The medical device industry is very hesitant in trying new materials- in fact, very few new polymeric materials get adopted by this industry. Two factors are mainly responsible for this - The quality of existing materials and high costs of introducing new materials. It is very rarely that totally new biomaterials are required for implantable medical devices � existing range of materials offer a spectrum of mechanical properties and acceptable biocompatibility. Specific properties can be improved by modification to chemical structure or composition, improved wear resistance can be achieved through cross-linking ultrahigh molecular weight polyethylene, improved mechanical properties can be obtained with a reduction in the grain size of alumina . Also, volumes of newer materials introduced for clinical uses are required at very low levels annually , hampering commercial developments.

The rigorous demands on biomaterials used in long term implantable devices, including, biostability and biocompatibility, coupled with creep, fatigue and wear resistance, have meant that many high performance materials are not appropriate for clinical use. As a result largely polyethylene, polypropylene, fluorocarbons and acrylics are used for most structural applications. However, sporadically, a new material is unearthed with distinct characteristics suggesting that significant performance improvements could be expected or that different clinical needs could be met. It is for this reason that the introduction of polyetheretherketone (PEEK) into the medical field has caused a great deal of interest in the past few years. PEEK is rapidly emerging as a fore runner for high performance implantable applications. It PEEK and carbon fiber reinforced PEEK are known to have the potential for good biocompatibility two decades ago, when their usage in implantable devices was also suggested. However, it took a long time for this potential to be realized on a practical basis because of the disincentives for investment in new biomaterials and the limited number of companies engaged in the development and production of PEEK.

PEEK is normally a semi crystalline polymer whose properties can be varied by the use of different processing methods. It is amenable to injection molding, where it is normally produced with a 30�35% degree of crystallinity and a molecular weight in the order of 100,000. PEEK polymer is an exceptionally strong engineering thermoplastic that retains its mechanical properties even at very high temperatures. The material is tough and abrasion resistant with high-impact strength and excellent flexural and tensile properties. It has a low coefficient of friction and resists attack by a wide range of organic and inorganic chemicals and solvents.
Implantable-grade PEEK polymer, which is suitable for long-term (greater than 30 days) implantation, shows significant benefits over traditional materials, such as polyethylenes, metallic alloys, and ceramics. Implantable-grade PEEK polymer is characterized by its biocompatibility�the suitability of a material for exposure to the body or bodily fluids, and its biostability�the ability of a material to maintain its physical and chemical integrity after implantation in living tissue. The combination of strength, stiffness, and toughness, along with the ability to be repeatedly sterilized without the degradation of its mechanical properties, makes it suitable for implantable medical device applications. I t is resistant to attack by all substances apart from concentrated sulphuric acid. It is unaffected by radiation, whether gamma or electron beam, and although it does absorb water slightly (0.5% by weight at equilibrium), it does not hydrolyze. Implantable-grade PEEK polymer is one of the most chemically resistant polymers available. It displays chemical resistance, confirmed by 30-day exposure to simulated body fluid environments utilizing a sodium chloride solution, glycerol, vegetable oil, and an alcohol, with no adverse influence on the material's mechanical properties. Similarly, compression tests after soaking in physiological saline for up to 5000 hours have confirmed the stability of the polymer under these exposure conditions.

On the basis of the known properties and characteristics of PEEK and its good performance in biological safety and preclinical in vivo studies, PEEK and carbon fiber PEEK are now promoted for applications in implantable devices, especially, but not exclusively, in structural situations in the musculoskeletal system. Spinal cages used to stabilize the anterior spinal column have been available for some time, the early versions using titanium for their construction. However, titanium is stiff and its radiopacity hinders the examination of bone growth within the device. PEEK and PEKEKK have been used for this application with success. Polymer, carbon fiber reinforced PEEK and hydroxyapatite reinforced PEEK have also been used. In addition, sometimes the cages have been combined with bone morphogenetic protein contained in a collagen sponge in attempts to enhance bone regeneration. An increasing number of products for spinal surgery are now commercially available.

Implantable-grade PEEK polymer compounds can be formulated using a variety of additives including carbon fibers, barium sulfate, and glass fibers. The additives satisfy various application-specific requirements. Because the polymer is naturally radiolucent, adding barium sulfate at varying concentrations enables the x-ray density of devices to be tailored from mild to strong radiopacity. It is also being used in the development of cardiovascular applications such as heart valves and intracardiac pumps; for suture anchors for arthroscopy; and for dental applications such as permanent dentures, bridges, abutments, and healing caps. Device manufactures have used the material in the development of spinal-fusion cages, finger joints, hip and femoral bone replacements, bone screws and pins, components for implanted cardiac pumps, and dental posts and caps.

In many other clinical areas, the potential for PEEK and carbon fiber reinforced PEEK has been recognized, but commercial products and clinical use are only just emerging. It is widely appreciated within orthopedic surgery that metallic femoral stems used in total hip replacement are not ideal biomechanically because of their much higher elastic modulus compared with that of bone. Attempts have been made to design lower modulus composite stems involving reinforced polymers, some of which have involved carbon fiber reinforced PEEK or PEKEKK. Some of these systems have regulatory approval, but long term clinical data are not yet available. Similarly, PEEK has been considered as an alternative to high density polyethylene for the bearing surface in total hip and knee replacement prostheses. Some short term data indicate that these materials may give good tribological performance in conventional types of prosthesis design and in hip resurfacing systems. One of the major problems in orthopedic surgery is the mismatch of stiffness between the bone and metallic or ceramic implant. The moduli of metals and ceramics are fixed at their inherently high levels, whereas the modulus of implantable-grade PEEK can be adapted. This adaptability reduces stress concentrations that can be transferred to the bone and stimulates the healing process.

Significant work is still required to see the commercial viability of this new development.