Formation and adhesion of biomimetic hydroxyapatite deposited on titanium substrates
Introduction
It has been reported that titanium oxidized to the rutile phase is bioactive [1], [2], [3]. This is a property first discovered for certain ceramics such as Bioglass® and sintered hydroxyapatite (HA). Bioactivity is a desirable property for implants that are to be integrated into bone. A layer of bone-like apatite forms on a bioactive surface when it is implanted in vivo. This apatite layer chemically bonds to the bioactive surface and acts as an intermediate layer between new bone and the implant. Unprocessed titanium is not bioactive but is used as a material for implants because of its biocompatibility and its mechanical properties. The native oxide that spontaneously forms on the titanium surface in ambient air is amorphous, very thin (2–6 nm) and works as a passivating layer [4]. Crystalline phases of TiO2 occur naturally as rutile, anatase and brookite. Rutile is the thermodynamically stable phase of crystalline TiO2 and can be formed on titanium by heat treatment. When rutile surfaces are exposed to body fluids, OH-groups adsorb to the titanium ions in the oxide [5]. The isoelectric point (IEP) of rutile is about 5.9 [6], which is lower than the pH of body fluids (∼7.4). This leads to a deprotonization of the rutile surface and negative Ti–O− groups are formed over it. These negative sites attract Ca2+ ions from the body fluid that bond to the surface [5]. A layer of amorphous calcium titanate is hence formed and the surface becomes slightly positively charged as the layer grows due to the Ca2+ ions. It will then attract negatively charged phosphate ions, which bond to the surface, and a metastable phase of calcium phosphate is formed. This layer is then crystallized into bone-like apatite because it is thermodynamically more favourable for the amorphous calcium phosphate to adopt a crystalline structure in a wet environment [7], [8]. Osteoblasts adhere to the apatite and produce a network of collagen fibres, which constitutes the organic phase of bone. This collagen network is mineralized by apatite and new bone is formed.
It is not fully understood why rutile is bioactive. The dissociation of water on rutile could explain the adsorption of OH-groups. However, this mechanism is controversial, and experimental and theoretical studies contradict each other [9]. Dissociation is a surface structure-dependent phenomenon and is often more regulated by the crystal structure than by the type of metal ions that are present at the surface [10].
To examine the bioactivity in this study, a biomimetic method was used. The bioactive substrate is exposed to an acellular solution with pH and ion concentrations similar to those of human blood plasma and HA is deposited on the surface due to differences in pH and IEP. The biomimetic method is a way to predict and to study how a layer of HA will form in vivo [11]. The method is also suitable for deposition of a HA coating on titanium oxide before implantation. Plasma spraying is currently the most common method for deposition of HA on orthopaedic implants; this has some disadvantages, such as poor step coverage and high deposition temperature, that may affect the substrate. The biomimetic method is an interesting alternative to plasma-spraying because it is a low-temperature process that reduces the risk of the coating flake off; it forms bone-like HA crystals with good bioactivity; it has good step coverage on implants; and it can incorporate bone-growth-stimulating factors.
Adhesion tests between biomedical coatings and substrates are normally performed using standard ASTM methods [12], [13]. These methods can be briefly described as follows: a specific glue (FM 1000) with strength properties equal to 34.5 MPa is used to bond two coated plates to each other and then the breakage strength is measured. The method can determine a coating adhesion value, as long as this is below 34.5 MPa. Within the surface and coating community, scratch testing of coatings is a widely used method to determine coating adhesion and failure modes [14], [15], [16]. Scratch testing is normally performed under controlled conditions (increasing load and constant speed or vice versa, or both constant) via scratching of a coated surface with a specified indenter. A diamond-shaped indenter is usually used and from an exact knowledge of all test parameters and by a measurement of the critical load when the coating fails, the adhesion can be estimated. In this study, a spherical indenter was used to better mimic the actual strain that a coating deposited on an implant is exposed to when inserted into bone.
The objective of the present paper is twofold: (i) to study the formation of biomimetic apatite on rutile surfaces; and (ii) to measure the adhesion of the coating using a scratch test.
Section snippets
Materials and methods
Plates of commercially pure titanium 25 × 25 × 0.5 mm were oxidized in a furnace at 800 °C for 1 h in order to transform the surfaces into rutile. Before the heat treatment, the plates were cleaned in an ultrasonic bath with acetone and ethanol. The furnace temperature was increased at 5 °C min−1 until it reached 800 °C. Subsequently the temperature was held constant for 1 h before the furnace was turned off with the plates left in the furnace to cool slowly. Afterwards the plates were cleaned with an
Results
The X-ray diffraction (XRD) analysis shows that the titanium surface has been totally transformed into rutile TiO2 after the heat treatment (see Fig. 1). Small faceted crystals that describe the topography (Fig. 2) constitute the rutile surface.
XRD and scanning electron microscopy (SEM) analysis showed that HA coatings had formed on the rutile surfaces after seven days of exposure (Fig. 3, Fig. 4). The coatings appeared to homogeneously cover the surfaces and were composed of needle-like
Discussion
The results in this study contradict somewhat the results of Wang et al. [19] whose similar 800 °C heat treatment of titanium did not induce apatite formation to the same extent. In the present study, after a few days of exposure to PBS, a dense coating of HA had formed on the rutile surfaces and after seven days, the surfaces were fully covered with a thick, rather porous HA coating. The critical pressure for the coating was estimated to be 2.4 ± 0.1 GPa; this can be compared with the hardness of
Conclusions
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Heat treatment of commercially pure titanium at 800 °C transforms the inert native oxide into rutile.
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Rutile proved to be highly bioactive. The rutile surfaces were covered with a layer of HA several micrometers thick after seven days of exposure to PBS.
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Dulbecco’s 1 × /PBS was found to function well as a simulated body fluid in the in vitro bioactivity test as well as being a biomimetic deposition source for HA.
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The modified scratch test was found to be a useful method for approximating the adhesion
Acknowledgement
The financial support from the Göran Gustafsson foundation for academic research is acknowledged.
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