Microstructure and Macroscopic Properties of Bioactive CaO–SiO 2 –PDMS Hybrids A. J. Salinas, 1 J. M. Merino, 1 F. Babonneau, 2 F. J. Gil, 3 M. Vallet-Regı´ 1 1 Dept. Quı ´mica Inorga ´ nica y Bioinorga ´ nica, Facultad de Farmacia, Universidad Complutense, Madrid 28040, Spain 2 Chimie de la Matie ` re Condense ´ e, UPMC/CNRS, UMR 7574, 4 place Jussieu, 75252 Paris cedex 05, France 3 Dpt CC Materiales e Ingenierı´a Metalu ´ rgica Universidad Polite ´ cnica de Catalun ˜ a, Barcelona, Spain Received 22 December 2005; revised 26 May 2006; accepted 6 June 2006 Published online 14 November 2006 in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/jbm.b.30663 Abstract: CaO–SiO 2 –PDMS (polydimethylsiloxane) hybrid materials were synthesized as crack- free monoliths presenting in vitro bioactivity, i.e. able to be coated with a calcium phosphate-rich layer after having been soaked in simulated body fluid (SBF). A wide physical–chemical characterization of these materials was carried out to relate their microscopic structure and macroscopic properties. The effect of PDMS and the amounts of water used for the tetraethoxysilane (TEOS) hydrolysis on the mechanical properties of hybrid materials was investigated by three-point bending tests. For a given amount of water, as PDMS content in hybrids increased, the elastic modulus decreased. Furthermore, keeping the PDMS content constant, when the amount of H 2 O decreased, the elastic modulus increased. Regarding in vitro bioactivity and mechanical properties, the hybrid material obtained with molar ratios H 2 O/TEOS = 2 and TEOS/PDMS = 3.5 proved to be the best candidate for either soft tissue substitution or metallic implant coating since the hybrid material would promote bond to bone formation, simultaneously dampening the mechanical charges. ' 2006 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater 81B: 274–282, 2007 Keywords: bioactive material; organic–inorganic hybrids; CaO–SiO 2 –PDMS system; in vitro tests; microstructure INTRODUCTION Materials able to bond with living tissues when implanted are usually denoted as ‘‘bioactive’’ and have been widely studied because of their potential use for repairing living tis- sues. 1 When in contact with physiological fluids, these mate- rials are coated with a carbonate hydroxyapatite (CHA) layer that favors the colonization by osteoblasts, followed by the proliferation and differentiation of cells to form new bone, that bonds the implant surface and the living tissues. 2 The apatite-like layer is also formed in vitro, when bioactive materials are soaked in a solution mimicking blood plasma. 3 Over the last decade, a growing interest was devoted towards the synthesis of bioactive organic–inorganic hybrid materials with mechanical properties close to those of con- nective tissues. With this purpose, SiO 2 –PDMS (polydime- thylsiloxane) organic–inorganic hybrid materials, exhibiting unusual rubber-like mechanical properties, 4 were modified with amounts of CaO to induce in vitro bioactivity. 5 In the last few years, aspects of the synthesis of bioactive CaO– SiO 2 –PDMS hybrids were studied, including the effect of precursor solution composition on the bioactivity and me- chanical properties of materials. 6–8 The influence of the cata- lyst on the microstructure and textural properties of hybrids and their in vitro bioactivity was also investigated. 6 However, the effect of PDMS and water contents upon the microstruc- ture of bioactive hybrids and the resultant effect on their mac- roscopic mechanical properties remains unknown. In this study, the synthesis conditions, particularly the effect of PDMS and H 2 O, on obtaining CaO–SiO 2 –PDMS monoliths exhibiting in vitro bioactivity will be investigated. A wide physical–chemical characterization of bioactive hy- brid materials will be carried out to relate their microstruc- ture, at atomic scale, with their macroscopic mechanical behavior. Finally, we will try to explain how the differences in the composition of precursor solutions influence the me- chanical properties of these hybrid materials. MATERIALS AND METHODS Synthesis of Hybrid Materials CaO–SiO 2 –PDMS organic–inorganic hybrid materials were synthesized via sol–gel by using tetraethoxysilane, (TEOS, Si(OCH 2 CH 3 ) 4, Aldrich), calcium nitrate tetrahydrate (Ca Correspondence to: A. J. Salinas (e-mail: salinas@farm.ucm.es) Contract grant sponsor: CICYT Spain; Contract grant numbers: MAT2004-03378 and MAT2005-01486 ' 2006 Wiley Periodicals, Inc. 274