The history of silicone rubber is synonymous with advancements in medical materials. Silicones, a family of biocompatible elastomers, provide an attractive balance of properties. For example, silicones offer thermal stability over a wide temperature range. They also provide electrical insulation and are resistant to ozone, water, ultraviolet (UV) light, and some chemicals. For medical applications, silicone rubber can also meet requirements from the U.S. Food and Drug Administration (FDA) and the U.S. Pharmacopeia (USP).
Yet, silicones can come with limitations. Although they’re available in softer durometers, silicones become harder and more brittle with repeated steam sterilization. They’re also susceptible to abrasion and tearing, and pigmented silicones may appear faded or dirty following long-term use. Because of this, silicones used with reusable instruments don’t last as long as the industry would prefer. Further, for instrument manufacturers, silicone rubber may limit design freedom for soft touch surgical instruments with thin overmoldings.
To enhance the durability and appearance of medical devices, designers are evaluating other elastomers — including polymers that are generally associated with aerospace and oil and gas exploration. Candidates include FKM fluoroelastomers, high-performance materials that are biocompatible and can withstand over 1,000 steam sterilization cycles while maintaining their mechanical properties over time. This material also offers greater chemical resistance compared to silicones, which have traditionally been the elastomer of choice for many healthcare applications.
FKM fluoroelastomers have not yet been used in commercial applications for healthcare; however, material converters and surgeons have already demonstrated interest in its potential for orthopedic use. Compared to silicones, FKM fluoroelastomers combine greater durability while avoiding embrittlement, color, and surface changes, as well as conditions such as tackiness and pilling. While silicones remain the elastomer of choice for most healthcare applications, recent innovations in orthopedic elastomers may now offer alternatives.
The history of silicone began with the discovery of silicon, a key building block for this synthetic elastomer, in the 1800s. Subsequent research demonstrated that silicones could be produced commercially. During the 1940s, the introduction of an insulating silicone grease helped Allied aircraft to fly at low temperatures in high altitudes during World War II. Union Carbide, some of whose assets later became part of Solvay, began silicone production later during this same decade.
Silicone’s usefulness in a wide range of applications contributed to its growing popularity during the post-war years. During the 1950s, silicone was used in one of history’s fastest selling toys, a novel leather treatment, and various aerospace applications. During the 1960s, silicone rubber was chosen for use in the boots that astronaut Neil Armstrong used to walk on the Moon. The material was also used for a medical catheter that reduced the incidence of urethritis and rates of infection. Later, silicone implants and prosthetics were developed.
The first silicones used peroxide as a catalyst to initiate curing. Peroxide, or, more specifically, the addition curing method is still used but requires post-cure oven baking to remove an acid residue that can leave a powder, or bloom, on the elastomer’s surface. By using platinum as a catalyst instead, silicone compounders can achieve faster cure rates and eliminate post-baking, which improves cycle times, reduces the equipment needed, reduces energy, and simplifies the process. Today, these higher purity silicones are used in a variety of medical applications and can achieve FDA approval and meet rigorous USP Class VI requirements, which certify that there are no harmful reactions or long-term bodily effects caused by chemicals that leach out of plastic materials.
Medical-grade silicones are used in valves and tubes, for external jacketing on wires and cables for hospital equipment, surgical tools, patient monitoring systems, imaging and diagnostic instruments, and for short- and long-term implantable devices. Additional processing is required, but medical-grade silicones are biocompatible, hypoallergenic, flexible, and durable. They’re also available in softer durometers and support pigmentation that is useful for color-coding surgical instruments.
Silicone rubber offers many advantages but has some important limitations as a medical material. For example, silicone can withstand autoclaving to a point but has a service life that is limited to several hundred cycles. Hydrolysis, the chemical breakdown of a compound due to reaction with water, occurs after just a few hundred cycles when silicone is subjected to repeated steam sterilization. The resulting embrittlement, hardening, and color and surface changes of the silicone-based part can shorten the life of orthopedic instruments.
Silicone’s service life is also commonly reduced by abrasion and tearing. In fast-paced environments like hospitals and clinics, silicone used with medical devices and equipment are susceptible to damage. Increasing the thickness of the silicone that is used can improve durability but at the expense of flexibility. There are other considerations as well. Silicones are relatively thick and viscous, which may affect processing with some medical device designs.
Silicone also has limitations in terms of manufacturing and re-use. As a thermoset material, it has strong molecular bonds and resists deformation. Polymerization, whether catalyzed by peroxide or platinum, creates interlinked chains that give cured silicone its permanent shape. This provides dimensional stability but prevents subsequent manipulation. For example, injection molded silicones cannot be melted and reprocessed once molding is complete. Silicones can be recycled, but depolymerization is required.
Silicones can cost more than some other elastomers, but higher performance materials generally come with a higher price point. Still, the healthcare industry would prefer to use materials that can withstand a high number of steam sterilization cycles. For brand owners, appearance is also a concern. Pigmented silicones can appear faded or dirty after fairly limited use, which may suggest that a medical technology is old and, by implication, out of date. Fortunately, there’s an emerging alternative to silicone that addresses both durability and appearance.
The FKM fluoroelastomers are fluoro-carbon-based rubbers that withstand high temperatures and aggressive chemicals while maintaining their mechanical properties. FKM is the ASTM designation for this group of elastomers. With their extremely strong bonds between atoms, these materials are unlikely to undergo changes that can shorten service life. Their high fluorine-to-carbon ratio provides increased environmental resistance and their single carbon backbone helps minimize the effects of chemical attacks.
FKM fluoroelastomers are well established and have a long history of use. The first product was developed in 1957 in response to the high-performance sealing needs of the aerospace industry. Today, FKM fluoroelastomers are produced by many of the world’s largest chemical corporations and used in industries that range from semiconductor manufacturing to oil and gas exploration, to automotive and transportation.
Medical applications aren’t generally associated with FKM fluoroelastomers, but this family of materials shows promise as an alternative to silicone when higher performance and better appearance are required. With their unmatched operational temperature range and chemical resistance, FKM fluoroelastomers can withstand the high heat and aggressive chemicals associated with the steam sterilization, chemical disinfection and cleaning procedures used for orthopedic surgical instruments.
Recently, Solvay presented technical arguments for using FKM fluoroelastomers at a meeting of the American Academy of Orthopedic Surgeons (AAOS). The potential uses for these materials are promising. Greene Tweed, a manufacturer of high-performance elastomers, is partnering with Solvay to formulate and analyze different FKM variants and perform validation work. The results of the initial testing that compared this FKM fluoroelastomer to silicone across various performance criteria are shown in Figures 1 and 2.
Performance Comparisons and Next Steps
The FKM fluoroelastomer and silicone materials were repeatedly steam sterilized, then compared in terms of mechanical properties, color, chemical resistance, and biocompatibility. After more than 1,000 steam sterilization cycles, the fluoroelastomer performed favorably for strength and elongation. In some cases, the silicone material did not retain its color as well as the FKM fluoroelastomer. The FKM fluoroelastomers provided a significant performance improvement in terms of chemical resistance, a key consideration for disinfecting surgical instruments. Testing and chemical data for biocompatibility are also available.
Today, the development of potentially higher performance alternatives to silicone is ongoing. Solvay is in discussion with original equipment manufacturers (OEMs) and brand owners about real-life simulations of this promising next-generation material. Industry-relevant validations with partners such as surgeons and engineers are also under way. The development of a supply chain to serve this segment of the medical device market is required, and the regulatory environment for medical materials is well known.
As the market for reusable surgical instruments continues to grow and pressures to control cost increase, demand for higher-performance orthopedic elastomers will rise. A material that preserves the many desirable properties of silicone, while improving performance in terms of steam sterilization and chemical disinfection and cleaning, has widespread potential for soft-touch surgical instruments. Brand owners are seeking more attractive materials, too. While FKM fluoroelastomers are not yet used in medical applications, they may represent a premium alternative to silicones within the healthcare space, just as they do in other markets that have benefitted from their use.
This article was written by Dane Waund, Global Marketing Manager for Healthcare, Solvay Specialty Polymers global business unit, Alpharetta, GA. For more information, visit here .