Identification and mapping of the amorphous phase in plasma-sprayed hydroxyapatite coatings using scanning cathodoluminescence microscopy

Synthetic amorphous calcium phosphates (ACPs): preparation, structure, properties, and biomedical applications

Amorphous calcium phosphates (ACPs) represent a metastable amorphous state of other calcium orthophosphates (abbreviated as CaPO₄) possessing variable compositional but rather identical glass-like physical properties, in which there are neither translational nor orientational long-range orders of the atomic positions. In nature, ACPs of a biological origin are found in the calcified tissues of mammals, some parts of primitive organisms, as well as in the mammalian milk. Manmade ACPs can be synthesized in a laboratory by various methods including wet-chemical precipitation, in which they are the first solid phases, precipitated after a rapid mixing of aqueous solutions containing dissolved ions of Ca²⁺ and PO₄^3- in sufficient amounts. Due to the amorphous nature, all types of synthetic ACPs appear to be thermodynamically unstable and, unless stored in dry conditions or doped by stabilizers, they tend to transform spontaneously to crystalline CaPO₄, mainly to ones with an apatitic structure. This intrinsic metastability of the ACPs is of a great biological relevance. In particular, the initiating role that metastable ACPs play in matrix vesicle biomineralization raises their importance from a mere laboratory curiosity to that of a reasonable key intermediate in skeletal calcifications. In addition, synthetic ACPs appear to be very promising biomaterials both for manufacturing artificial bone grafts and for dental applications. In this review, the current knowledge on the occurrence, structural design, chemical composition, preparation, properties, and biomedical applications of the synthetic ACPs have been summarized.

Preparation and Degradation Characteristics of MAO/APS Composite Bio-Coating in Simulated Body Fluid

In this work, ZK60 magnesium alloy was employed as a substrate material to produce ceramic coatings, containing Ca and P, by micro-arc oxidation (MAO). Atmospheric plasma spraying (APS) was used to prepare the hydroxyapatite layer (HA) on the MAO coating to obtain a composite coating for better biological activity. The coatings were examined by various means including an X-ray diffractometer, a scanning electron microscope and an energy spectrometer. Meanwhile, an electrochemical examination, immersion test and tensile test were used to evaluate the in vitro performance of the composite coatings. The results showed that the composite coating has a better corrosion resistance. In addition, this work proposed a degradation model of the composite coating in the simulated body fluid immersion test. This model explains the degradation process of the MAO/APS coating in SBF.

Thermal and electron stimulated luminescence of natural bones, commercial hydroxyapatite and collagen

The luminescence (cathodoluminescence and thermoluminescence) properties of natural bones (Siberian mammoth and adult elephant), commercial hydroxyapatite and collagen were analyzed. Chemical analyses of the natural bones were determined using by Electron Probe Micro-Analysis (EMPA). Structural, molecular and thermal characteristics were determined by X-ray Diffraction (XRD), Raman spectroscopy and Differential Thermal and Thermogravimetric analysis (DTA-TG). Cathodoluminescence (CL) spectra of natural bones and collagen showed similar intense broad bands at 440 and 490 nm related to luminescence of the tetrahedral anion [Formula: see text] or structural defects. A weaker luminescence exhibited at 310 nm could be attributed to small amount of rare earth elements (REEs). Four luminescent bands at 378, 424, 468 and 576 nm were observed in the commercial hydroxyapatite (HAP). Both natural bones and collagen samples exhibited natural thermoluminescence (NTL) with well-defined glow curves whereas that the induced thermoluminescence (ITL) only appears in the samples of commercial hydroxyapatite and collagen. Additional explanations for the TL anomalous fading of apatite, as a crucial difficulty performing dosimetry and dating, are also considered.

Nano-indentation on amorphous calcium phosphate splats: effect of droplet size on mechanical properties

Droplet processing technologies and many biological processes use disk-like or hemispherical shapes for construction or the design of surfaces. The ability to tune the characteristics and properties of a surface is important at the micro- and nano-scale. The influence of size on the mechanical properties is presently unknown. This work set out to produce splats from different droplet sizes (20-40 μm, 40-60 μm and 60-80 μm), and then determine the deposit characteristics and mechanical properties. All splats produced by melting particles in a flame and depositing onto a polished titanium surface were amorphous, as determined by Raman micro-spectrometry. The topography shown in an optical and scanning electron microscope and topographically mapped using the scanning mode of the nano-indenter revealed a flattened hemispherical deposit. The critical nano-indentation load for determining the true hardness decreased with increasing splat size; for 20-40 μm, 40-60 μm and 60-80 μm splats the critical load was 19, 16, 11 mN respectively compared to 30 mN for sintered hydroxyapatite. Higher loads are required to cause cracking and delamination in smaller splats. A load between 40 and 60 mN was required for delamination of the splat. Delamination of the splats could offer a new means to determine the adhesion of splats on low roughness surfaces.

Osteoclast resorption of thermal spray hydoxyapatite coatings is influenced by surface topography

Coating characteristics such as composition, crystallite features and topography collectively impact the cell response. The influence from splats has not yet been assessed for hydroxyapatite (HAp) thermal spray coatings. The objective of this work is to (a) survey the topography on commercial implants, (b) ascertain topography formation from single splats, and (c) determine the osteoclast resorption pattern on a topographically refined coating compared to dentine. Coatings on dental implants, an orthopedic screw, a femoral stem and a knee implant were studied for reference. The effects of substrate pre-heat, roughness, spray distance and particle size on the coating roughness and topography were studied. Human-derived osteoclasts were placed on a coating with refined topography and compared to dentine, a polished coating and polished sintered HAp. A pre-heat of at least 200°C on titanium was required to form rounded splats. The greatest influence on coating roughness and topography arose from particle size. A 2-fold increase in the mean particle size from 30 to 72 μm produced a significant difference (P<0.001) in roughness from 4.8 and 9.7 μm. A model is shown to illustrate topography formation, nanostructure evolution on single splats, and the topography as seen in commercial implants. Osteoclasts showed a clear preference for activity on coatings with refined topography. A one-way ANOVA test revealed a significantly greater pit depth (P=0.022) for dentine (14 μm) compared to the as-sprayed and polished coating (5 μm). Coatings with topography display a similar number of resorption pits with dentine, but a 10-fold greater number than polished coatings, emphasizing the importance of flattened droplet topography on implant surfaces.

Kinetics and the role of off-stoichiometry in the environmentally driven phase transformation of commercially available zirconia femoral heads

The low-temperature polymorphic transformation behavior of two types of commercially available femoral head, both made of 3 mol.% Y(2)O(3)-stabilized tetragonal ZrO(2) polycrystals (3Y-TZP), was examined by in vitro experiments. Both materials contained a small amount (0.25 wt.%) of Al(2)O(3), but they differed slightly in their SiO(2) impurity content, in the morphology and crystallinity of the dispersed Al(2)O(3) phase, and in grain size. In vitro experiments were conducted in a water-vapor environment at temperatures in the range 90-134°C and for periods of time up to 500 h. Despite the materials having the same nominal composition, quite different behaviors were found in the hydrothermal environment for the two types of femoral head investigated. A phenomenological description of the kinetics of monoclinic nuclei formation/growth led to the experimental determination of activation energy values for the environmentally driven polymorphic transformation. From the material physics viewpoint, cathodoluminescence spectroscopy enabled us to rationalize the role of surface stoichiometry on the mechanisms leading to polymorphic transformation. Spectroscopic experiments unveiled some new relevant aspects of surface off-stoichiometry, which lie behind the different phase transformation kinetics experienced by the investigated femoral heads.

Micromechanical properties of single crystal hydroxyapatite by nanoindentation

Knowledge of the intrinsic properties of hydroxyapatite (HAp) single crystals is important for the design of natural systems and will assist further improvements of manufactured biomaterials. Nanoindentation provides a useful tool for determining mechanical properties such as the hardness, elastic modulus and fracture toughness of small samples. A Berkovich indenter was placed on the side and basal planes of a natural single crystal of Durango HAp. The hardness and elastic modulus values obtained revealed higher values for the base (7.1 and 150.4GPa) compared to the side (6.4 and 143.6GPa). The cracking threshold, i.e., the load at which cracking initiates, revealed earlier crack formation on the base (at 8mN) compared to the side (at 11mN). Fracture toughness was measured as 0.45+/-0.09 and 0.35+/-0.06MPam(1/2) for the side and basal plane, respectively. These results suggest that crystals are less prone to cracking and resist microcrack events better on the side, which is useful in bone, while exposing the base, the hardest face, to minimize mass loss from abrasion with teeth.

Influence of experimental parameters on spatial phase distribution in as-sprayed and incubated hydroxyapatite coatings

In the present study, the behavior and properties of plasma-sprayed hydroxyapatite coatings [Ca(10)(PO(4))(6)(OH)(2), HAp] were investigated in relation to the spraying process. The experiments were focused on the influence of type of feedstock and spray power on the phase composition and distribution within the coatings. Depth profiles of the coatings were investigated before and after incubation in revised simulated body fluid (SBF) by X-ray diffraction and infrared spectroscopy. Besides HAp, the coatings contain oxyapatite (OAp) and carbonate apatite (CAp). Additionally, tricalcium phosphate (TCP), tetracalcium phosphate (TTCP), CaO, and an amorphous phase were detected in the coatings. The HAp content directly depends on the used spray powder and spray power, where the influence of spray powder is much higher than the influence of the spray power. The grain size range of the spray powder strongly influences the HAp content in the coating and the formation of CaO. The in vitro behavior of the coatings in simulated body fluid mainly depends on the contents of CaO and amorphous calcium phosphate, respectively. The formation of portlandite due to the reaction of the coating with the SBF is strongly influenced by the porosity of the coatings and can be used as an indicator for the depth of interaction between fluid and coating.

Cathodoluminescence as a Method of Microstructure Characterization of Biphasic Ceramics Composed of Hydroxyapatite and β-Tricalcium Phosphate

This paper describes the investigation into the use of cathodoluminescence for distinguishing HA and β-TCP phases within the biphasic calcium phosphate ceramic microstructure. Polished samples were scanned using SEM-CL at an accelerating voltage in the range of 10-15 keV with a beam current of 5-10 nA. The grayscale images produced reveal distinctive patterns for each composition. EDS and EPMA suggest chemical differences among the contrasting regions. Image analysis of voxel values indicate that areas of bright contrast correspond to β-TCP grains with phase amounts confirmed by XRD.

Influence of fluorapatite on the properties of thermally sprayed hydroxyapatite coatings

Thermally sprayed hydroxyapatite has been the widely used on orthopaedic prosthesis to induce bone growth and facilitate bone attachment. However, hydroxyapatite has a greater affinity for the formation of an amorphous phase in the thermally sprayed coating that results in the release of excessive amount of mineral ions from the implant coating leading to a saturated environment in the immediate vicinity of the bone cells. Fluorapatite however is highly crystalline and offers the potential for lower mineral ion release by dissolution. Thus study investigates the influence of fluorapatite in a thermally sprayed hydroxyapatite coating. Mechanical blends of fluorapatite with hydroxyapatite were thermally sprayed, characterized with X-ray diffraction, SEM, FTIR, optical microscopy for microstructure, roughness and tested for solubility. Cathodoluminescence microscopy was used to examine the resorbed coating surface. Fluorapatite coatings crystallized more readily and produce a greater coating roughness. The roughness in fluorapatite coatings arises from less flattened droplets that show a tendency for finger formation. Addition of fluorapatite increases coating crystallinity. The use of slower resorbing fluorapatite produces less particle release which favors improved osseointegration. Less change in the surface topography during resorption can be used to an advantage to control the coating surface presented to cells and extra cellular matrix proteins.