äéããéàÑçõâ ÜìêçÄã, 2009, ÚÓÏ 71, ‹ 6, Ò. 775–783 775 1. INTRODUCTION Calcium phosphates constitute the major inorganic phase of human hard tissues like the bone and teeth, are bioactive and can be rapidly integrated into the human body [1, 2]. Therefore, they are of high relevance to mate- rial science, biology and medicine. Calcium phosphate bioceramics with low reliability under tensile load can be used as powders, or as small, unloaded implants such as in the middle ear, dental implants with reinforcing metal posts, coatings on metal implants, low-loaded porous im- plants where bone growth acts as a reinforcing phase, and as the bioactive phase in a composite [3]. Calcium orthophosphates are salts of tribasic phosphoric acid, H 3 PO 4 and its ionic compounds. Hydroxyapatite Ca 5 (PO 4 ) 3 (OH), octacalcium phosphate Ca 8 H 2 (PO 4 ) 6.5 H 2 O, monetite CaHPO 4 · 2H 2 O, and brushite CaHPO 4 · 2H 2 O are different crystalline orthophosphates that have been exten- sively studied for their relevance in biological mineraliza- tion [4]. Brushite has raised considerable interest amongst researchers in bone substitution due to their fast resorption as it is a metastable compound when used under physiolog- ical conditions [5–7]. It is a suitable matrix as osteoconduc- tive bone grafts as it serves as a precursor phase [8]. The bio- compatibility and degradation of brushite have been demon- strated in several in vitro and in vivo studies [9–11]. Following immersion in aqueous media, brushite may, de- pending on conditions, be stable, disintegrate or dissolve and precipitate as hydroxyapatite [12]. As the natural bone is made of calcium phosphate par- ticles scattered in the organic matrix [13], it is believed that deliberate tailoring of the crystallite size, morphology, stoichiometry and composition of calcium phosphates could lead to improved properties such as bioactivity, bio- compatibility, surface area, chemical and physical stabili- ties and mechanical properties by adjusting the concentra- tion of reactants, type and concentration of surfactants, pH and temperature [14–16]. The different forms of calcium phosphate particles manufactured and sold in the market include irregular multifaceted particles and smooth rounded particles, with solid or porous structure [17]. They are osteoconductive and help in growth and attachment of bone [18, 19], and are clinically used in oral surgical procedures to augment the alveolar ridge [20] and in periodontal repair [21]. Kent et al. [22] reported improved denture stability and reten- tion when dense calcium phosphate particles have been used for augmentation of atrophic mandibular and maxil- lary alveolar ridges. Many researchers have explored fabrication of calci- um phosphate particles of various morphologies such as lath [16], rod [23], belt [24], sheet, needle [25], wire [26], and cone [27]. Among the processing routes used to fabri- cate the particles are sol-gel [14], solid state [28], biosyn- thesis [29], chemical precipitation [4], hydrothermal [30, 31], hard templating [23], emulsion [32, 33], and micro- emulsion [16]. PREPARATION AND CHARACTERIZATION OF BRUSHITE CRYSTALS USING HIGH INTERNAL PHASE EMULSION © 2009 H. N. Lim*, A. Kassim*, N. M. Huang**, M. A. Yarmo***, P. S. Khiew****, W. S. Chiu**** *Department of Chemistry, Faculty of Science, Universiti Putra Malaysia, 43400 UPM Serdang, Selangor Darul Ehsan, Malaysia **Physics Department, Faculty of Science, University Malaya, 50603 Kuala Lumpur, Malaysia ***School of Chemical Sciences and Food Technology, Faculty of Science and Technology, Universiti Kebangsaan Malaysia, 43000 Bandar Baru Bangi, Selangor, Malaysia ****Faculty of Engineering and Computer Science, Nottingham University, Jalan Broga, 43500 Semenyih, Selangor Darul Ehsan, Malaysia èÓÒÚÛÔË· ‚ ‰‡ÍˆË˛ 11.01.2009 „. Various morphologies of brushite crystals were successfully synthesized using oil-in-water high internal phase emul- sion stabilized by surfactants with various polyoxyethylene chain lengths for the first time. Rheological measure- ments had proven that such emulsion was a highly stable reaction media for the synthesis of particulate and porous brushite crystals. Rheological properties of the emulsion with and without the presence of crystal growth were com- pared and discussed. Morphologies of brushite crystals comprised of leaflet-like particles to continuous dendritic net- work structure which gave rise to porosity in submicron size with increasing polyoxyethylene chain length. The mechanism leading to the various types of morphologies produced is discussed schematically based on small angle X-ray scattering results. ìÑä 541.18