Get ready to shake up your preconceived notions regarding what materials work best in medical devices. Materials are available for medical applications that didn't even exist until recently. PEEK, for example, now comes in grades for permanent implantation. And certain metal alloys can now be injection molded, displacing plastics for lighter and stiffer products. That's why John Porter, business manager of Phillips Plastics Corp.'s Design Development Center in Hudson, Wisc. (phillipsplastics.com), suggests that during the design phase, it's most important to keep an open mind regarding material selection.

He points to just one emerging trend: PEEK replacing titanium. A reason for it is that PEEK is radiolucent — X-rays can see through it. That lets doctors understand how, for example, a bone is healing. Titanium, on the other hand, blocks X-rays. Also, PEEK has a flexual modulus that promotes bone cellular growth and healing, whereas titanium's rigidity doesn't let bone fractures heal well. Best yet, according to Porter, PEEK can be machined or injection molded less expensively than machining titanium.

In other cases new metal alloys are replacing plastics. “An example is a medical device recently designed to be all polycarbonate,” says Porter. “The device lacked the necessary structural rigidity to protect its large LED lens. Our first approach was to add stiffening ribs, but then the hand-held device got too thick. With an open mind towards material, we selected thixotropic magnesium, a form of magnesium for injection molding. Using this, we got the customer a part with a low profile for the hand, while still rigid enough to protect the LED. Thixotropic magnesium works with thinner wall sections and gives denser parts and better surface finishes as compared to die-cast magnesium.”

Alloy for high-performance hips

A HIPed alloy made with a special powder-metal process features a fine microstructure and uniformly distributed carbides of <10 µm for superior wear properties, higher strength, and better corrosion resistance as compared to conventionally processed CoCrMo alloys. HIPed alloy is used in metal-to-metal hip, knee, elbow, shoulder, ankle, and finger prosthetic joints that are said to withstand wear better than metal-to-polyethylene joints.

The alloy is made with Carpenter Technology's Micro-Melt process, which forms a consolidated powder-metal billet rather than conventionally wrought cast ingot billet. The technology eliminates microstructural defects and porosity typical with casting wrought alloys. It also eliminates defects in standard as-cast ingot such as carbide segmentation, which makes the material difficult to hot work.

Micro-Melt involves gas atomization of the alloy and yields a fine-powder particle size (<150 µm) with a homogenous composition. The powder alloy is then hot isostatically pressed (HIPed) to produce a fully dense compact, which is thermomechanically processed to a required production form. Properties of the ally exceed minimum requirements of ASTM F 1537 for wrought material used in medical devices.

Carpenter Technology Inc., 2 Meridian Blvd., Wyomissing, PA 19610, (610) 208-2000,
ims.info.ca/5569-225
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Compound guards against static attraction

Vu-Stat Y-20 is an acrylic multipolymer compound for injection-molding applications that provides permanent electro-static dissipative (ESD) protection. It works on medical devices where static attraction is a concern, such as needle hubs for bracheotherapy and spacers for dry powder pharmaceutical inhalation. The company claims unlike compounds that rely on topical ESD coatings, its compound offers static protection that can not be compromised by surface scratches or abrasions. Additionally, the compound is not dependent on moisture or humidity for its effectiveness and retains its ESD properties even after repeated washing cycles. The company says the compound provides a balance of mechanical, thermal, and flow properties. Available in pellet form, it can be sterilized by EtO, e-beam, or gamma radiation without affecting appearance. The optically clear material can be processed in all types of injection molding presses.

CYRO Industries, 100 Enterprise Drive, Rockaway, NJ 07866, (800) 631-5384, ims.info.ca/5569-226
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Vinyl elastomers stay tough and flexible

Medical-grade 3500 NT Flexalloy vinyl elastomers are said to be tougher than the flexible vinyl widely used in medical devices. They perform like rubber at a lower cost than polyolefin and styrenic TPEs. The compounds remain flexible at low temperatures. The elastomers also remain fatigue-resistant even after prolonged flexing in applications such as peristaltic-pump tubing. The company claims the elastomers beat polyolefin and styrenic TPEs in compression set, tear, and tensile strength, and resistance to flexural fatigue. Additionally, they provide better resistance to oils and fats and wider formulation versatility than TPEs. When compared with flexible vinyl compounds of the same hardness levels, the elastomers exhibit superior compression set, tear strength, and abrasion resistance and provide the melt strength necessary for blow molding. Flexalloy vinyl elastomers have Shore A hardness levels from 30 to 80 and are provided in grades with excellent clarity.

Teknor Apex Co., 505 Central Avenue, Pawtucket, R I 02861, (401) 725-8000, ims.info.ca/5569-227Circle 227

Thermoplastic alloy enhances enclosure design

Kydex T is a proprietary thermoplastic alloy from Kleerdex Co. that carries a UL Std. 94 V-0/5V fire rating and has an impact resistance of 15 ft-lbs/in (801 J/m). In one application, the alloy was used to enhance the enclosure design of a recently developed nuclear diagnostic imaging system. Original housings were painted and textured-fiberglass panels, difficult and costly to manufacture and assemble.

System developers worked with San Diego-based pressure former Specialty Manufacturing Inc. (SMI), which recommended the alloy because it provides good repeatability of the sheet. The enclosure redesign called for precise tongue-and-groove connections to simplify assembly. The connections' tabs and receiving slots, formed into the panels during processing, maintains tolerances within 0.015 in. And the natural memory of the polymer holds the part shape after the tabs and slots engage.

SMI says the thermoplastic sheet provides the color repeatability associated with a precision color house and accurately reproduces the acid-etched texture and mold-finish of aluminum pressure-forming molds, even with deep-drawn parts. Additionally, the firm says the alloy provides better protection against impact from wheelchairs and gurneys than other thermoplastics.

Kleerdex Co. LLC, (800) 325-3133, ims.info.ca/5569-228
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Foam urethanes keep their shape

Durethane M and F foam-urethane products maintain their shape, physical characteristics, coefficient of friction, and chemical resistance, while preventing glazing and buildup of various materials typical of solid elastomers. In addition the foam urethanes are durable compared to other light-duty froth-foamed processed materials. Durethane M is a mechanically mixed, microcellular foam with a uniform, mostly open, cell structure for products such as pick pads, actuator assemblies, rollers, and idler belts. Durethane F is a chemically blown, cellular foam with a closed cell structure suited for items such as self-cleaning paper-feed rollers. The developer says both materials have an excellent compression set, a high coefficient of friction, and special self-cleaning properties. They can be made conductive for static dissipation in high-speed digital-equipment applications.

MPC Inc., 16 Western Industrial Dr., Cranston, RI, (401) 946-4400, ims.info.ca/5569-229
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Material makes prototypes for flexible end-products

FullCure Tango rubber-like, photopolymer modeling materials work in Stratasys' Eden333 PolyJet rapid prototyping system to make prototypes for flexible end-products. Applications include discs for vertebrae, cartilage for knees or hips, and “muscles,” which can be used to emulate and study muscle movement in the joint. TangoBlack provides maximum flexibility with hardness of Shore A. TangoGrey is still flexible but with a hardness of 75 Shore A. Both offer high elongation at break.

Stratasys Inc., Minneapolis, (952) 937-3000, ims.info.ca/5569-230
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Two PEEKs withstand repeated sterilization

Two biocompatible polyetheretherketone (PEEK) polymers include an implantable and a medical grade. PEEK-Optima polymer is for medical implants having blood or tissue contact for more than 30 days. The material has strength and toughness, extensive biocompatibility, ideal imaging properties, and optimal modulus. PEEK-Classix is a thermoplastic for medical applications requiring blood or tissue contact of less than 30 days.

Both materials are chemically resistant and can be repeatedly sterilized using conventional sterilization methods, including steam, gamma radiation, and ethylene oxide processes without the degradation of their mechanical properties.

Invibio, (866) 468-4246, ims.info.ca/5569-231
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Foams safe for permanent skin contact

Poron Medical Urethanes and Bisco Medical Silicones meet internationally recognized FDA G95 and ISO 10993 standards for surface medical devices with permanent skin contact.

The Poron urethanes are biocompatible and contain no PVC, latex, or solvents. They are also medium density, open-celled, breathable, and fungal-resistant. The flexible materials resist collapse and offer comfort and durability. Their shock-absorption capabilities makes them well-suited for protective-garment padding, monitoring-device cushioning, and rehabilitative-brace padding. Poron urethanes come in thicknesses of 1.5 to 12.7 mm.

The Bisco silicones, with open and closed cell structures, maintain dimensional stability even when exposed to dry heat, steam, electron beam, ethylene oxide, alcohol, or gamma radiation. Applications include extreme-condition padding, sonography surfacing, and sterile, cleanroom surfaces. The silicones come in thicknesses of 0.5 to 12 mm, and meet FDA 21 CFR 177.2600 standards covering rubber articles for repeated use.

Rogers Corp., One Technology Dr., PO Box 188, Rogers, CT 06263, (860) 774-9605, ims.info.ca/5569-232
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Natural-rubber latex safe for allergy sufferers

U.S.-produced Natural Rubber Latex is safe for healthcare workers and patients who suffer from tropical latex allergy. The natural latex is derived from Guayule, a desert plant found in the U.S. Southwest. It is suited for manufacturers of surgical gloves, condoms, catheters, and other latex-base medical products seeking an alternative to lower-performing synthetic-latex materials.

Yulex Corp., 1945 Camino Vida Roble, Suite C, Carlsbad, CA, (760) 476-0320, ims.info.ca/5569-233
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Composites shield against radiation

Several grades of Gravi-Tech polymer-metal composites from PolyOne are used in radiation shields. They provide protection similar to lead-based materials but with 37% less weight. Other grades are available that match the density of lead.

Spring City, Pa.-based AMICI Inc. uses the polymer-metal composites to manufacture its nuclear lung-ventilation diagnostic devices called the Swirler UltraShield. The composites let the company injection mold a complex, radiation-shielded part. The alternative would have been to cast lead. Part weight dropped from nine to five lb, while providing the same shielding against low-level radiation isotopes. The composites enhance design flexibility because the device can be injection molded into unusual shapes rather than the larger and more standard ones if lead had been used.

PolyOne Corp., 33587 Walker Rd., Avon Lake, OH 44012, (440) 930-1000, ims.info.ca/5569-234
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