Effect of pores formation process and oxygen plasma treatment to hydroxyapatite formation on bioactive PEEK prepared by incorporation of precursor of apatite

Abstract

When bioinert substrates with fine-sized pores are immersed in a simulated body fluid (SBF) and the pH value or the temperature is increased, fine particles of calcium phosphate, which the authors denoted as ‘precursor of apatite’ (PrA), are formed in the pores. By this method, hydroxyapatite formation ability can be provided to various kinds of bioinert materials. In this study, the authors studied fabrication methods of bioactive PEEK by using the above-mentioned process. First, the fine-sized pores were formed on the surface of the PEEK substrate by H₂SO₄ treatment. Next, to provide hydrophilic property to the PEEK, the surfaces of the PEEK were treated with O₂ plasma. Finally, PrA were formed in the pores by the above-mentioned process, which is denoted as ‘Alkaline SBF’ treatment, and the bioactive PEEK was obtained. By immersing in SBF with the physiological condition, hydroxyapatite formation was induced on the whole surface of the substrate within 1 day. The formation of PrA directly contributed to hydroxyapatite formation ability. By applying the O₂ plasma treatment, hydroxyapatite formation was uniformly performed on the whole surface of the substrate. The H₂SO₄ treatment contributed to a considerable enhancement of adhesive strength of the formed hydroxyapatite layer formed in SBF because of the increase of surface areas of the substrate. As a comparative study, the sandblasting method was applied as the pores formation process instead of the H₂SO₄ treatment. Although hydroxyapatite formation was provided also in this case, however, the adhesion of the formed hydroxyapatite layer to the substrate was not sufficient even if the O₂ plasma treatment was conducted. This result indicates that the fine-sized pores should be formed on the whole surface of the substrate uniformly to achieve high adhesive strength of the hydroxyapatite layer. Therefore, it is considered that the H₂SO₄ treatment before the O₂ plasma and the ‘Alkaline SBF’ treatment is an important factor to achieve high adhesive strength of hydroxyapatite layer to the PEEK substrate. This material is expected to be a candidate for next-generation implant materials with high bioactivity.