Intrinsic factors of apatite influencing its amorphization during plasma-spray coating

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.

Application of femtosecond laser microfabrication in the preparation of advanced bioactive titanium surfaces

The surface activation of titanium plays a key role in the biological properties of titanium implants as bone repair materials. Improving the ability to induce apatite precipitation on the surface was a well-accepted titanium bioactivation route. In this study, advanced femtosecond laser microfabrication was applied to modify titanium surfaces, and the effect of femtosecond laser etching on apatite precipitation was investigated and compared with popular titanium modification methods. Meanwhile, the mechanism of apatite formation after femtosecond laser modification was interpreted from the point of materials science. The surface physical-chemical characterization results showed that femtosecond laser etching can improve the surface hydrophilicity and increase the surface energy. Compared with traditional abrasive paper and acid-alkali treatment, this method increased the contents of active sites including titanium oxide and titanium-hydroxyl on titanium surfaces. TiO2 on the surface was transformed to TiO after femtosecond laser treatment. The samples etched with 0.3 W and 0.5 W femtosecond lasers had a better ability to induce apatite deposition than those treated with traditional mechanical treatment and popular acid-alkali modification, which would lead to better bioactivity and osteointegration. Considering the technical advantages of femtosecond lasers in microfabrication, it provides a more efficient and controllable scheme for the bioactivation of titanium. This research would improve the application potential of femtosecond laser treatment, such as micropattern preparation and surface activation, in the field of biomaterials.

Induction suspension plasma sprayed biological-like hydroxyapatite coatings

Substituted hydroxyapatite coatings with different ions (Mg, Na, K, Cl, F) have been developed by the induction suspension plasma spray process. Suspensions were prepared with sol–gel. The main objective of this study was to demonstrate that induction suspension plasma spray technology possesses high material composition flexibility that allows as-sprayed coatings to closely mimic natural bone composition. Long-term in vitro behaviour of as-sprayed substituted coatings was evaluated with simulated body fluid. Data on the suspensions showed the formation of a pure hydroxyapatite phase. Transmission electron microscopy characterized various preparation stages of the suspensions. As-sprayed samples were distinguished by X-ray diffraction and scanning electron microscopy. Substituted elements were quantified by neutron activation. A well-crystallized hydroxyapatite phase was produced with concentration in various substitutions very close to natural bone composition. Ca/P and (Ca + Mg + Na + K)/P ratios provided evidence of the introduction of different cations into apatite structures. The immersion of samples into simulated body fluid led to the nucleation and growth of a flake-like octacalcium phosphate crystal layer at the surface of as-sprayed coatings after one week. Proof of octacalcium phosphate transformation and its partial dissolution and direct re-precipitation into apatite was disclosed by local energy dispersive spectroscopy and microstructure observation. Formation of a Ca/P ratio gradient from the precipitated layer surface to the as-sprayed coatings interface was observed after four weeks once the octacalcium phosphate crystals reached a critical size, resulting in the formation of a rich apatite layer at the interface after six weeks. A set of mechanisms has been proposed to explain these findings.

Preparation and characterization of bioactive composites and fibers for dental applications

OBJECTIVES

The present study was carried out to create composites and fibers using polyurethane (PU) with hydroxyapatite (HA) that could be used for dental applications.

METHODS

Composites with varying HA concentration were prepared by solution casting technique. Similarly, PU-HA fibers with varying PU hard and soft segments and fixed HA concentration were also prepared. Various characterization techniques, such as, X-ray diffractometry, differential scanning calorimetry, scanning electron microscopy and Fourier transform infrared spectroscopy in conjunction with photo-acoustic sampling cell were employed to study the composites and fibers for changes in their physicochemical properties before and after immersion in artificial saliva at 37°C for up to 5 days.

RESULTS

The results indicated formation of amorphous apatite layers with maximum amorphicity in composites containing highest amount of HA with 5 days of immersion in artificial saliva. Similarly, fibers with more PU hard segment resulted in better transformation of crystalline HA to its amorphous state with increasing immersion time thus confirming the bioactive nature of the HA-PU fibers.

SIGNIFICANCE

Concentrations of HA and PU hard segment along with the duration of immersion in artificial saliva are two major factors involved in the modification of solid-state properties of HA. The amorphous apatite layer on the surface is known to have tendency to bind with living tissues and hence the use of optimum amount of HA and PU hard segment in composites and fibers, respectively could help in the development of novel dental filling material.

Micro-Raman spectroscopy shows how the coating process affects the characteristics of hydroxylapatite

The diversity in the structural and chemical state of apatites allows implant manufacturers to fine-tune implant properties. This requires suitable manufacturing processes and characterization tools to adjust the amorphous phase and hydroxyl content from the source hydroxylapatite. Hydroxylapatite was processed by high-velocity oxy-fuel spraying, plasma spraying and flame spraying, and primarily analyzed by Raman spectroscopy. Investigation of rounded splats, the building blocks of thermal spray coatings, allowed correlation between the visual identity of the splat surface and the Raman spectra. Splats were heat-treated to crystallize any remaining amorphous phase. The ν1 PO4 stretching peak at 950-970 cm(-1) displayed the crystalline order, but the hydroxyl peak at 3572 cm(-1) followed the degree of dehydroxylation. Hydroxyl loss was greatest for flame-sprayed particles, which create the longest residence time for the melted particle. Higher-frequency hydroxyl peaks in flame- and plasma-sprayed splats indicated a lower structural order for the recrystallized hydroxylapatite within the splats. Crystallization at 700 °C has shown potential for revealing hydroxyl ions previously trapped in amorphous calcium phosphate. This work compares Fourier transform infrared and Raman spectroscopy to measure the hydroxyl content in rapidly solidified apatites and shows that Raman spectroscopy is more suitable.

Physical properties and cellular responses to calcium phosphate coating produced by laser rapid forming on titanium

In order to improve the surface bioactivity of titanium implants, CaCO₃ and CaHPO₄·2H₂O powder was used to fabricate a calcium phosphate (CaP) coating using laser rapid forming (LRF) technology. The surface characterization showed that a porous and beta-tricalcium phosphate (beta-TCP) layer with small amount of alpha-TCP was formed on commercial pure titanium (Ti). The bonding strength between the coating and the Ti substrate was above 40.17 MPa measured by the means of pull-off test. The elastic modulus and the average microhardness of the coating were 117.61 GPa and 431.2 HV₀.₁, respectively. Through the static immersion test, it was proved that the coating could not only prevent the corrosion of Ti but also promote the redeposition of beta-TCP in artificial saliva. Osteoblasts possessed good attachment performance and strong proliferation ability on the surface of LRF coating (p < 0.05) in our cell experiments. This result demonstrated that the LRF coating could improve the surface cytocompatibility of titanium. Using scanning electron microscopy observation, it was found that osteoblasts grown on LRF coating formed multiple layers in pours. The result of reverse transcription PCR analysis demonstrated that the expressions of ITGβ1 and BMP-2 were significantly (p < 0.05) upregulated on the LRF coating in a time-dependent manner, compared with uncoated Ti. These findings suggested that the LRF technology might be a promising potential treatment for fabricating CaP coatings on titanium implants.

Electrochemical and mechanical behavior of laser processed Ti-6Al-4V surface in Ringer's physiological solution

Laser surface modification of Ti-6Al-4V with an existing calcium phosphate coating has been conducted to enhance the surface properties. The electrochemical and mechanical behaviors of calcium phosphate deposited on a Ti-6Al-4V surface and remelted using a Nd:YAG laser at varying laser power densities (25-50 W/mm(2)) have been studied and the results are presented. The electrochemical properties of the modified surfaces in Ringer's physiological solution were evaluated by employing both potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) methods. The potentiodynamic polarizations showed an increase in the passive current density of Ti-6Al-4V after laser modification at power densities up to 35 W/mm(2), after which it exhibited a decrease. A reduction in the passive current density (by more than an order) was observed with an increase in the laser power density from 25 to 50 W/mm(2). EIS studies at the open circuit potential (OCP) and in the passive region at 1.19 V showed that the polarization resistance increased from 8.274 × 10(3) to 4.38 × 10(5) Ω cm(2) with increasing laser power densities. However, the magnitudes remain lower than that of the untreated Ti-6Al-4V at OCP. The average hardness and modulus of the laser treated Ti-6Al-4V, evaluated by the nanoindentation method, were determined to be 5.4-6.5 GPa (with scatter <±0.976 GPa) and 124-155 GPa (with scatter <±13 GPa) respectively. The corresponding hardness and modulus of untreated Ti-6Al-4V were ~4.1 (±0.62) and ~148 (±7) GPa respectively. Laser processing at power densities >35 W/mm(2) enhanced the surface properties (as passive current density is reduced) so that the materials may be suitable for the biomedical applications.

Compositionally graded hydroxyapatite/tricalcium phosphate coating on Ti by laser and induction plasma

In this study we report the fabrication of compositionally graded hydroxyapatite (HA) coatings on Ti by combining laser engineering net shaping (LENS) and radio frequency induction plasma spraying processes. Initially, HA powder was embedded in the Ti substrates using LENS, forming a Ti-HA composite layer. Later, RF induction plasma spraying was used to deposit HA on these Ti substrates with a Ti-HA composite layer on top. Phase analysis by X-ray diffraction indicated phase transformation of HA to β-tricalcium phosphate in the laser processed coating. Laser processed coatings showed the formation of a metallurgically sound and diffused substrate-coating interface, which significantly increased the coating hardness to 922 ± 183 Hv from that of the base metal hardness of 189 ± 22 Hv. In the laser processed multilayer coating a compositionally graded nature was successfully achieved, however, with severe cracking and a consequent decrease in the flexural strength of the coating. To obtain a structurally stable coating with a composition gradient across the coating thickness a phase pure HA layer was sprayed on top of the laser processed single layer coatings using induction plasma spray. The plasma sprayed HA coatings were strongly adherent to the LENS-TCP coatings, with adhesive bond strength of 21 MPa. In vitro biocompatibility of these coatings, using human fetal osteoblast cells, showed a clear improvement in cellular activity from uncoated Ti compared with LENS-TCP-coated Ti and reached a maximum in the plasma sprayed HA coating.

Induction Plasma Sprayed Nano Hydroxyapatite Coatings on Titanium for Orthopaedic and Dental Implants

This paper reports preparation of a highly crystalline nano hydroxyapatite (HA) coating on commercially pure titanium (Cp-Ti) using inductively coupled radio frequency (RF) plasma spray and their in vitro and in vivo biological response. HA coatings were prepared on Ti using normal and supersonic plasma nozzles at different plate powers and working distances. X-ray diffraction (XRD) and Fourier transformed infrared spectroscopic (FTIR) analysis show that the normal plasma nozzle lead to increased phase decomposition, high amorphous calcium phosphate (ACP) phase formation, and severe dehydroxylation of HA. In contrast, coatings prepared using supersonic nozzle retained the crystallinity and phase purity of HA due to relatively short exposure time of HA particles in the plasma. In addition, these coatings exhibited a microstructure that varied from porous and glassy structure at the coating-substrate interface to dense HA at the top surface. The microstructural analysis showed that the coating was made of multigrain HA particles of ~200 nm in size, which consisted of recrystallized HA grains in the size range of 15- 20 nm. Apart from the type of nozzle, working distance was also found to have a strong influence on the HA phase decomposition, while plate power had little influence. Depending on the plasma processing conditions, a coating thickness between 300 and 400 μm was achieved where the adhesive bond strengths were found to be between 4.8 MPa to 24 MPa. The cytotoxicity of HA coatings was examined by culturing human fetal osteoblast cells (hFOB) on coated surfaces. In vivo studies, using the cortical defect model in rat femur, evaluated the histological response of the HA coatings prepared with supersonic nozzle. After 2 weeks of implantation, osteoid formation was evident on the HA coated implant surface, which could indicate early implant- tissue integration in vivo.

Amorphous Calcium Phosphates

Amorphous calcium phosphates (ACPs) represent a unique class of biomedically relevant calcium orthophosphate salts, in which there are neither translational nor orientational long-range orders of the atomic positions. Nevertheless, the constancy in their chemical composition over a relatively wide range of preparation conditions suggests the presence of a well-defined local structural unit, presumably, with the structure of Ca9(PO4)6 – so-called Posner’s cluster. ACPs have variable chemical but rather identical glass-like physicochemical properties. Furthermore, all ACPs are thermodynamically unstable compounds and, unless stored in dry conditions or doped by stabilizers, spontaneously they tend to transform to crystalline calcium orthophosphates. Although some order within general disorder is the most distinguishing feature of ACPs, the solution instability of ACPs and their easy transformation to crystalline phases might be of a great biological relevance. Namely, the initiating role ACPs play in matrix vesicle biomineralization raises the importance of this phase from a mere laboratory curiosity to that of a key intermediate in skeletal calcification. Furthermore, ACPs are very promising candidates to manufacture artificial bone grafts.

Amorphous calcium phosphates: synthesis, properties and uses in biomaterials

This review paper on amorphous calcium phosphates (ACPs) provides an update on several aspects of these compounds which have led to many studies and some controversy since the 1970s, particularly because of the lack of irrefutable proof of the occurrence of an ACP phase in mineralised tissues of vertebrates. The various synthesis routes of ACPs with different compositions are reported and the techniques used to characterise this phase are reviewed. We focus on the various physico-chemical properties of ACPs, especially the reactivity in aqueous media, which have been exploited to prepare bioactive bone substitutes, particularly in the form of coatings and cements for orthopaedic applications and composites for dental applications.

Macroporous hydroxyapatite scaffolds for bone tissue engineering applications: physicochemical characterization and assessment of rat bone marrow stromal cell viability

In this work, a new methodology is reported for developing hydroxyapatite (HA) scaffolds using an organic sacrifice template. The novelty of work consists of possibility of obtaining porous and highly interconnected scaffolds mimicking the sacrificial component. Our purpose consisted of evaluating the physicochemical properties of the HA scaffolds by means of Fourier transform infra-red spectroscopy, X-ray diffraction analysis, and scanning electron microscopy (SEM) attached with an X-ray detector. The HA scaffolds obtained possess a porosity of approximately 70%, and macropores diameter in the range of 50-600 microm. In contrast, results regarding the microcomputed tomography analysis have demonstrated both high pore uniformity and interconnectivity across the scaffolds. The compressive strength of the HA scaffolds was found to be 30.2 +/- 6.0 MPa. Bioactivity of the HA scaffolds was assessed by immersion into a simulated body fluid solution, in vitro. SEM observations have showed a deposition of apatite on the surface of the HA scaffolds, with a "cauliflower-like" morphology after 1 day, and tend to be more pronounced with the immersion time. The changes in calcium and phosphorus concentration were monitored by inductively-coupled plasma optical emission spectrometry. Cytotoxicity of the HA scaffolds was preliminarily investigated by carrying direct observation of mouse fibroblasts cells (L929 cell-line) death in the inverted microscope, and then cell viability was determined by means of carrying out a MTS assay. Complementarily, a luminescent cell viability assay based on the quantification of adenosine triphosphate was performed using rat bone marrow stromal cells (RBMSCs). A LIVE/DEAD assay and SEM analysis allowed the visualization of the RBMSCs adhesion and proliferation on the surface of the HA scaffolds. According to the results obtained from 3D architecture, mechanical properties, biocompatibility, and adhesion tests, it is suggested that HA scaffolds has potential to find applications in bone tissue engineering scaffolding.

Laser processing of bioactive tricalcium phosphate coating on titanium for load-bearing implants

Laser-engineered net shaping (LENS), a commercial rapid prototyping (RP) process, was used to coat titanium with tricalcium phosphate (TCP) ceramics to improve bone cell-materials interactions. During LENS coating process, the Nd:YAG laser melts the top surface of Ti substrate in which calcium phosphate powder is fed to create a TCP-Ti composite layer. It was found that an increase in laser power and/or powder feed rate increases the thickness of the coating. However, coating thickness decreased with increasing laser scan speed. TCP coating showed columnar titanium grains at the substrate side of the coating and transitioned to equiaxed titanium grains at the outside. When the scan speed was reduced from 15 to 10mms(-1), coating hardness increased from 882+/-67 to 1049+/-112Hv due to an increase in the volume fraction of TCP in the coating. Coated surfaces showed uniformly distributed TCP particles and X-ray diffraction data confirmed the absence of any undesirable phases, while maintaining a high level of crystallinity. The effect of TCP coating on cell-material interaction was examined by culturing osteoprecursor cells (OPC1) on coated surfaces. The results indicated that TCP coating had good biocompatibility where OPC1 cells attached and proliferated on the coating surface. The coating also initiated cell differentiation, ECM formation and biomineralization.

In vitro dissolution of plasma-sprayed hydroxyapatite coatings with different characteristics: experimental study and modeling

The dissolution of plasma-sprayed hydroxyapatite (PHA) coatings with different characteristics, produced by various spraying conditions, in a Tris-buffered solution at pH 7.4 was experimentally studied through the measurement of the release of calcium ions. The phase composition of the coatings at surface and interface, and the porosity were evaluated. The analytical modeling revealed that the calcium dissolution process was composed of two stages. The first stage was found to be both surface and diffusion controlled. The second stage was an exactly diffusion-controlled dissolution. In the first stage, the rate of dissolution and the solubility of the coatings with minimum contents of impurity phases were mainly influenced by the contents of recrystallized HA (RHA) and amorphous calcium phosphate (ACP). It is suggested that the optimized values of the ACP and the RHA at the coating surface can tend to encourage the early fixation properties of the PHA coatings.

Microstructures and bond strengths of plasma-sprayed hydroxyapatite coatings on porous titanium substrates

Hydroxyapatite (HA) coating was carried out by plasma spraying on bulk Ti substrates and porous Ti substrates having a Young's modulus similar to that of human bone. The microstructures and bond strengths of HA coatings were investigated in this study. The HA coatings with thickness of 200-250 microm were free from cracks at interfaces between the coating and Ti substrates. XRD analysis revealed that the HA powder used for plasma spraying had a highly crystallized apatite structure, while the HA coating contained several phases other than HA. The bond strength between the HA coating and the Ti substrates evaluated by standard bonding test (ASTM C633-01) were strongly affected by the failure behavior of the HA coating. A mechanism to explain the failure is discussed in terms of surface roughness of the plasma-sprayed HA coatings on the bulk and porous Ti substrates.

Evaluation of the expression of collagen type I in porous calcium phosphate ceramics implanted in an extra-osseous site

The aim of the present study is to demonstrate the newly formed tissue in calcium phosphate (Ca/P) ceramics after extra-osseous implantation by histological and immunohistochemistry (IHC) methods. Synthesis porous Ca/P ceramics without adding any growth factor and living cell were implanted in the dorsal muscle of dogs for 1 and 2 months. Undecalcified and decalcified sections were stained by hematoxylin and eosin (H&E), and IHC, respectively. The histological results showed the beginning of osteogenesis and angiogenesis after being implanted for 1 month and the obvious new bone formation after being implanted for 2 months. IHC were conducted via the avidin-biotin peroxidase complex (ABC) method and the primary antibody was collagen type I. IHC results indicated that collagen type I was expressed within osteoblast-like cells and newly formed bone-like tissue in Ca/P ceramics after 1 month, and in the mineralized matrix of newly formed bone and osteoblasts, some osteocytes and some lacunae after 2 months. No cartilage and chondrocytes were observed in the histological and IHC-stained sections. Evidence of intramembranous osteogenesis was confirmed.

Lack of OH in nanocrystalline apatite as a function of degree of atomic order: implications for bone and biomaterials

Using laser Raman microprobe spectroscopy, we have characterized the degree of hydroxylation and the state of atomic order of several natural and synthetic calcium phosphate phases, including apatite of biological (human bone, heated human bone, mouse bone, human and boar dentin, and human and boar enamel), geological, and synthetic origin. Common belief holds that all the studied phases are hydroxylapatite, i.e., an OH-containing mineral with the composition Ca10(PO4)6(OH)2. We observe, however, that OH-incorporation into the apatite crystal lattice is reduced for nanocrystalline samples. Among the biological samples, no OH-band was detected in the Raman spectrum of bone (the most nanocrystalline biological apatite), whereas a weak OH-band occurs in dentin and a strong OH-band in tooth enamel. We agree with others, who used NMR, IR spectroscopy, and inelastic neutron scattering, that-contrary to the general medical nomenclature-bone apatite is not hydroxylated and therefore not hydroxylapatite. Crystallographically, this observation is unexpected; it therefore remains unclear what atom(s) occupy the OH-site and how charge balance is maintained within the crystal. For non-bone apatites that do show an OH-band in their Raman spectra, there is a strong correlation between the concentration of hydroxyl groups (based on the ratio of the areas of the 3572 deltacm(-1) OH-peak to the 960 deltacm(-1) P-O phosphate peak) and the crystallographic degree of atomic order (based on the relative width of the 960 deltacm(-1) P-O phosphate peak) of the samples. We hypothesize that the body biochemically imposes a specific state of atomic order and crystallinity (and, thus, concentration of hydroxyl) on its different apatite precipitates (bone, dentin, enamel) in order to enhance their ability to carry out tissue-specific functions.

Physicochemical study of plasma-sprayed hydroxyapatite-coated implants in humans

This study represents the first report of the physical and chemical changes occurring in coatings of failed hydroxyapatite (HA)-coated titanium implants obtained from a comprehensive, multicenter human dental implant study. A total of 53 retrieved samples were obtained and compared with unimplanted controls with the same manufacturer and similar manufacture dates. Forty-five retrieved implants were examined for surface characteristics and bulk composition. Implants were staged based on implantation history: stage 1 (implants retrieved between surgical placement and surgical uncovering), stage 2 (implants retrieved at surgical uncovering and evaluation), stage 3 (implants retrieved between surgical uncovering evaluation and occlusal loading), and stage 4 (implants retrieved after occlusal loading). Scanning electron microscopy showed progressive coating thinning with implantation time. At later stages, bare Ti metal was detected by energy-dispersive X-ray analysis and electron spectroscopy for chemical analysis. Increases in Ti and Al (2-7.5 atm % each) were detected at the apical ends of all stage 4 samples. In unimplanted coatings, X-ray diffraction analysis demonstrated the presence of amorphous calcium phosphate, beta-tricalcium phosphate, tetracalcium phosphate, and calcium oxide in addition to large hydroxyapatite crystals (c axis size, D002 = 429 +/- 13 A; a axis size, D300 = 402 +/- 11 A, a/c aspect ratio 0.92). The nonapatitic phases disappeared with increased implantation time, although there was a persistence of amorphous calcium phosphate. Bulk coating chemical analysis showed that Ca/P ratios for implant controls (1.81 +/- 0.01) were greater than stoichiometric HA (1.67) and decreased for implant stages 3 and 4 (1.69 +/- 0.09 and 1.67 +/- 0.09, respectively), explained by the dissolution of the non apatitic phases. Crystal sizes also changed with implantation times, being smaller than the control at all but stage 4. Fourier transform infrared analyses agreed with these results, and also indicated the accumulation of bone (protein and carbonate-apatite) in the retrieved coatings. The accumulation of bone was not stage dependent. These findings indicate that there was some biointegration with the surrounding bone, but the greatest changes occurred with the HA coating materials, their loss, and chemical change.

Effect of vapor-flame treatment on plasma sprayed hydroxyapatite coatings

A vapor-flame treatment was developed to modify the crystallinity of the as-sprayed hydroxyapatite (HA) coatings. The effects of the treatment on composition, structure, and properties of HA coatings were investigated. Results showed that the vapor-flame treatment is simple and efficient to adjust the crystallinity of as-sprayed HA coating. Its crystallinities can be raised from 53.5 to 98.7% in 3-7 min. The porosities of coatings increased with an increase in the vapor-flame treating time. The microhardness of coating decreased as a result of this treatment. It may be explained in terms of the extent of microcracks caused by recrystallization of amorphous HA and relaxation of stress of the coating. The porosity, bonding strength, and hardness of HA coatings treated for 7 min were 15.7%, 32.0 MPa (300 microm thickness), and 1.9 GPa, respectively.

Vibrational spectroscopic characterization of new calcium phosphate bioactive coatings

In this work calcium phosphate (CaP) compounds with different PO(3-)(4)/HPO(2-)(4) R molar ratios in the 0.65-149 range were synthesized. In fact, all these CaPs contain different amounts of HPO(2-)(4) and PO(3-)(4) ions as well as the amorphous precursors (tricalcium phosphate and octacalcium phosphate) of hydroxyapatite deposition, which was shown by in vitro and in vivo measurements. Spectroscopical IR and Raman results showed the presence of bands whose intensity ratio can be related to the molar ratio R; in particular, the Raman I(962)/I(987) and the IR I(1035)/I(1125) intensity ratios were characterized as markers of the molar ratio. For these CaP compounds a nucleation model, which was based on the ability of HPO(2-)(4) ions to form strong H bonds with PO(3-)(4) ions, was proposed.

Structural and morphological study of pulsed laser deposited calcium phosphate bioceramic coatings: influence of deposition conditions, laser parameters, and target properties

Calcium phosphate (CaP) bioceramics, especially hydroxyapatite (HA), have been used as coatings on implants owing to their biocompatible properties. The commercial practice for applying HA coating, plasma spraying, has some disadvantages which limit the long-term stability of the implants. Pulsed laser deposition (PLD) is being investigated as an alternative technique. The purpose of this research was to systematically study the effect of various parameters of the PLD process on the properties of CaP coatings. In this study, three types of HA targets and two laser wavelengths were used to make six categories of coatings. Predominantly crystalline HA coatings were produced under all six categories at optimum conditions, although small amounts of minor phases sometimes were found. Sufficient coating/substrate bond strength was also obtained. A wide variety of coating morphologies was obtained, from rather dense and uniform to rough and porous. The important factors that affected the morphology included target properties, vacuum level, deposition temperature, and laser wavelength and energy density. PLD's ability to produce both amorphous and crystalline, and both smooth/dense and rough/porous coatings may be a unique advantage.

XPS, EDX and FTIR analysis of pulsed laser deposited calcium phosphate bioceramic coatings: the effects of various process parameters

Many techniques have been used to produce calcium phosphate, especially hydroxyapatite (HA), coatings on metallic implant surfaces for improved biocompatibility. Although some techniques have produced coatings used clinically, the long-term stability of the coating/implant is still questionable. As a new technique for making HA coatings, pulsed laser deposition (PLD) shows some advantages in controlling the coatings' crystal structure and composition. In this study, three types of HA target and two wavelengths of laser were used to produce calcium phosphate coatings. Despite PLDs ability to improve the crystal structure by incorporating water vapor into the deposition process, the characterization with EDX and XPS showed that coatings had different Ca/P ratios from that of the pure HA targets, which almost assured the presence of non-HA phases. FTIR spectra also showed differences in phosphate bands of coatings and targets although the difference in data collecting modes might have been a factor. The observed differences might be related to the differences between the surface and bulk chemistries of the coatings. Nevertheless, when evaluating the suitability of the PLD technique for making HA coatings, the possibility of the formation of non-HA phases cannot be excluded, although it may not necessarily be a negative factor.

Differences in microstructural characteristics of dense HA and HA coating

Two implant types of hydroxyapatite (HA) currently are available for dental implants: dense HA-cemented titanium (Ti) and HA-coated. It has been shown in previous reports that there are differences in the chemical and mechanical stabilities between the dense HA and HA coated. The differences are thought to be due to structural differences between the two ceramic types. The aim of this study was to investigate the differences in microstructural characteristics of currently available dense HA and HA coated implants before implantation and at periods of 3 weeks and 10 months after implantation in canine bone. X-ray diffractometry, infrared analysis, transmission electron microscopy, and energy dispersive X-ray analysis were used. The dense HA is composed of crystal grains, with a well crystallized structure of HA, closely bound to each other and approximately 0.4-0.6 micron in size. Implantation did not change the original sintered structure of the dense HA. The HA coating was composed of an amorphous phase with a Ca/P ratio of 1.46 and a crystal phase consisting of oxyhydroxyapatite, tricalcium phosphate, tetracalcium phosphate, and CaO, with a Ca/P ratio of 1.57. In the amorphous phase, compared to other portions in the amorphous phase, there were some layers with lower atomic density and with no significant difference in Ca/P ratio. After implantation, the crystallization of super fine crystals of approximately 4-5 nm in thickness occurred in the amorphous phase, and with time it progressed and spread from the surface to the deeper portion of the HA coating. A Ca/P ratio of 1.58 in the crystallized portion was close to the ratio (1.60) in the dense HA, suggesting that the super fine crystals were HA. This crystallization cannot significantly decrease the solubility of the amorphous phase portion and poses risks of stress accumulation within the coating and a decrease of binding strength between the HA coating and the substrate.

Studies on diffusion maximum in x-ray diffraction patterns of plasma-sprayed hydroxyapatite coatings

Study of an amorphous phase in plasma-sprayed hydroxyapatite (HA) coatings is important owing to its unique characteristics and nonnegligible amount of the amorphous phase compared to crystalline HA. However, little is known about the component parts of an amorphous phase. It is known that amorphous phase usually appears as the diffusion maximum (Dmax) in X-ray diffraction (XRD) patterns. Analyzing Dmax, including the position (Pmax) and area of Dmax, we can indicate the component parts of an amorphous phase and their transitions. In this study, the variation of Dmax in XRD patterns of the coatings during plasma spraying, in postheating, and in dissolving in vitro was studied with the aid of XRD. It was found that component parts of the amorphous phase in the coating varied with increasing thickness, consisting of two part represented by Dmax1, located between 29.4 and 29.8 degrees (2 theta), and Dmax2, located between 31.0 and 31.4 degrees (2 theta). It was concluded that Dmax3, located between 32.0 and 32.4 degrees (2 theta), should be referred to as nanocrystals of HA. In addition, the particle size of the starting powder may affect the component parts of the amorphous phase in the coating in addition to thickness. With vacuum heating (650 degrees C) and water vapor treatment at a low temperature (125 degrees C) in a saturated vaporic atmosphere, transition of the amorphous components was not as efficient as that at 490 degrees C with water vapor. The reason might be that the amorphous-to-crystalline HA conversion is dependent on both temperature and water vapor pressure. It was found that amorphous components were transformed completely into crystalline HA after heating at 490 degrees C with a partial water vapor pressure of 0.01 MPa for 2 h. It was concluded that the unstable amorphous components (Dmax1, Dmax2) converted into more stable nanocrystals of HA (Dmax3). Degradation in vitro showed that Dmax3 was more stable than Dmax1 and Dmax2. It was concluded that nucleation of apatite in vitro should be attributed to nanocrystals of HA (Dmax3) except for the amorphous components. It is recommended that the optimal phasic contents of the plasma-sprayed HA coating be mainly composed of crystalline HA and nanocrystals of HA (Dmax3) in terms of the stability and biocompatibility of the coating.

The process of physical weakening and dissolution of the HA-coated implant in bone and soft tissue

Hydroxyapatite (HA)-coated implants were developed to promote osseointegration of titanium implants and to overcome the mechanical drawbacks of solid HA implants. Although many clinical reports on the prognosis of HA-coated implants have reported high success rates, the risks of dissolution and weakening of the coating have been noted. We hypothesized that the chemical and mechanical stability of HA coating are affected by its microstructural characteristics. The present study investigates differences in the microstructures of available HA-coated implants, before and after implantation into the coxal bones of dogs for periods ranging from 3 weeks to 10 months and under the coxal periosteum of dogs for 10 months. The results of transmission electron microscopy and energy-dispersive x-ray analysis revealed that crystallization of super-fine HA crystals occurred in the amorphous phase of the HA coating and progressed over time. This crystallization weakens HA-coated implants by making the amorphous phase brittle, causing stress accumulation within the coating, and causing a decrease in the binding strength between the coating and the substrate. Furthermore, the HA coating dissolved in soft tissue. Dissolution started with the super-fine HA crystals in the crystallized portion that was originally part of the amorphous phase.

Amorphous phase formation in plasma-sprayed hydroxyapatite coatings

The amorphous phase content of air plasma-sprayed hydroxyapatite coatings is dependent upon spraying and deposition conditions. X-ray diffraction and optical microscopy were used to investigate the influence of spray parameters on the formation of the amorphous phase. Results show three factors which most influence the formation of the amorphous phase: dehydroxylation of the molten particle during flight, the cooling rate as it impinges onto the metal substrate, and the substrate temperature. Crystalline regions were identified as unmelted particles and elongated recrystallized areas. Amorphous phase regions vary throughout the coating but are more commonly found at the coating-substrate interface, i.e., the regions decrease toward the surface of the coating. Such an inhomogeneous distribution of phase content is expected to affect the clinical process of bone deposition, and therefore successful implant fixation.

Water vapour-treated hydroxyapatite coatings after plasma spraying and their characteristics

A novel way to enhance the ability of hydroxyapatite (HA) coatings in resisting degradation was revealed. The as-received plasma sprayed HA coatings were kept in water vapour at 125 degrees C, with a pressure of 0.15 MPa for 6 h; most of the amorphous phase in the coating was converted into crystalline HA and enhanced the crystallinity significantly. Meanwhile, the alpha-tricalcium phosphate, tetracalcium phosphate and CaO which decomposed from HA during plasma spraying were also transformed into crystalline HA. The dissolution experiment in distilled water at room temperature showed that the post-water vapour-treated coatings were more stable than post-heat-treated ones. The average interfacial tensile bond strength between HA and substrate before and after water vapour treatment was 45.0 and 39.1 MPa, respectively.