From the chemical point of view bones consist of 60-65 wt% of calcium phosphate. Therefore synthetic compounds of calcium phosphate are suitable bone replacement materials. Compounds that are already widely used for bone replacement are for example alpha-tricalcium phosphate (α-TCP), beta-tricalcium phosphate (ß-TCP) and hydroxyapatite (HA) as well as composites of ß-TCP and HA so called biphasic calcium phosphates (BCP). These materials are prepared by high temperature solid state reactions yielding in compact, highly crystalline structures which form crystallite sizes of several micrometers. However, compared to natural bone mineral, conventionally prepared sintered porous or non-porous hydroxyapatite implant materials exhibit much lower resorption rates under in vivo conditions. For clinical applications the bone replacement material has to act as a temporary substitute that can be resorbed by cellular mechanisms and dissolution processes so that a simultaneous ingrowth of natural bone into the defect site is possible.
A substancial progress in developing fast-resorbing calcium phosphates was achieved by the synthesis of nanocrystalline apatites having crystallite sizes of less then 200 nm in dimension. These bone replacement materials show a high similarity in their chemical composition and morphology to native bone. The small crystallites possess a high surface area to volume ratio, which leads to an increased solubility and resorbability compared to sintered HA.
We are able to synthesize nanocrystalline HA and nanocrystalline carbonated hydroxylapatites (CHA) in a cost-effective reproducible process (precipitation reaction) in a large scale. By varying the reaction conditions such as temperature and time we are able to synthesize well-defined crystallites in a broad size range (15-50 x 30-150 nm, width x length). An additional incorporation of carbonate ions into the crystal lattice of hydroxylapatite leads to a further decrease of the crystallite sizes and to an increased solubility.
The phase composition, lattice constants and crystallite sizes were determined by X-ray diffraction analysis, further characterization was performed by IR spectroscopy, ICP, elemental composition, SEM and TEM.
Dr. Matthias Schnabelrauch