Immersion behavior of RF magnetron-assisted sputtered hydroxyapatite/titanium coatings in simulated body fluid

Preparation and Characterization of Lanthanum-Incorporated Hydroxyapatite Coatings on Titanium Substrates

Titanium (Ti) has been widely used in clinical applications for its excellent biocompatibility and mechanical properties. However, the bioinertness of the surface of Ti has motivated researchers to improve the physicochemical and biological properties of the implants through various surface modifications, such as coatings. For this purpose, we prepared a novel bioactive material, a lanthanum-incorporated hydroxyapatite (La-HA) coating, using a dip-coating technique with a La-HA sol along with post-heat treatment. The XRD, FTIR and EDX results presented in this paper confirmed that lanthanum was successfully incorporated into the structure of HA. The La-HA coating was composed of rod-like particles which densely compacted together without microcracks. The results of the interfacial shear strength test indicated that the incorporation of lanthanum increased the bonding strength of the HA coating. The mass loss ratios under acidic conditions (pH=5.5) suggested that the La-HA coatings have better acid resistance. The cytocompatibility of the La-HA coating was also revealed by the relative activity of alkaline phosphatase, cellular morphology and cell proliferation assay in vitro. The present study suggested that La-HA coated on Ti has promising potential for applications in the development of a new type of bioactive coating for metal implants.

Surface properties and biocompatibility of nanostructured TiO2 film deposited by RF magnetron sputtering

Nanostructured TiO2 films are deposited on a silicon substrate using 150-W power from the RF magnetron sputtering at working pressures of 3 to 5 Pa, with no substrate bias, and at 3 Pa with a substrate bias of -50 V. X-ray diffraction (XRD) analysis reveals that TiO2 films deposited on unbiased as well as biased substrates are all amorphous. Surface properties such as surface roughness and wettability of TiO2 films, grown in a plasma environment, under biased and unbiased substrate conditions are reported according to the said parameters of RF power and the working pressures. Primary rat osteoblasts (MC3T3-E1) cells have been cultured on nanostructured TiO2 films fabricated at different conditions of substrate bias and working pressures. The effects of roughness and hydrophilicity of nanostructured TiO2 films on cell density and cell spreading have been discussed.

Functional Coatings or Films for Hard-Tissue Applications

Metallic biomaterials like stainless steel, Co-based alloy, Ti and its alloys are widely used as artificial hip joints, bone plates and dental implants due to their excellent mechanical properties and endurance. However, there are some surface-originated problems associated with the metallic implants: corrosion and wear in biological environments resulting in ions release and formation of wear debris; poor implant fixation resulting from lack of osteoconductivity and osteoinductivity; implant-associated infections due to the bacterial adhesion and colonization at the implantation site. For overcoming these surface-originated problems, a variety of surface modification techniques have been used on metallic implants, including chemical treatments, physical methods and biological methods. This review surveys coatings that serve to provide properties of anti-corrosion and anti-wear, biocompatibility and bioactivity, and antibacterial activity.

Characterisation of calcium phosphate/titanium dioxide hybrid coatings

The role of titanium dioxide (TiO2) as a means to engender enhanced stability into calcium phosphate (Ca-P) coatings has been well recognised. Several different methods have been used to create such Ca-P/TiO2 hybrid layers on a range of substrates. This paper reports the properties of a Ca-P/TiO2 system created by the sputter deposition of hydroxyapatite onto a titanium surface and the subsequent thermal diffusion of TiO2 through the porous Ca-P layer. The role of temperature in determining the surface contribution from TiO2 has been determined. Coatings annealed up to 600 degrees C did not exhibit any hybrid nature in the uppermost surface, however the coatings annealed to 700 degrees C did show the presence of both HA and rutile TiO2. The surfaces annealed to 800 degrees C were predominantly rutile TiO2. It was also observed that the Ca/P ratio decreased with increasing annealing temperature and that the coating annealed to 700 degrees C had a value of 1.82 +/- 0.07, which was closest to stoichiometric HA. Furthermore, the coatings that were annealed to 700 degrees C displayed a Ca-P/TiO2 hybrid nature, specifically in their uppermost surface and supported the growth and proliferation of osteoblast-like cells more readily when compared to the HA coatings or the rutile TiO2 surfaces.

Characterization of functionally graded hydroxyapatite/titanium composite coatings plasma-sprayed on Ti alloys

Bioceramic coatings like hydroxyapatite (HA) have shown promising bioactive properties in load-bearing implant applications. The aim of this work is to deposit functionally graded HA/Ti layers consisting of an underlying Ti bond coat, the alternating layer, and an HA top-layer on Ti6Al4V substrates using plasma spray to improve the coating-substrate interface properties. The alternating layers were created by means of changing the feeding rate and input power of Ti and HA powders, which gradually decrease Ti content with increasing depth from the Ti bond-coat. The major consideration is to examine the stability of the graded coatings. Experimental results indicated that surface chemistry and morphology of the graded coatings were similar to those of monolithic HA coatings. The bond strength values of the as-sprayed graded coatings were much superior to those of monolithic HA coatings. The cyclic fatigue did have a statistically significant effect on bond strength of monolithic HA coatings, with a decrease of 23%. However, the graded coatings were able to survive 1 million cycles of loading in air without significantly reduced bond strength. The in vitro electrochemical measurement results also indicated that the graded coatings had a more beneficial and desired behavior than monolithic HA coatings after fatigue.

Evaluating interface strength of calcium phosphate sol-gel-derived thin films to Ti6Al4V substrate

The interface shear strength of Ca-P thin films applied to Ti6Al4V substrates have been evaluated in this study using a substrate straining method--a shear lag model. The Ca-P films were synthesized using sol-gel methods from either an inorganic or organic precursor solution. Strong interface bonding was demonstrated for both film types. The films were identified as non-stoichiometric hydroxyapatite but with different Ca/P ratios. The Ca-P films were 1-1.5 microm thick and testing and analysis using the shear lag approach revealed a shear strength of approximately 347 and 280 MPa for Inorganic and Organic Route-formed films, respectively. Overall, the exceptional mechanical properties of Ca-P/Ti6Al4V system along with the inherent advantages of sol-gel processing support continued studies to utilize this technology for bone-interfacing implant surface modification.

Osteoclastic resorption of calcium phosphate coatings applied with electrostatic spray deposition (ESD),in vitro

Calcium phosphate (CaP) coatings have been applied on titanium implants to improve the bioactivity in order to favor the initial bone healing response. Recently, a new technique has been developed to apply CaP coatings: electrostatic spray deposition (ESD). Although ESD-derived coatings have several benefits, it is not known whether they are degradable. This study was designed to examine the cell-mediated degradation of two ESD-derived coatings with different chemical compositions, that is, beta-tricalcium phosphate (beta-TCP) and carbonate apatite (CA). First, coatings were deposited and analyzed physiochemically. Subsequently, rat bone marrow-derived osteoclastlike cells were seeded on the coatings, and analyzed with osteoclast-specific markers, scanning electron microscopy, and transmission electron microscopy. Results showed that both coatings exhibited porous morphologies, with an average pore size of less than 1 microm (beta-TCP), or larger than 1 microm (CA). After heat treatment, both coatings were crystalline in structure. The Ca/P ratios were 1.4 to 1.5 for the beta-TCP coating, and 1.8 to 2.0 for the CA coating. After 8 and 12 days of culture, multinucleated osteoclastlike cells were observed on both coatings. The osteoclast phenotype was confirmed by tartrate resistant acid phosphatase (TRAP) staining, and immunostaining against the calcitonin receptor. Using scanning electron microscopy, numerous resorption lacunae were observed in both coatings. Finally, transmission electron microscopy of TRAP-positive cells confirmed the osteoclastlike aspect of the cells revealing multiple nuclei and a ruffled border. In conclusion, CaP coatings produced with the ESD process can be degraded by osteoclasts.

Osseointegration of a hydroxyapatite-coated multilayered mesh stem

A new type of porous coating for hip prostheses called "multilayered mesh" was tested under weight-bearing conditions. The surface of the stem is constructed of titanium mesh produced by etching. The hip stems of hydroxyapatite (HA)-coated multilayered mesh and conventional beads were implanted into canine right hips, and animals were killed 3, 6 and 10 weeks and 6 and 12 months after implantation. Shear strength between the implant and the bone was evaluated by the push-out test. Bone ingrowth was calculated from backscattered electron imaging-scanning electron microscopy (BEI-SEM) images of transverse sections. Toluidine blue stained sections and the BEI-SEM images were evaluated histologically. The break sites of the specimens after the push-out test were evaluated on BEI-SEM images of longitudinal sections. The mean push-out strength of the HA-coated multilayered mesh samples was greater than that of the beads-coated samples every time tested, and the HA-coated multilayered mesh implants had significantly stronger push-out strength at 3 and 6 weeks (p<0.05). The strength of the HA-coated multilayered mesh implants was even greater at 6 and 12 months, whereas the strength of the beads-coated samples decreased. The HA-coated multilayered mesh implants showed significantly higher percentages of bone ingrowth than the beads-coated implants every time tested, except at 6 months (p<0.05). At 6 and 12 months, the bone ingrowth data for the HA-coated multilayered mesh implants increased, whereas it decreased for the beads-coated implants. The new bone formation had reached the bottom of the porous area of the HA-coated multilayered mesh surface by 3 weeks, but not had reached the bottom of the conventional beads surface. At 6 and 12 months, the smaller pores of the bead surface stopped the thickening of trabecular bone, and at 12 months, the break sites were at the bone-implant interface of the bead surface, whereas they were on the bone side of the HA-coated multilayered mesh surface. The difference between the break sites was significant at 12 months (p<0.05). The HA-coated multilayered mesh stem provided faster, stronger, and more durable osseointegration than the conventional bead stem.

Increased osteoblast adhesion on titanium-coated hydroxylapatite that forms CaTiO3

CaTiO(3) is a strong candidate to form at the interface between hydroxylapatite (HA) and titanium implants during many coating procedures. However, few studies have compared the cytocompatibility properties of CaTiO(3) to HA pertinent for bone-cell function. For this reason, the objective of the present in vitro study was to determine the ability of bone-forming cells (osteoblasts) to adhere on titanium coated with HA that resulted in the formation of CaTiO(3). To accomplish the formation of CaTiO(3), titanium was coated on HA discs and annealed either under air or a N(2)+H(2) environment. Materials were characterized by X-ray diffraction (XRD), Rutherford backscattering spectroscopy (RBS), and atomic force microscopy (AFM). These characterization techniques demonstrated the formation of a nanometer rough CaTiO(3) layer as a consequence of interactions between HA and titanium during coating conditions. Results from cytocompatibility tests revealed increased osteoblast adhesion on materials that contained CaTiO(3) compared to both pure HA and uncoated titanium. The greatest osteoblast adhesion was observed on titanium-coated HA annealed under air conditions. Because adhesion is a crucial prerequisite to subsequent functions of osteoblasts (such as the deposition of calcium containing mineral), the present in vitro results imply that orthopedic coatings that form CaTiO(3) could increase osseointegration with juxtaposed bone needed for increased implant efficacy.

Structure and immersion behavior of plasma-sprayed apatite-matrix coatings

The microstructure and properties of a series of plasma-sprayed coatings from sinter-granulated powders fabricated from SiO2, CaO, P2O5 and Na2O-containing HA composite powders on Ti-6Al-4V substrate were reported. The immersion behavior of these coatings in a simulated body fluid (SBF) was also investigated. The results showed that sinter-granulated apatite-matrix powders were irregularly shaped and appeared quite similar. XRD patterns showed that during fabrication of the powders, P2O5 and SiO2 enhanced the decomposition of HA structure, while CaO and Na2O did not. Reasonably high bond strengths (45-50 MPa) were obtained from all coatings. The plasma spray process itself enhanced the decomposition of apatite and chemical reactions among different phases. When immersed in SBF, the intensities of such phases as alpha- and beta-TCP in all coatings decreased with immersion time and an apatite precipitation took place on all coating surfaces. The immersed SiO2- and CaO-containing HA (HSC) coating had the highest rate of apatite precipitation among all coatings. The variations in calcium ion concentration in simulated body fluid indicated that the HSC-immersed solution reached its maximal Ca concentration the earliest, while the HSCP (HA, SiO2, CaO and P2O5)-immersed solution reached its maximum the latest.