Titanium (Ti) and its alloys have been used for dental implants because of their high biocompatibility and excellent resistance to corrosion. On the other hand, these materials are also stiffer than bone, which can prevent bone-implant osseointegration, as well as an equal load distribution between the oral implant and the hard tissue, leading to stress shielding and consequent bone resorption, causing failure of the dental implant. There is a need, therefore, to develop dental implants with superior osseointegration. One candidate material for this is Polyetheretherketone (PEEK). PEEK has previously been applied for dental and medical restorations, particularly since its stiffness can be manipulated to match the mechanical properties of bone using appropriate filler materials. Despite its promising properties, however, PEEK is a relatively bioinert material which affects osseointegration. Numerous studies have tried to solve this problem, either by combining PEEK with bioactive filler ingredients to support bone growth, such as hydroxyapatite (HA), nano-hydroxyapatite (nHA), and titanium or by surface coatings.
This study represents a contribution to this body of research. It develops, functionally graded bioactive PEEK samples for dental implant applications by applying a combination of injection-moulding of PEEK with bioactive materials and Physical Vapour Deposition (PVD) coating technologies to produce bioactive PEEK samples for load-bearing applications. More specifically, these bioactive PEEK composites were manufactured via injection-moulding technique by incorporating 0, 20, 40, 50, and 60 weight percentage (wt.%) of nHA synthesised via sol-gel process, and for comparison, commercial HA. In addition, three innovative bioactive PEEK composites were also fabricated by injection moulding. These contained 50 wt.% PEEK and, respectively, i) 25 wt.% titanium alloy Ti-6Al-4V and 25 wt.% nHA or HA; ii) 50 wt.% anatase titanium dioxide (TiO2); and, iii) 50 wt.% high-quality ultra-fine pearl. All the above bioactive PEEK composites were evaluated using Fourier Transform Infrared Spectroscopy (FTIR) and Scanning Electron Microscopy (SEM). Resazurin Reduction (RR) assay was applied to evaluate cell viability in vitro using human osteosarcoma (MG-63) and mouse osteoblast (MLO-A5) cell lines exposed over different periods 1, 4, 7, 14, 21, and 28 days. Alizarin Red Staining (ARS) and Sirius Red Staining (SRS) assays were performed, to evaluate calcium and collagen matrix formation. X-ray diffraction (XRD) patterns, tensile strength testing, and water contact angle (wettability) evaluations were also performed in this study. The FTIR spectroscopic analysis indicated that strong organisational bonds were established between the PEEK and the bioactive ingredients. The XRD pattern analysis showed the formation of crystalline PEEK and HA in the XRD spectra. The bioactive composites created in this study exhibited improved biocompatibility and bioactivity, encouraging cellular attachment and mineralised matrix deposition. Amongst the bioactive composites, samples made up of 20 wt.% nHA and 25 wt.% nHA:25 wt.% Ti-6Al-4V showed the highest mechanical strength for use as dental implants. Wettability measurements for bioactive composite samples showed that the hydrophilicity was significantly improved compared with the neat PEEK (control), indicating that combining these the bioactive ingredients with the PEEK polymer matrix increased the hydrophilicity of the surface. This improved hydrophilicity was another factor, in the enhanced cell attachment at the interfaces.
The study also used a plasma-assisted Physical Vapour Deposition (PVD) coating technique to enhance bioactivity by functionalising the surface of the PEEK. Thin layers of Ti and titanium-niobium (Ti-Nb) alloys were coated on the surface of PEEK. EDX analysis indicated that the titanium-coated PEEK samples had an elemental composition of atomic percentage (at.%) 100 at.% pure Ti whereas Ti-Nb coated PEEK samples had an elemental composition of 53.84 at.% Ti concentration and 46.16 at.% Nb. Structural characterisation showed that the Ti coating had a stable α-Ti phase, while the Ti-Nb coating had a metastable β-Ti phase. Nano-indentation testing confirmed significant differences between the hardness and modulus of elasticity of PEEK before and after coating with Ti and Ti-Nb alloys. Ti coated PEEK and Ti-Nb coated PEEK samples exhibited enhancement in osteoblast cell proliferation after 28-days in vitro as well as showing more bone extracellular matrix deposition than that on uncoated PEEK samples. In conclusion, the injection-moulding and PVD coating techniques both successfully produced bioactive PEEK composites with enhanced biological and mechanical structures that could be used for dental implants in load-bearing applications.
