Coralline apatites can be derived from sea coral. Coral is composed of calcium carbonate in the form of aragonite. As coral is a naturally occurring structure and has optimal strength and structural characteristics. The pore structure of coralline calcium phosphate produced by certain species is similar to human cancellous bone, making it a suitable material for bone graft applications (Figure 1).
Figure 1. Comparison of an Australian coral (a) in original state and (b) after hydrothermal conversion.
Coralline Implants in Clinical Applications
Coral and converted coralline hydroxyapatite have been used as bone grafts and orbital implants since the 1980s, as the porous nature of the structure allows in-growth of blood vessels to supply blood for bone, which eventually infiltrates the implant.
Pore interconnection sizes are of utmost importance when hard and soft tissue in-growth is involved. Kühne et al., showed that implants with average pore sizes of around 260 µm had the most successful in-growth as compared to no implants (simply leaving the segment empty). It was further reported that the interaction of the primary osteons between the pores via the interconnections allows propagation of osteoblasts.
Hydrothermal Conversion of Coralline Apatite to Hydroxyapatite
The hydrothermal method was first used in 1974, for hydroxyapatite formation directly from corals by Roy and Linnehan. It was reported that complete replacement of aragonite (CaCO3) by phosphatic material was achieved less than 533 K and 103 MPa by using the hydrothermal process. In 1996, hydroxyapatite derived from Indian coral using hydrothermal process was reported. However, the resultant material was in the form of a powder and required further forming and sintering.
During the hydrothermal treatment hydroxyapatite replaces the aragonite whilst preserving the porous structure. The following exchange takes place:
The resulting material is known as coralline hydroxyapatite, whether in the porous coralline structure or in powdered form.
Microwave Conversion of Coralline Apatite to Carbonated Hydroxyapatite
Aragonite to carbonate hydroxyapatite was achieved by using microwave processing technique. Higher extents of conversion were reported.
Hu et al., succeeded to convert Australian coral to monophasic hydroxyapatite by using a two-stage process where the hydrothermal method was followed by a patented hydroxyapatite sol-gel coating process based on alkoxide chemistry. They reported 120% increase in the biaxial strength of the double-treated coral in comparison to only converted one.