J. G. C. Wolke

Formation and characteristics of the apatite layer on plasma-sprayed hydroxyapatite coatings in simulated body fluid

Plasma-sprayed hydroxyapatite (HA) coatings were incubated in simulated body fluids (SBFs) for different periods of time to investigate the nucleation and growth of apatite on their surface. The layer that formed was recognized as having similarities to bone apatite because it is poorly crystallized, non-stoichiometric or calcium deficient, and contains carbonate and magnesium. Scanning electron microscopy (SEM) and infrared spectroscopy (IR) were employed to investigate the morphological changes of the coating surface and the structure of the grown layer respectively. In the first few hours, calcium and phosphate ions dissolved from the coatings so as to increase their local supersaturation to a higher degree, thereafter followed by the nucleation and growth of apatite. The nucleation occurred firstly on the recessed regions, inside pores and cracks where the higher supersaturation was readily maintained. Only after 24 h incubation was a complete layer formed on the surface of the coating. There is no obvious interface between the grown layer and the underlying coating. Heat treatment in the air made the apatite transform into biphasic calcium phosphate of HA and tricalcium phosphate, with a blue colour because of trace manganese ions. The heat-treated HA coating showed no dissolution by SEM observation. This resulted in no precipitation on the surface. When SBF was used with two-fold higher ion concentrations, the apatite layer formed slowly in 72 h without dissolution of the coating surface. This may mean that the microenvironment with a sufficiently high degree of supersaturation of calcium and phosphate ions is crucial for apatite to nucleate and grow in SBF, while the HA crystalline structure is not critical in the nucleation process, as expected.

Influence of annealing temperature on RF magnetron sputtered calcium phosphate coatings

The effect of different annealing temperatures on the characteristics of thin calcium phosphate coatings fabricated by radiofrequency magnetron sputtering was studied. Annealing of the as-sputtered films was necessary to change the amorphous coating to a crystalline coating. The films were annealed for 2 and 4 h at 400, 600, 800, 1000 and 1200 degrees C under dry argon or argon and water vapour flow. After annealing, the structure and the chemical composition of these films were characterized with incident light microscopy, Rutherford backscattering spectrometry (RBS), X-ray diffraction (XRD), and Fourier transform infrared absorption spectrometry (FTIR). Incident light microscopy showed cracks in the coatings annealed at a higher temperature than 400 degrees C. RBS revealed that the as-sputtered coatings had a high Ca/P ratio which decreased with increasing annealing temperature. After annealing at a temperature of 600 degrees C or more the XRD showed crystalline hydroxyapatite (HA) coatings. However, the second phase, present in the coatings, changed from tetra-calcium phosphate to calcium oxide to beta-tri-calcium phosphate with increasing annealing temperature. FTIR measurements showed the existence of OH- and PO- bonds in all coatings, although the PO- bonds varied for different annealed coatings, from the PO- bonds due to HA to PO- bonds due to other calcium phosphates. From the results of this study we suggest that 600 degrees C is probably the best annealing temperature to obtain a better characterization and understanding of the coating.

Fast precipitation of calcium phosphate layers on titanium induced by simple chemical treatments.

A simple two-step chemical treatment, i.e. etching with HCl and H2SO4 followed by immersion in boiling dilute NaOH solution, has been developed by our group to prepare bioactive microporous titanium surfaces allowing fast deposition of a calcium phosphate layer (CPL) from an in vitro supersaturated calcification solution (SCS). In this work, a precalcification (Pre-Ca) procedure was applied by soaking the two-step treated titanium in Na2HPO4 and then saturated Ca(OH)2 solution before immersion in SCS to accelerate further the CPL precipitation. The treated titanium surfaces with Pre-Ca were characterized after 1, 2, 4, 8 and 16 h of immersion in SCS by means of scanning electron microscopy together with energy dispersive X-ray analysis, X-ray diffraction and infrared absorption analysis. It was observed that the CPL precipitation rate with Pre-Ca averaged 1 microm h-1, twice as fast as without Pre-Ca. No precipitation was observed on untreated titanium with Pre-Ca up to day 14 of immersion in the SCS.

Long-term in vivo study of plasma-sprayed coatings on titanium alloys of tetracalcium phosphate, hydroxyapatite and α-tricalcium phosphate

Abstract In order to study the interaction of calcium phosphate coatings with bone tissue, coated titanium plugs of standard size were implanted in dog femora. The bone bonding and bone formation of hydroxyapatite, α-tricalcium phosphate (α-TCP) and tetracalcium phosphate plasma-sprayed coatings were evaluated by mechanical push-out tests and histological observations after 3, 5, 15 and 28 months of implantation. During this time all coating types degraded. α-TCP showed the most significant degradation after 3 months of implantation. Hydroxyapatite and tetracalcium phosphate showed significant signs of degradation after about 5 months of implantation. All coatings showed a small increase in bone bonding after 5 months of implantation. In general, all types of implants showed similar bone response, some bone contact and several remodelling lacunae along the surfaces after long-term implantation.

In vivo dissolution behavior of various RF magnetron-sputtered Ca-P coatings on roughened titanium implants

RF magnetron sputter deposition was used to produce 0.1, 1.0 and 4.0 microm thick Ca-P coatings on TiO(2)-blasted titanium discs. Half of the as-sputtered coated specimens were subjected to an additional infrared heat treatment for 30s at 425-475 degrees C. X-ray diffraction demonstrated that infrared radiation changed the amorphous 4 microm sputtered coatings into an amorphous-crystalline structure, while the amorphous 0.1 and 1 microm changed in a crystalline apatite structure with the presents of tetracalciumphosphate as a second phase. Scanning electron microscopically examination of the sputtered coatings revealed that annealing of the 4 microm thick coatings resulted in the appearance of small cracks. Subsequently, the discs were implanted subcutaneous into the back of rabbits. After 1, 4, 8 and 12 weeks of implantation, the implants were retrieved and prepared for histological and physicochemical evaluation. Histological evaluation revealed that the tissue response to all coated implants was very uniform. A very thin connective tissue capsule surrounded all implants. The capsule was usually free of inflammatory cells. At the interface, there was a close contact between the capsule and implant surface and no inflammatory cells were seen. Physicochemical evaluation showed that the 0.1 and 1 microm thick amorphous coatings had disappeared within 1 week of implantation. On the other hand, the 4 microm thick amorphous phase disappeared during the implantation periods, which was followed by the precipitation of a crystalline carbonate apatite. Further, at all implantation periods the heat-treated 1 and 4 microm thick coatings could be detected. Occasionally, a granular precipitate was deposited on the heat-treated 4 microm thick coating. Fourier transform infrared spectroscopy showed the formation of carbonate apatite (CO(3)-AP) on the 4 microm thick amorphous coating and on the heat-treated specimens. On basis of our findings, we conclude that 1 microm thick heat-treated Ca-P sputter coating on roughened titanium implants appear to be of sufficient thickness to show bioactive properties, under in vivo conditions.

In vivo dissolution behavior of various RF magnetron sputtered Ca‐P coatings

Radiofrequency magnetron sputter deposition was used to deposit Ca-P sputter coatings on titanium discs, and these coatings were implanted subcutaneously into the backs of rabbits. Half of the as-sputtered coatings were subjected to additional heat treatment for 2 h at 500 degrees C. X-ray diffraction (XRD) demonstrated that annealing at 500 degrees C changed the amorphous sputtered coating into an amorphous-crystalline apatite structure. Scanning electron microscopic (SEM) examination of the sputtered coatings showed excellent coverage of the substrate surface. Annealing of the 4-microm-thick coatings resulted in the appearance of small cracks. SEM demonstrated that until 4 weeks of implantation, all heat-treated coatings were present and all amorphous coatings were completely or mostly dissolved. Fourier transform infrared spectroscopy showed the formation of carbonate apatite (CO3-AP) on these specimens. Furthermore, XRD analysis showed that these CO3-AP precipitated coatings disappeared after 8 weeks of implantation. On the other hand, SEM inspection of these specimens revealed that the 4-microm heat-treated coating was still partially maintained and that small Ca-P crystals were present on the titanium substrate. On the basis of these results, we conclude that apparently 0.1 microm heat-treated Ca-P sputter coating is of sufficient thicknesses to stimulate carbonate apatite deposition under in vivo conditions.

Evaluation of hydroxylapatite/poly(l-lactide) composites: physico-chemical properties

The aim of this in vitro study was to examine the physico-chemical behaviour of hydroxylapatite/poly(l-lactide) (HA/PLLA) composites in solution tests. The polymer PLLA, the composites 30 wt% HA/PLLA (C30) and 50 wt% HA/PLLA (C50) and a one-side HA-coated PLLA (HAcP) were evaluated. Rectangular specimens were incubated in various acellular aqueous buffer solutions [citrate, Gomoris and phosphate-buffered saline (PBS)] up to 24 weeks. Data for cumulative release of calcium, phosphate and l-lactate release in solutions containing C30 or C50 showed linear patterns. Release data for solutions containing HAcP combined with scanning micrographs, X-ray microanalysis and X-ray diffraction patterns of the specimens in time showed that the plasma-sprayed HA coating on PLLA dissolves significantly, progressively in the first weeks and almost completely within the tested period of 24 weeks in vitro. A precipitate of scaly crystallites (calcium phosphates) was observed at the HA coating-PBS interface. After 24 weeks incubation all materials were still above their initial weight, indicating that swelling still exceeded dissolution. Application of C30, C50 and HAcP as implant materials seems interesting where initial stabilization through bone bonding is needed or where the linear release of constituents is a requirement. HAcP has the advantage that the HA coating acts as a hydrolysis barrier and consequently delays the degradation of PLLA in vitro.

Studies on the thermal spraying of apatite bioceramics

Hydroxylapatite (HA) coatings on metal substrates have been investigated for many years. These coatings have proved to be compatible with bone. The degree of crystallinity of HA changed, and sometimes dissociation was observed with respect to the plasma spray process. However, the plasma spray process hardly altered the crystallographic structure, with only line broadening visible. Thein vitro solubility is dependent on the degree of crystallinity of the coating. Tensile strength measurements on the strength of the coating-substrate interface using various adhesives revealed a significant difference between epoxy resin and methacrylate. The failure mode of this tensile test was dependent on the coating thickness and surface texture (polished versus nonpolished). In animal studies, the fixation of hydroxylapatite plasma- spray coated cylinders as well as noncoated Ti- 6A1- 4V cylinders (Ti) in cortical bone was evaluated using pushout tests. It appeared that HA- coated implants showed higher push- out strengths in the first months than the titanium implants, because of the earlier bone formation against the HA coating.

Microstructure and crystallinity in hydroxyapatite coatings

Plasma spraying was used to produce hydroxyapatite (HA) coatings on metal. The microstructure of these coatings, sprayed with two powder particle size distributions, was examined by scanning electron microscopy (SEM) and showed scattered HA particles with completely melted and unmelted or partially melted cores. Röntgen diffraction shows that the crystallinity increases after a vacuum heat treatment at 600 degrees C. SEM reveals that the amorphous phase recrystallized and new crystalline grains were formed at the surface of the crystalline cores.

Influence of Ar pressure on r.f. magnetron-sputtered Ca5(PO4)3OH layers

Abstract Currently, medical and dental implants are often provided with thin calcium phosphate ceramic coatings (preferably Ca 5 (PO 4 ) 3 OH (HA)) to improve their biological behaviour. Previous studies have demonstrated that radio-frequency (r.f.) magnetron sputtering is a suitable technique for depositing such thin Ca−PO layers. Although X-ray diffraction showed that the deposited films had an HA structure, the Ca/P ratio of the layers was higher than the theoretical value of 1.67 for HA. In the present study the influence of the argon pressure and input power on the structure and chemical composition of the sputtered layers, especially the Ca/P ratio, is investigated. Rutherford backscattering spectrometry (RBS), stylus profilometer (alpha-step), X-ray diffraction spectrometry (XRD), and IR spectrometry (FTIR) were used to characterise the coatings. Although the Ca/P ratio became lower when sputtered at lower input power, it was still higher than the value of 1.67 for HA. The density measured for the films sputtered at 200 and 400 W did not show a simple relation with the argon pressure. For all the films after annealing XRD and FTIR showed an HA-like structure and HA bonds. However, the OH bond appeared to be dependent on the partial pressure of water vapour during sputtering.