40 Interceram 01/2013 high-performance ceramics 1 Introduction Stoichiometric hydroxyapatite (HA) has the unique capability of binding to natural bone through biochemical bonding, which pro- motes the interaction between host bone and grafted material. Pure HA has a relative- ly high dissolution rate in simulated body fluid (SBF), which affects its long-term sta- bility. A high dissolution rate may lead to disintegration of the coatings, which hin- ders fixation of the implant to the host tis- sue in HA coating applications [1]. Nano- carbonated apatite has an important impact on cell–biomaterial interaction because of its better resorption and close surface con- tact with the surrounding tissue [2]. The addition of citric acid (C 6 H 8 O 7 ) as a chelating reagent prevents the agglomera- tion of HA powders and makes more car- bonate (CO 3 2– ) groups which enter the HA crystal structure and substitute for phos- phate (PO 4 3– ) groups. This makes the ob- tained HA closer in terms of composition to the mineral constituents of the bone [3]. Citric acid is also used to increase the me- chanical properties of chitosan by increas- ing bonding strength between particles [4]. The three carboxyl groups of citric acid pro- vide more nucleation sites for the formation of ultra-fine nano-sized carbonate apatite, and the increase of citric acid benefits bone resorption and ossification through the for- mation of dissociated calcium citrate com- plexes in the surrounding body fluid [5]. The ability of chitosan to support cell at- tachment and proliferation is attributed to its chemical properties. Chitosan has a high resistance to heat due to its intra-molecular hydrogen bonds [6]. Chitosan is considered as an appropriate functional material for biomedical applications because of its high biocompatibility, biodegradability, non-an- tigenicity and adsorption properties [7]. Al- so, it is biocompatible and can be degraded by enzymes in the human body and the deg- radation product is non-toxic [8]. Gelatin is a biodegradable polymer with many attractive properties, such as excellent biocompatibility, non-antigenicity, plastici- ty and adhesiveness, and it is widely used in biomedical and pharmaceutical fields. Thus, gelatin was selected as a suitable candidate to be blended with chitosan. Also, gelatin is derived from the skin, connective tissue, and bones of animals and it contains carboxyl groups on its chain backbones and has the potential to mix with chitosan due to its ability to form hydrogen bonding with chi- tosan [9]. Gelatin is an amphoteric polyelec- trolyte as it contains both anionic and cati- onic groups that can absorb ions. The ad- sorption could be driven by electrostatic and/or by hydrophobic interaction, depend- ing on the nature of the surface and the me- dium [10]. The combination of gelatin and chitosan was chosen due to the benefits achieved from several synergistic effects and for their clinical applications. The hydroxyapatite (HA)/polymer composites have been widely used in bone tissue engineering due to their chemical similarity to natural bone and the K.R. Mohamed*, H. Orban**, A.M. Mohamed** Characterization and In-vitro Properties of Composite Materials The present work is concerned with the effect of chitosan polymer con- centration on the properties of hydroxyapatite/chitosan–gelatin compos- ite materials. The prepared composites were characterized by Fourier transform infrared spectroscopy, thermo-gravimetric analysis, and scan- ning electron microscopy. Also, in vitro degradation and swelling tests were performed. The results showed that the increase of chitosan con- tent in the composites in the presence of citric acid improved its com- pressive strength. The swelling of the composites increased with the increase of chitosan proving high water affinity. The degradation of the composites was directly proportional to the chitosan content but it re- duced in the presence of citric acid due to an increase of the binding strength. In vitro data proved that the high content of chitosan with a fixed content of gelatin and citric acid in the composites enhanced the formation of carbonated hydroxyapatite on their surfaces; therefore, these biocomposites materials are promising for bone grafting and bio- applications. composites, chitosan, hydroxyapatite, degradation, scanning electron microscope Interceram 62 (2013) [1] The corresponding author, Khaled Rezk Mohamed Ibrahim, earned his M.Sc. and Ph.D. in Medical Biophysics from Facul- ty of Science of Cairo Univer- sity (Egypt) in 1997 and 2004, respectively. His interest is concentrated on biophysics and biomaterials in bone grafting applications. Till now he is working as Associate Professor in the Biomaterials Department at the National Research Center, Cairo. He is member of several scientific societies like the Scientific syndicate of Egypt, Biophysics & Bioengineering Society of Egypt, Arabic of Material Science Society. Fur- thermore he is reviewer for the Journal Materials Science Engineering C and co-editor of the Jour- nal of Advanced Genetic Engineering and Bio- technology. E-Mail: kh_rezk1966@yahoo.com The auThor absTracT Keywords * Biomaterials Dept., National Research Centre, Dokki, Cairo (Egypt) ** Medical Biochemistry, National Research Centre, Dokki, Cairo (Egypt)