Plastics infiltrate medical-device redesigns

DN Staff

January 8, 1996

10 Min Read
Plastics infiltrate medical-device redesigns

The proposed "Clinton Health Plan" may have died, but its siren call to cripple runaway health-care costs continues. This unrelenting economic pressure challenges medical OEMs to create more innovative and cost-effective products. The result: a heightened symbiosis between plastics manufacturers, fabricators, and medical OEMs, who, more frequently than ever, are turning to plastics to save weight, consolidate parts, raise performance and safety levels, and reduce product costs.

"Everybody has a tremendous interest in reducing the cost as opposed to the past several years when that wasn't so much of an issue, plus this was a high margin-level industry," explains Edmundo Vallejo, emerging markets manager at GE Plastics. "In contrast, today we all see a tremendous drive towards cost reduction."

Nancy Hermanson echoes Vallejo. She's chairperson of the Society of Plastics Engineers Medical Products Div., and a medical markets technical leader at Dow Plastics. "This cost-conscious environment has caused a lot of changes in how device manufacturers market and develop new products. Because price pressures exist, you're looking for ways to make the medical process safe and effective, yet bring the device in at a lower cost."

Market prognosticator, The Freedonia Group, Cleveland, foresees medical plastics growing from a 1993 level of 2.6 billion pounds to 3.37 billion pounds in 2003. The 23% spurt in growth is fueled primarily by the increased use of plastics in product component--not packaging--applications, Freedonia claims. The group also notes that polyvinyl chloride (PVC) and polypropylene (PP) with improved infection-resistance and sterilization properties should retain their top market positions. This, says Freedonia, is due to their prevalence in non-invasive component and packaging applications.

The conversion from glass and metal to plastic should also continue, and engineered resins with favorable cost-performance advantages, such as polycarbonates, thermoplastic copolyesters, and advanced styrenic resins (high-impact polystyrene, or HIP) should become more commonplace. Advanced engineering resins, such as sulfones, will turn up in orthopedic implants, internal catheters, and other invasive devices, due to their biocompatiblity, radiation tolerance, and bioabsorbable properties.

Where are plastics making a big difference? Here's a clue. Look where the systemic push for health-care cost reductions and service improvements leads:

Shorter hospital stays.

  • Less critical care.

  • Fewer doctors.

  • More off-site, outpatient, and home health care with an emphasis on wellness.

  • Improved product safety.

  • Simplified diagnostic, testing, and surgical procedures.

  • Large, consolidated medical care entities with tremendous buying power.

These needs pressure the medical products industry to create replacement products that offer equal or better performance at a lower price. They can even result in new and more costly high-tech product designs that shorten procedure and hospital stays, leading to overall cost savings by improving health-care efficiency. What follows are examples of products and materials that address both ends of the medical products marketplace.

Plastics in the operating room. First, let's look at minimally invasive surgery (MIS). Equipment required for surgery is sophisticated and costly. Also, since the surgeon can't work as fast, operating-room time increases. But, with MIS's smaller wounds, patients tend to recover much faster than they would under conventional surgical practices. As a result, less time is spent in the hospital, cutting overall costs.

Harnessing plastics' advantages to redesign more traditional medical devices can point the way to such savings. Take the "Villalta retractor" from Advanced Surgical Instruments, Indianapolis, IN, for example. The new design replaced a complicated, 50-year-old, stainless-steel device used in 3 to 4 million laporoscopic procedures each year in the U.S. Its genesis began when surgeon and OB/GYN specialist Dr. Josue Villalta became frustrated with the old design's inefficiency.

Villalta and engineer John Passmand initially developed a better metal prototype. Still dissatisfied, they switched to plastic with design assistance from Metro Plastics Technologies, Inc., Nobelsville, IN. Part consolidation dropped the final part count from 49 to 8 for the first all-plastic, self-retaining, disposable abdominal retractor. It weighs only 1.1 pounds and measures 18 cm by 15 cm.

The lightweight retractor is molded entirely out of 40% glass-filled Isoplast(R) 2540 polyurethane from Dow Plastics. The glass fill gives the retractor's blades and frame the needed strength, rigidity, and impact resistance. At 50% glass-loading, the polyurethane's flexural modulus exceeds 2 million psi, while notched Izod impact resistance tops 15 ft-lb/in. The engineering resin has a class VI USP rating, helping to speed FDA approval.

In the operating room, the prepackaged, preassembled, and presterilized retractor cuts procedural time and eliminates one person. Advanced Surgical's Mike Einterz claims the retractor's patented ratchet system allows a surgeon to easily adjust the four retractor blades in about two minutes. The old design took up to 15 minutes. Ten minutes saved in a typical operating room amounts to between $200 to $400 in cost savings.

Plastics lower blood-test costs. Other common, yet critical medical procedures can benefit from plastics and interdisciplinary engineering acumen. For instance, i-STAT's blood analyzer eliminates a costly step: the lab. The device is not only simple to use, but it can perform eight standard blood tests. And plastic abounds in its cellular-phone-sized base unit and disposable cartridge system.

With just two drops of oxygenated arterial blood, the analyzer can calculate, display, and store the results of a typical "oxygen, pH, and carbon dioxide" blood-gas test in two minutes or less--at the patient's bedside. Even in emergencies, a blood lab needs 20 minutes to turn a test around. Normally, the tests take two hours.

Each blood test requires its own cartridge. According to Joe Rogers, i-STAT's director of assembly and manufacturing, a key issue involved the choice of plastics for the high-volume, injection-molded cartridges. The cover had to be clear to show the blood sample and to allow examination of the tiny sensors' alignments during production. Meanwhile, to meet precise tolerance, the base required sufficient rigidity and dimensional stability. Of special concern was a molded point that punctures a hole that allows blood to pass during testing.

I-STAT met all concerns using conventional engineering thermoplastics from GE Plastics. The cartridge cover is molded from Lexan(R) HP1 polycarbonate, the cartridge base and analyzer from Cycolac(R) ABS. Other smaller internal cartridge components employ Valox(R) polyester, picked for its electrical properties. Both the Lexan and Cycolac resins satisfied i-STAT's moldability requirements for the intricately channeled cartridge's base and cover. And, because the resins are "reasonably priced," they solved i-STAT's cost equation.

Another switch in materials enabled Xylum Corp., Scarsdale, NY, to improve the design of its CSA Clot Signature Analyzer. The unit analyzes blood coagulation and platelet response by recreating the body's hemostasis processes. As with i-STAT's blood analyzer, the unit's sealed plastic cassette is thrown away after use.

When Xylum originally tried to injection mold the cassette's clear acrylic, stress cracks developed upon ejection from the tooling, resulting in ugly streaking. To overcome this problem, Xylum turned to ICI Acrylics' Perspex(R) CP-927G HF impact acrylic polymer. The company found that the easy flowing acrylic also had a high melt index that could navigate the sharp corners, edges, and points in the cassette's 11 complex injection-molded parts. ICI claims that Perspex, a USP Class VI resin, offers seven times the impact strength of general-grade acrylics.

Environmental advantage. Switching materials to meet environmental and waste-management concerns and regulations can pay unexpected dividends. Such was the case when the Kendall Co., Mansfield, MA, redesigned its disposable sequential compression sleeve (SCD), called Green-Sleeves. The sleeves are placed around a patient's legs to help prevent deep-venous thrombosis before, during, and following surgery.

In redesigning the SCD, Kendall had two objectives: lowering the sleeve's weight, and eliminating chlorine emissions during incineration. By supplanting PVC with chlorine-free, non-reactive, metallocene-catalyzed Exact(R) plastomers from Exxon Chemical Co., Kendall not only met its environmental goals, but wound up with a better performing product.

The new SCDs perform like their predecessors, but at 12 ounces each, weigh 25% less. That translates into less waste to dispose of or incinerate. Perhaps more significant, thanks to the softness of the plastomer, GreenSleeves provide a more comfortable, quieter environment for patients than the crinkly PVC products they replace.

Once under the scalpel, the body instructs the blood to clot. Since circulation slows greatly in leg veins, clots may form there more frequently. By sequentially inflating its six annular compression collars, GreenSleeves force the venous blood back to the heart before clots can form.

The changeover to the Exact polymers challenged Kendall. "The old classic manufacturing techniques used in vinyl don't apply, and we had to create new ones," explains John Dye, senior engineering associate at Kendall. "We're still on a learning curve, but that's normal." GreenSleeves' higher performance currently commands a 10% price premium that offsets early manufacturing inefficiencies. Kendall uses a proprietary RF-welding process to join the Exact films.

Award-winning form. When Denver Instruments set out to design a low-cost personal centrifuge with features rivaling laboratory models, cost, performance, and appearance pointed to plastics. The reward: The Force series centrifuge walked off with Business Week's Gold Award for Industrial Design. The magazine cited the product for its "handsome combination of rounded and angular shapes, functionality, and price."

At the design's outset, Denver Instruments led an interdisciplinary engineering effort with Phillips Plastics Corp. and Design Continuum on a fixed-cost-per-part basis. The team specified GE Plastics' Lexan 141 for its chemical resistance to cleaning solvents and cleansers, tensile and impact strength, and ease of processing. Mitch Houston, Denver Instrument's product manager, says the 4.5-pound product achieves a 4-to-1 weight savings over metal.

To control rotor deflection at rotations up to 10,000 rpm, a 20% glass-fill strengthens the polycarbonate. The plastic's lightness meant that a smaller, lower-energy motor could be used.

Advancing on metals. Advanced medical-grade polysulfones, like Amoco's Rydel(R) R and GE Plastics' Ultem(R) HTX, are also bumping metals aside in some tough equipment reuse applications. For instance, Polyvac, Manchester, NH, now makes surgical trays from Jadex(R), a proprietary derivative of Raydel R. The polyphenylsulfone is transparent, lighter than stainless steel, provides excellent impact strength, and is sterilizable by all commercial methods.

Polyvac trays contain custom-labelled slots designed to hold and organize surgical instruments. The trays help manage the operating-room environment, and, in turn, improve surgical-procedure efficiency. The trays tend to be abused, and, since they are reused, must be tough. They also must resist chemicals such as autoclave steam that comes from a hospital's heating system and contains corrosive boiler additives. Jadex stands up under all of these conditions.

Ease of processing and the ability to survive sterilization also came into play when ValleyLab chose 30% glass-filled Vectra(R) resin from Hoechst Celanese for its reusable electrosurgical pencil. James Ladtkow, VallyLab product engineer, finds the liquid-crystal polymer (LCP) "a nice, easy material to process. It shoots like water, fills the six-inch mold easily, and there's no toxicity." The pencil's injection-molded body is ultrasonically welded to seal out saline and protect the internal electronics.

Finally, a pneumatic bellows made from Stevens polyurethane sheet from JPS Elastomerics Corp., Holyoke, MA, helps rebuild muscle strength and mobility for patients recovering from strokes, broken limbs, and other injuries. The polyurethane combines high puncture and abrasion resistance with good memory, high flex fatigue resistance, and durability. During the materials selection process, PVC was eliminated as a candidate because its plasticizers could leach to the surface and cause brittleness.

Many more exciting medical plastics applications are on tap. Unfortunately, several resin manufacturers refused to disclose what they are due to their customers' reluctance to reveal "trade secrets." Once they are ready to talk, Design News readers will be the first to learn about them.

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