Development and characterization of titanium-containing hydroxyapatite for medical applications J. Huang a, * , S.M. Best a , W. Bonfield a , Tom Buckland b a Department of Materials Science and Metallurgy, University of Cambridge, Pembroke Street, Cambridge CB2 3QZ, UK b ApaTech Ltd., 370 Centennial Avenue, Centennial Park, Elstree, Hertfordshire, WD6 3TJ, UK article info Article history: Received 26 February 2009 Received in revised form 29 May 2009 Accepted 29 June 2009 Available online 3 July 2009 Keywords: Substituted hydroxyapatite Titanium Microstructure In vitro bioactivity Human osteoblast cells abstract Hydroxyapatite containing levels of titanium (TiHA) of up to 1.6 wt.% has been produced via a chemical co-precipitation route. The distribution of Ti was seen by transmission electron microscopy/energy-dis- persive X-ray analysis to be uniform throughout as-prepared nanosized TiHA particles (20 nm  100 nm). The incorporation of Ti into the HA structure was found to influence the ceramic microstructure on sin- tering and the grain size was found to decrease from 0.89 lm with HA to 0.63 lm with 0.8 wt.% TiHA (0.8 TiHA) and 0.45 lm with 1.6 wt.% TiHA (1.6 TiHA). Rietveld refinement analysis showed that there was a proportional increase in both the a and c axis with incorporation of Ti into the HA lattice structure, lead- ing to an increase in the cell volume with the addition of Ti. Fourier transform-Raman analysis showed a slight increase in the ratio of O–H/P–O peaks on TiHA, in comparison with HA. A bone-like apatite layer was formed on the surface of TiHA after immersion in simulated body fluid for 3 days, which demon- strated the high in vitro bioactivity of TiHA. In vitro culture with primary human osteoblast (HOB) cells revealed that TiHA was able to support the growth and proliferation of HOB cells in vitro, with a signif- icantly higher cell activity being observed on 0.8 TiHA after 7 days of culture in comparison with that on HA. Well-organized actin cytoskeletal protein was developed after 1 day of culture, and an increase in cell filopodia (attachment) was observed on TiHA sample surfaces. The results indicate that TiHA has great potential for biomedical applications. Ó 2009 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. 1. Introduction Hydroxyapatite (HA) is one of the most extensively used syn- thetic calcium phosphates for bone replacement because of its chemical similarities to the inorganic component of bone and tooth [1,2]. HA, with a chemical formula of Ca 10 (PO 4 ) 6 (OH) 2 and a Ca/P molar ratio of 1.667, shows good stability over time in vivo under normal physiological conditions. Biological apatites are characterized by nanometer-sized crys- tals, poor crystallinity, nonstoichiometry and a variety of ionic (cationic and anionic) substitutions, such as Mg for Ca, CO 3 for PO 4 or OH, F for OH, etc. The type and amount of the ionic substi- tutions in the apatite phase varies from the wt.% level (e.g. 3–8 wt.% CO 3 ) to the ppm–ppb level (e.g. Mg or Sr). The substitution and incorporation of ions affects the properties of apatite, such as lattice parameters, crystal size and crystallinity, which in turn influence the stability and solubility characteristics of HA. The fluoride substitution (F À for OH À ) has the consequence of increasing the crystallinity, crystal size and stability of the apatite, thus reducing its solubility. Its presence in enamel crystals increases stability, which helps it resist dissolution in the acidic oral environment [3]. Aluminum, vanadium and titanium ions were also found to inhibit the dissolution and the formation of HA [4]. Carbonate can substitute for either hydroxyl groups (Type A) or phosphate groups (Type B). An important effect of carbonate sub- stitution in hydroxyapatite is on the crystal size and morphology [5]. An increase in carbonate content leads to changes in the size and shape of the apatite crystal, and carbonate-substituted apatites are more soluble than carbonate-free synthetic apatites. A recent study found that in vitro and in vivo bioactivity was enhanced with the incorporation of silicon into the HA lattice [6– 9]. Silicon-substituted HA (SiHA) is now successfully used as bone graft material in spinal fusion. By replacing SiO 4 4À for PO 4 3À in the HA structure, SiHA was more negatively charged in comparison with stoichiometric HA. The question then arises of which factor plays the dominant role in the bioactivity and biocompatibility of SiHA: the material chem- istry or the surface charge. It is well known that for a bioactive glass, the formation of Si–OH (silanols) on the surface after immersion in physiological 1742-7061/$ - see front matter Ó 2009 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.actbio.2009.06.032 * Corresponding author. Present address: Department of Mechanical Engineering, University College London, Torrington Place, London WC1E 7JE, UK. Tel.: +44 207 679 7183; fax: +44 207 388 0180. E-mail address: jie.huang@ucl.ac.uk (J. Huang). Acta Biomaterialia 6 (2010) 241–249 Contents lists available at ScienceDirect Acta Biomaterialia journal homepage: www.elsevier.com/locate/actabiomat