STRONTIUM-SUBSTITUTED HYDROXYAPATITE THIN FILMS GROWN BY PULSED LASER DEPOSITION C. CAPUCCINI1*, F. SIMA2, E. AXENTE2, E. BOANINI1, M. GAZZANO3, A. BIGI1, AND I.N. MIHAILESCU2 1Biomimetics and Materials Chemistry Laboratory, Department of Chemistry, Bologna, ITALY 2Laser-Surface-Plasma Interactions Laboratory, Lasers Department, National Institute for Lasers, Plasma and Radiation Physics, PO Box MG-54, Bucharest-Magurele, ROMANIA 3ISOF-CNR, c/o Department of Chemistry “G. Ciamician”, Bologna, ITALY Abstract Strontium substitution for calcium in the hydroxyapatite structure has lately attracted growing interest due to its beneficial effects on both bone formation and prevention of bone resorption. Coating Ti implants with Sr2+-substituted hydroxyapatite is expected to enhance the bioactivity of the surface and stimulate bone apposition. To this end, we deposited thin films of hydroxya-patite with different substitutions of Sr2+ for Ca2+ on Ti substrates by Pulsed Laser Deposition (PLD). Solid solutions of Sr-Ca hydroxyapatites ?Ca10-xSrxHA (x = 01)? were prepared by direct synthesis in aqueous medium at 90C. Sr2+ insertion led to a decrease of crystallinity degree, which accounted for the simultaneous reduction of the crystal dimensions. For PLD experiments, we used an UV excimer (KrF*) laser source (248 nm, 7.4 ns) operating at a repetition rate of 2 Hz. The fluence during target irradiation was set at 2.4 J/cm2, and substrate temperature kept at 400C. The depositions were per-formed from HA at different degrees of Sr2+ substitution for Ca2+ (x = 0; 0.1; 0.5; 1). All structures were post-treated in a H2O enriched atmosphere for 6 h. The results of structural and morphological characterizations carried out on the obtained structures indicated that the coatings, which adhered well to the substrates, were made of crystalline HA and contained strontium with a (Ca + Sr)/P molar ratio close to the stoichiometric value of HA. _ * To whom correspondence should be addressed: C. Capuccini, email: chiara.capucciniunibo.it A. Vaseashta and I.N. Mihailescu (eds.), Functionalized Nanoscale Materials, Devices and Systems. Springer Science + Business Media B.V. 2008 389C. CAPUCCINI ET AL. Keywords: Isomorphous substitution, strontium, hydroxyapatite, PLD 1. Introduction Apatites are widely spread in nature; in particular, the inorganic phase of the hard tissues of vertebrates can be assimilated to the synthetic hydroxyapatite Ca10(PO4)6(OH)2, CaHA. The similarity with biological apatites accounts for the high biocompatibility of synthetic CaHA. This bioactive ceramic material does not possess acceptable mechanical properties, as it is brittle in bulk. However, it does demonstrate significant potential for use as a coating on metallic ortho-paedic and dental prostheses.1 At present, titanium and its alloys due to their excellent mechanical and biomedical properties are the most widely used materials for the production of metal implants.2 The presence of a CaHA coating improves osteointegration and creates a barrier to the release of metallic elements from the implant. Pulsed laser deposition has proved to be a compete-tive technique for growing thin calcium phosphate structures on metallic sub-strates. PLD utilizes a short, generally UV pulsed laser beam that is focused onto a rotating target placed inside a reaction chamber, where a controlled atmosphere can be maintained. The species that are expulsed by each subse-quent laser pulse form the coating as they reach the substrate, which can also be heated to a fixed temperature. The stoichiometry and crystallinity of the deposited material can be selected by a proper choice of the ablation and deposition parameters.37 The high stability and flexibility of the hydroxyapatite structure justify the wide variety of possible ionic substitutions.6 Among the bivalent cations that can replace calcium in CaHA, strontium has attracted a remarkable interest for its potential biological role. Strontium is present in the mineral phase of the bone, especially in the regions of high metabolic turnover,8 and its beneficial effect in the treatment of osteoporosis is well known.9 In vitro, strontium pro-motes the proliferation of osteoblasts and decreases the number and activity of osteoclasts10,11; in addition, strontium administration reduces bone resorption and stimulates bone formation.1214 Strontium can replace calcium in the HA struc-ture over the whole range of composition. The solid solutions that have been obtai-ned by hydrothermal methods or by treatment at high temperatures, display a linear variation with composition in the lattice parameters, whereas different data have been reported on the preferential substitution site of Sr for Ca in CaHA.1517 To better clarify the interaction with HA structure of Sr, we previously synthesized and characterized Sr-Ca-HA solid solutions across the whole range of concentrations and proved that Sr when in low concentration showed an unexpected pr