Structural changes of thermally sprayed hydroxyapatite investigated by Rietveld analysis

Novel hydroxyapatite nanorods improve anti-caries efficacy of enamel infiltrants

OBJECTIVES

Enamel resin infiltrants are biomaterials able to treat enamel caries at early stages. Nevertheless, they cannot prevent further demineralization of mineral-depleted enamel. Therefore, the aim of this work was to synthesize and incorporate specific hydroxyapatite nanoparticles (HAps) into the resin infiltrant to overcome this issue.

METHODS

HAps were prepared using a hydrothermal method (0h, 2h and 5h). The crystallinity, crystallite size and morphology of the nanoparticles were characterized through XRD, FT-IR and TEM. HAps were then incorporated (10wt%) into a light-curing co-monomer resin blend (control) to create different resin-based enamel infiltrants (HAp-0h, HAp-2h and HAp-5h), whose degree of conversion (DC) was assessed by FT-IR. Enamel caries lesions were first artificially created in extracted human molars and infiltrated using the tested resin infiltrants. Specimens were submitted to pH-cycling to simulate recurrent caries. Knoop microhardness of resin-infiltrated underlying and surrounding enamel was analyzed before and after pH challenge.

RESULTS

Whilst HAp-0h resulted amorphous, HAp-2h and HAp-5h presented nanorod morphology and higher crystallinity. Resin infiltration doped with HAp-2h and HAp-5h caused higher enamel resistance against demineralization compared to control HAp-free and HAp-0h infiltration. The inclusion of more crystalline HAp nanorods (HAp-2h and HAp-5h) increased significantly (p<0.05) the DC.

SIGNIFICANCE

Incorporation of more crystalline HAp nanorods into enamel resin infiltrants may be a feasible method to improve the overall performance in the prevention of recurrent demineralization (e.g. caries lesion) in resin-infiltrated enamel.

Laser surface modification of titanium substrate for pulsed laser deposition of highly adherent hydroxyapatite

Biomedical implant devices made out of titanium and its alloys are benefited by a modified surface or a bioactive coating to enhance bone bonding ability and to function effectively in vivo for the intended period of time. In this respect hydroxyapatite coating developed through pulsed laser deposition is a promising approach. Since the success of the bioactive ceramic coated implant depends mainly on the substrate-coating strength; an attempt has been made to produce micro patterned surface structure on titanium substrate for adherent hydroxyapatite coating. A pulsed Nd-YAG laser beam (355 nm) with 10 Hz repetition rate was used for surface treatment of titanium as well as hydroxyapatite deposition. The unfocussed laser beam was used to modify the substrate surface with 500-18,000 laser pulses while keeping the polished substrate in water. Hydroxyapatite deposition was done in a vacuum deposition chamber at 400 °C with the focused laser beam under 1 × 10⁻³ mbar oxygen pressure. Deposits were analyzed to understand the physico-chemical, morphological and mechanical characteristics. The obtained substrate and coating surface morphology indicates that laser treatment method can provide controlled micro-topography. Scratch test analysis and microindentation hardness values of coating on laser treated substrate indicate higher mechanical adhesion with respect to coatings on untreated substrates.

An organic matrix-mediated processing methodology to fabricate hydroxyapatite based nanostructured biocomposites

An amorphous calcium phosphate precursor phase, which forms by adding orthophosphoric acid to a calcium hydroxide suspension, is transformed into crystalline hydroxyapatite by introducing polymer solutions. The nanostructured composite films formed by a solvent casting technique from the concentrated hybrid suspension are characterised for structure and mechanical properties.

Porous titanium and silicon-substituted hydroxyapatite biomodification prepared by a biomimetic process: characterization and in vivo evaluation

Porous titanium with a pore size of 150-600 microm and a porosity of 67% was prepared by fiber sintering. The porous titanium had a complete three-dimensional (3D) interconnected structure and a high yield strength of 100 MPa. Si-substituted hydroxyapatite (Si-HA) was coated on the surface by a biomimetic process to improve the surface bioactivity. X-ray diffraction results showed that Si-HA coating was not well crystallized. New bone tissue was found in the uncoated porous titanium after 2 weeks of implantation and a significant increase (p<0.05) in the bone ingrowth rate (BIR) was found after 4 weeks of implantation, indicating the good osteoconductivity of the porous structure. The HA-coated and Si-HA-coated porous titanium exhibited a significantly higher BIR than the uncoated titanium at all intervals, highlighting the better surface bioactivity and osteoconductivity of the HA- and Si-HA coatings. Also, the Si-HA-coated porous titanium demonstrated a significantly higher BIR than the HA-coated porous titanium, showing that silicon plays an active role in the surface bioactivity. For Si-HA-coated porous titanium, up to 90% pore area was covered by new bone tissue after 4 weeks of implantation in cortical bone. In the bone marrow cavity, the pore spaces were filled with bone marrow, displaying that the interconnected pore structure could provide a channel for body fluid. It was concluded that both the 3D interconnected pore structure and the Si-HA coating contributed to the high BIR.

Chitosan-mediated crystallization and assembly of hydroxyapatite nanoparticles into hybrid nanostructured films

The synthesis and subsequent assembly of nearly spherical nano-hydroxyapatite (nHA) particles in the presence of trace amounts of the polysaccharide chitosan was carried out employing a wet chemical approach. Chitosan addition during synthesis not only modulated HA crystallization but also aided in the assembly of nHA particles onto itself. Solvent extraction from these suspensions formed iridescent films, of which the bottom few layers were rich in self-assembled nHA particle arrays. The cross-section of these hybrid films revealed compositional and hence structural grading of the two phases and exhibited a unique morphology in which assembled nHA particles gradually gave way to chitosan-rich top layers. Transmission electron microscope and selected area electron diffraction studies suggested that the basal plane of HA had interacted with chitosan, and scanning electron microscope studies of the hybrid films revealed multi-length scale hierarchical architecture composed of HA and chitosan. Phase identification was carried out by X-ray diffraction (XRD) and Rietveld analysis of digitized XRD data showed that the basic apatite structure was preserved, but chitosan inclusion induced subtle changes to the HA unit cell. The refinement of crystallite shape using the Popa method clearly indicated a distinct change in the growth direction of HA crystallites from [001] to [100] with increasing chitosan concentration. The paper also discusses the likelihood of chitosan phosphorylation during synthesis, which we believe to be a pathway, by which chitosan molecules chemically interact with calcium phosphate precursor compounds and orchestrate the crystallization of nHA particles. Additionally, the paper suggests several interesting biomedical applications for graded nHA-chitosan nanostructured films.

The chalcogenide phosphate apatites Ca10(PO4)6S, Sr10(PO4)6S, Ba10(PO4)6S and Ca10(PO4)6Se

Four new apatitic phases were prepared and their structures determined. The structure of Ca10(PO4)6S was refined from single crystal X-ray data and the structures of Sr10(PO4)6S, Ba10(PO4)6S and Ca10(PO4)6Se from powder X-ray data using the Rietveld method.

The four apatites are isostructural and crystallize in the trigonal space group P-3 with the chalcogenide ion positioned at (0 0 ½). The sulfoapatites show no ability to absorb H2S in the way that oxyapatite absorbs H2O at elevated temperatures. This can be attributed to the position of the sulfide ion and the way it influences the crystal structure around the vacant chalcogenide position at (0 0 0).

Electrophoretic deposition of silicon substituted hydroxyapatite coatings from n-butanol-chloroform mixture

Silicon Substituted Hydroxyapatite (Si-HA) coatings were prepared on titanium substrates by electrophoretic deposition (EPD). The stability of Si-HA suspension in n-butanol and chloroform mixture has been studied by electricity conductivity and sedimentation test. The microstructure, shear strength and bioactivity in vitro has been tested. The stability of Si-HA suspension containing n-butanol and chloroform mixture as medium is better than that of pure n-butanol as medium. The good adhesion of the particles with the substrate and good cohesion between the particles were obtained in n-butanol and chloroform mixture. Adding triethanolamine (TEA) as additive into the suspension is in favor of the formation of uniform and compact Si-HA coatings on the titanium substrates by EPD. The shear strength of the coatings can reach 20.43 MPa after sintering at 700 degrees C for 2 h, when the volume ratio of n-butanol: chloroform is 2:1 and the concentration of TEA is 15 ml/L. Titanium substrates etched in H(2)O(2)/NH(3) solution help to improve the shear strength of the coatings. After immersion in simulated body fluid for 7 days, Si-HA coatings have the ability to induce the bone-like apatite formation.

Biocompatibility of electrophoretical deposition of nanostructured hydroxyapatite coating on roughen titanium surface:In vitroevaluation using mesenchymal stem cells

A nano hydroxyapatite (HAp) layer was coated on a roughen titanium surface by means of electrophoretic deposition with an acetic anhydride solvent system. The objectives of this current study are to investigate whether nano-HAp can improve mechanical strength at a lower sintering temperature and biocompatibility. Densification temperature was lowered from usual 1000 to 800 degrees C. The coating interfacial bonding strength, phase purity, microstructure, and biocompatibility were investigated. Degradation of HA phase was not detected in XRD. A porous TiO2 layer acts as a gradient coating layer with an intermediate thermal expansion coefficient between hydroxyapatite and titanium that reduces the thermal stress. From SEM image, the coating does not contain any crack. Mesenchymal stem cell (MSC) is the progenitor cell for various tissues in mature animals, which can improve integration of bone tissue into implant. In this in vitro study, rabbit MSCs culture indicated that the HAp/Ti nanocomposite biomaterial had good biocompatibility and bioactivity. Around materials and on its surface cell grew well with good morphology. Proliferation of the MSCs on the nano-HAp coating was higher than its micron counterpart in XTT assay. These properties show potential for the orthopaedic and dental applications.

Early bone in-growth ability of alumina ceramic implants loaded with tissue-engineered bone

To enhance early bonding of an alumina ceramic implant to bone, we evaluated a method of seeding the implant surface with bone marrow mesenchymal cells that differentiated to osteoblasts and bone matrix prior to implantation. The usefulness of the method was evaluated in Japanese white rabbits. In our study, an alumina ceramic test piece loaded with differentiated osteoblasts and bone matrix by a tissue engineering technique was implanted into rabbit bones. Three weeks after the procedure, evaluation of mechanical bonding and histological examination were performed. Histological examination of the noncell-loaded implant surfaces showed no bone infiltration into the implant gap. However, the cell-loaded implant surfaces exhibited new bone infiltration into the implant gap with mechanical bonding. In the mechanical test, the average failure load was 0.60 kgf for the noncell-loaded side and 1.49 kgf for the cell-loaded side. Preculturing mesenchymal cells on the surface of the alumina ceramic prior to implantation increased the debonding strength by two and half times. The present findings indicate early bonding between the implant and bone three weeks after the procedure.

Electrophoretic Deposition of Hydroxyapatite Nano Coating on Etched Titanium Surface

Hydroxyapatite (Ca10(PO4)6(OH)2, HAp) is biocompatible and bioactive, however, it is relatively brittle. The development of HAp coatings on medical metal surface is a good way to improve the mechanical properties of HAp. In the present study, a HAp coating with nano-structure on a roughened titanium surface was developed by electrophoretic deposition process. To decrease sintering temperature HAp nanoparticles synthesized by a wet chemical method was used. It was observed that the coating was uniform and showed no cracks. After sintering the HAp coating still remained nano structured. The surface treatment of Ti was applied to form a distribution of small pits and a TiO2 thin layer on the Ti surface that improves the adhesion of coating to the Ti substrate. It was shown that the bonding strength of coating was 18 ± 2.5MPa. The hardness and Young’s modulus were 40.6 and 0.42 GPa, respectively.

A new way of incorporating silicon in hydroxyapatite (Si-HA) as thin films

Bioactive silicon-containing hydroxyapatite (Si-HA) thin films that can be used as coatings for bone tissue replacement have been developed. A magnetron co-sputtering technique was used to deposit Si-HA films up to 700 nm thick on titanium substrates, with a silicon level up to 1.2 wt%. X-ray diffraction demonstrated that annealing transformed the as-deposited Si-HA films which were amorphous, into a crystalline HA structure. A human osteoblast-like (HOB) cell model was used to determine the biocompatibility of these films. HOB cells were seen to attach and grow well on the Si-HA films, and the metabolic activity of HOB cells on these films was observed to increase with culture time. Furthermore, mineralisation of the cell layers was observed after 8 weeks of culture. Based on the present findings, Si-HA of different film compositions demonstrate bioactive properties in-vitro, and indicate the potential as biocoatings for a wide variety of medical implants including load-bearing applications such as the femoral stem of hip replacement implants.

Hydroxyapatite-coated metals: interfacial reactions during sintering

Electrophoretic deposition (EPD) is a low cost flexible process for producing HA coatings on metal implants. Its main limitation is that it requires heating the coated implant in order to densify the HA. HA typically sinters at a temperature below 1150 degrees C, but metal implants are degraded above 1000 degrees C. Further, the metal induces the decomposition of the HA coating upon sintering. Recent developments have enabled EPD of metathesis-synthesised uncalcined HA which sinters at approximately 1000 degrees C. The effects of temperature on HA-coated Ti, Ti6Al4V, and 316L stainless steel were investigated for dual coatings of metathesis HA sintered at 1000 degrees C. The use of dual HA coatings (coat, sinter, coat, sinter) enabled decomposition to be confined to the "undercoat" (HA layer 1), with the surface coating decomposition free. The tensile strength of the three metals was not significantly affected by the high sintering temperatures (925 degrees C < T < 1000 degrees C). XRD/SEM/EDS analyses of the interfacial zones revealed that 316L had a negligible HA:metal interfacial zone (approximately 1 microm) while HA:Ti and HA:Ti6Al4V had large interfacial zones (>10 microm) comprising a TiO2 oxidation zone and a CaTiO2 reaction zone.

Bone-like apatite layer formation on hydroxyapatite prepared by spark plasma sintering (SPS)

Hydroxyapatite (HA) compacts with high density and superior mechanical properties were fabricated by spark plasma sintering (SPS) using spray-dried HA powders as feedstock. The formation of bone-like apatite layer on SPS consolidated HA compacts were investigated by soaking the HA compacts in simulated body fluid (SBF) for various periods (maximum of 28 days). The structural changes in HA post-SBF were analyzed with scanning electron microscopy, grazing incidence X-ray diffraction and X-ray photoelectron spectroscopy. It was found that a layer consisting microcrystalline carbonate-containing hydroxyapatite was formed on the surface of HA compacts after soaking for 24h. The formation mechanism of apatite on the surface of HA compacts after soaking in SBF was attributed to the ion exchange between HA compacts and the SBF solution. The increase in ionic concentration of calcium and phosphorus as well as the increase in pH after SBF immersion resulted in an increase in ionic activity product of apatite in the solution, and provided a specific surface with a low interface energy that is conducive to the nucleation of apatite on the surface of HA compacts.

A new electrochemically graded hydroxyapatite coating for osteosynthetic implants promotes implant osteointegration in a rat model

Hydroxyapatite (HAP) is widely used as an osteoconductive coating for orthopedic implants. So far standard coating methods like plasma spraying produce a relatively thick coating layer (>30 microm). In addition, the chemical structure of the HAP may be altered because of the heating throughout the coating process. This may have negative effects on the coating stability, implant fixation, and induction of bone formation. The relatively thick layer may detach from the implant with the risk of wear debris. In the present study the potential of a newly developed HAP coating of implants on osteointegration was investigated in a rat model. The coating method, based on an electrochemical process, is applied in a graded manner and results in a biodegradable HAP coating with a thickness of approximately 2 mum. Coated versus uncoated titanium Kirschner wires (1.4-mm diameter) were inserted into the medullary cavity of the right femora of 5-month old female Sprague Dawley rats (n=36) in a retrograde fashion. Throughout an experimental period of 2 months the osteointegration was traced radiologically. After this time the animals were sacrificed and the implant integration was tested biomechanically with the use of a push-out test. To analyze the bone-implant interface, histological sections (80 mum) were investigated with an image analyzing system. The biomechanical testing revealed a significantly higher implant fixation in the group treated with the HAP-coated implant (shear strength: 27.8 +/- 6.7 MPa) compared to control (shear strength: 8.08 +/- 3.4 MPa). The histological analyses demonstrated a better ingrowth of the implants in the HAP group with significantly more direct bone-implant contacts compared to the control group. The results demonstrate that the HAP coating promotes implant osteointegration in a rat model.

Stem cell technology and bioceramics: from cell to gene engineering

Mesenchymal stem cells reside in bone marrow and, when these cells are incorporated into porous ceramics, the composites exhibit osteo-chondrogenic phenotypic expression in ectopic (subcutaneous and intramuscular) or orthotopic sites. The expressional cascade is dependent upon the material properties of the delivery vehicle. Bioactive ceramics provide a suitable substrate for the attachment of the cells. This is followed by osteogenic differentiation directly on the surface of the ceramic, which results in bone bonding. Nonbioactive materials show neither surface-dependent cell differentiation nor bone bonding. The number of mesenchymal stem cells in fresh adult bone marrow is small, about one per one-hundred-thousand nucleated cells, and decreases with donor age. In vitro cell culture technology can be used to mitotically expand these cells without the loss of their developmental potency regardless of donor age. The implanted composite of porous ceramic and culture-expanded mesenchymal stem cells exhibits in vivo osteo-chondrogenic differentiation. In certain culture conditions, these stem cells differentiate into osteoblasts, which make bone matrix on the ceramic surface. Such in vitro prefabricated bone within the ceramic provides immediate new bone-forming capability after in vivo implantation. Prior to loading of the cultured, marrow-derived mesenchymal stem cells into the porous ceramics, exogenous genes can be introduced into these cells in culture. Combining in vitro manipulated mesenchymal stem cells with porous ceramics can be expected to effect sufficient new bone-forming capability, which can thereby provide tissue engineering approaches to patients with skeletal defects in order to regenerate skeletal tissues.

Solution ripening of hydroxyapatite nanoparticles: effects on electrophoretic deposition

Electrophoretic deposition is a low-cost, simple, and flexible coating method for producing hydroxyapatite (Hap) coatings on metal implants. However, densification requires heating the coated metal to high temperatures, which, for commercial HAp powders, generally means at least 1200 degrees C. At such temperatures, the metal tends to react with the HAp coating, inducing decomposition, and the strength of titanium and stainless steel implants is severely degraded. With the use of raw uncalcined nanoparticulate Hap, densification can occur at 900 degrees -1050 degrees C; however, such coatings are prone to cracking due to the high drying shrinkage. This problem was solved by precipitating nanoparticulate HAp by the metathesis process [10Ca(NO3)2 + 6NH4H2PO4 + 8NH4OH] and optimizing the approximately 30 nm of nanoprecipitates by an Ostwald ripening approach, that is, by boiling and/or ambient aging in the mother liquor. While the as-precipitated nanoparticles produced severely cracked coatings, 2 h of boiling or 10 days of ambient aging ripened the "gel-like" mass into unagglomerated nanoparticles, which produced crack-free coatings. Since boiling enhanced particle size but ambient aging did not, crack elimination probably was due to the transition from the highly agglomerated gel-like state to the dispersed nanoparticulate state rather than to particle growth. Furthermore, boiling only reduced the amount of cracking whereas aging completely eliminated cracking.

Solution ripening of hydroxyapatite nanoparticles: effects on electrophoretic deposition

Electrophoretic deposition is a low-cost, simple, and flexible coating method for producing hydroxyapatite (Hap) coatings on metal implants. However, densification requires heating the coated metal to high temperatures, which, for commercial HAp powders, generally means at least 1200 degrees C. At such temperatures, the metal tends to react with the HAp coating, inducing decomposition, and the strength of titanium and stainless steel implants is severely degraded. With the use of raw uncalcined nanoparticulate Hap, densification can occur at 900 degrees -1050 degrees C; however, such coatings are prone to cracking due to the high drying shrinkage. This problem was solved by precipitating nanoparticulate HAp by the metathesis process [10Ca(NO3)2 + 6NH4H2PO4 + 8NH4OH] and optimizing the approximately 30 nm of nanoprecipitates by an Ostwald ripening approach, that is, by boiling and/or ambient aging in the mother liquor. While the as-precipitated nanoparticles produced severely cracked coatings, 2 h of boiling or 10 days of ambient aging ripened the "gel-like" mass into unagglomerated nanoparticles, which produced crack-free coatings. Since boiling enhanced particle size but ambient aging did not, crack elimination probably was due to the transition from the highly agglomerated gel-like state to the dispersed nanoparticulate state rather than to particle growth. Furthermore, boiling only reduced the amount of cracking whereas aging completely eliminated cracking.