Materials Chemistry and Physics 80 (2003) 725–730 A novel technique to synthesize hydroxyapatite at low temperature T.K. Anee a , M. Ashok a , M. Palanichamy b , S. Narayana Kalkura a,∗ a Crystal Growth Centre, Anna University, Chennai 600025, India b Department of Chemistry, Anna University, Chennai 600025, India Received 24 August 2002; received in revised form 24 October 2002; accepted 14 January 2003 Abstract Hydroxyapatite (HAP), a well known biomaterial, was synthesized by a novel technique using agarose gel in a high alkaline medium at a relatively low temperature 85 ◦ C. The XRD analysis of the as-synthesized material revealed the absence of either brushite or  or  tricalcium phosphate phases. Sintering studies at 750 and 1200 ◦ C revealed the conversion of HAP into pyrophosphate without leaving any additional phases. A considerable decrease in the transformation temperature of HAP phase to that of -calcium pyrophosphate (-CPP) phase was found compared to the reported values. © 2003 Elsevier Science B.V. All rights reserved. Keywords: Biomaterials; Hydroxyapatite; Phase transformation; Agarose 1. Introduction Among calcium phosphates, hydroxyapatite (HAP, Ca 10 (PO) 6 (OH) 2 ) is known to be the major constituent (69%) of the bones. It is also an attractive material for hard tissue implants [1–5]. Hence there has been persistent ef- forts in the development of this material in appropriate form for various applications as a biomaterial. Calcium pyrophos- phate (CPP, Ca 2 P 2 O 7 ) has wide applications in the manu- facture of oral care products like paste and also it is sold as a mineral supplement [6,7]. Further CPP can be used as a coating material for bone and teeth implants [8]. There are several reports dealing with the precipitation of HAP with varying Ca:P stoichiometries from aqueous solutions [9–15]. Slip casting slurries containing mixtures of tricalcium phos- phate and calcium hydroxide or calcium carbonate in a molar ratio of 3:1 in plaster moulds and subsequent sintered were reported to produce apatite [16,17]. Ramachandra Rao et al. [18] reported formation of pure HAP by mixing tricalcium phosphate and calcium hydroxide in molar ratios 3:2 and 3:1.5 and treated at 1000 ◦ C. However, there are limitations for these powder composition-sintering techniques due to the developed materials are often weak and composed of other phases and pores. Jarcho et al. [19] reported synthesis of ∗ Corresponding author. Present address: Institut für Medizinische Biochemie und Molekularbiologie, Arbeitsgruppe für Makromolekulare Strukturanalyse, c/o DESY, Geb. 22a, Notkestraße 85, 22603 Hamburg, Germany. Tel.: +49-40-8998-4749; fax: +49-40-8998-4747. E-mail address: Kalkura@unisgi1.desy.de (S. Narayana Kalkura). HAP by precipitation technique at sintering temperature of 1100 ◦ C. They reported presence of three phases in the fired materials. Various batch precipitation methods yielding HAP crystals in the nanometric, sub-nanometric and micrometric size range have been developed [20–24]. But these materi- als are calcium deficient and incorporate carbonate ions in the crystal lattice. Liu et al. [25] have reported the synthe- sis of HAP at a temperature as low as 350 ◦ C by sol–gel route using two solvents, water and anhydrous ethanol, as diluting media. Varma et al. [26] reported a polymeric precursor route for the preparation of calcium phosphate compounds. They observed after being treated at 1000 ◦ C, co-existence of a mixture of two phases, -tricalcium phos- phate, HAP and transition from -tricalcium phosphate to HAP. All these reported methods employed high temper- ature to synthesize HAP. To the best of our knowledge there are no previous reports on the synthesis of HAP us- ing agarose as a medium. Here we report the preparation of HAP by a sol–gel process at 85 ◦ C using agarose and also the sintering characteristics of the as-synthesized HAP powder. 2. Materials and methods 2.1. Synthesis and processing Calcium nitrate (Ca(NO 3 ) 2 ·4H 2 O) and di-ammonium hy- drogen phosphate ((NH 4 ) 2 HPO 4 , Merck), ammonia solution 0254-0584/03/$ – see front matter © 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0254-0584(03)00116-0