MATERIAL MATTERS COLUMN
The medical background to PEEK
Professor David Williams DSc, FREng
is Professor of Tissue Engineering at the University of Liverpool and Director of the UK Centre for Tissue Engineering located in the Universities of Liverpool and Manchester. He is Editor-in-Chief of Biomaterials, the leading journal in the biomaterials field. He is Scientific Director of STEPS, the European Commission Framework VI Programme on a Systems Approach to Tissue Engineering Products and Processes. Professor Williams is also a Managing Partner of Morgan & Masterson LLC, a consulting partnership that focusses on global health-care issues.
As alluded to many times before in this column, there are extremely few occasions when totally new biomaterials are required for implantable medical devices. We already have a good collection of materials with a range of mechanical properties and acceptable biocompatibility. In addition, the high cost of introducing new materials for clinical uses, where volumes of the material required per annum are relatively low, militate against commercial developments. Sometimes specific properties can be improved by refinements to chemical structure or composition, as with the improved wear resistance achieved through cross-linking ultrahigh molecular weight polyethylene, or improved mechanical properties can be obtained with a reduction in the grain size of alumina. However, occasionally a new material emerges with distinct characteristics that suggest that significant improvements in performance could be expected or that different clinical needs could be met.
One area where there have been limited developments over the last several decades, but where improvements are still desirable, concerns the use of thermoplastic polymers. Uses of these polymers in engineering in general have of course increased enormously in recent years. There is available a range of high performance materials and methods to manufacture them and they compete with metals under many circumstances. The automotive and aerospace industries have been transformed by the availability of these materials. However, 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 of these materials are not appropriate for clinical use. As a result we are still largely using polyethylene, polypropylene, fluorocarbons and acrylics for most structural applications. 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 one of the family of polymers known as polyaryletherketones. The molecular backbone of these polymers is aromatic with combinations of ketone and ether functional groups appearing between the aryl rings. PEEK has ether-ether-ketone sequences and the other prominent member of the family has ether-ketone-ether-ketone-ketone (PEKEKK) sequences. This type of chemical structure provides good mechanical properties coupled with exceptionally good chemical resistance and stability at high temperatures, a combination that is particularly attractive in high performance situations, for example, in aircraft systems. It is also amenable to fibre reinforcement, with glass or carbon fibres, which increases the versatility of this group of materials even more.
It was known that PEEK and carbon fibre reinforced PEEK had the potential for good biocompatibility twenty years ago, when I published the first paper on the subject,1 and there were several suggestions that these materials could well find applications in implantable devices. However, it took a long time for this potential to be realised on a practical basis. This was partly associated with the disincentives for investment in new biomaterials mentioned above and the limited number of companies engaged in the development and production of this group of materials. The main company that was involved was Imperial Chemicals Industries (ICI) Ltd in the UK, but it specifically excluded the medical sector from the areas of applications in which it invested. ICI sold its PEEK business in 1993 and it was not until the establishment of Victrex Ltd that this position changed. Victrex launched a medical grade of PEEK, known as PEEK-OPTIMA in 1998, and then established a separate company Invibio Biomaterials Solutions in 2001 to actively promote clinical applications.
The essential characteristics of PEEK
PEEK is normally a semicrystalline polymer whose properties can be varied by the use of different processing methods. It is amenable to injection moulding, where it is normally produced with a 30–35% degree of crystallinity and a molecular weight in the order of 100,000. The sequence of ether and ketone groups interspersed between the aryl rings of the backbone provides exceptional chemical stability: it 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 hydrolyse.
The flexural modulus of nonreinforced PEEK is approximately 4 GPa compared with 1 GPa for high density polyethylene, and this increases to 20 GPa when reinforced with 30% chopped carbon fibre. The tensile strength is close to 100 MPa and the elongation at break is 30–40%.
A variety of biocompatibility studies have been published, as reviewed extensively by Kurtz and Devine.2 There is general agreement that biological safety with respect to the usual array of preclinical tests is assured; that is to say that no cytotoxicity, mutagenicity, immunogenicity and so on should be expected.
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 fibre PEEK are now promoted for applications in implantable devices, especially, but not exclusively, in structural situations in the musculoskeletal system. A good place to start the discussion of applications is in the area of spinal surgery, which was the subject of a recent column.3 Spinal cages used to stabilise the anterior spinal column have been available for some time, the early versions relying on titanium for their construction. However, titanium is stiff and its radiopacity hinders the examination of bone growth within the device, thus thermoplastic polymers are considered as alternatives. PEEK and PEKEKK have been used for this application with success.4 As well as the nonreinforced polymer, carbon fibre 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.2
In many other clinical areas, the potential for PEEK and carbon fibre reinforced PEEK has been recognised, but commercial products and clinical use are only just emerging. It is widely appreciated within orthopaedic 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 fibre 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 Again the results have to be considered to be preliminary and long term data are not available.
The PEEK future
These are early days for the clinical applications of PEEK and other polyaryletherketones, both reinforced and nonreinforced. Data on clinical performance in a wide range of situations are required before these materials can truly be considered to be effective and reliable biomaterials. The signs are good, however, and we may anticipate an increase in their applications in the future should their undoubted potential be confirmed by good clinical outcomes.
1. D.F. Williams, A. McNamara and R.M. Turner, “Potential of Polyetheretherketone (PEEK) and Carbon-Fibre Reinforced PEEK in Medical Applications,” J. Mater. Sci. Letters, 6, 188–94 (1987).
2. S.M. Kurtz and J.N. Devine, “PEEK Biomaterials in Trauma, Orthopaedic and Spinal Implants,” Biomaterials, 28, 4845–4869 (2007).
3. D.F. Williams, “Cages and Chondrocytes: Techniques To Replace and Regenerate the Troublesome Intervertebral Disc,” Medical Device Technology, 18, 7, 8–10 (2007).
4. S.J. Ferguson, J.M. Visser and A. Polikiet, “The Long Term Mechanical Integrity of Nonreinforced PEEK-OPTIMA Polymer For Demanding Spinal Applications,” Eur. Spine J., 15, 149–56 (2006).