Hydroxylapatite coatings on ZrO2 and Al2O3 ceramics by bio-mimetic method

author: Irena Pribošič, Condensed Matter Physics, Jožef Stefan Institute
published: Feb. 12, 2008,   recorded: October 2007,   views: 765
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Description

The materials that can be used for the weight-carrying bone implants are: titanium, Al2O3 or ZrO2 ceramics. All those materials, with appropriate mechanical properties are bio-inert. When bio-inert material is implanted into the living body, a fibrous capsule is developed to isolate the implant from the surrounding tissue. When a bio-inert implant is loaded such that interfacial movement can occur (implantation of a hip or a knee), the fibrous tissue can become very thick and the implant loosens quickly. The problem can be solved by use of bio-active materials (bio-active glasses, glass-ceramics composites, hydroxylapatite, etc.). These materials stimulate the growth of the bone or the soft tissue directly on their surface and are strongly bonded to the surrounding tissue. However, the low mechanical strength of the known bio-active materials does not permit these materials to be used as weight-carrying bone implants. An alternative for weight-carrying bone implants is the use of bio-active coatings on bio-inert material with a high mechanical strength. The plasma coating of the hydroxylapatite is mostly used. Still, there are some problems connected to this method: it is difficult to control the thickness of the coating and the compounds formed during the process; due to the high temperature during the synthesis the biological active molecules cannot be incorporated into the coating. With the use of bio-mimetic methods, which imitate the crystallization process of the bone, the monophase hydroxylapatite coating with a uniform thickness can be prepared. Using this method we can also incorporate the biologically active molecules, e.g., proteins that stimulate the bone formation. Compared to plasma spraying the bio-mimetic methods are also much cheaper. When the coated implants are used the adhesion between the implant and its coating becomes very important, which is the main problem with the use of the bio-mimetic method. The bio-mimetic methods are based on the soaking of the implant in a supersaturated solution of calcium and phosphate ions. Besides other solutions the simulated body fluid (SBF) and its higher concentrations are most commonly used. In our work the coatings of bio-active calcium phosphate were prepared on the surface of ZrO2 and Al2O3 ceramics using, from the literature already known, Ca/P supersaturated solution with the ion concentrations: Na+ 25.5, Ca2+ 2.5, Cl- 5.0 HCO3- 18.0 and PO43- 2.5 mM. The hydroxylapatite coating on the surface of Al2O3 ceramic, which is identical on ZrO2 ceramic. The adhesion between the coating and the ceramic substrate was improved by a heat treatment at 1050 °C for 1 hour. Before and after the heat treatment the coating was analysed by XRD and a TEM equipped with an EDS detector and a parallel EELS spectrometer. In order to minimise any possible microstructural changes due to the electron beam irradiation in the TEM the cooling holder was used. All the EDS and EELS measurements were carried out when the temperature in the cold stage was stabilized (approximately 100 K). After precipitation the coating was composed polycrystal plate-like particles, orientated perpendicularly to the substrate surface. Each individual plate-like particle was composed of poorly crystalline elongated nano crystals. The heat treatment increased the coating’s crystallinity and particles grow isotropic up to 200 nm. Using XRD, EDS and EELS, the crystal structure was determined to be apatite. The simple in-vitro bio-activity test using SBF solution proved the bio-activity of the “as-prepared” coatings and after a heat treatment as well.
After soaking the coated substrates in the SBF solution for 7 days the layer of hydroxylapatite was formed on the coated surface of Al2O3 and on ZrO2 ceramics. The bio-mimetic method of soaking in the Ca/P solution is simple, fast and with the use of it we produced the uniform bio-active coating on the bio-inert material, which can be used as weight-carrying bone implants.

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