By Kevin S. Marshall
While many medical products are still produced in off-the-shelf plastics, new thermoplastic technologies target medical applications specifically. Combined with a variety of specialty modifiers, they address the issues of biocompatibility, chemical resistance, sterilization compatibility, and processability. They can also reduce the hazards associated with electrostatic buildup, add lubricity, improve ergonomics, provide gripping surfaces, absorb X-rays, and boost stiffness/flexibility/tearability.
But where medical products are concerned, the whole field of additives comes under scrutiny. Thermoplastic formulas often start with ingredients that are considered safe for food contact as detailed in Title 21 of the Code of Federal Regulations (21 CFR) promulgated by the U.S. Food and Drug Administration (FDA). But where bodily contact is involved, the health-care industry tends to use materials that comply with either USP Class VI or ISO 10993-1 test criteria assuming this reduces the biological and legal risks.
It is interesting to review some recent advances in technologies that have established important medical uses.
Electrically conductive modifiers added to thermoplastics permanently protect against static accumulation and electrostatic discharge (ESD). Conductive thermoplastics continuously dissipate static rather than let it accumulate and discharge rapidly. ESD can damage sensitive electronic components and initiate explosions in flammable environments. Accumulated static charges can also halt mechanical processes by clogging the flow of materials. Conductive thermoplastics come in a wide variety of colors and, in some cases, retain transparency.
New pharmaceutical delivery systems, such as inhalation devices, currently incorporate conductive thermoplastics to facilitate accurate drug dosages for powders/mists. Conductive compounds stabilize the static effect so each use of the device has a stable environment in which to operate. Without conductive plastics, dosages would be inaccurate from either too little medicine (microparticles attracted to the walls) or too much medicine (medication builds up over time and suddenly releases).
Wear-resistant additives lower the coefficient of friction and reduce wear rates. A glucometer, for example, benefits from wear-resistant additives in that material lubricity extends the life of moving components such as the cover that moves back and forth to protect its display. The glucometer also contains critical color matches between mating parts made from dissimilar materials, which is challenging because of the way resins and additives impact color appearance.
Even high-volume, disposable applications benefit from lubrication technology. There are industry requirements to reduce needlesticks and other "sharps" injuries that can expose medical personnel to bloodborne pathogens. New product designs incorporate needles which retract or get covered after use. Some of these designs incorporate lubricated materials to provide a consistent "feel" and make the safety cover slide smoothly.
Long-fiber-reinforced nylon provides the right balance of burst strength plus chemical, creep, and permeation resistance in a compressed gas canister. The canister is used in a "needleless" drug injector. It introduces drugs in liquid form into the body using compressed air to move a plunger. This action forces medication through a micro-orifice into tissue under the skin without a needle. The injectors are said to deliver medicine with little or no pain and minimize infections. Lubricated elastomer stoppers and glassfilled polycarbonate plungers also offer opportunities for engineered thermoplastics.
Laser marking involves the use of a laser beam to create a permanent graphic by permeating the surface of a thermoplastic part. Unlike hot stamping or pad printing, the contrasting mark becomes integral to the part, making it more durable and less affected by solvents. Laser graphics can be applied to complex or inaccessible geometries unlike direct contact methods. One application is a reusable insulin pen, where the dark gray dosage numbers are laser marked into a white cylindrical surface. Laser marking can also permanently apply manufacturing lot numbers, dates, or unique identification codes.
Thermoplastic elastomers let designers create unique, "rubberlike" parts with customized color, softness, and other attributes. Two-shot overmolding techniques add an ergonomic, slip-resistant, grip to medical devices such as surgical tools. This process consists of two molding operations where the first molded material provides the rigidity, and the overmolded elastomer gives the finished product a soft touch. Elastomers can also be overmolded onto nonplastics or coextruded.
High-specific-gravity thermoplastics duplicate the "feel of metal" while retaining the processability of thermoplastics. These high-density materials also serve as an environmentally friendly replacement for lead. In some cases the additives render the thermoplastic radiopaque. Injection-molded parts, tubing, or sheet made from radiopaque compounds absorb Xrays and are not transparent to radiation. During radiation therapy or surgical procedures, flexible radiopaque sheets can selectively protect equipment and personnel from scattered or indirect X-rays. Catheter sheaths made from thermoplastics compounded with barium sulfate let the doctor see the catheter inside the body via X-ray.
Catheters are also a good example of one application combining several different properties. A designer may desire a soft, flexible tip so the catheter can follow the contours of the arterial system. Yet, stiffness is a must so the catheter can be pushed into position inside the body. Lubricated materials make the patient more comfortable during insertion, while other technologies promote a smoother tearing tube or radiopacity.
Even color technologies have evolved in recent years. For medical applications, designers can select from FDA-sanctioned pigments and nonmigratory pigments, which will not migrate out of the plastic. Glowin-the-dark pigments can be tailored for glow longevity and brightness. One hand-held medical device on the market utilizes a glow-in-the-dark thermoplastic bezel to provide visibility in low light.
Suppliers are responsible not only for new material technologies, but also for providing consistent, traceable, and quality products through current good manufacturing practice techniques. Thermoplastic processors expect less material variation, such as tight melt-flow indices and consistent pellet sizes. This results in fewer machine adjustments, moreconsistent part dimensions, and repeatable cycle times. When a product involves human health, any change in material or process could have profound effects.
Despite the perception of leading edge material technologies, medical markets change slowly due to the long regulatory validation process. Many of the "new" medical technologies currently in development already have a successful track record in other industries, giving medical designers a refreshing new palette of properties and options. While material cost is important, many specialty thermoplastics offer cost savings through manufacturing process improvements and parts consolidation.