Surface modification of bioceramics: chemically enhanced laser surface microstructuring of hydroxyapatite

Abstract

Bioceramics have been developed for implants to repair damaged tissues of the human musculo-skeletal system. The clinical success of a bioceramic implant depends largely on the chemical response at the implant interface in addition to the sufficiency of the mechanical properties for the application. The present study combines the developments in the fields of bioceramic materials and laser surface micro structuring of materials. Bioceramic hydroxyapatite powders (HA, Formula: CaIO(PO4)6(OH)2) have been produced by emulsion technology and freeze-drying methods exhibiting BET specific surface areas >148 m2 /g and particle sizes <13 nrn prior to thermal treatment. The powder yield has been doubled using an increased reaction temperature of 25 *C from 17 *C, with a small increase (< 4nm) in the average particle size. HA discs that were >95.5 % dense have been achieved after isostatic pressing with pressure of 0.59 MPa and pressurelesss intering at 1200 *C for 2 hours. No chemical decomposition was detected using X-ray Diffraction Analysis (XRD). Methods of chemically enhanced laser-assisted etching have been developed to produce microstructural features on the surface of bioceramic HA discs that were 78.5 % dense (2482.95 kr , /M3 measured density). The use of 10 MPa SF6 at laser fluencies in the range of 14.50-15.20 W/M2 produced a columnar topography with individual structures featuring 10-20 pm height and 8-12 pm width as characterised by Scanning Electron Microscopy (SEM). Chemical characterisation by X-ray microdiffraction, Energy Dispsersive, X-ray analysis (SEM-EDS), Fourier Transformed Infrared spectroscopy (FTIR) and Raman spectroscopy (Raman) found the microtopography to be composed of fluorine-substituted HA (FHA). Alternatively the use of 80 MPa NH3 at laser fluencies in the range of 17.17-18.50 kj/m 2 produced an irregular surface of scattered porous hillocks that remained chemically unchanged in composition but exhibited four times as many surface hydroxyl groups. In both cases the mechanical and chemical stability of the bulk composition is maintained and the surface of increased surface area, in addition to the presence of concavities and pores is likely to be of enhanced osteoconductivity.