affect the chemistry of the silicone and inhibit or poison the silicone cure system. Many drugs are also unstable at the elevated temperatures required for the cost-effective vulcanization of silicone components. The Specialty Silicone Fabricators process immerses cut sections of pre-vulcanized silicone rubber components into solutions containing drugs, which are then absorbed into the components. “This process will provide a broad and important platform suitable for numerous other antibiotic drugs and active pharmaceutical ingredients,” said Mazelin. “It also opens up many possibilities for improving existing product lines, as well as developing new opportunities.”
To achieve and reproduce such precise component dimensions and tolerances, extruders must use highly accurate measurement instruments and control equipment. Manufacturing is supported by in-line controls such as laser microscopes or ultrasound devices, which are essential for maintaining critical dimensions such as inner diameter (ID), outer diameter (OD), and wall dimensions and documenting a repeatable, reliable, and stable extrusion process. Repeatability and tolerance control are essential for validation and regulatory approval. Ultrasonic gaging, which has advanced considerably in recent years, is gaining popularity as a process for measuring extruded tubing in-line, especially for tighter tolerances on smaller parts with thinner walls.
“Until recently, the ability of ultrasonic gaging to accurately measure thin walls was very limited, especially with some of the engineering resins in use today,” said Mike Badera, president of Precision Extrusion Inc., a Glens Falls, N.Y.-based subsidiary of Pexco that manufactures advanced medical tubing and catheter products. “It requires more time to set up and fine tune, but for long runs, it makes production more efficient. For short runs, it may not always be a good tradeoff to increase set-up time with less inspection time.”
Material Advances
Medical device manufacturers are increasingly interested in making products from highly engineered thermoplastic resins with enhanced physical properties, such as polyether ether-ketone (PEEK). Some PEEK products can be extruded with walls as thin as 0.006 inches and tolerances of about ±0.0015 ID/OD. Material producers continue to release new hybrid and blended materials with various additives that enhance characteristics such as strength, heat, chemical resistance, flexibility, and durometer.
Although they have attractive physical properties, filled materials that contain radiopaque fillers, lubricous additives, and antimicrobial agents do not always process as easily as their base compounds. Many new materials are also highly heat- or moisture-sensitive and require additional care in handling and processing to maintain material integrity. “This requires newer and better drying equipment, and sometimes changing the extrusion equipment to minimize residence times,” said Badera.
“We design extruder screws and tooling using our specialized machine shop and analytical software, along with custom downstream material handling, to accommodate some of these challenging materials,” added Hill.
Silicone is another material that can be engineered to improve physical characteristics. It is available as long-term-implant, medical, and pharmaceutical grade and can be extruded in several forms, including single and multiple lumen and braided/reinforced tubing. Product applications include feeding tubes, catheters, implants, minimally invasive devices, and sealants. High-consistency rubber (HCR) silicone is becoming more popular because it can be custom-formulated by introducing additives to produce specific physical properties, or improve performance. An increasing number of extruders of silicone tubing are using closed-loop feedback measurement systems to adjust extrusion processes on-line to ensure immediate correction to variations in dimensions such as ID, OD, wall thickness, and concentricity.
“In the past, these measurements were made off-line at regular time intervals, which increased the probability for variation within the system,” said Louay Aboushady, an extrusion project engineer with Freudenberg Medical, a Carpinteria, Calif.-based developer and manufacturer of specialty components and minimally invasive solutions.
Curing silicone with ultraviolet (UV) light is a good approach for high-rate extrusions and thick injection-molded components. The main advantage over the traditional platinum-curing process is higher extrusion speed and output at lower temperatures. “With UV curing, customers will benefit from faster processing, higher yield, and the ability to produce new combination products by eliminating the constraints of conventional heat-cured silicones,” added Aboushady.
OEMs are also very interested in combining the physical properties of silicone and thermoplastics through a co-extrusion process. The outer silicone material provides flexibility and strong bonding characteristics with fittings, while the thermoplastic lining provides barrier properties. For example, with peristaltic pumps, a silicone outer layer provides outstanding pump characteristics and excellent resilience for pump tubing applications; the thermoplastic inner layer (commonly polypropylene), with its distinct hydrophobic properties and low gas permeability, allows almost no interaction with the flow medium running through the tube. One of the most challenging aspects of co-extrusion is creating a strong bond between the silicone and the increasing variety of substrates that OEMs request for multilayered designs. Sometimes a primer or surface preparation using plasma or corona etching is required to enhance bonding.
Advanced extrusion materials include PTFE Moldflon tubing, a medical-grade material that combines polytetrafluoroethylene (PTFE) properties and thermoplastic processing capabilities manufactured by Raumedic AG, a Mills River, N.C.-based provider of extrusion tubing, molding, and assembly services for the medical device industry. PTFE Moldflon has physical properties that are very similar to standard PTFE, such as high temperature and chemical resistance, tensile strength, and very low coefficient of friction—making it ideal for catheter sheathing, wire coating, stent delivery, endotracheal devices, or any applications that require material with a very low coefficient of friction, high abrasion resistance, and insulation properties. Because it is pelletized, it can run continuously on a standard extrusion line, whereas traditional PTFE is a batch process that utilizes non-continuous RAM extrusion lines. PTFE Moldflon can therefore be run on micro-extrusion lines for ultra-thin wall thicknesses down to 0.0005 inches to produce inner catheter liners, without the need to be drawn/necked down, as is required with standard PTFE.
“The biggest advantage, however, stems from its thermoplastic processing capabilities, as this allows additional thermoforming operations on the tube such as tipping and flaring, without showing common manufacturing related issues such as shearing, cracking, or flaking,” said Rudi Gall, managing director for Raumedic.
It can also be easily co-extruded with barium sulfate stripes, which are widely used in IV catheter tubing. Very fine platinum/iridium lead wires of 0.001 inches in diameter can also be continuously insulated/coated with PTFE Moldflon, thereby protecting the physical properties of the fragile wires (for example, cochlear implants that electrically stimulate the cochlea).
Regulatory Considerations
As devices become smaller and more complex, with tolerances within ten-thousandths of an inch, regulatory agencies are taking a closer look at the validation of extrusion processes.
“Validation at this scale means demonstrating that a process can run within a narrow range of an already microscopic specification limit window, and that features of such scale can be measured with sufficient accuracy,” said Mazelin. “To control and measure these complex features, we must continuously develop custom manufacturing and inspection processes. We are often involved early in the design phase when such guidance is most beneficial.”
Regulators are showing increased scrutiny over tooling. For example, the same set of tooling (tip and die) can create many different sizes of tubing (ID and OD). The gap between the tooling and the cooling bath allows for the tubing dimensions to be modified/changed by simply reducing or increasing the gap.
“Regulators, however, can sometimes get too focused on very minor changes to tooling—something that should not be a big concern,” said Declan Whyte, quality, regulatory, and environmental manager for VistaMed.
The other main challenge, he notes, is validation of the extrusion process itself. “A significant number of regulators/auditors come with some knowledge of injection molding, and seem to want to instill the rules around validating an injection molding process onto an extrusion process,” he added. “This is not an ideal scenario, as they are very different processes and need to be approached with a complete different understanding of how best to validate such a process, and what the key parameters are.”
MDMs are moving away from PVC tubing and chlorine-based materials for several reasons. The first is that PVC may interfere with drug dosing efficacy. Independent studies using nitroglycerin show that PVC is prone to adsorb drugs and critical drug components, which raises concerns about the dosing accuracy of drugs supplied to the patient through PVC-based medical devices. The second reason is that PVC may be eventually severely restricted or banned from medical devices for toxicity concerns.
As a result, especially for high-volume PVC tubing demands, companies are moving away from traditional plasticizers such as diethylhexyl phthalate (DEHP) to alternative plasticizers such as diisononyl ester, di-2-ethylhexyl-terephtalate, or tri-(2-ethylhexyl)-trimellate. One reason for this shift is that the European Union (EU) has classified DEHP under the category of carcinogens, mutagens, reprotoxins (CMRs), and endocrine-disrupting substances under its Registration, Evaluation, Authorization, and Restriction of Chemicals regulation. The EU issued a sunset date set of February 2015 for most applications on the use of DEHP.
“Although medical devices fall under the EU’s Medical Device Directive, this does not prevent EU members from enforcing much stricter laws, such is the case of France, which has put in place a ban of DEHP for neonatal and nursing-mother applications that started in July 2015,” said Gall. “On the EU level, there are also first calls to replace CMRs and endocrine disruptors with substitute materials in all medical devices by 2026.”
California’s Proposition 65 was the first in the continental United States to require companies to clearly identify and label DEHP as an ingredient with a cause for concern. The Environmental Protection Agency (EPA) initiated an endocrine disruptor screening program to also review DEHP. However, the U.S. Food and Drug Administration (FDA) has pointed out that it did not have a cause for concern for the continued use of PVC or DEHP for most medical devices. “Nevertheless, this discussion has been ongoing for many years and it has not gone away, so a change of regulatory requirements might be looming,” Gall added.
Therefore, more companies are looking for alternatives to DEHP plasticizer, or even PVC overall. PVC substitutes include polypropylene (PP) blends and thermoplastics elastomers (TPE), which have been developed and are ready available. TPEs, however, are not easy to bond to other substrates, such as polycarbonate or acrylonitrile butadiene styrene connectors, which are widely used in standard IV tubing sets. Good bonding characteristics can be achieved with PP tubing using tetrahydrofuran.
“Although these materials closely approach the performance of PVC, they may require a different approach to design and manufacturing when it comes to bonding to other substrates,” said Gall.
Moving Forward
Extruders are becoming accustomed to the nonstop pace of their business—especially the urgency for higher performance, faster turnaround, longer lifecycle, reduced cost, and inspection/validation. Medical device manufacturers count on their extruders to take the lead in finding the right materials to deliver the proper physical characteristics for their tubing, such as flexibility, torque transmission, and kink resistance, as well as having the technical ability to extrude extremely complex multi-lumen structures and unique profiles. It can be especially challenging to find USP Class IV or ISO 10993 materials that meet all the customer’s requirements and produce the quality required for medical use. “To do this, we rely on our extensive material test history—including mechanical, electrical, and thermal data—to choose one or several materials and/or reinforcements that meet the customer’s specification,” said Hill.
Demand for hybrid tubing and co-extrusions will continue to increase as OEMs seek to cut costs, which may require extruders to invest in specialized equipment. Extrusion is both a science and art. As extrusion becomes more complex, and starts to push the limits of technology, it becomes less a standardized process and more an art form that requires extremely high skill levels and insight by the operator. To push new boundaries, extruders must continue to eliminate variations as much as possible through in-line controls and full loop systems, where real-time data is evaluated immediately to adjust extrusion parameters, if needed.
“It is of great benefit for any OEM to include an experienced extrusion partner of medical-grade materials in early stage product development,” said Gall. “By sharing decades of experience in thermoplastics extrusion and compounding, the extruder will be able to provide a very wide range of material and extrusion know-how and resources. Tapping into such knowledge allows for quick development time, avoids unnecessary surprises, and allocates resources efficiently and wisely to optimize overall positive outcome for time to market of any new medical device.”
Mark Crawford is a full-time freelance business and marketing/communications writer based in Madison, Wis. His clients range from startups to global manufacturing leaders. 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.
Meeting these requirements pushes the limits of extrusion technology and material performance. Extruders must be experts at their game. To be a “go to” provider, they must understand and apply the latest science, have expert knowledge of material behavior across a range of production settings, and be able to adapt and finesse their equipment (or design new proprietary in-house equipment) to meet customer design and performance specifications—all in a way that can be verified and meets regulatory demands.
As tolerances continue to approach zero, medical device manufacturers (MDM) rely more on higher-performance polymer materials and sophisticated extrusion methods. These include multi-lumen extrusion, co-extrusion, jacketing, tapered/bump extrusions, and braid-reinforced extrusions, as well as the integration of some of these technologies together. Accurate and relevant data capture is also essential to verify the accuracy and repeatability of the extrusion process and the quality of the extruded product. The need for increasingly specific performance criteria for MDMs also raises the demand for better lot-to-lot consistency.
All these expectations exert constant pressure on component suppliers to maintain the best knowledge of materials and extrusion technology and equipment. They must find ways to use specialized tooling and production processes to create more complex and multifunction tubing—for example, hybrid tubing, which incorporates conductors in the walls of tubing.
“Imaging, electro-surgery, heating, measuring, and other functionalities can all be incorporated into a smaller cross section when bundled into a multi-lumen or hybrid tube configuration,” said Tucker Hill, manufacturing manager for New England Catheter, a Lisbon, N.H.-based provider of custom braid or spiral reinforced tubing, lubricious lined catheter shafts, single/multi-lumen tubing, and hybrid configurations for the medical electronics and device industries.
In contrast to the intense focus on smaller components, there is a growing need for large diameter shafts—up to 35F—that can maintain dimensional stability and concentricity. “In addition to large diameters, customers want shafts that combine engineered elements as thin walls, braid and coil configurations, marker bands, and tri-layer tips,” said Jim McCormack, manager of global marketing communications for Gurnee, Ill.-based Teleflex Medical OEM, a provider of design, development, and manufacturing services for the medical device industry.
Pushing the Limits
“Customers are constantly asking us to make things smaller,” said Chris Mazelin, marketing manager for Specialty Silicone Fabricators, a Tustin, Calif.-based contract manufacturer that specializes in silicone fabrication, assembly packaging, sterilization, and drug-eluting combination products. “For example, customers want lumens of tubing with a diameter of 0.004 inches. This is in response to the increased demand for miniaturized, less-invasive surgical techniques, which is pushing the technological limits of tubing manufacturing.”
“It is not uncommon for us to manufacture extrusions with lumens less than 0.012 inches in diameter, reinforced tubing walls less than 0.005 inches in thickness, or miniature coaxial cables less than 0.01 inches in diameter,” added Hill.
It is not just size—tubing designs such as multi-durometer tubing, multilayer tubing, and material blends/fillers test extruder capabilities on a daily basis. New extrusion equipment continues to show improved digital control. Servo-driven pullers and extruders with pressure control provide greatly improved line speed and output control; traversing spool pay-offs and take-ups, as well as the use of ultrasonic dancers, provide gentler material handling.
“New England Catheter has used these advances to develop in-house pressure control, etching, pre-heating, straightening, guiding, and other equipment made specifically for products and processes,” said Hill.
Extruder manufacturers serving the medical industry have also developed downsized extruders capable of extruding small medical tubes with tight tolerances. These machines are sometimes just smaller versions of larger extruders, which lead to higher efficiencies when extruding small tubing sizes.
“These micro-extruders, with screw diameters as small as 0.25 inches, are very useful in addressing issues of the polymer’s residence time in the extruder,” said Dwain Tarmey, project manager for VistaMed Ltd., an Ireland-based Freudenberg Medical company that develops and manufactures complex extrusions and innovative catheter systems for the medical device market. “Polymers will degrade once melted, so it is important to pump the polymer through the extruder efficiently, maintaining consistent back pressure that is slow enough to have manageable haul-off speeds, but fast enough not to degrade the resin.”
If needed, extruders also invent their own innovative equipment to deliver the functionality and performance that OEMs want in new products—for example, equipment that can change the shape of the extrusion, or the materials being used, during the extrusion process. Specialty Silicone Fabricators has also developed the ability to add pharmaceutical compounds to an extruded tube after the tube is made. The current approach involves adding the pharmaceutical to the melt, which can affect the chemistry of the silicone and inhibit or poison the silicone cure system. Many drugs are also unstable at the elevated temperatures required for the cost-effective vulcanization of silicone components. The Specialty Silicone Fabricators process immerses cut sections of pre-vulcanized silicone rubber components into solutions containing drugs, which are then absorbed into the components. “This process will provide a broad and important platform suitable for numerous other antibiotic drugs and active pharmaceutical ingredients,” said Mazelin. “It also opens up many possibilities for improving existing product lines, as well as developing new opportunities.”
To achieve and reproduce such precise component dimensions and tolerances, extruders must use highly accurate measurement instruments and control equipment. Manufacturing is supported by in-line controls such as laser microscopes or ultrasound devices, which are essential for maintaining critical dimensions such as inner diameter (ID), outer diameter (OD), and wall dimensions and documenting a repeatable, reliable, and stable extrusion process. Repeatability and tolerance control are essential for validation and regulatory approval. Ultrasonic gaging, which has advanced considerably in recent years, is gaining popularity as a process for measuring extruded tubing in-line, especially for tighter tolerances on smaller parts with thinner walls.
“Until recently, the ability of ultrasonic gaging to accurately measure thin walls was very limited, especially with some of the engineering resins in use today,” said Mike Badera, president of Precision Extrusion Inc., a Glens Falls, N.Y.-based subsidiary of Pexco that manufactures advanced medical tubing and catheter products. “It requires more time to set up and fine tune, but for long runs, it makes production more efficient. For short runs, it may not always be a good tradeoff to increase set-up time with less inspection time.”
Material Advances
Medical device manufacturers are increasingly interested in making products from highly engineered thermoplastic resins with enhanced physical properties, such as polyether ether-ketone (PEEK). Some PEEK products can be extruded with walls as thin as 0.006 inches and tolerances of about ±0.0015 ID/OD. Material producers continue to release new hybrid and blended materials with various additives that enhance characteristics such as strength, heat, chemical resistance, flexibility, and durometer.
Although they have attractive physical properties, filled materials that contain radiopaque fillers, lubricous additives, and antimicrobial agents do not always process as easily as their base compounds. Many new materials are also highly heat- or moisture-sensitive and require additional care in handling and processing to maintain material integrity. “This requires newer and better drying equipment, and sometimes changing the extrusion equipment to minimize residence times,” said Badera.
“We design extruder screws and tooling using our specialized machine shop and analytical software, along with custom downstream material handling, to accommodate some of these challenging materials,” added Hill.
Silicone is another material that can be engineered to improve physical characteristics. It is available as long-term-implant, medical, and pharmaceutical grade and can be extruded in several forms, including single and multiple lumen and braided/reinforced tubing. Product applications include feeding tubes, catheters, implants, minimally invasive devices, and sealants. High-consistency rubber (HCR) silicone is becoming more popular because it can be custom-formulated by introducing additives to produce specific physical properties, or improve performance. An increasing number of extruders of silicone tubing are using closed-loop feedback measurement systems to adjust extrusion processes on-line to ensure immediate correction to variations in dimensions such as ID, OD, wall thickness, and concentricity.
“In the past, these measurements were made off-line at regular time intervals, which increased the probability for variation within the system,” said Louay Aboushady, an extrusion project engineer with Freudenberg Medical, a Carpinteria, Calif.-based developer and manufacturer of specialty components and minimally invasive solutions.
Curing silicone with ultraviolet (UV) light is a good approach for high-rate extrusions and thick injection-molded components. The main advantage over the traditional platinum-curing process is higher extrusion speed and output at lower temperatures. “With UV curing, customers will benefit from faster processing, higher yield, and the ability to produce new combination products by eliminating the constraints of conventional heat-cured silicones,” added Aboushady.
OEMs are also very interested in combining the physical properties of silicone and thermoplastics through a co-extrusion process. The outer silicone material provides flexibility and strong bonding characteristics with fittings, while the thermoplastic lining provides barrier properties. For example, with peristaltic pumps, a silicone outer layer provides outstanding pump characteristics and excellent resilience for pump tubing applications; the thermoplastic inner layer (commonly polypropylene), with its distinct hydrophobic properties and low gas permeability, allows almost no interaction with the flow medium running through the tube. One of the most challenging aspects of co-extrusion is creating a strong bond between the silicone and the increasing variety of substrates that OEMs request for multilayered designs. Sometimes a primer or surface preparation using plasma or corona etching is required to enhance bonding.
Advanced extrusion materials include PTFE Moldflon tubing, a medical-grade material that combines polytetrafluoroethylene (PTFE) properties and thermoplastic processing capabilities manufactured by Raumedic AG, a Mills River, N.C.-based provider of extrusion tubing, molding, and assembly services for the medical device industry. PTFE Moldflon has physical properties that are very similar to standard PTFE, such as high temperature and chemical resistance, tensile strength, and very low coefficient of friction—making it ideal for catheter sheathing, wire coating, stent delivery, endotracheal devices, or any applications that require material with a very low coefficient of friction, high abrasion resistance, and insulation properties. Because it is pelletized, it can run continuously on a standard extrusion line, whereas traditional PTFE is a batch process that utilizes non-continuous RAM extrusion lines. PTFE Moldflon can therefore be run on micro-extrusion lines for ultra-thin wall thicknesses down to 0.0005 inches to produce inner catheter liners, without the need to be drawn/necked down, as is required with standard PTFE.
“The biggest advantage, however, stems from its thermoplastic processing capabilities, as this allows additional thermoforming operations on the tube such as tipping and flaring, without showing common manufacturing related issues such as shearing, cracking, or flaking,” said Rudi Gall, managing director for Raumedic.
It can also be easily co-extruded with barium sulfate stripes, which are widely used in IV catheter tubing. Very fine platinum/iridium lead wires of 0.001 inches in diameter can also be continuously insulated/coated with PTFE Moldflon, thereby protecting the physical properties of the fragile wires (for example, cochlear implants that electrically stimulate the cochlea).
Regulatory Considerations
As devices become smaller and more complex, with tolerances within ten-thousandths of an inch, regulatory agencies are taking a closer look at the validation of extrusion processes.
“Validation at this scale means demonstrating that a process can run within a narrow range of an already microscopic specification limit window, and that features of such scale can be measured with sufficient accuracy,” said Mazelin. “To control and measure these complex features, we must continuously develop custom manufacturing and inspection processes. We are often involved early in the design phase when such guidance is most beneficial.”
Regulators are showing increased scrutiny over tooling. For example, the same set of tooling (tip and die) can create many different sizes of tubing (ID and OD). The gap between the tooling and the cooling bath allows for the tubing dimensions to be modified/changed by simply reducing or increasing the gap.
“Regulators, however, can sometimes get too focused on very minor changes to tooling—something that should not be a big concern,” said Declan Whyte, quality, regulatory, and environmental manager for VistaMed.
The other main challenge, he notes, is validation of the extrusion process itself. “A significant number of regulators/auditors come with some knowledge of injection molding, and seem to want to instill the rules around validating an injection molding process onto an extrusion process,” he added. “This is not an ideal scenario, as they are very different processes and need to be approached with a complete different understanding of how best to validate such a process, and what the key parameters are.”
MDMs are moving away from PVC tubing and chlorine-based materials for several reasons. The first is that PVC may interfere with drug dosing efficacy. Independent studies using nitroglycerin show that PVC is prone to adsorb drugs and critical drug components, which raises concerns about the dosing accuracy of drugs supplied to the patient through PVC-based medical devices. The second reason is that PVC may be eventually severely restricted or banned from medical devices for toxicity concerns.
As a result, especially for high-volume PVC tubing demands, companies are moving away from traditional plasticizers such as diethylhexyl phthalate (DEHP) to alternative plasticizers such as diisononyl ester, di-2-ethylhexyl-terephtalate, or tri-(2-ethylhexyl)-trimellate. One reason for this shift is that the European Union (EU) has classified DEHP under the category of carcinogens, mutagens, reprotoxins (CMRs), and endocrine-disrupting substances under its Registration, Evaluation, Authorization, and Restriction of Chemicals regulation. The EU issued a sunset date set of February 2015 for most applications on the use of DEHP.
“Although medical devices fall under the EU’s Medical Device Directive, this does not prevent EU members from enforcing much stricter laws, such is the case of France, which has put in place a ban of DEHP for neonatal and nursing-mother applications that started in July 2015,” said Gall. “On the EU level, there are also first calls to replace CMRs and endocrine disruptors with substitute materials in all medical devices by 2026.”
California’s Proposition 65 was the first in the continental United States to require companies to clearly identify and label DEHP as an ingredient with a cause for concern. The Environmental Protection Agency (EPA) initiated an endocrine disruptor screening program to also review DEHP. However, the U.S. Food and Drug Administration (FDA) has pointed out that it did not have a cause for concern for the continued use of PVC or DEHP for most medical devices. “Nevertheless, this discussion has been ongoing for many years and it has not gone away, so a change of regulatory requirements might be looming,” Gall added.
Therefore, more companies are looking for alternatives to DEHP plasticizer, or even PVC overall. PVC substitutes include polypropylene (PP) blends and thermoplastics elastomers (TPE), which have been developed and are ready available. TPEs, however, are not easy to bond to other substrates, such as polycarbonate or acrylonitrile butadiene styrene connectors, which are widely used in standard IV tubing sets. Good bonding characteristics can be achieved with PP tubing using tetrahydrofuran.
“Although these materials closely approach the performance of PVC, they may require a different approach to design and manufacturing when it comes to bonding to other substrates,” said Gall.
Moving Forward
Extruders are becoming accustomed to the nonstop pace of their business—especially the urgency for higher performance, faster turnaround, longer lifecycle, reduced cost, and inspection/validation. Medical device manufacturers count on their extruders to take the lead in finding the right materials to deliver the proper physical characteristics for their tubing, such as flexibility, torque transmission, and kink resistance, as well as having the technical ability to extrude extremely complex multi-lumen structures and unique profiles. It can be especially challenging to find USP Class IV or ISO 10993 materials that meet all the customer’s requirements and produce the quality required for medical use. “To do this, we rely on our extensive material test history—including mechanical, electrical, and thermal data—to choose one or several materials and/or reinforcements that meet the customer’s specification,” said Hill.
Demand for hybrid tubing and co-extrusions will continue to increase as OEMs seek to cut costs, which may require extruders to invest in specialized equipment. Extrusion is both a science and art. As extrusion becomes more complex, and starts to push the limits of technology, it becomes less a standardized process and more an art form that requires extremely high skill levels and insight by the operator. To push new boundaries, extruders must continue to eliminate variations as much as possible through in-line controls and full loop systems, where real-time data is evaluated immediately to adjust extrusion parameters, if needed.
“It is of great benefit for any OEM to include an experienced extrusion partner of medical-grade materials in early stage product development,” said Gall. “By sharing decades of experience in thermoplastics extrusion and compounding, the extruder will be able to provide a very wide range of material and extrusion know-how and resources. Tapping into such knowledge allows for quick development time, avoids unnecessary surprises, and allocates resources efficiently and wisely to optimize overall positive outcome for time to market of any new medical device.”
Mark Crawford is a full-time freelance business and marketing/communications writer based in Madison, Wis. His clients range from startups to global manufacturing leaders. 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.
