Plasma-sprayed Hydroxylapatite Coatings Research Articles

Functional plasma-sprayed hydroxylapatite coatings for medical application: Clinical performance requirements and key property enhancement

Functional hydroxylapatite (HAp) coatings deposited by atmospheric plasma spraying on parts of hip and knee endoprosthetic implants, artificial dental roots, spinal implants, and other medical devices serve to aid in osseointegration by providing a biocompatible and osseoconductive/osseoinductive template for bone growth-supporting actions of cytokines and noncollagenous proteins and proteoglycans, mediated by transmembrane proteins such as integrins. To be successful in this task, HAp coatings need to be carefully designed and optimized by closely controlling key properties such as phase composition, degree of crystallinity, crystallographic texture, thickness, adhesion strength to the implant surface, porosity, pore size distribution, surface nanostructure and roughness, residual coating stresses, and dissolution kinetics during in vivo contact with extracellular fluid. As this contribution discusses salient aspects of design, properties, and application of HAp coatings, it attempts to chart ways toward improving their in vivo performance and, thus, may be considered a helping hand and guiding manual of instruction for their successful deposition. In this review, much contextual recourse has been taken to the work of the present author and his research group during the last two and a half decades.

Plasma-Sprayed Hydroxylapatite Coatings as Biocompatible Intermediaries Between Inorganic Implant Surfaces and Living Tissue

The present contribution discusses critical aspects of the thermal alteration that HAp particles undergo when passing along the extremely hot plasma jet. This heat treatment leads to dehydroxylated phases such as oxyhydroxylapatite/oxyapatite as well as thermal decomposition products such as tri- and tetracalcium phosphates, and quenched phases in the form of amorphous calcium phosphate (ACP) of variable composition. The contribution also includes studying the influence bioinert TiO2 bond coats have on adhesion, crystallinity, and composition of HAp coatings. Moreover, the question is being addressed whether oxyapatite might exist as a (meta)stable phase or whether its occurrence is merely an ephemeral event. In addition, the article deals with the role that HAp coatings are playing during in vitro interaction with simulated body fluid (SBF) resembling the composition of extracellular fluid (ECF). The biological and biomechanical advantages of using HAp coatings for medical implants as well as salient aspects of their biomineralization and osseointegration will be discussed in some detail.

Tracking the thermal decomposition of plasma-sprayed hydroxylapatite

In modern orthopedics, plasma-sprayed hydroxylapatite coatings are applied routinely to metallic parts of hip and knee prostheses, as well as to dental root implants to render them osseoconductive, that is, able to assist the body in creating new bone by ingrowth of bone cells, blood capillaries, and soft tissue. In this work, hydroxylapatite coatings were deposited by atmospheric plasma spraying on titanium alloy substrates, and characterized by synchrotron radiation X-ray diffraction and solid-state nuclear magnetic resonance spectroscopy. The deposition parameters were varied using a statistical design of experiments methodology. Depending on the degree of heat input and heat transfer rates, (1) dehydroxylated hydroxylapatite phases such as oxy- and oxyhydroxylapatite, (2) thermal decomposition phases (tri- and tetracalcium phosphates, CaO), and (3) amorphous calcium phosphate were formed. Implications of this research are that oxyapatite appears to be unstable at ambient conditions, and that proper selection of intrinsic plasma-spray parameters is key to the chemical stability and mechanical performance of the coating.

Laser‐Raman and Nuclear Magnetic Resonance (NMR) studies on plasma‐sprayed hydroxylapatite coatings: Influence of bioinert bond coats on phase composition and resorption kinetics in simulated body fluid

AbstractThe structure and phase composition of HAp coatings deposited onto Ti6Al4V coupons (50x20x2mm) by atmospheric plasma spraying (APS) were studied by laser‐Raman spectroscopy, 31P‐ and 1H‐MAS‐NMR and 2D‐31P/1H HETCOR‐CP‐NMR spectroscopy, and XRD with Rietveld refinement. The samples investigated comprised APS HAp coatings with and without a TiO2 bond coat as well as coatings incubated for different lengths of time (up to 12 weeks) in simulated body fluid (SBF) under physiological conditions. In APS coatings the presence of a bond coat increased the proportion of well‐ordered crystalline HAp at the expense of distorted apatite‐like structures such as oxyHAp and oxyapatite, and thermal decomposition products such as tricalcium phosphate (TCP) and tetracalcium phosphate (TTCP), and also decreased the amount of amorphous calcium phosphate (ACP). Incubation in SBF further advanced the proportion of crystalline HAp since the disordered structures, the thermal decomposition products, and ACP exhibit substantially higher solubility.

Structural characterization of plasma-sprayed hydroxylapatite coatings

Plasma-sprayed hydroxylapatite coatings, widely used on metallic surgical implants to improve their adhesion to bone, are formed by rapid quenching of molten, or partly molten, particles which impact the substrate at high velocity. the performance of these coatings in the body depends upon their structure, which is not well understood. Coatings prepared under a range of spraying conditions have been studied by X-ray diffraction (XRD). differential thermal analysis (DTA), thermogravimetric analysis (TGA) and solid-state nuclear magnetic resonance spectroscopy (NMR). The results suggest that particles partly melt and lose combined water at lower plasma torch input powers forming a glass, by quenching of the liquid phase, and an OH-depleted hydroxylapatite residual crystalline phase. At higher power inputs an increasing amount of P2O5 is also lost and the coatings contain CaO and Ca4P2O9. Heat treatment of coatings in air at 600°C results in crystallization of the glass phase and reaction with water vapour to form hydroxylapatite. The results show that XRD is relatively insensitive to some of the structural details of hydroxylapatite coatings which may be significant to their performance. NMR provides more structural information and is a significant tool for coating characterization.

The influences of plasma spraying parameters on the characteristics of hydroxyapatite coatings: a quantitative study

All laboratory-made plasma-sprayed hydroxylapatite coatings (HACs) were found to undergo, to different degrees, changes in phase composition, crystallinity, morphology and roughness dependent on plasma spraying parameters (PSPs). The PSPs, which were systematically varied, included the plasma atmosphere, the spraying current and the stand-off distance. Through the determinations of the concentration of impurity phase (CIP) and the index of crystallinity (IOC), the extent of phase purity and the degree of crystallinity of HACs were quantitatively assessed, respectively. Coatings consisting of at least 50% (IOC>50%) of the original crystalline structure and almostly 95% (CIP<5%) apatite with barely detectable extra phases were obtained. The microstructure of HACs exhibited great deviations both in morphology from molten to partial molten state and in roughness from coating of high irregularity (R a=14.48μm) to a smoother (R a=4.46 μm) one, dominantly influenced by the spraying atmosphere. As the terms of CIP and IOC are defined and established, the biological responses related to phase purity and crystallinity of HACs can be further evaluated in vitro and in vivo.

Study of the surface characteristics of magnetron-sputter calcium phosphate coatings.

Plasma-sprayed hydroxylapatite coatings on metals such as titanium have been investigated for many years and have shown a good biocompatibility when implanted in bony tissues. Radiofrequency magnetron sputtering was used as an alternative method to deposit thin films of hydroxylapatite on titanium substrates. X-ray diffraction demonstrated that the sputtered layer was crystalline with a preferred (001) crystallographic orientation with the C-axis perpendicular to the substrate surface. Scanning electron microscopy showed that the deposited films had a uniform and dense structure. The calcium phosphate ratio varied between 1.5 and 2.0, as determined by energy-dispersive X-ray analysis. The in vitro dissolution appeared to be determined by the degree of the coating's crystallinity.

The effect of dissolution on plasma sprayed hydroxylapatite coatings on titanium

Plasma sprayed hydroxylapatite (HA) coated titanium specimens were immersed into Ca-free Hank's balanced salt solution for periods of l, 2, 4, and 6 weeks. At each of the respective time intervals the HA coatings were analyzed with X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), and Raman spectroscopy to determine the effect of dissolution on the structure and composition of the coatings. At the 2- and 6-week intervals, additional samples were removed from solution and shear tested to evaluate the effect of dissolution on the coating-to-substrate interfacial bond strength. XRD results revealed that the plasma sprayed coatings retained their basic apatitic structure throughout the 6-week period in solution. There was, however, a trend towards a more definitive baseline, suggesting a loss of amorphous material by dissolution. FTIR and Raman analyses of the as-sprayed and dissolution specimens showed that the phosphate groups were not lost during the time in solution; however, there was a decrease in hydroxyl content as a result of dissolution mechanisms. FTIR also revealed an increase in carbonate content within the coating during immersion in the simulated physiological balanced salt solution. The average shear bond strengths for the as-sprayed, 2-, and 6-week dissolution specimens were 14·82 ± 3·52 MPa, 12·51 ± 3·41 MPa, and 12·54 ± 2·02 MPa, respectively. Duncan's multiple range test confirmed that the shear bond strength was not significantly reduced after 6 weeks in solution, but there was evidence that suggested a decrease in interfacial bond strength as a result of dissolution mechanisms.

Adhesion Properties of Plasma Sprayed Hydroxylapatite Coatings for Orthopaedic Prostheses

Using Air Plasma Spraying (APS) and Vacuum Plasma Spraying (VPS) techniques, hydroxylapatite (HA) and mixtures of HA and titanium (Ti) were deposited on a Ti6A14V alloy (and on an AISI 316L steel) subjected to different surface treatments. The deposits were investigated for their crystallinity, thickness, and adhesion properties. Higher adhesion values were obtained with VPS rather than with APS. By utilising VPS, the deposition conditions were selected in order to achieve crystallinity values between 70 and 90%. The adhesion results depend on the crystallinity (increasing with its decrease), on the thickness (decreasing slightly with its increase) and especially on the surface finish of the metallic substrate. A porous Ti precoat was more effective than either chemical etching in HCl or sandblasting; sandblasting being the least effective. In particular, the double deposits consisting of a porous Ti precoat and a successive layer of HA proved to be most interesting for their higher adhesion properties and for their capability of providing primary stability due to the presence of the HA and secondary stability, in the case of its reabsorption, due to the porous metal.