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Chapter 390leading to increased wear and eventual implant failure.(81,84) To overcome this problem, several techniques are under research. Jiang et al.(82) made use of a gradient coating in which the carbon concentration gradually increases towards the surface. While only C-C bonds can be found on the surface and Ti-C bonds can be found deeper in the Ti, a hybrid layer of Ti-C and C-C bonds can be seen between these two layers. This not only improved adhesion but also increased wear resistance. Yetim et al.(86) applied a duplex surface technique to Ti6Al4V, in which the surface of the Ti alloy was first treated with plasma nitriding, after which a DLC layer was applied using magnetron sputtering. This produces a diffusion layer below the DLC coating, which acts as a hardened case, giving more support to the DLC layer, while retaining a low friction coefficient. As a result, wear properties were superior to those seen with a single treated surface. Furthermore, adhesion of the DLC layer increased, although only by a moderate amount.(86) PolyetheretherketoneDespite the recent advances in Ti alloy development, mismatch with the elastic modulus of bone remains a serious problem. Non-metallic fiberreinforced composites are currently being considered as an alternative for load-bearing implant applications.(87) These materials can combine a low elastic modulus, characteristic of the polymer matrix, with some excellent mechanical properties, depending on the nature and volume fraction of the reinforcing fibers. One of the materials of interest is polyetheretherketone (PEEK), a semi-crystalline polyaromatic linear polymer and part of the polyaryletherketones (PAEKs).Interest in polyaromatic polymers grew in the 1980s in an attempt to design hip stems and fracture plates with an elastic modulus close to that of bone.(88) After extensive studies of its biological and mechanical properties, PEEK was first used for implantation by Brantigan and Steffee in 1989.(89) This was a 2-year clinical study during which PEEK was used as spine cage for lumbar fusion. PEEK was commercialized as a biomaterial in 1998 and has been used since then in the field of spine surgery.(88) Nikolas de Meurechy NW.indd 90 05-06-2024 10:14