Polymeric materials provide new options to medical device designers

The medical industry, like most other industries, is forced to deal with market trends and changing regulatory environments. In addition to rapid and costly changes, today's medical device manufacturers are challenged by increased competition, new regulations, higher quality standards and the need for more flexibility.

While advanced engineering plastic materials are still "new" when compared to materials that engineers typically use, they can offer many benefits that help address these critical issues. For example, Quadrant Engineering Plastic Products, thought to be the world's largest manufacturer of machinable plastics, has been working hand-in-hand with customers, helping designers replace traditional materials like stainless steel and ceramics. As a result, customers have witnessed several advantages with the machinable polymeric materials including biocompatibility, ease of sterilization and cleaning, X-ray transparency, self-lubrication (for wear applications), ductility and durability and lighter weight.

One of the most interesting developments in machinable plastics is the availability of tested and certified stock shapes that meet the stringent criteria – not just the resin, or starting materials used to make the machining stock. According to Quadrant, this new compliance adds to the compelling reasons for the designer to consider the use of machinable plastics.

The following summary on the advantages of advanced engineering plastic material offers insights into application considerations.

Biocompatibility
Depending on the type and duration of body contact, different regulatory compliances may be required. Knowing that engineers select materials that meet the demanding USP and ISO standards, manufacturers such as Quadrant have developed a range of materials that are pre-tested by an independent, internationally recognized testing organization. These pre-tested materials are offered as designated Life Science Grades. This product range meets various portions of the United States Pharmacopoeia (USP) and ISO 10993-1 guideline requirements for Biocompatibility Testing of Materials.

These pre-tested and certified Life Science Grades (LSG) not only save money, they also reduce time to market for new equipment and devices. Materials like Quadrant's LSG range are produced from specific raw masterial lots and come with full traceability from resin to finished machining stock.

Sterilization and cleaning
Typically, parts for single use applications must be injection-molded in large series, in order to meet targeted price guidelines. For devices that have repeated use, machining becomes a viable process because of the high cost/value often designed into reusable medical devices. For these demanding applications, cleaning and sterilizing can require up to 1,000 autoclave cycles. To avoid early and costly failure, consideration of the sterilization procedure and frequency are key factors in the material selection process. Graph 2 outlines the suitability of the different materials for sterilization.

Quadrant's testing shows that most of the Advanced Engineering Plastics are rated good or very good when exposed to different sterilization methods. Steam sterilization, dry heat sterilization and gamma sterilization can cause entry-level materials like polycarbonate (PC) and acetal (POM) some difficulties.

Samples of these materials should be utilized to simulate the whole cleaning process, which is a combination of cleaning; drying and sterilizing; and using specified cleaning agents, concentrations and procedures which can best dictate material selection.

It is important to identify chemicals present in the application environment early in the design phase in order to select materials that can handle and excel in that environment. Manufacturers like Quadrant keep detailed databases of chemical resistance data, and an e-mail or phone call can help to answer questions. Graph 3 gives a brief overview of the chemical resistance of materials frequently specified for use in the Life Science environment. It indicates the broad capability range of many plastic materials.

X ray transparency
The good X-ray transparent behavior of plastic materials, when compared to traditional materials like stainless and specialty steels, has fueled the adoption of these new products. Fixtures made of thermoplastic materials will not cause a "shadow" on an X-ray, giving them a distinct advantage in use. The material's transparency varies with cross-sectional thickness and materials used in the specific device. For example, tests run with a 12mm thick sample indicate a transparency between 65% and 80%. In another test, a 30mm thick test piece of the very rigid Ketron PEEK CA-30 LSG material displayed a value of more than 50%, making it an ideal choice for fixation or targeting devices with X-ray transparency requirements.

Self lubrication (Wear applications)
The majority of machinable plastics are used for dynamic applications where extreme wear resistance is required. Many medical and pharmaceutical applications would benefit from the same wear-resistance gains that other designers have enjoyed; however, medical equipment engineers have been unable to benefit from this feature due to the restriction of the required regulatory guidelines. Using the materials like Quadrant's Life Science Grades allows the designer to meet or exceed the engineering targets while meeting the demanding regulations. The wear data presented was generated in Quadrant's laboratories and shows the performance gains possible with some of the extreme wear resistant grades developed by Quadrant's material scientists.

Stiffness
In addition to the wear resistance that plastic materials can offer, structural parts can also benefit from designs that incorporate this new generation of plastics. While not as stiff as metals or ceramics, some plastics can offer remarkable strength. With an E-modulus of almost 10,000 MPa, Ketron PEEK CA30 LSG can be used in applications like fixation, targeting devices, grips and handles. In many cases, this unique combination of rigidity and durability can lengthen the service life of devices.

Dimensional Stability
Dimensional stability or the precision to which a part can be machined and maintained is important in the material selection process. It is a combination of thermal coefficient of expansion (CLTE) and moisture absorption. This is a key design criteria for applications that require frequent steam sterilization.

Selecting the correct material will insure that parts are delivered in tolerance and remain useful throughout their projected service life. Ketron PEEK LSG materials demonstrate the best dimensional stability within the pre-tested and certified range. A broad range of other materials, not currently part of Quadrant's Life Sciences program can demonstrate very precise tolerance control.

Conclusion:
These few key factors offer a brief introduction into the material selection process. There are many criteria that drive the selection process. These are determined by the type and use of materials in the application. Once service temperature, part loading and chemical resistance concerns are addressed, other special factors can be considered. Quadrant and other materials manufacturers provide technical teams that can offer assistance.

Marc Knebel is the Market Segment Manager of Life Science Products for Quadrant Engineering Plastic Products. He is located in their Sinsheim, Germany facility. In addition, the company maintains a U.S. Headquarter Office in Reading, PA. Quadrant EPP employs an estimated 2,000 people worldwide with 1,000 people located at U.S. sites.

Quadrant Engineering Plastic Products
Reading, PA
quadrantplastics.com