Hydroxylapatite Coatings Research Articles

Study on the influence of plasma spray processes and spray parameters on the structure and crystallinity of hydroxylapatite coatings

AbstractHydroxylapatite (HA) is one of the most important materials for human hard tissue implants. Until now, HA coatings are widely deposited on implants using atmospheric plasma spraying (APS). Due to the decomposition of HA and formation of amorphous phase during spraying, the crystallinity of APS coatings is usually below 70 %, although coarser spray powders such as one with a size distribution of –150 +45 μm are used. In this study a finer HA powder –80 +50 μm was sprayed using both a conventional APS process and the new “microplasma spraying” (MPS) process. The coatings were characterized in terms of their microstructure and crystallinity. It was found that the spray parameters influenced strongly the coating structure and phase composition. Despite the smaller particles, the HA content of APS coatings could be increased from 27 % to 72 % only by changing the spray parameters. Higher HA contents up to over 85 % could be obtained by microplasma spraying. It was found that textured coatings could form under certain conditions during both spray processes.

Growth Behaviour of Rat Bone Marrow (RBM) Cells on RF Magnetron Sputtered Hydroxyapatite and Dicalcium Pyrophosphate Coatings

The aim of this study was to evaluate the osteogenic properties of magnetron sputtered dicalcium pyrophosphate (DCPP) and hydroxylapatite (HA) coatings. Therefore, DCPP and HA coatings were deposited on grit-blasted titanium discs. The substrates were used as-prepared or received an additional heat treatment with changed the amorphous coating structure to a crystalline structure. Subsequently, rat bone marrow stromal cells were cultured for 1-24 days on the various substrates. DNA and alkaline phosphatase activity was determined after 1, 3, 5, 8 and 12 days of incubation. Osteocalcin expression was evaluated after 8, 12, 16 and 24 days of incubation. Scanning electron microscopical analysis of cell morphology and coating characteristics was done after 8 and 16 days of incubation. All assays were done in duplicate and in each assay all specimens were present in fourfold. Results demonstrated that the cells did not proliferate and differentiate on all amorphous coatings. SEM revealed that the amorphous coatings showed significant dissolution. On the crystalline DCPP and HA coatings an increase in DNA and alkaline phosphatase activity was seen starting at day 8 of incubation. Osteocalcin expression on the crystalline coatings started to increase at day 16 of incubation. SEM showed that the growth and differentiation of the cells was associated with extensive collage fiber formation and surface mineralization in the form of globular accretions. Further, statistical testing revealed that proliferation and differentiation of the rat bone marrow stromal cells started significantly earlier on the crystalline HA coatings than on the crystalline DCPP coatings. These results demonstrate that the rat bone marrow stromal cells proliferated and differentiated only on crystalline magnetron sputtered DCPP as well as HA coatings, which warrants the further in vivo analysis of the bone healing supporting properties of these coatings.

Growth behavior of rat bone marrow cells on RF magnetron sputtered hydroxyapatite and dicalcium pyrophosphate coatings

The aim of this study was to evaluate the osteogenic properties of magnetron sputtered dicalcium pyrophaosphate (DCPP) and hydroxylapatite (HA) coatings. Therefore, DCPP and HA coatings were deposited on grit-blasted titanium discs. The substrates were used as-prepared or received an additional heat treatment which changed the amorphous coating structure to a crystalline structure. Subsequently, rat bone marrow stromal cells were cultured for 1-24 days on the various substrates. DNA and alkaline phosphatase activity was determined after 1, 3, 5, 8, and 12 days of incubation. Osteocalcin expression was evaluated after 8, 12, 16, and 24 days of incubation. Scanning electron microscopical analysis of cell morphology and coating characteristics was done after 8 and 16 days of incubation. All assays were done in duplicate and in each assay all specimens were present in fourfold. Results demonstrated that the cells did not proliferate and differentiate on all amorphous coatings. SEM revealed that the amorphous coatings showed significant dissolution. On the crystalline DCPP and HA coatings an increase in DNA and alkaline phosphatase activity was seen starting at day 8 of incubation. Osteocalcin expression on the crystalline coatings started to increase at day 16 of incubation. SEM showed that the growth and differentiation of the cells was associated with extensive collagen fiber formation and surface mineralization in the form of globular accretions. Further, statistical testing revealed that proliferation and differentiation of the rat bone marrow stromal cells started significantly earlier on the crystalline HA coatings than that on the crystalline DCPP coatings. These results demonstrate that the rat bone marrow stromal cells proliferated and differentiated only on crystalline magnetron sputtered DCPP as well as HA coatings, which warrants the further in vivo analysis of the bone healing supporting properties of these coatings.

Bioactive coatings obtained at room temperature with hydroxyapatite and polysiloxanes

Low temperature Hydroxylapatite (HA) coatings were obtained on metal substrates by depositing HA particles suspended in a ormosil polymer gel made of an equimolar mixture of amino and epoxy substituted silanes. The new coatings were analysed by thermal analysis, infrared and solid state NMR, as well as X-ray diffraction and SEM observations. Results show the HA powder conserves its characteristic physicochemical properties during the coating process. Furthermore, the coatings induce the formation of a biomimetic HA layer when soaked in simulated body fluids. This room temperature process may lead to novel calcium phosphate coatings with better control of crystallinity and porosity and with the possible incorporation of biofunctional organic molecules.

Biological stability and osteoconductivity in rabbit tibia of pulsed laser deposited hydroxylapatite coatings

A comparative study of the biological stability and the osteoconductivity of hydroxylapatite (HA) coatings produced by pulsed laser deposition (PLD) and plasma spraying (PS) was conducted. Three different implant groups were used: grit-blasted titanium rods coated with HA-PLD (2-microm-thick), grit-blasted titanium rods coated with HA-PS (50-microm-thick), and uncoated. Implantation took place into the proximal tibia of 12 mature New Zealand White rabbits for 24 weeks. Samples were evaluated using descriptive histology and histomorphometry. While HA-PS implants showed considerable instability and reduction in thickness after 24 weeks, but no statistical difference to the titanium group, the HA-PLD group showed a significant higher amount of bone apposition (Scheffé test, p<0.05) than the other two groups, without signs of degradation or dissolution. Remarkably, after 6 months, the almost intact thin pulsed laser deposited coating could be observed by electron microscopy in extended areas.

Subcutaneous evaluation of RF magnetron-sputtered calcium pyrophosphate and hydroxylapatite-coated Ti implants

The in vivo behavior of infrared-heated, RF magnetron-sputtered hydroxylapatite (HA) and calcium pyrophosphate (DCPP) coated titanium discs was investigated. The discs were implanted subcutaneously in the back of six goats for 2, 4, 8 and 12 weeks. At the end of the study, coated discs were removed and examined on their physicochemical properties by X-ray diffraction (XRD) and scanning electron microscopy (SEM), including energy dispersive spectroscopy (EDS). Also, implants were prepared for light microscopical evaluation of the tissue response. The results showed that heat-treated HA coatings showed a stable behavior, i.e. no changes in the XRD pattern occurred during implantation. Also, no dissolution of the coating was observed by SEM. EDS revealed that the Ca/P ratio of the HA coatings remained stable during implantation. In contrast, heat-treated DCPP coatings showed a compositional change into apatite and tricalcium phosphate (TCP) during implantation. This was confirmed by the SEM and EDS analysis. The Ca/P ratio of the DCPP coatings changed from 0.8 to 1.52 during implantation. Finally, histology showed that both heat-treated HA and DCPP coatings showed no adverse tissue response, as characterized by the presence of thin, dense fibrous tissue capsule. Consequently, it can be concluded that 2 mum thick heat-treated, RF magnetron-sputtered HA and DCPP coatings are of sufficient thickness to withstand dissolution during 12 weeks of implantation in a subcutaneous location in goats. In addition, both coatings showed a biocompatible tissue behavior. Further, heat-treated DCPP coatings revealed a gradual compositional change into apatite and TCP.

Preparation and characterization of RF magnetron sputtered calcium pyrophosphate coatings

CaP ceramic has been widely used as coating on metals in orthopedics and oral dentistry. Variations in CaP composition can lead to different dissolution/precipitation behavior and may also affect the bone response. In the present study calcium pyrophosphate and hydroxylapatite coatings were successfully prepared by RF magnetron sputtering deposition. The phase composition, morphological properties, and the dissolution in SBF were characterized by using XRD, FTIR, EDS, SEM, and spectrophotometry. The results showed that all the sputtered coatings were amorphous and changed into a crystal structure after IR-radiation. The temperature for the crystallization of the amorphous coatings is lower for the hydroxylapatite coating (550 degrees C), compared to the calcium pyrophosphate coating (650 degrees C). All sputtered amorphous coatings were instable in SBF and dissolved partially within 4 wks of incubation. The heat-treated coatings appeared to be stable after incubation. These results showed that magnetron sputtering of calcium pyrophosphate coating is a promising method for forming a biocompatible ceramic coating.

A contemporary snapshot of the use of hydroxyapatite coating in orthopaedic surgery

The capacity for biological materials to combine with inorganic salts is well established. Bio-mineralisation was an evolutionary milestone, and a key contributor to the ‘cladogenesis of life’ which occurred in the early Cambrian period, 570 million years ago.[1][1] A protracted ice age in the

Pulsed laser deposition of hydroxylapatite thin films on biomorphic silicon carbide ceramics

Hydroxylapatite (HA) coatings were produced by pulsed laser deposition (PLD) on biomorphic silicon carbide ceramics (Bio-SiC) by ablation of non-sintered HA discs with an ArF excimer laser (193 nm, 25 ns, 4.2 J cm −2) at different conditions of water vapour pressure and substrate temperature. The characterization of the coatings performed by X-ray diffractometry (XRD), X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared spectroscopy (FTIR) techniques showed changes of the crystallinity of the coatings and the substitution of phosphate by carbonate groups in the HA structure, induced by the variations of the pressure and the substrate temperature.

In vitro and in vivo performance of Ti6Al4V implants with plasma-sprayed osteoconductive hydroxylapatite-bioinert titania bond coat "duplex" systems: an experimental study in sheep.

To evaluate the in vivo performance of "duplex" hydroxylapatite top coat/TiO(2) bond coat systems, cylindrical Ti6Al4V rods of 130 mm in length and 11-13 mm in diameter were coated by atmospheric plasma spray (APS) technique with both a standard hydroxylapatite (HAp) layer and a HAp+TiO(2) bond coat "duplex" layer. In this pilot study coated and uncoated rods serving as controls were implanted into the femur of sheep so that their distal ends were freely suspended in the medulla of the femur. After an observation time of six months it was found that bone apposition and bone ingrowth were considerably increased in the presence of a osteoconductive coating. In particular, in vivo spalling and delamination frequently observed with HAp coatings was virtually absent in duplex coatings owing to the strong adhesion of the bond coat to the HAp top coat that anchored the latter solidly to the metallic surface of the implant. Some tentative mechanisms leading to this improved coating adhesion will be discussed.

Mechanical Evaluation of the Use of a Buffer Layer in Hydroxylapatite Coatings Produced by Pulsed Laser Deposition at High Temperature

Mechanical Evaluation of the Use of a Buffer Layer in Hydroxylapatite Coatings Produced by Pulsed Laser Deposition at High Temperature

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.

Laser ablation rate of hydroxylapatite in different atmospheres

Hydroxylapatite (Ca 10(PO 4) 6(OH) 2) is a calcium phosphate used as coating for dental and orthopaedic implants, because its composition and structure are similar to the mineral part of bone. Pulsed laser deposition has been applied as an alternative to the commercial technique for the production of hydroxylapatite coatings: plasma spraying. Hydroxylapatite targets were ablated at 0.9 J cm −2 using an ArF excimer laser (193 nm) at 20 Hz in order to investigate the ablation rate of hydroxylapatite in different atmospheres: water vapour, Ar and O 2. The ablation rate was measured by profilometry for different pressures in a range of 15–80 Pa. The ablation rate depends on the backscattering of the ablated particles by the molecules of the gas, which produces different amounts of re-deposited material on the target surface for each gas. The ablation rate in a water vapour atmosphere presents a particular behaviour due to the formation of different calcium phosphate phases from the original hydroxylapatite under ArF excimer laser irradiation as compared those formed in other ambient gases.

In vitro bioactive behavior of hydroxylapatite‐coated porous Al2O3

To produce bioactive materials for bone substitutes, two major deposition methods, suspension method and thermal deposition method, were employed to develop bioactive, mechanically strong, and porous ceramics. Hydroxylapatite (HA) has been uniformly coated onto inner pore surfaces of reticulated alumina substrates. It has been found that the in vitro bioactivity of HA coatings was affected by both structural crystallinity and specific surface area. Well-crystallized HA heat-treated at high temperatures has resulted in reduced bioactivity. The bio-reaction rate was found to increase with the surface area of HA. We have found that the stability of the well-crystallized HA is associated with the high driving force required for the formation of hydroxy-carbonate apatite (HCA) phase. © 2000 John Wiley & Sons, Inc. J Biomed Mater Res (Appl Biomater) 53: 457–466, 2000

In vitro bioactive behavior of hydroxylapatite-coated porous Al(2)O(3).

To produce bioactive materials for bone substitutes, two major deposition methods, suspension method and thermal deposition method, were employed to develop bioactive, mechanically strong, and porous ceramics. Hydroxylapatite (HA) has been uniformly coated onto inner pore surfaces of reticulated alumina substrates. It has been found that the in vitro bioactivity of HA coatings was affected by both structural crystallinity and specific surface area. Well-crystallized HA heat-treated at high temperatures has resulted in reduced bioactivity. The bio-reaction rate was found to increase with the surface area of HA. We have found that the stability of the well-crystallized HA is associated with the high driving force required for the formation of hydroxy-carbonate apatite (HCA) phase.

Effects of hydroxylapatite coating crystallinity on biosolubility, cell attachment efficiency and proliferation in vitro

A hydroxylapatite (HA) coating with ∼97% crystalline HA content (MP-1 treated HA coating, MP-HA) was tested in vitro for its biosolubility and cellular biocompatibility. The MP-HA coating was compared with a standard HA coating with ∼63% crystallinity (SHA) and an amorphous HA coating with ∼25% crystallinity (AHA), as well as a titanium (Ti) surface without HA coating as a control. The topographic study with scanning electron microscopy indicated that MP-HA appeared more coarse, with projected nodules which altered the shape of cells attached to the substrate. Biosolubility study indicated that MP-HA had the least effect on the culture medium pH, while AHA ( P<0.01) and SHA ( P<0.05) significantly raised the medium pH up to 8.2 and 7.75, respectively. X-ray diffraction (XRD) analysis showed essentially unchanged levels of the total soluble phases of all coatings after incubation with culture medium, except that the CaO phase was rapidly dissolved from AHA coatings and completely eliminated from SHA coatings. Cultures of human gingival fibroblasts on these HA coatings showed that MP-HA and SHA had about the same cell attachment efficiency which was relatively lower than that of AHA coatings. MP-HA generated significant higher cell proliferation rate relative to AHA ( P<0.01) and SHA ( P<0.05). This study indicated that surface chemistry and topography of lower crystallinity might be favorable to cell attachment, but that elevated medium pH might result in a cytotoxic effect that inhibits the proliferation of attached cells on coating surfaces.

Degradation of hydroxylapatite, fluorapatite, and fluorhydroxyapatite coatings of dental implants in dogs.

Calcium phosphate coatings on dental implants enhance integration of the material. Resorption of the ceramic coatings has raised some concern about the behavior of the bone-implant interfaces after the coating disappearance. Substitution of the OH- ions by fluoride in the hydroxylapatite (HA) lattice makes the calcium phosphate more stable. We investigated the degradation rate of dental implants with 50- and 100-microm coatings of HA, fluorapatite (FA), or fluorhydroxylapatite (FHA). The implants were inserted in dog jaws and retrieved for histological analysis after 3, 6, and 12 months. The thickness of the calcium phosphate coatings was evaluated using an image analysis device. A relative resorption index and its standard deviation were studied. HA and FA coatings (even at 100-microm thickness) were almost totally degraded within the implantation period. In contrast, the FHA coatings did not show significant degradation during the same period. The standard deviation showed that the resorption process for FHA with thicknesses of 50 or 100 microm was the same. Such a difference was not observed between the 50- and 100-microm thick coatings of FA and HA. In conclusion, the FHA coatings showed good integration in the bone tissue and lasted much longer than classic calcium phosphate coatings.

Degradation of hydroxylapatite, fluorapatite, and fluorhydroxyapatite coatings of dental implants in dogs

Degradation of hydroxylapatite, fluorapatite, and fluorhydroxyapatite coatings of dental implants in dogs

Healing of large (2 mm) gaps around calcium phosphate-coated bone implants: a study in goats with a follow-up of 6 months.

Plasma-sprayed hydroxylapatite (HA) coatings are known for their ability to demonstrate osseointegration with bone. Recently it was found that the amount of bone apposition was strongly reduced 6 weeks after implantation in a goat model if gaps of two millimeters between bone and apatite coating existed. Stability of the apatite coatings examined did not influence the gap-healing ability. This study investigated whether a longer follow-up period of 24 weeks would be sufficient for the restoration of bone apposition on apatite coatings in an identical surgical model with 2 mm gaps, and whether bone apposition on the apatite coatings is influenced by the coating stability. Three coatings were investigated: 25-30% crystalline HA (aHA), 60-63% crystalline HA (cHA), and 85-90% crystalline fluorapatite (FA). Uncoated Ti-6A1-4V implants were used as controls. Implants were inserted in the femoral condyles of both femora of eight goats. Each goat received four implants. Histology revealed that bone formation on each of the apatite coatings remained low and did not increase with an extended follow-up period of 24 weeks. The coatings showed significantly (P < 0.01) more bone contact than the uncoated control implants. The three different coatings did not show significant differences in bone apposition. The aHA coating in most cases had disappeared completely after 24 weeks. Despite the disappearance of the aHA coating, bone contact was seen on the substrate surface without fibrous tissue interposition. The cHA coating showed minor signs of degradation while the FA coatings showed no visible degradation. It is concluded that non-press-fit implantation of apatite-coated implants leads to more bone apposition as compared to uncoated Ti-6A1-4V implants. However, it is suggested by these results that the upper limit of gaps around apatite implants is 2 millimeters in a non-weight-bearing model in goats. Bone apposition will not increase by extending the follow-up period more than six weeks, nor will it be altering the stability of the apatite coatings used.

The effect of biological environment on the surface of titanium and plasma-sprayed layer of hydroxylapatite.

The solubility of titanium samples with different surface coatings, i.e., hydroxylapatite (HA) powders, a two-layer coating of ZrO5+HA on a titanium substrate in solution and of tooth implants after long-term functioning in the human organism, was studied. A minimum difference in solubility of titanium samples with different surface finishes (polished or grit blasted) was established. For the HA powders and coatings, the lowest solubility was observed with a coarse-grained HA-B powder and a coating made of that powder. Clinical tests of tooth implants after long implantation times were performed. A titanium implant (implantation 12 y), a titanium implant with a two-layer coating of ZrO5+HA-A (implantation time 4 y) and a titanium implant with a two-layer coating of (Al5O3+3% TiO2)+HA-A (implantation time 6 y) were studied. The results show that the titanium surface and HA-A layers were dissolved. Nevertheless, after 6 y implantation, total removal of HA-A coating from that part of implant set into the bone, was not observed.