STRUCTURAL INVESTIGATION OF ELECTRODEPOSITED HYDROXYAPATITE ON TITANIUM SUPPORTS*

BRANDUŞA GHIBAN, GABRIELA JICMON, GEORGETA COŞMELEAŢA

“Politehnica” University of Bucharest

Received December 21, 2004

The present paper presents results concerning electrodeposition hydroxyapatite on titanium supports. Two methods are used for deposition of hydroxyapatite:electrolytic deposition (ELD), using solutions as electrolytes and ions as charge carriers and electrophoretic deposition (EDP), using suspensions of powder that needs to be deposited in organic or aqueous organic media. Samples of hydroxyapatite were treated at 300°C/1 hour. Deposited samples were characterized by optical and scanning electron microscopy, microhardness and surface toughness.

1. INTRODUCTION

Biomaterials have received much interest and intensive research during the last few decades, due to their obvious use as replacements of various body parts or even organs. Their use and the improvement of their reliability and life span would undoubtedly improve the quality of human life in more than one aspect.

The prosthetic devices commonly used to replace human bone are currently obtained from biocompatible metals and alloys, such as stainless steel, Ti and its alloys with Al, Fe and V, gold, platinum, CoCr and so on. These metals are used mainly due to their mechanical properties (mechanical resistance, Young’s modulus, corrosion resistance), however they do not exhibit any biointegration properties, meaning that the human organism accepts these implants only by isolating them, and sometimes rejection reactions occur.

The use of bulk ceramic biocompatible materials, such as zirconia, calcium phosphates and mainly hydroxyapatite (Ca5(PO4)3OH) is limited to very small implants, without being subjected to high loads, due to the low values of their mechanical properties, especially fracture toughness and elasticity modulus (too high).

* Paper presented at the 5th International Balkan Workshop on Applied Physics, 5–7 July 2004, Constanţa, Romania.

Rom. Journ. Phys., Vol. 51, Nos. 1–2, P. 187–196, Bucharest, 2006

Branduşa Ghiban, Gabriela Jicmon, Georgeta Coşmeleaţa 2 188

The next step in improving the bone implants properties is the coating of metallic implants, having good mechanical properties, with thin biocompatible ceramic films, in order to improve the osteointegration of these implants. The main problem of these coatings is raised by their poor adhesion to the metallic substrates, due to the sometimes large difference between their thermal coefficients, causing the appearance of tensile stresses at the interface. This causes the coatings obtained by high-temperature processes to fail either during the mounting procedure, or during service in the human body.

The commercial processes of obtaining these ceramic coatings are: the dip coating method, the sol-gel process, pulsed laser deposition and, most of all, the plasma spraying process, used for a wide range of materials.

One of the most interesting alternatives of obtaining these biocompatible coatings is by electrodeposition. This process is developed at room temperature, and has many advantages over other, more commercial deposition processes, such as: uniformity of the deposited layer, availability of various substrate shapes other than plates, good control of the deposition thickness and quality, low energy consumption and also the fact that it is an environmentally friendly process.

2. MATERIALS AND EXPERIMENTAL PROCEDURE

Among the electrodeposition processes, we can distinguish two different methods: electrolytic deposition (ELD), using solutions as electrolytes and ions as charge carriers, and electrophoretic deposition (EPD), using suspensions of the powder that needs to be deposited in organic or aqueous-organic media. This last method is used for high-molecular-weight species, difficult to obtain by ionic transfer, and for polymeric species.

Samples HAP 1 – 6 consist of titanium substrates deposited by EPD with hydroxyapatite powders. The variables of these samples are: the electrodeposition media, time, current density and substrate preparation.

These samples have been thermally treated in the METLAB laboratory at a temperature of 3000C for 1 hr. and subsequently characterized by optical and scanning electron microscopy, microhardness and surface roughness.

3. EXPERIMENTAL RESULTS AND INTERPRETATION

The sintered samples of HAp-deposited titanium have been analyzed by optical microscopy, and micrographs have been taken of both the surface and the cross-section of these.

The surfaces of the 6 samples have different appearances, from very smooth, small-pored aspects to rough, island-like ones. These differences are mainly due to the different deposition conditions of the samples.

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Fig. 1 – Sample 1 – surface.

Fig. 2 – Sample 5 – surface.

In the case of good-quality deposits, micrographs of higher resolution have also been performed, in order to observe the structure of the coatings.

The higher-resolution micrographs have shown the different types of coating structures, ranging between microporous, continuous layers, to large open-porosity layers, lacking micropores.

The samples have been cut transversally, mounted in hot mounting resin and subsequently grinded and polished up to a 1 µm surface finish, using Struers polishing cloths and 1-µm diamond paste. Micrographs of these samples have then been obtained.

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Fig. 3 – Surface of sample 1 (x500).

Fig. 4 – Surface of sample 6 (x500).

The cross-section micrographs have evidenced continuous, thin (~5µm) layers of hydroxyapatite deposited on the titanium substrate, proving that the electrophoretic deposition process produces constant-thickness deposits on substrates, regardless of the substrate shape or surface profile.

In order to better understand the difference between the hydroxyapatite deposits obtained by EPD and by other commercial processes, such as plasma spraying, a sample of hydroxyapatite deposited by plasma spraying on titanium has been cut, resin-mounted and polished up to a 1 µm surface finish. A micrograph of this sample has been obtained and compared with an EPD sample micrograph. The plasma – deposited coating is highly discontinuous; it has a large porosity, much larger thickness (~50–60 µm) than the EPD coating (~5 µm), which varies along

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the interface. Also, the adherence of this deposit is poorer than the EPD coating, due to the high-temperature process by which it has been obtained and the consequent internal stresses that occurred during the cooling stage.

Fig. 5 – Cross-section of sample 1.

Fig. 6 – Cross-section of sample 6.

A scanning electron microscopy analysis has been performed on several of the electrodeposited samples, together with elemental analysis. This has provided supplementary information regarding the chemical composition and the morphological structure of the deposits. The structure has a sponge-like appearance, being continuous and not agglomerated; the deposited grains have

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bounded during the sintering process, and the layer has little or no evidence of cracks. The data available in literature shows that this sponge-like structure is favorable to good osteointegration of the implant, due to the easy conversion of the HAp structure to bone structure by osteoblasts.

The chemical elemental analysis shows the presence of Ca and Ti peaks, and thus the calcium phosphates composition of the deposited layer on the Ti substrate. The phosphorus peaks are not present, probably due to their overlapping with Au peaks (Au was used as a conductive film on the sample surface, due to the coating’s oxide, insulating nature).

Fig. 7 – Sample of HAp on Ti deposited by plasma.

Fig. 8 – Sample of HAp on Ti (no 6) deposited by EPD.

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The cross-section micrograph performed on the same sample shows the thickness of the coating, and also a crack at the deposit-resin interface, probably caused by the sample preparation procedure (polishing). The chemical analysis performed shows TiO2 on the substrate and calcium phosphates on the coating layer, as expected.

Fig. 9 – Coating structure of sample HAP 6.

Fig. 10 – Cross-section SEM of sample HAP 6.

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Microhardness tests have been performed on the surface of the samples, using a Vickers microindentor with loads of 100g and load time of 5 seconds. Samples 3-5 could not be tested due to the low adhesion of the coatings and low hardness. The values of the other coatings are good, and come very close to the hardness values for the bulk materials.

Fig. 11 – Microhardness of the EPD samples.

The roughness of the samples has also been measured, using a Mituyoto Surftest surface roughness tester. The values of the roughness parameter Ra (average value of the roughness) are presented in the subsequent table.

Fig. 12 – Surface roughness of the EPD samples.

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These values show that samples 3-5 have a higher surface roughness than the others, meaning a higher porosity and discontinuities on the coating surface of the samples. The island-like appearance of these deposits, caused by the deposition parameters, cause a high roughness and also small hardness values, as noted earlier.

CONCLUSIONS

Although the analyzed samples have provided various results, several tendencies are notable: samples 1, 2 and 6, deposited on electrochemically oxidized titanium substrates, show a better adhesion and improved structure of the coating layer. The other samples (3–5), which were obtained on clean, non-oxidized titanium substrates, have island-like deposits, probably due to the spalling of the deposited ceramic particles during the deposition process. The monoatomic interfacial layer of TiO2 obtained on the Ti substrates for samples 1, 2 and 6 improves the adhesion of the ceramic particles during electrodeposition, and favors the oxygen diffusion during the sintering of the samples. Moreover, since the thermal coefficient of TiO2 is closer to the one of the ceramic deposit, it acts as a buffer layer and thus a smoother gradient of the dilatation coefficient along the cross-section of the sample is obtained, reducing the internal stresses at the ceramic/metal interface.

The electrical insulating properties of the ceramic deposit causes the increase of the electrical resistivity through the sample during electrodeposition, and thus a lower current intensity in the electrochemical cell. As a result, the current passes through the areas with the least electrical resistivity, namely with the thinnest ceramic layer deposited, therefore the ceramic layer grows with a uniform thickness throughout the surface, without being influenced by the substrate shape.

The electrophoretic deposition of biocompatible and bioactive ceramics on metallic substrates is a step in the direction of the improvement of bone and dental implants, mainly of the adhesion of the coating on the substrate and therefore of the osteointegration and lifespan of the implant in the human body.

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