While many engineers and designers have a solid understanding of metals in long-term implantable applications, there is limited in-depth understanding of implantable-grade polymers since, by comparison, they are relatively new biomaterials.

Despite their relative newness, the use of implantable-grade polymers is growing because medical-device designers are discovering they possess characteristics that let the polymers significantly outperform traditional materials in the aggressive environment of the human body. These characteristics include mechanical properties that can be tailored to meet specific needs. For example, a polymer's strength and stiffness can be varied by using carbon-fiber additives. Additionally, polymers are radiolucent on x-ray and do not produce artifacts on MRI or CT films, allowing a clear post-operative inspection of the implant site. Alternatively, specific additives such as barium sulphate can be incorporated into a polymer to produce varying levels of optical density for applications requiring implant visibility in post-operative images.

Implantable-grade polymers also offer manufacturing flexibility. Most can be injection molded and machined to extremely small dimensions and tight tolerances. The injection molding process may also reduce manufacturing expenses for high-volume products compared to traditional machining. However, not all polymers are successfully processed by all techniques. Therefore, comprehensive details on stability and advice on processing temperatures and parameters should be available from the polymer supplier.

As with any novel and industry-changing technology, there is a learning curve associated with using implantable-grade polymers. By recognizing the key engineering success factors and by involving experienced biomaterial suppliers early in development, medical-device designers can efficiently increase their expertise and understanding of polymers in long-term medical applications.

Polyurethanes, polyethylenes and polyetheretherketones (PEEK) are polymeric biomaterials currently used for existing long-term implantable devices and in the development of new devices. Engineers and designers should carefully research the polymer selection to ensure the appropriate properties for a particular device, which should include biostability, chemical resistance, elastic modulus, and impact strength. It's also important to evaluate:

Sterilization Because all implants must be sterilized before use, it is critical to carefully consider the effects of various sterilization methods on a polymer's mechanical properties. Some polymers are more resistant to certain sterilization methods than others. In addition, consider the effects of repeated sterilization. In all cases, final-property testing should be performed on a sterilized final part.

Wear concerns when used as a bearing surface Some polymers can be used as bearing surfaces, those surfaces that move against other surfaces while in motion. In this case, it is critical to understand the long-term affects of wear on the polymer. The type of motion (uni-axial vs. bi-axial) can significantly alter the size, shape, and number of wear particles that are generated and potentially released into the body.

Manufacturing Manufacturing methods can potentially alter a polymer's biocompatibility, so it is critical to understand which manufacturing techniques should be used for which polymers.

It is also important to carefully select a materials supplier. Several factors should be specifically considered when selecting a polymeric biomaterial supplier:

Security of supply Given the often long and costly product development timelines and regulatory approval process, it is essential that biomaterial suppliers commit to continued technical support and long-term availability of their raw materials.

Manufacturing control Determining the biocompatibility of new materials can be costly and lengthy. A costly requalification process can usually be avoided by selecting a polymer supplier with a stable and consistent manufacturing process. Manufacturing processes should be certified to Good Manufacturing Practice (GMP) standards wherever possible.

Accelerating regulatory approval The sterilization and biocompatibility characteristics of metals and ceramics are well established. This is not always the case for polymeric biomaterials. Therefore it is critical device designers seek out biomaterial suppliers that can provide comprehensive technical data and support including FDA master files. These should describe all critical and pertinent information regarding the materials' manufacturing details and extensive testing data. This would include biocompatibility, sterilization testing, ISO 10993 testing data, shelf life, and mechanical performance data.

Having this information available can eliminate the expense of repeating these tests for regulatory approval. Additionally, the ability of a supplier to directly answer questions regarding material performance from regulatory authorities like the FDA and notified bodies, can be a powerful tool for ensuring that device approval is as efficient as possible.

Commitment to quality Selecting polymer suppliers that demonstrate an on-going commitment to quality and that meet the strict standards employed by the medical-device industry is crucial to receiving consistent biomaterials. Adherence to GMP guidelines provides a transparent, auditable production process that brings peace of mind to device designers and regulatory bodies.

Michael Callahan is the president of Invibio Inc., a biomaterial provider to the medical-device market, and the world-wide manufacturer and distributor of PEEK-OPTIMA, PEEK-CLASSIX, and ENDOLIGN polymers.