Conversion of sea urchin spines to Mg-substituted tricalcium phosphate for bone implants

The skeleton of sea urchin spines is composed of large single crystals of Mg-rich calcite, which have smooth, continuously curved surfaces and form a three-dimensional fenestrated mineral network. Spines of the echinoids Heterocentrotus trigonarius and Heterocentrotus mammillatus were converted by the hydrothermal reaction at 180 °C to bioresorbable Mg-substituted tricalcium phosphate (β-TCMP). Due to the presence of Mg in the calcite lattice, conversion to β-TCMP occurs preferentially to hydroxyapatite formation. The converted β-TCMP still maintains the three-dimensional interconnected porous structures of the original spine. The main conversion mechanism is the ion-exchange reaction, although there is also a dissolution–reprecipitation process that forms some calcium phosphate precipitates on the surfaces of the spine network. The average fracture strength of urchin spines and converted spines (β-TCMP) in the compression tests are 42 and 23 MPa, respectively. In vivo studies using a rat model demonstrated new bone growth up to and around the β-TCMP implants after implantation in rat femoral defects for 6 weeks. Some new bone was found to migrate through the spine structural pores, starting from the outside of the implant through the pores at the edge of the implants. These results indicate good bioactivity and osteoconductivity of the porous β-TCMP implants.