Surface nanocrystallization of hydroxyapatite coating
Introduction
Plasma spraying is one of the most widely used methods for the fabrication of biomedical hydroxyapatite (HA) coatings, and it is generally agreed that HA decomposes partially during air plasma spraying [1], [2], [3], [4], [5]. The coating process results in structural alterations of HA including: (i) the formation of amorphous calcium phosphate; (ii) a loss of OH− groups; and (iii) the formation of secondary calcium phosphate phases [2]. Plasma-sprayed HA coatings have been proved to contain quite a large amount of amorphous phase, being an oxyapatite, and hydroxyapatite was proved to be an oxyhydroxyapatite with a small quantity of HA [3]. Meanwhile, as the plasma power level is increased, the crystallinity and OH− ion content of the coatings decrease, while the amount of non-HA calcium phosphate compounds increases [4]. Thermal decomposition of apatite in coatings is catalyzed by underlying titanium during plasma spraying and post-heat treatment [6].
The amorphous phase, which is a product of the structural destruction of HA during plasma spraying, is more commonly found at the coating–substrate interface [5], and is deemed to have a detrimental effect on the long-term stability of the coating–substrate interface after implantation, because the amorphous phase could be rapidly dissolved when exposed to body fluid. Extra phases, such as tricalcium phosphate (TCP), tetracalcium phosphate (TTCP) and CaO, might have a similar effect. At the same time, thermal decomposition of HA will intensify its structural difference from bone apatite, which may reduce the bonding efficiency between them.
Post-heat treatment has been reported to be effective in improvement not only in the structural integrity [7], [8], [9] but also in the mechanical properties of HA [10], [11]. A stable HA coating with high crystallinity not only promotes cell proliferation and bio-integration [12], [13], but also is beneficial to the long-term effectiveness of the coating, especially when used to shield the release of metallic ions. Many factors affect the restoration of structural integrity of the coating during post-heat treatment. In our previous work, factors influencing phase composition and structure of plasma-sprayed hydroxyapatite coatings during post-heat treatment were systematically investigated [14].
At the same time, ultrafine nanosized HA particles have been found on traditionally post-heat treated HA coatings [15]. Compared to conventionally crystalline HA, in addition to improving the bone bonding to HA, nanostructured HA is speculated to enhance the proliferation of osteoblasts, synthesis of intracellar proteins, alkaline phosphatase activity and deposition of calcium-containing mineral, thus enhancing the bonding of orthopedic/dental implants to juxtaposed bone and improving the overall implant efficacy [16], [17]. Cytotoxicity results with human osteoblast cells showed excellent cell attachment and cell spreading on compacts made from HA nanopowders [18]. Nanocrystalline SiHA coating on titanium (Ti) caused a significant increase in human osteoblast-like cell growth density with culture time as compared with the uncoated Ti. Also, rapid dissolution of the coating is not favorable to very early cells attachment [19]. Meanwhile, high crystallinity of the HA coating [20] and stoichiometric elements content in dense HA ceramic [21] are more beneficial to cell growth.
In this work, surface nanocrystallization of plasma-sprayed HA coatings was achieved by use of conventional post-heat treatment; attention is focused on the controllability of the nanocrystalline surface of the HA and the cell response on nanocrystallized coating surfaces.
Section snippets
Specimen fabrication
Fully crystallized HA powder with particle sizes of 38–75 μm were produced in our laboratory and were used as the starting materials. The HA powder was synthesized by a wet chemical method, sintered at 900 °C for 0.5 h, milled and sieved to the required sizes. Commercially pure titanium buttons, with a diameter of 10 or 13 mm and a thickness of 2 mm, were used as the substrate materials. Here we chose pure Ti instead of Ti–6Al–4V as the substrate because (i) potential adverse biological effects of
Phase evolution
Fig. 1 shows the XRD patterns of the as-sprayed HA coatings and those post-heat treated at 650 °C for various time periods. The as-sprayed coating contains a large amount of amorphous phase, together with the decomposed phases TTCP, TCP and CaO (Fig. 1a). As compared to the as-sprayed coating, the crystallinity of HA in post-heat-treated coatings (Fig. 1b–d) increased obviously and crystalline HA with minor CaO was finally obtained. The calculated crystallinities of samples corresponding to Fig.
Discussion
Positive effects of nanoscale microstructures on osteoblast behavior have been emphasized recently [16], [17], [18], [19]. The results shown in this work indicate that formation of nanoparticles on plasma-sprayed HA coatings is controllable. Moreover, the nanocrystallized surface has excellent cell compatibility, which is thus expected to promote bone formation during implantation surgery.
The surface nanocrystallization of plasma-sprayed HA coatings was achieved through conventional post-spray
Conclusions
Surface nanocrystallization, a novel change on the HA coating surface during post-heat treatment, was revealed in this study. An increase in holding time led to an increase in surface nanocrystals and intensified their aggregation, indicating that the surface nanocrystallization is controllable to a great extent. The nanocrystallized surfaces have excellent osteoblastic cell compatibility. Because surface nanocrystallization affects the cell response, it should be taken into account in
Acknowledgements
This work is financially supported by the Department of Science and Technology of Shandong Province, China. The authors thank Dr. Ke-Tao Wang and Feng Ma, from Qilu Hospital, Shandong University, for their technical assistance in cell culture.
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