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Articles from 1998 In March


Double-Coated Adhesive Tape Eliminates a Sticky Operation

Double-Coated Adhesive Tape Eliminates a Sticky Operation

Polyethylene foam tape allows a cleaner, more efficient process as compared to hot-melt

A manufacturer of disposable stimulation electrodes used for defibrillation, ECG monitoring, and external pacing wished to update the design of its R2 pads to enhance quality and performance and, at the same time, simplify the manufacturing process.

Design engineers at Cardiotronics Systems Inc. (Carlsbad, CA) determined they could improve the pad assembly process by substituting a double-coated foam adhesive tape for the messy hot-melt adhesive and single-sided vinyl adhesive tape combination the company had previously been using.

Cardiotronics sought a double-coated foam adhesive tape that would provide the performance required. It also had to be biocompatible and capable of adhering reliably to human skin. With these imperatives in mind, Cardiotronics approached the Medical Products Group of Scapa Tapes (Windsor, CT), knowing that the firm specializes in customizing products for specific applications.

Scapa custom designed a cross-linked, double-coated polyethylene foam tape for the Cardiotronics R2 pad. Not only is the double-coated tape easier to apply, cleaner, and more environmentally friendly than the hot-melt process, but it also provides uniform bonding and stress distribution on the pad. The tape features a hypoallergenic acrylic adhesive formulated for reduced skin sensitivity on direct contact. Because the tape has adhesive on both sides of the foam substrate, it eliminates the time-consuming and messy hot-melt adhesive step, thereby simplifying the production process and increasing product quality.

The outer border of the newly designed pad comprises a 1-in.-wide ring of double-sided foam tape. One side is joined to a similar ring of single-coated foam tape, which is bonded to electrically conductive tin. The other side of the double-coated foam tape retains its liner until it is ultimately used on a patient's skin.

By incorporating Scapa's double-coated polyethylene foam tape into the manufacturing process, Cardiotronics reduced the assembly time required for each pad. Production of the R2 pad still requires some hand assembly; however, instead of applying a messy and time-consuming hot-melt adhesive to bond the two substrates, a production worker simply removes the liner from one side of the double-coated tape and quickly and easily adheres the tape directly onto the single-coated vinyl tape. The foam tape also provides a stronger, more enduring bond than is possible with a hot-melt process.

According to Tim Way, general manager of Cardiotronics, "Scapa's Medical Products Group worked with us to develop a superior product which met our needs. By replacing hot-melt adhesives with the high-performance medical foam tape, we simplified the bonding process and virtually guaranteed that a consistent amount of adhesive is applied every time a pad is manufactured. Our R2 pads are now even higher-quality products, and they're produced faster than ever before."

Scapa Tapes' Medical Products Group's ability to customize a specialized adhesive tape product resulted in a more efficient manufacturing process and a sturdier construction of Cardiotronics's R2 pad.

For more information on tapes and adhesives from Scapa Tapes' Medical Products Group, call 860/688-8000.

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Products Featured on the March 1998 issue of MPMN

Products Featured on the March 1998 issue of MPMN

Video cables custom designed for x-ray machines

In keeping with its tradition of customizing cables for medical equipment (such as pacemakers and ultrasound machines), a company offers video cables for use in x-ray machines. According to the manufacturer, the cables can last up to 20 times longer than conventional cable products. The UL-approved x-ray cables withstand more than 1 million bending cycles at a bend radius as small as five times the cable diameter. Specially designed shielding provides signal protection against high frequencies and magnetic fields. Elocab Tailor-Made Cables, 258 McBrine Dr., Kitchener, ON N2R 1H8, Canada.



Turnkey contract manufacturing services offered

A fully integrated FDA-registered and ISO 9002­certified company offers contract manufacturing of a variety of products, including IV infusion sets, trocars, and blood transfer sets. All development, regulatory, and manufacturing steps are included. Turnkey services include material procurement and incoming inspection, cleanroom assembly, packaging, and sterilization. The company has experience with a wide range of materials, process requirements, packaging configurations, and sterilization methods. Horizon Medical Inc., 1719 S. Grand Ave., Santa Ana, CA 92705.



Prototypes available from investment casting manufacturer

An investment casting company offers direct casting of models made of a thermoplastic material similar to, but with greater mechanical strength than, wax. The CAD files used to build the patterns can be developed from customer specifications on paper or 2-D drawings. Tolerances of ±0.002 in. x-y-z can be achieved using a 3-D model maker, as can very fine detail and surface finishes. The company also produces high-precision metal parts with good performance and functional qualities. Northern Precision Casting Co., Hwy. 120 N., P.O. Box 580, Lake Geneva, WI 53147.


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Editor's Page

Editor's Page

Speeding Product Development: Plan to Succeed

Shortening the product development cycle is without a doubt one of the most important parts of your job as a medical device manufacturer, because getting a product to market as quickly as possible can have a huge impact on the success of the product, and subsequently on your bottom line.

Delays in getting a device to market can be costly and can erode your product's life cycle and future market share. One study estimates that a six-month delay in launching a product can reduce its life-cycle profits by as much as 33% (McKinsey & Co.).

How device manufacturers can avoid this delay was the topic of a conference held during Medical Design & Manufacturing West (January 19­22, 1998) in Anaheim, CA. Bill Evans, president of Bridge Design Inc. (San Francisco), explained that concentrating on the little things is important. "Managers tend to look for the big fix," he said. "But often many small changes will bring results."

One of your first steps should be to analyze the schedule of a recently completed project. "If you find out that failure of early production samples caused delays, you may need to invest in more product validation tools such as finite element analysis or go to outside specialists," Evans explained.

Other tips include opening the communication lines between the marketing teams and the engineers, encouraging the design team to take risks, trusting engineers to fix the problems that arise, and using the right tools. "As early as possible in the product definition stage, test run and think through your CAD data flow," Evans said, "Then select CAD tools that allow this back-and-forth process."

Rapid prototyping can also take weeks, even months, off the product development cycle. "It has so much potential for improving your schedule, it can pay to invest in some in-house rapid prototyping machinery as a competitive advantage," Evans emphasized. You should also explore ways to shorten tooling time and production ramp-up by involving the manufacturing team from the beginning and by starting pilot evaluations and associated documentation early.

Clearly, the more planning you can do prior to initiating a new project, the better your chances of success.

Ursula Jones

ursula.jones@cancom.com

Equipment News: Focus on Surface Modification Equipment

Equipment News: Focus on Surface Modification Equipment

Coating equipment

A supplier of PVD coatings and technology to the medical device industry offers coating equipment for the deposition of biocompatible coatings. Coatings are deposited using various processes, including Ion Bond cathodic-arc PVD, enhanced-arc PVD, and unbalanced magnetron sputtering. The coatings adhere at temperatures as low as 150°F; therefore, all grades of stainless steel, Co-Cr-Mo alloys, and titanium alloys can be coated without affecting the metals' properties or dimensions. Ion Bond coatings provide increased wear and corrosion resistance to such medical devices as orthopedic implants, surgical blades and instruments, and dental handpieces. The high ionization rate and ion energy created by the plasma in the coating systems enable an intermixing of coating material into a device's surface to provide excellent adhesion. Multi-Arc Inc., 200 Roundhill Dr., Rockaway, NJ 07866.



Plasma system

A plasma surface modification system is designed for medical device applications. The system can include custom vacuum chambers and electrode configurations. The company manufactures the system with high- and low-RF generators, computer and manual control systems, process temperature control, multiple gas flows, mass-gas-flow controllers, and automated vacuum chamber doors. Plasma Etch Inc., 3522 Arrowhead Dr., Carson City, NV 89706.



Plasma treatment system

A plasma treatment system is designed to increase the adhesion characteristics of almost any nonconductive material, including plastics, silicone rubber, resins, fluoropolymers, and Teflon. The PT-2000 is suitable for applications that require gluing, potting, marking, painting, or coating of a nonconductive material. The plasma generated is precisely controlled by a built-in programmable solid-state generator. The system operates in an open ambient environment, as opposed to the cumbersome vacuum chambers required for some plasma systems. Typical applications include the treatment of catheters, syringes, biomedical sensors, orthopedic and dental implants, electrical conductor shells, connector inserts, and PC boards. The turnkey system is available in several configurations specifically tailored to the medical industry. Tri-Star Technologies, 2201 Rosecrans Ave., El Segundo, CA 90245.



Corona treating equipment

A company manufactures corona treating equipment that can be used to treat such medical products as petri dishes, test panels, tubes, and other disposable labware. The single-headed MultiDyne 1 corona treating unit is easy to use, safe, and reliable, according to the manufacturer. The MultiDyne system offers treatment on polymer materials prior to printing, gluing, and laminating. The generator is connected to a single-phase power line of 100­240 V, and it can be used as a stand-alone unit or interfaced to a manufacturing line. 3DT Inc., N. 114 W. 18850 Clinton Dr., Germantown, WI 53022.



Plasma cleaning unit

A plasma system performs ultracleaning in many medical devices including angioplasty balloons, catheters, filter materials, syringe hubs, and intraocular lenses. The B-Series machine features a patented gas-reversal function that ensures uniform surface exposure. The chamber size is custom designed to meet customer requirements. Because minimal power is required for plasma processing, the system has low operating costs and high productivity. The company's specialized software, the P2CIM, can be used to monitor temperatures and process values. Advanced Plasma Systems Inc., 12000 28th St. N., St. Petersburg, FL 33716.


Vacuum deposition system

A vacuum deposition system is used for the application of parylene film to a variety of substrates, including medical components, implantables, and surgical hardware. The PDS-2060 automated system is designed for accurate and repeatable operation. The coating sequence is controlled by a programmable logic controller. Coating parameters, such as temperatures, vacuum levels, and dwell times, are continuously monitored by a fault alarm system, and a printed status chart is provided for each coating run. The system features interchangeable chamber modules to suit various coating applications. Coating chamber capacity ranges from 25 to 100 L. Specialty Coating Systems Inc., 5707 W. Minnesota St., Indianapolis, IN 46241.


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Hotline

Hotline

Product Marking

Laser Marking System Simplifies Product Marking and Cutting

System uses all-digital technology

Laser marking has come of age in the last five years. Efficient CO2 lasers have replaced more expensive YAG lasers in many applications. The sealed CO2 lasers are more versatile and capable of marking a wider range of materials.

Synrad (Mukilteo, WA), a manufacturer of sealed RF-excited CO2 lasers, has introduced a laser marking system based on all-digital technology. The system makes the advantages of laser marking available to both large, high-end manufacturers and end-users doing low-volume custom work.

The system, which is sold as a partially assembled, self-contained kit of components, is designed to be easy to assemble and use. Devices can be reliably marked without fear of contamination. An operator using a desktop computer can mark a company logo on a wide range of materials, including plastics, wood, leather, glass, rubber, stainless steel, and titanium. Because the lasers are sterile, medical filters and other devices can be reliably cut and marked.

The laser can be expected to perform according to specification for 35,000 continuous hours. Because it is sealed, there are no consumable parts, and no maintenance is required. When the laser ultimately depletes its gas, it can be sent back to the manufacturer for a refill.

Using the laser is facilitated with system control software. Created in a Windows format, the system features pull-down menus, tool bars, and dialog boxes that guide the user through the set up and execution of virtually any laser marking application. While supporting thousands of True Type fonts, the software also can import more than 40 different file formats including BMP, GIF, TIF, JPEG, DXF, DWG, and PLT.

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Wire Components

Multifilament-Drawn Wire Can Replace Solid Stainless Steel and Nitinol

Wire technology enables the mixing of different properties into single wire

As an alternative to stainless-steel and Nitinol solid wire, Cathguide Corp. (Miami) has designed a multifilament-drawn (MFD) wire that features high flexibility and torqueability.

The company's new wire technology allows the integration of two or more different materials (with different physical properties) into a single wire structure, thereby opening the door to new wire blends. When a wire blend of stainless steel and platinum-iridium is coiled, it provides a good distal tip because of the combined radiopacity and high tensile strength. In this case, a wire hybrid reduces manufacturing costs while providing a good balance between stiffness and suppleness, making the idea of a guidewire distal tip a reality.

Some of the key properties of MFD wire that differentiate it from solid wire are fatigue strength, high flexibility, high torsional stiffness, and low resistivity. The company can incorporate radiopacity into small-diameter wires, eliminating the need for marker bands and radiopaque coatings.

MFD wires are available in a wide variety of dimensions from combinations of metals such as stainless steels, nickel-titanium, and nonferrous and precious metals. Medical device applications include stents, guidewire cores and coils, pacer leads, stylets, flexible shafts, and snares.

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Sterilization

Irradiation System Available for In-House Operations

On-site system can reduce shipping costs involved in sterilization

Recognizing a need in the device industry for more centralized sterilization operations, SteriGenics International (Fremont, CA) has developed an irradiation system that can serve both in-house and contract sterilization purposes.

The MiniCell system is suitable for device manufacturers with high-volume production schedules that want to maintain an in-house operation. According toSteriGenics, research shows that many device manufacturers ship their products several hundred miles to be sterilized. A MiniCell system installed in a customer's own plant could lower transportation costs, expedite turnaround, and reduce inventory requirements for that company.

All the long-standing benefits of gamma processing--simplicity, reliability, and the high flexibility of products that can be processed--are provided by the MiniCell. In addition, the issue of multiple-dose ranges is resolved by using a high-efficiency batch-type system to deliver target doses at any range without ever increasing process complexity in scheduling.

But the true advantage of the MiniCell system is the shield design and manufacturing method. The traditional poured-in-place-concrete design has been modified to virtually eliminate the construction contractor. The entire design is prefabricated and assembled before being shipped to the site.

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Conference Focuses on Medical Laser Technology

MPMN Industry News

Conference Focuses on Medical Laser Technology

ICALEO meeting discusses advances in lasers used in medical devices and for device manufacturing

The recent International Congress on Applications of Lasers & Electro-Optics (ICALEO) placed special emphasis on the impact that lasers are having in the medical device industry, both for use in manufacturing and as medical devices.

The conference, held in San Diego and organized by the Orlando-based Laser Institute of America (LIA), began with a keynote address by Michael Berns (pictured above), the president and CEO of the Beckman Laser Institute (Irvine, CA). Berns discussed the use of lasers in medical applications. One wing of the Beckman Laser Institute is devoted to research, while the other uses newly developed laser devices to treat patients. "Delivery systems, miniaturization of systems, and microscale technologies are areas for development," Berns noted. Lasers are being developed for use in diagnosing cancer, determining the depths of burns, and as "laser scissors" and "laser tweezers" to cut and move chromosomes and to fuse cells.

The first day's medical session was titled "Issues in Bringing Laser Technology to Medical Devices Manufacturing." The session's lectures covered such topics as installing lasers, ensuring laser safety, and determining the benefits and costs of bringing a laser into the manufacturing process.

Of particular importance for device manufacturers who are considering using lasers in their manufacturing process was the lecture titled "Making Laser Systems Pay: The Tangible and Intangible Benefits and Costs." William Lawson of Laser Machining Inc. (Somerset, WI) said, "In our experience, it is normal for a company adding a new laser technology to take six months to a year to feel comfortable enough to use the technology they've bought." Lawson said that with good planning and training before installation, most systems will produce good parts at speed in a few weeks. "However, it will take much longer for your entire manufacturing process to be able to function well and to know how to handle normal problems."

Lawson noted that key factors for success include meeting installation requirements, having a schedule for regular preventive maintenance and operators who are committed and well trained, having the early involvement of all personnel and the strong commitment of upper management, and planning enough time to get up to speed rather than expecting instant miracles.

The second day's morning session was focused on laser micromachining, while the afternoon session was concerned with the surgical applications of lasers. Highlights included the lecture by C. Paul Christensen of Potomac Photonics (Lanham, MD), who noted, "The biggest benefits that we see in using lasers for micromachining are that lasers surpass mechanical limits, that they work with difficult materials, and that they allow flexibility in manufacturing." Helen Ward of the Medical Health and Research Centre presented a study that her institution had conducted on treating deep-seated brain tumors with lasers. She said that lasers minimize the damage normally done in brain tumor surgery.

A workshop on the third day was titled "FDA Requirements for Laser Manufacturers." The lecturer, Robert Handren of Handren Associates, related his experiences in working for FDA and later as a regulatory consultant helping device manufacturers. He explained that separate bureaus within FDA regulate medical devices and electronic products that emit radiation. A laser used in medical device manufacturing would have to meet FDA regulations pertaining to lasers and laser systems, whereas a laser used as a medical device would fall under the medical device regulations.

For more information on upcoming LIA events, call 407/380-1553.

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Judson A. Smith Doubles Manufacturing Space

Extra space made available for fabrication of miniature tubing

Judson A. Smith Co. (Boyertown, PA) is adding 20,000 sq ft of manufacturing space to its facility. The addition doubles the manufacturing area at the facility, significantly expanding the company's capabilities for the fabrication of miniature tubing.

"We are adding screw machining, laser cutting, and laser welding equipment to the range of services currently offered," says Duane Ottolini, general manager of Judson A. Smith. "This is the first step in a new five-year strategic plan that will continue to advance our efforts in meeting the most complex requirements for highly specialized small-diameter metal tubing and miniature tubing parts."

For more information, contact Judson A. Smith Co. at 610/367-2021.

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Product Genesis Wins Design Award

Steerable forceps recognized for design innovation

Product Genesis Inc. (Cambridge, MA), a high-technology product development and engineering firm, has won a contest sponsored by Design News for its design of steerable forceps for Endius Inc. (Plainville, MA). The product design, which allows surgeons to easily remove blockage in the sinus cavity and perform less-invasive surgery, was recognized for its innovation and overall design excellence.

When accepting the award with Product Genesis, Endius CEO Tom Davidson reflected on the strategic product development partnership between the two companies and the challenges the design team overcame. "Product Genesis essentially served as Endius's product development department. The demands were high. As a medical device, the product had to be highly reliable yet cost-effective, and to maneuver in small endoscopic spaces. It had to be both sturdy and flexible."

In Product Genesis's design of the steerable forceps, a stack of injection- molded vertebrae with centrally located pivots provides both the necessary strength and movement. A thumbwheel is used to flex the spine of the device, while the index-finger-controlled trigger moves the jaw of the forceps. The use of injection-molded parts allowed Endius to achieve the necessary target cost for this disposable product, while the simplicity of the design of the main housing helped to keep the manufacturing process efficient and the parts count low.

For more information, contact Product Genesis Inc. at 617/661-3552.

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Zevex Receives EN 46001 Certification

Allows company access to sell and distribute products in Europe

Zevex International Inc. (Murray, UT) announced that it has received EN 46001 certification from the National Standards Association of Ireland. The EN 46001 designation goes beyond ISO 9001 requirements, allowing a medical device company to perform its own CE testing and marking.

"The EEC applies stringent quality requirements to designated medical device manufacturers," said Dean Constantine, Zevex's president and CEO. "To achieve this designation, we have had to demonstrate a continuing commitment to quality management and an ability to be self-regulating."

Zevex designs and manufactures medical devices such as surgical systems, device components, and sensors for medical technology companies. It also designs, manufactures, and markets its own medical devices using proprietary technologies.

For more information, call Zevex at 801/264-1001.

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Laparoscopic Surgery Improved with Monitoring System

Laparoscopic Surgery Improved with Monitoring System

Laparoscopic electrosurgery is a combination of two techniques--monopolar electrosurgery and laparoscopy. Monopolar electrosurgery allows surgeons to both cut and coagulate tissue by passing an electrical current through the tissue mass. The current produces heat that cuts tissue at above 100°C and dries the tissue between 70° and 100°C.

Electrosurgery is frequently used in combination with laparoscopy, a minimally invasive form of surgery that allows surgeons to see the operating area on a video monitor. By inserting the laparoscope, a small camera, into an incision, the doctor can manipulate tissue with remote electrosurgical instruments.

Although both electrosurgery and laparoscopy have been used successfully for years, the combination of electrosurgery plus a closed environment creates a potential for patients to be burned outside of the surgical area if there is a malfunction of the electrosurgical equipment.

Despite occurrences of unintended patient burns, laparoscopic electrosurgery is becoming a safer surgical option because of products such as the Active Electrode Monitoring System (AEM) from Electroscope Inc. (Boulder, CO). Consisting of a monitoring device and integrated surgical instruments, the system has all but eliminated the risk of accidentally burning patient tissue outside of the designated surgical area.

Monitoring System Eliminates Potential Complications

During laparoscopic electrosurgery, burns can occur as a result of either insulation failure in the electrosurgical instrument or capacitive coupling. Insulation failure can occur when the insulation is worn away after repeated use or if it is cut by another instrument. When electrical current travels from the electrosurgical generator through a surgical instrument, any imperfection in the insulation can cause the current to exit not only at the end of the electrode, but also at the break in the insulation. Many breaks in insulation are too small to see and therefore pose a serious problem in electrosurgery where the instruments are frequently reused.

Capacitive coupling occurs when the electric field around the active electrode transfers current to tissue or objects in close proximity to the electrode. This type of burn may occur even if the insulation around the electrode is intact. Burns from capacitive coupling may be found in the vicinity of the cannula, a plastic or metal cylinder used to hold the incision open.

Electroscope's AEM system monitors for stray current on the internal conductive shield of the instrument's 35-cm shaft. When a fault is detected, the AEM shuts down the electrosurgical generator, protecting the patient from burns outside the view of the surgeon performing laparoscopy. The system operates on the principle of monitoring the internally shielded laparoscopic instrument and ensures that 100% of the energy is delivered to the patient wirhin the surgeon's view or target site.

Continuing Improvements

Electroscope has refined its monitoring system and surgical instruments. For example, it has developed universal cords that simplify the setup between the monitor and generator. In addition, the company has reengineered several components in its electrosurgical instruments including the 5-mm laparoscopic handle used to hold jaw inserts such as scissors or graspers.

The new handle is injection molded by Upchurch Scientific (Oak Harbor, WA) using PEEK, a high-performance thermoplastic material manufactured by Victrex Inc. (West Chester, PA). According to Electroscope mechanical design engineer Rich Schneider, "We needed a material that could provide strength and rigidity to the handle so that surgeons could squeeze it with a potentially high amount of force and not have it flex or distort excessively. PEEK polymer provides us with a handle that meets these requirements. It also gives the handle enough impact strength to withstand being dropped on the floor or other abuses."

PEEK grade 450G was used. This is a general-purpose grade for injection molding and extrusion. Because up to 3000 V are applied to the handle, the material must have good dielectric strength. This high voltage is also why uniform molding is so important. The grade used has a dielectric strength of 190 kV/cm and a melt point of 644°F. A critical aspect of any surgical device is its ability to be sterilized without severe degradation. The PEEK polymer material enables the handle to resist the extreme heat and moisture of autoclave sterilization.

Schneider says, "You can certainly find other materials that are just as strong as the PEEK polymer. And, you can find other materials that are just as fracture resistant, or that are just as dielectrically strong. However, it was the combination of these properties plus the polymer's sterilizability that gave us what we needed for the handle."

In addition to using PEEK polymer for the handle, Electroscope used it in the redesign of other system components. "We also used the polymer for the index knob that rotates the jaw insert within the handle."

The instrument redesign was concurrently engineered by Electroscope and Upchurch Scientific. "They had the professional expertise to provide valuable input into the design of the part and to mold PEEK polymer the way it should be molded. The handles are smooth, fully crystalline, and consistent in color," Schneider says.

Molder Aids Redesign

From the design and development through validation and production, Upchurch Scientific performed an integral role in the instrument redesign.

In addition, Upchurch Scientific designed, prototyped, and insert molded several stainless-steel parts into the PEEK polymer handle, and this helped decrease cost and ensure electrical insulating properties.

The result of the collaboration, says Schneider, is that "Electroscope now has a handle that is esthetically appealing, resists degradation from repeated sterilization, provides excellent electrical insulation, and has good structural integrity."

For more information on Upchurch Scientific, call 800/426-0191. To learn more about the PEEK polymer, call Victrex at 610/696-3144.

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Spotlight on Pressure-Sensitive Adhesives

Spotlight
on Pressure-Sensitive Adhesives

Medical component converting

Medical-grade PSAs are die-cut for use in device components at a company's GMP-compliant facility. Such capabilities as die-cutting, laminating, printing, slitting, and sheeting allow for functional handling of multiple webs. Pressure-sensitive foams, films, nonwovens, and foil laminations can be converted for use in wound- and burn-care dressings; grounding pads and electrodes; diagnostics; IV administration; urological, ostomy, and personal-care products; and transdermal applications. Promedicon, 905 Pennsylvania Blvd., Feasterville, PA 19053.



Pressure-sensitive adhesives

A manufacturer's pressure-sensitive adhesives are used for bonding devices such as electrodes to the skin. The company has developed noncurable PSAs dispersed in three different solvents: ethyl acetate; 1,1,1 trichloroethane; and hexamethyldisiloxane. The dispersion is applied either to the skin or the device. The solvent evaporates, leaving a tacky silicone layer that can be adhered to another object. The company has also developed a curable, cross-linking PSA that cures to a solvent-resistant tacky coating. Nusil Technology, 1050 Cindy Ln., Carpinteria, CA 93013.



PSA products

Nearly 30 standard products specifically designed for use in diagnostic, medical device, and wound-care applications are offered to meet the demand for medical-grade pressure- sensitive adhesives. The manufacturer specializes in the development of custom PSA tapes for new or unique applications. One of the standard products, ARcare 8383, is suitable for wound-care applications that call for an acrylic PSA tape with a low moisture-vapor transmission rate. ARcare 8544 can increase the speed of diagnostic testing; using the manufacturer's MA-54 adhesive, this product increases lateral flow, providing faster results and improving user satisfaction. Adhesive Research Inc., P.O. Box 100, Glen Rock, PA 17327.



Enhancer-compatible PSA

A high-performance PSA for transdermal drug-delivery systems uses an enhancer-compatible component. This special component resists detackifying effects of certain enhancers by preventing their excessive migration to the bond interface. Furthermore, the Duro-Tak 87-4677 adhesive's high molecular weight and other characteristics allow the polymer to resist the plasticizing effects of various enhancers. National Starch & Chemical Co., P.O. Box 6500, Bridgewater, NJ 08807.



PSA components

A contract manufacturer of PSA components specializes in flexographic printing, precision rotary and flat-bed die-cutting, multilayer lamination, island placement, design and prototyping, subassemblies, and packaging. The company converts nonsensitizing adhesive-coated papers, films, foils, nonwovens, foams, fabrics, hydrocolloids, mineral oil polymers, and hydrogels. Applications include ECG, EMG, EEG, TENS, iontophoresis, wound care, catheter securement, laparoscopy, cardiac stimulation, and oximetry/gas monitoring. Acutek, 540 N. Oak St., Inglewood, CA 90302



Custom PSA components/assemblies

A company offers custom PSA components and assemblies. Its capabilities include rotary and reciprocating die-cutting, laminating, slitting, level winding, and custom-sterilizable packaging in a cleanroom environment. The company is ISO 9001 and EN 46001 certified. It has experience sourcing PSA films, foils, and double- and single-coated foams, as well as conductive adhesives, hydrogels, and hydrocolloids. Its technical development functions include prototyping, short clinical evaluation runs, design of experiments, process validation, and first-article capabilities to support customers throughout the design and regulatory approval phases and into full production. Tapemark Medical Fabricating Div., 1685 Marthaler Ln., West St. Paul, MN 55118.



Medical foam tapes

Pressure-sensitive foam tapes are designed for the manufacture of medical devices such as electrodes and grounding pads. The Bioflex Rx216V and Rx232V incorporate soft, closed-cell polyethylene foams that are from 1/16 to 1/32 in. thick. The foam is designed for comfort, conformity to body contours, and repellency against liquids such as water and sweat. The polyethylene foam is coated on one side with a nonsensitizing acrylic adhesive that is formulated to provide good adhesion to human skin and yet remove easily without causing skin trauma. The tapes are covered with a heavy lay-flat release liner that aids in die-cutting operations. Scapa Tapes/Coating Sciences Inc., 111 Great Pond Dr., Windsor, CT 06095.



PSAs and other products

A company specializing in die-cutting, laminating, slitting, and converting manufactures pressure-sensitive adhesives, foams, foils, conductive and antistatic materials, and more. Its manufacturing facility is capable of meeting demanding production requirements from clinical trials to mass production. The company offers an experienced engineering and technical staff and is ISO 9000 certified and FDA registered. Pacific Die Cut Industries, 3399 Arden Rd., Hayward, CA 94545.



Medical adhesive systems

A company supplies wound- care, transdermal, electromedical, ostomy, diagnostic, and surgical drape markets with pressure-sensitive fastening and bonding systems. Medical-grade adhesive formulations are supplied as foam tapes, film tapes, nonwoven tapes, unsupported transfer tapes, and double-coated film tapes. Its research and development staff includes polymer scientists, chemists, and chemical engineers. They understand the manufacturing and converting operations and can help design a product that will run smoothly on a client's equipment and streamline production. Avery Dennison, Specialty Tape Div., 250 Chester St., #5M, Painesville, OH 44077.



Wound-care products

Rotary die-cutting, laminating, printing, and packaging are offered by a contract manufacturer of medical disposable components and finished devices. Some of the materials processed include hydrogels, nonwovens, films, foams, adhesives, and many other nonmetallic materials. Finished devices such as island dressings, hydrogel dressings, and other wound-care products are offered. The company also offers flatbed die-cutting and heat sealing. Small prototype runs are performed to validate materials, device design, and manufacturing processes. Precision Converters Inc., 10969 Shady Trail, Ste. 101, Dallas, TX 75220.



Custom die-cutting

An ISO 9002­certified manufacturer specializes in custom die-cutting of skin-contact adhesives. Wound dressings, tube holders, suture strips, hydrogel dressings, and many other disposable devices are routinely fabricated. In addition, the company offers prototyping, contract packaging services, and agreements with various sterilization firms nationwide for a complete turnkey package. Materials regularly converted include adhesive-backed films, foils, wovens, nonwovens, foams, and hydrogels. Heat- or cold-seal single-use pouches are produced from papers, films, Tyvek, and foils. TTL Medical, 10537 Lexington Dr., Knoxville, TN 37932.


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Deformation, Morphology, and Wear Behavior of Polyethylene Used in Orthopedic Implants

Medical Plastics and Biomaterials Magazine
MPB Article Index

Originally published March 1998

TECHNICAL PAPER SERIES

For more than 30 years, ultra-high-molecular-weight polyethylene (UHMWPE) has been used as a bearing material in total-joint-replacement prostheses.1 Such orthopedic implants usually comprise a metal (typically a cobalt-chromium alloy) or ceramic component that articulates against a UHMWPE component during in vivo use. It has been well established that the longevity of such implants depends on the wear performance of the UHMWPE components.1—4 The presence of particulate wear debris of UHMWPE that is generated due to the sliding of UHMWPE components against the metal or ceramic counterface has been linked to complications such as tissue inflammation, bone loss (or osteolysis), and implant loosening.2—7 Osteolysis resulting from wear of UHMWPE is recognized as the leading problem in orthopedic surgery today. Although UHMWPE has superior wear characteristics compared with those of other polymers,1 its resistance to wear must be improved for increased lifetime of joint-replacement prostheses.

New formulations of UHMWPE components have been developed in the past, with the goals of reducing creep and wear rates. For example, UHMWPE has been blended with carbon fibers to fabricate total-joint-replacement components known as Poly Two (Zimmer Inc., Warsaw, IN).8 However, while the devices manufactured using this blend had excellent resistance to creep, there was a decrease in fatigue resistance. In addition, no improvement in wear resistance was observed,8 and the material was ultimately discontinued for use in joint-replacement devices. More recently, high-pressure crystallization was employed to produce UHMWPE components with an increase in mechanical properties such as yield stress and Young's modulus.9 However, this material, known as Hylamer (DePuy-Dupont Orthopaedics, Newark, DE), has again not shown any improvement in laboratory wear tests,10 despite enhanced creep resistance and an increase in resistance to fatigue crack growth.11 Early clinical results also indicate that Hylamer does not demonstrate increased resistance to wear in total-hip-replacement prostheses compared with conventional UHMWPE.12,13

In recent years, a new approach has been adopted to improve the wear performance of UHMWPE. Instead of using novel processing methods such as high-pressure crystallization or physical blending, UHMWPE components have been modified via chemical methods. Cross-linking of UHMWPE macromolecules has been performed using cross-linking agents such as peroxides,14 and through gamma15—17 or electron-beam irradiation.18,19

The cross-linking of UHMWPE results in an interpenetrating network of high-molecular-weight polyethylene chains, with the potential benefit of increased strength in the interfacial region between resin particles of polyethylene components. Incomplete consolidation of resin particles has been observed in components of UHMWPE, and is believed to contribute to wear.1 While there have been previous investigations of the effects of cross-linking on polyethylene morphology and mechanical properties,20,21 these are the first studies that demonstrate the advantage of cross-linking UHMWPE for use in total-hip-replacement prostheses. Laboratory hip-simulator wear tests have shown that there is a decrease in UHMWPE wear rate corresponding to an increase in the degree of cross-linking.14—19 However, it should be noted that these studies have not yet addressed the resistance of cross-linked UHMWPE to third-body wear, which is abrasion that results from the presence of hard particles that do not originate from either the UHMWPE or the metallic articulating component (e.g., bone chips, bone-cement particles, or debris from the metallic stem).

Although cross-linking of UHMWPE has been shown to improve performance in hip-simulator wear tests, mechanical tests conducted on cross-linked material have shown a reduction in several mechanical properties including Young's modulus, yield stress, ultimate tensile stress and strain-to-break.19 These results appear contradictory, since it is generally believed that the toughness of a polymer correlates with its wear performance. A better understanding of the relationship between the mechanical properties and wear performance of UHMWPE is required for the development of new wear-resistant polymeric components to be used in total-joint-replacement prostheses. In this study, the mechanical properties of three types of polyethylene with vastly different resistance to wear were compared. The goals were to identify the mechanical behavior and morphology of polyethylene that correspond to increased wear resistance. Because high-density polyethylene (HDPE) is known to have lower resistance to wear compared with UHMWPE,22 we used HDPE, UHMWPE, and cross-linked UHMWPE to identify those mechanical properties that correlate with improved wear performance.

EXPERIMENTAL

Pellets of HDPE (Petrothene LS 606-00) were obtained from the USI Division of Quantum Inc. (Cincinnati). The molecular weight of the HDPE was 55,000, with a polydispersity of Mw/Mn = 4.8 and a melt flow index of 9—11 g/10 min (ASTM D 1238). The pellets were compression-molded at 180°C and 10,000 psi into sheets of 1—2-mm thickness, using a Carver hydraulic press. The sheets were then slowly cooled to room temperature and annealed for 6 hours at 80°C. Molded sheets of GUR 415 resin (Hoechst Celanese, Houston, TX) were obtained from several commercial sources. Each sheet was heated in a vacuum oven to 180°C and slowly cooled to room temperature in order to remove any residual stresses from high pressures imposed during processing and to replace the previously unknown thermal history with a controlled thermal history.

Cross-linking of UHMWPE was performed using the electron-beam irradiation facility at the High-Voltage Research Laboratory of the Massachusetts Institute of Technology. UHMWPE sheets of 2-mm thickness were subjected to doses of 2.5, 5.0, 10.0, and 20.0 Mrd (25, 50, 100, and 200 kGy) at room temperature. The sheets were then placed in a vacuum oven, evacuated, and heated to 180°C for 30 minutes in a nitrogen gas—filled environment to initiate the cross-links and quench the free radicals generated in the polymer by the electron beam. Finally, the sheets were slowly cooled to room temperature.

The HDPE, UHMWPE, and cross-linked UHMWPE sheets were machined into dog bone—shaped tensile specimens (ASTM D 638M-III) using an Omax water-jet cutting machine. Tensile and strain-recovery tests were performed using an Instron-4201 tensile tester, with a crosshead speed of 10 mm/min used for all experiments. Four to six specimens were used for each tensile test to calculate experimental error. In the case of the strain-recovery tests, initial nominal strains of 75, 150, and 225% were imposed on each specimen prior to strain recovery.

Differential scanning calorimetry (DSC) was performed on each specimen using a Perkin-Elmer DSC-7 to obtain the percentage crystallinity for each type of polyethylene. Each sample was heated from room temperature to 170°C, using a heating rate of 10°C/min. A heat-of-fusion value of 293 J/g was used for 100% crystalline polyethylene to calculate the percentage crystallinity in each specimen.

Specimen Degree of Crystallinity (±3%)
HDPE70.3%
UHMWPE, 0 Mrd 58.5
UHMWPE, 2.5 Mrd 43.9%
UHMWPE, 5.0 Mrd 43.8%
UHMWPE, 10.0 Mrd 43.3%
UHMWPE, 20.0 Mrd 42.4%


Table I. Degree of crystallinity for samples of HDPE, UHMWPE, and cross-linked UHMWPE.

RESULTS AND DISCUSSION

A combination of DSC, tensile tests, and strain-recovery tests showed that—in addition to differences in wear characteristics—there were discernible differences in the morphology and mechanical properties of HDPE, UHMWPE, and cross-linked UHMWPE. DSC measurements of HDPE, UHMWPE, and cross-linked UHMWPE showed that the degree of crystallinity in UHMWPE was significantly lower than that of HDPE (see Table I). This is attributed to the large number of entanglements present in UHMWPE. Because polyethylene has a high rate of crystallization, the long, entangled chains of UHMWPE do not have sufficient time to disentangle and fold into the growing crystallites, thereby resulting in a larger number of tie molecules and entanglements in the amorphous region between crystallites. There is a concomitant reduction in the number of loose chain folds in the crystallites of UHMWPE compared with those of HDPE (as depicted in Figure 1). It is well known that an increase in the number of tie molecules leads to an increase in toughness in polyethylene.23 Therefore, the larger number of tie molecules and chain entanglements associated with high-molecular-weight polyethylenes are believed to be the reason for their improved resistance to wear.

Figure 1. A schematic showing the arrangement of macromolecules of polyethylene and the crystallites of HDPE (left), UHMWPE (center), and cross-linked UHMWPE (right).



Cross-linking of UHMWPE led to a further lowering of the degree of crystallinity (see Table I). The reduction in crystallinity from 0-Mrd UHMWPE to 2.5-Mrd UHMWPE was large, followed by a minor reduction in crystallinity with higher doses of radiation (or a higher degree of cross-linking). This suggests that constraints imposed on the crystallizing chains due to the formation of a cross-linked network structure had a larger role in reducing the degree of crystallinity in UHMWPE than did the presence of cross-links themselves.

Figure 2. An engineering stress-versus-nominal-strain plot for UHMWPE subjected to E-beam doses of 0, 2.5, 5.0, 10.0, and 20 Mrd. Each curve has been offset by 10% for clarity.



Tensile tests performed on UHMWPE and cross-linked UHMWPE showed that there was a monotonic reduction in the yield stress, ultimate tensile stress, and strain-to-break with an increase in the degree of cross-linking (see Figure 2). This finding is in agreement with a previous study on the effects of radiation cross-linking on mechanical properties of UHMWPE.19 The systematic reduction in toughness (area under the stress-strain curve) with increasing degrees of cross-linking suggests that cross-linked UHMWPE should be less resistant to wear. However, hip-simulator tests have shown that there is a monotonic decrease in wear rates with increasing doses of radiation. One plausible explanation for these seemingly contradictory results is that the toughness of UHMWPE—defined by the area under the stress-strain curve—correlates with third-body wear, a mechanism of wear that was not present in the hip-simulator wear tests performed on cross-linked UHMWPE.

Figure 3. An engineering stress-versus-nominal-strain plot for HDPE, UHMWPE (m415), and cross-linked UHMWPE (x415).



To determine the relationship between mechanical behavior, morphology, and wear resistance of polyethylene, we compared the tensile behavior of HDPE, UHMWPE, and 5-Mrd-irradiated, cross-linked UHMWPE, as shown in Figure 3. It can be observed that HDPE, which has the lowest resistance to wear, stretched to higher strains with little or no strain hardening compared with UHMWPE and cross-linked UHMWPE. A similar deformation behavior has been observed in solution-crystallized UHMWPE, although the strain-to-break is substantially higher in solution-crystallized UHMWPE compared with HDPE.24 The primary reason for the similarity between the deformation behavior of solution-crystallized UHMWPE and melt-crystallized, low-molecular-weight HDPE is that, in both cases, there are a lower number of chain entanglements and a higher number of chain folds compared with melt-crystallized, cross-linked UHMWPE.

A study in which UHMWPE was crystallized from solutions of various concentrations showed that the material's strain-to-break was reduced and its strain hardening increased with an increase in polymer concentrations.25 Since higher polymer concentrations contain a larger number of chain entanglements, it was deduced that entanglements were responsible for strain hardening and the reduction in strain-to-break. Based on these previous studies and our observations on HDPE and UHMWPE, it can be concluded that entanglements play an important role in wear resistance and must be increased to improve the wear performance of UHMWPE. The results also suggest that if a polyethylene specimen demonstrates a large degree of strain hardening in a tensile test, it is likely to exhibit high wear resistance as well.

Figure 4. An engineering stress-versus-nominal-strain plot showing strain recovery after a 75% nominal strain imposed in HDPE, UHMWPE, and cross-linked UHMWPE.



It has been observed that the multidirectional motion of metal against UHMWPE leads to higher wear rates in hip-simulator wear tests.26 One theory proposes that the mechanism of wear involves orientation of UHMWPE followed by fracture of the oriented UHMWPE when the wear path changes. The observations led us to hypothesize that the ability of polyethylene to recover imposed strain should be beneficial for wear performance. Strain-recovery tests showed that cross-linked UHMWPE recovered a larger amount of strain compared with UHMWPE (see Figure 4). Also, both of the UHMWPEs recovered a much larger amount of strain than did the HDPE. The strain recovery behavior persisted even at higher imposed strain, as shown in Figure 5, although the ability of HDPE to recover strain fell sharply with increasing applied strains. The strain recovery behavior of the more wear-resistant UHMWPE can also be attributed to the presence of entanglements, which behave as physical cross-links in such high-rate deformation tests. The strain energy stored in this network structure upon application of large, plastic strain leads to recovery upon release of the load. Oriented HDPE, which contains very few chain entanglements, is unable to recover strain to the same extent as does UHMWPE.

Figure 5. A plot of percentage strain recovery versus imposed strain for HDPE, UHMWPE (m415), and 5-Mrd cross-linked UHMWPE (x415).



CONCLUSION

Wear performance of polyethylene can be predicted by its mechanical behavior and morphology. This study indicated that increased amounts of strain hardening and strain recovery correlate with an increase in wear resistance of polyethylene in hip-simulator wear tests. Since a higher number of chain entanglements lead to strain hardening and strain recovery, processing methods that maximize the number of chain entanglements should be used to fabricate polyethylene components that are intended for load-bearing applications. However, it must be noted that whereas a higher number of entanglements can result in improvements in wear performance, the concomitant reduction in crystallinity leads to a reduction in resistance to creep, which may be undesirable for certain applications.

ACKNOWLEDGMENTS

The authors wish to thank Professor Robert E. Cohen of the Massachusetts Institute of Technology and Professor Myron Spector of Harvard Medical School/Brigham & Women's Hospital for their valuable insights on the morphology and wear performance of UHMWPE. Kenneth Wright of the High-Voltage Research Laboratory of the Massachusetts Institute of Technology is gratefully acknowledged for his assistance with the electron-beam irradiation. An earlier version of this paper was presented at the 23rd Annual Meeting of the Society for Biomaterials, held in New Orleans, April 30—May 4, 1997.

REFERENCES

1. Li S, and Burstein AH, "Current Concepts Review: Ultra-High Molecular Weight Polyethylene," J Bone Joint Surg, 76-A:1080—1090, 1994.

2. Landy MM, and Walker PS, "Wear of Ultra-High Molecular Weight Polyethylene Components of 90 Retrieved Knee Prostheses," J Arthroplasty (suppl), S73—S85, 1988.

3. Wright TM, Rimnac CM, Stulberg SD, et al., "Wear of Polyethylene in Total Joint Replacements: Observations from Retrieved PCA Knee Implants," Clin Orthop Rel Res, 276: 126—133, 1992.

4. McKellop HA, Campbell P, Park S-H, et al., "The Origin of Submicron Polyethylene Wear Debris in Total Hip Arthroplasty," Clin Orthop Rel Res, 311:3—20, 1995.

5. Willert H-G, and Semlitsch M, "Reactions of the Articular Capsule to Wear Products of Artificial Prostheses," J Biomed Mat Res, 11:157—164, 1977.

6. Dannenmaier WC, Haynes DW, and Nelson CL, "Granulomatous Reaction and Cystic Bony Destruction Associated with High Wear Rate in a Total Knee Prosthesis," Clin Orthop, 198:224—230, 1985.

7. Howie DW, "Tissue Response in Relation to Type of Wear Particle around Failed Hip Arthroplasties," J Arthroplasty, 5:337—348, 1990.

8. Wright TM, Fukubayashi T, and Burstein AH, "The Effect of Carbon Reinforcement on Contact Area, Contact Pressure, and Time-Dependent Deformation in Polyethylene Tibial Components," J Biomed Mat Res, 15:719—730, 1981.

9. Li S, and Howard EG Jr, Process of manufacturing ultra high molecular weight polyethylene shaped articles, U.S. Pat. 5,037,928, 1991.

10. McKellop H, Lu B, and Li S, "Wear of Acetabular Cups of Conventional and Modified UHMW Polyethylenes Compared on a Hip Joint Simulator," Trans Orthop Res Soc, Rosemont, IL, Orthopaedic Research Society, p 356, 1992.

11. Champion AR, Li S, Saum K, et al., "The Effect of Crystallinity on the Physical Properties of UHMWPE," Trans Orthop Res Soc, Rosemont, IL, Orthopaedic Research Society, p 585, 1994.

12. Chmell MJ, Poss R, Thomas WH, et al., "Early Failure of Hylamer Acetabular Inserts Due to Eccentric Wear," J Arthroplasty, 11:351—353, 1996.

13. Livingston BJ, Chmell MJ, Spector M, et al., "Complications of Total Hip Arthroplasty Associated with the Use of Hylamer Acetabular Components," J Bone Joint Surg, October 1997.

14. Shen F-W, McKellop HA, and Salovey R, "Irradiation of Chemically Crosslinked Ultra High Molecular Weight Polyethylene," J Polym Sci: Polym Phys Edn, 34: 1063—1077, 1996.

15. Oonishi H, Ishimaru H, and Kato A, "Effect of Cross-Linkage by Gamma Radiation in Heavy Doses to Low-Wear Polyethylene in Total Hip Prostheses," J Mat Sci: Mat in Med, 7:753— 763, 1996.

16. Oonishi H, Kuno M, Tsuji E, et al., "The Optimum Dose of Gamma-Radiation-Heavy Doses to Low-Wear Polyethylene in Total Hip Prostheses," J Mat Sci: Mat in Med, 8:11—18, 1997.

17. Clarke IC, Good V, Williams P, et al., "Simulator Wear Study of High-Dose Gamma-Irradiated UHMWPE Cips," in Transactions of the Society for Biomaterials Annual Meeting, Minneapolis, Society for Biomaterials, p 71, 1997.

18. Premnath V, Merrill EW, Jasty M, et al., "A New Polyethylene: The Concept," presented at the Hip Course: Continuing Medical Education, Boston, October 1996.

19. Muratoglu OK, Bragdon CR, Jasty M, et al., "The Effect of Radiation Damage on the Wear Rate of UHMWPE Components," presented at the Symposium on Characterization and Properties of Ultra-High Molecular Weight Polyethylene, ASTM conference, New Orleans, November 1995.

20. Dijkstra DJ, Hoogsteen W, and Pennings AJ, "Cross-Linking of Ultra-High Molecular Weight Polyethylene in the Melt by Means of Electron-Beam Radiation," Polym, 30:866—873, 1989.

21. Hendra PJ, Peacock AJ, and Willis HA, "The Morphology of Linear Polyethylenes Cross-Linked in Their Melts. The Structure of Melt-Crystallized Polymers in General," Polym, 28: 705—709, 1987.

22. Anderson JC, "High-Density and Ultra-High Molecular Weight Polyethylenes: Their Wear Properties and Bearing Application," Tribol Int, 15:43—47, 1982.

23. Knight GW, "Polyethylene," chap 7 in Polymer Toughening, Arends CB (ed), New York, Marcel Dekker Inc., 1996.

24. Smith P, and Lemstra PJ, "Ultradrawing of High Molecular Weight Polyethylene Cast from Solution," Colloid Polym Sci, 258:891—894, 1980.

25. Smith P, Lemstra PJ, and Booij HC, "Ultradrawing of High-Molecular-Weight Polyethylene Cast from Solution. II. Influence of Initial Polymer Concentration," J Polym Sci: Polym Phys Edn, 19:877—888, 1981.

26. Bragdon CR, O'Connor DO, Lowenstein JD, et al., "The Importance of Multidirectional Motion on the Wear of Polyethylene," Proc Instn Mech Engrs, 210:157—165, 1996.

Seema H. Bajaria is a mechanical engineer in the commercial space division of Lockheed Martin Missiles and Space (Sunnyvale, CA). She performed this study at the Massachussetts Institute of Technology, where she obtained a BS in mechanical engineering. Anuj Bellare, PhD, received his doctorate through the program in polymer science and technology at MIT and spent two years as a postdoctoral research associate at Princeton University. He is currently an instructor of orthopedic surgery (biomaterials) at Harvard Medical School and Brigham & Women's Hospital in Boston.


Copyright ©1998 Medical Plastics and Biomaterials

Medical Plastics Failures from Heterogeneous Contamination

Medical Plastics and Biomaterials Magazine
MPB Article Index

Originally published March 1998

Michael T. K. Ling, Stanley P. Westphal, Chuan Qin, Samuel Ding, and Lecon Woo

In the health-care industry, failure criteria are often considerably more stringent than in other plastics market sectors. This vigilance is necessary because even minor, seemingly innocuous device defects—especially those biological in origin—could have devastating consequences for the patient. As a result, many visual inspections are built into both the manufacturing process and clinical protocols. Failures detected in these inspections are frequently cosmetic in nature and have no impact on the functionality of the medical device or drug container. However, because of the industry's precautionary practices, most of the defects are deemed unacceptable, and the affected products are rejected. Given today's cost-driven health-care environment, identifying and minimizing these cosmetic defects is an important part of the overall quality process.

The source of these cosmetic defects is often contamination from external sources. To implement a truly fundamental corrective action, the root cause of the defect must be unequivocally identified. Microscopic morphological analysis is an indispensable tool for this effort.

Besides cosmetic defects, another class of failures that originate from heterogeneous contamination involves inclusions in device components. If the inclusion is of a different modulus from the matrix material, it can act as a stress concentrator and cause premature mechanical failure well below the designed stress of the device.

There is yet a third class of failures that are due to external sources: those arising from the uneven distribution of additives and modifiers in the polymer. Since many additives are designed to protect the polymer against oxidative degradation, an uneven distribution can result in part of the product being unprotected during long-term aging, which can lead to premature failures.

In the experience of the authors, failures attributable to heterogeneous external contaminants constitute a significant part of total device failures. A detailed examination of the origin of these failures can help in implementing effective countermeasures. This article will present examples of the different types of failures and propose possible preventive measures to reduce or eliminate them.

EXPERIMENTAL

In dealing with heterogeneous structures and contamination, perhaps the most powerful tool is the optical microscope. Use of a simple stereomicroscope with moderate magnification and a long working distance is invariably the preferred first step for detailed examinations. If the samples are optically transparent, equipment with polarized light capability is also very useful. Many embedded particles are surrounded by molded-in stresses from the modulus mismatch; the stress causes massive birefringence, which can aid in detection and quantization.

Contaminated samples were microtomed to expose their surface for chemical-identification analysis via scanning electron microscopy (SEM) and energy-dispersive spectroscopy (EDS). In some cases in which microtoming was not feasible due to possible cross-contamination or the risk of losing the particles, samples were exposed to SEM under higher voltage—for deeper x-ray penetration—in order to identify the elements. The samples were sputter-coated with palladium to render them conductive for SEM observation. Fracture surfaces of the suspect specimens were also examined by SEM, and the exposed contaminants identified by EDS.

RESULTS AND DISCUSSION

Polyvinyl Chloride Contamination in Polyolefins. The contamination of olefin products by polyvinyl chloride is inevitable in a shared facility where both types of resins are processed. The two main sources of contamination are airborne PVC powder and PVC compound residues in the barrel and in the screen pack of the processing equipment. Airborne PVC particles with no stabilizers char rapidly at elevated temperature during polyolefin processing. The charred PVC particles can manifest themselves as dark specks embedded in the finished products. Even though these brown or black specks are often considered a merely cosmetic defect, their presence is not acceptable in the medical industry because of the perception of poor product quality. To reduce or eliminate airborne PVC powder particles, total segregation of the PVC and olefin processing areas is required.

The second source of PVC contamination arises from residual material in the barrel and in the screen pack of the processing equipment. When a production changeover from a previous PVC run is enacted without a thorough equipment cleaning, cross-contamination can result. Although PVC compounds are generally quite stable, long residence times at processing temperatures can degrade residual PVC to brown or black specks and gels that can appear in polyolefin products such as film and tubing. This mode of contamination is more serious than the specks caused by airborne particles, because the larger-sized gels embedded in relatively thin polyolefin films can bring about mechanical failure. For example, during a blood-component harvesting process, film containers are subjected to high centrifugal forces. Gels are the weakest points in the entire film, and dimpling and fracturing can occur under such conditions. To totally eliminate these kinds of failures, shared machinery should be disassembled and completely cleaned between changeovers. Ideally, PVC and polyolefins should each be run on their own dedicated machinery on separate production lines.

Figure 1. Optical microscope image of an ~100-µm medium-density polyethylene (MDPE) film (MDPE-1) oven aged at 120°C for 1080 hours. The dotted line represents the front of the ductile/brittle border.

Inadequate Dispersion of Antioxidants. Poor dispersion and distribution of antioxidant in a resin often leads to unpredictable product shelf life. When an ~10-µm-thick, medium-density polyethylene (MDPE) film was subjected to accelerated heat aging at 120°C, inadequate antioxidant dispersion resulted in a pattern of very uneven film embrittlement, as seen in Figure 1. Color changes in the film from clear to white translucent to yellow were observed. It was also very interesting to note that the degradation grew in all directions with a whitish front. The white areas were very brittle, whereas the yellowish areas tended to be less so. The darkest areas were those of yellowish color, and the lightest areas were the white front, marked by a dashed line in Figure 1. Infrared spectra suggested that the yellow and white areas were severely degraded—with the evidence of massive C=O carbonyl contents—as compared with the ductile, translucent area (see Figure 2). This was confirmed by results examining oxidative induction time (OIT), with negligible OIT detected at the yellow and white areas, indicating total degradation (see Figure 3).

Figure 2. IR spectra of MDPE film (MDPE-1) comparing ductile and brittle areas.

Figure 3. Oxidative induction time (OIT) of MDPE film (MDPE-1). The film was tested isothermally at 200°C in an air-purged DSC cell.

MDPE that has undergone chain-scission degradation will show evidence of embrittlement, accompanied by a white, translucent color. Scanning electron micrographs showed no sign of surface crazes, indicating that the whitish area was purely bulk color shift from light scattering. The yellow color was suspected to be the degraded hindered phenol antioxidants forming quinone structures. Because antioxidant was used as a sacrificial free-radical scavenger, it degraded to form quinone structures that were frequently deep yellow.

Partially degraded samples were further aged at 120°C for an additional 160 hours in order to observe the growth of the degradation, the change in the color, and the initiation of white spots. Figures 4 and 5 show this progression: the white front, which was very brittle, grew to a much larger size, and light yellow color started to form, increasing in size from the earliest white area seen in Figure 4. Several additional white translucent degradation spots were also initiated. The uneven initiation was an indication of uneven antioxidant dispersion and distribution.

Figure 4. Optical microscope image at an ~100-µm MDPE film (MDPE-2) oven aged at 120°C for 426 hours.

Figure 5. The film sample shown in Figure 4 after an additional 160 hours of aging.

Inadequate dispersion and distribution of antioxidants can shorten product shelf life unpredictably. The dispersion of various additives—including colorants, antistatic agents, impact modifiers, processing modifiers, fillers, etc.—in different polymers requires the proper selection of machinery and careful attention to process conditions.

Catalyst Residue and Processing-Machinery Transition-Metals Contamination. It is known that the thermal oxidation of polyethylene is often catalyzed by transition metals, presumably through the promotion of hydroperoxide decomposition. Polypropylene is also very susceptible to thermal oxidation, even at ambient temperatures, so much so that it always comes with an antioxidant additive package. Catalyst residues from polymerization—for example, titanium—and the presence of transition-metal impurities are frequently the accelerators for hydroperoxide thermal decomposition in polypropylene. Transition-metal contamination can come from different sources, including tools, machinery wear, contaminated additive packages, and rust transfer from autoclave equipment. Figures 6 and 7 show the evidence of metal contamination in MDPE film. Many yellow spots of approximately 1 mm or smaller were seen. High-voltage SEM and EDS analyses identified these yellow spots with dark-yellow core centers as iron, copper, and other organic compounds (see Figure 8). Since transition metals are prooxidants, they initiate the MDPE autocatalytic oxidation.

Figure 6. Oven-aged (120°C) MDPE film showing evidence of metal contamination. Magnification is 4.5x.

Figure 7. Spots shown in Figure 6 at magnification of 38x.

Figure 8. EDS spectrum of a contaminating particle in MDPE film. Iron (FE), copper (CU), and other organic compounds were detected.

It is interesting to observe that the fractal pattern of the degraded material flows away from the center when aged at 140°C (see Figure 9). From the flow pattern, it is reasonable to assume that the center is the initiation site, since the low-viscosity degradation products aggregated into fingers while the reaction progressed radically outward. The center yellow-spot contaminant appears to be prooxidant and responsible for the heterogeneous initiation of the degradation. This type of contamination is often ignored because the adverse effects do not become evident in the short term. However, depending on the product's shelf-life requirements, the long-term effects of such contamination need to be considered.

Figure 9. MDPE film (MDPE-3) aged at 140°C for 50 hours.

Embedded Particulate Contamination in Rigid Components. Particles embedded in a rigid plastic can severely reduce the ductility of the material. We have studied the effect of contamination in a polycarbonate tensile specimen, the elongation-to-break of which before and after steam sterilization can be seen in Table I.

% Elongation
Material Non-sterilized Steam-sterilized
at 250°F for
45 minutes
(a) Mw=19,900, MFR=48 Contamination not seen 19748
(b) Contaminated (a) 4430
(c) Mw=23,500, MFR=21 Contamination not seen 213N/A
(d) Contaminated (c)227N/A


Table I. Effect of contamination in polycarbonate tensile specimen.

It is well known that steam sterilization can cause a loss in ductility in polycarbonate materials. This occurs because the amorphous polycarbonate resin undergoes free-volume relaxation at autoclaving temperatures, and elongation is a measure of ductility. Table I shows that contaminants affect the lower-molecular-weight polycarbonate dramatically. The authors found it very surprising that particles as large as 1 mm could be seen on the fracture surface. For example, Figure 10 shows the fracture surface of a tensile specimen containing a particle ~0.4 mm in size; stress concentration had caused the specimen to break in a brittle manner. Contaminants, identified by microscope IR, included a wide range of materials, among them polyethylene, PVC, nylon, aluminum fragments, chromium titanium, silicone, sulfur, and so on. These specks could have been introduced from the room environment or during any of the manufacturing processes, including polymerization, compounding, pigment formulation, material handling, and injection molding.

Figure 10. Fracture surface of a polycarbonate (Mw=19,900, MFR=48) tensile specimen at magnification of 64x.

CONCLUSION

Plastics failure from heterogeneous contamination was examined. The contaminants were from degraded PVC, inadequate dispersion and distribution of antioxidants, transition-metal prooxidants, and particle inclusions in rigid plastic. PVC contamination is normally purely cosmetic, though it can cause problems if large gels are included in thin plastic films. Poor dispersion and distribution of antioxidants can often shorten in an unpredictable manner the shelf life of a finished plastic component. Transition-metal prooxidants catalyze thermal degradation and also photodegradation, and are the most deleterious species for many plastics. An inclusion—whether a rigid or soft particle—acts as a stress concentrator for rigid plastics. Depending on the size and shape of the particle, the plastic specimen or part could reach a critical stress and fail prematurely.

BIBLIOGRAPHY

Barr-Kumarakulasingshe SA, "Modeling the Thermal Oxidative Degradation Kinetics of Polyethylene Film Containing Metal Pro-oxidants," Polymer, 35(5):995, 1994.

Encyclopedia of Polymer Science and Engineering: Degradation, vol 4, Kroschwitz J (ed), New York, Wiley, 4:630—696, 1986.

Kudoh H, "Application of Target Theory for the Radiation Degradation of Mechanical Properties of Polymer Materials," J Mat Sci Letters, 15:666—669, 1996.

Plastic Additives and Modifiers Handbook, Edenbaum J (ed), New York, Van Nostrand Reinhold, 1992.

All of the authors are current or former employees of Baxter Healthcare Corp. (Round Lake, IL). Michael T. K. Ling is an engineering specialist concentrating on medical product design and troubleshooting and on developing applications for polymeric materials. Stanley P. Westphal, now retired, was an engineering specialist in polymer morphology and rheology. Chuan Qin, PhD, is an engineering specialist involved in biomedical process development for devices and drug-delivery systems. A senior engineering specialist, Samuel Ding, PhD, works in biomedical polymer development. Lecon Woo, PhD, is a Baxter distinguished scientist, specializing in biomedical polymer development and polymer rheology and processing.


Copyright ©1998 Medical Plastics and Biomaterials