Novel route for rapid sol-gel synthesis of hydroxyapatite, avoiding ageing and using fast drying with a 50-fold to 200-fold reduction in process time Basam A.E. Ben-Arfa, Isabel M. Miranda Salvado ⁎, José M.F. Ferreira, Robert C. Pullar ⁎ Department of Materials and Ceramic Engineering, CICECO – Aveiro Institute of Materials, University of Aveiro, 3810-193 Aveiro, Portugal abstract article info Article history: Received 1 June 2016 Received in revised form 23 August 2016 Accepted 24 September 2016 Available online 28 September 2016 We have developed an innovative, rapid sol-gel method of producing hydroxyapatite nanopowders that avoids the conventional lengthy ageing and drying processes (over a week), being 200 times quicker in comparison to conventional aqueous sol-gel preparation, and 50 times quicker than ethanol based sol-gel synthesis. Two differ- ent sets of experimental conditions, in terms of pH value (5.5 and 7.5), synthesis temperature (45 and 90 °C), dry- ing temperature (60 and 80 °C) and calcination temperature (400 and 700 °C) were explored. The products were characterised by X-ray diffraction (XRD) Fourier-transform infrared spectroscopy (FTIR), scanning electron mi- croscopy (SEM) and specific surface area (SSA) measurements. Pure hydroxyapatite (Ca 10 (PO 4 ) 6 (OH) 2 , HAp) was obtained for the powders synthesised at pH 7.5 and calcined at 400 °C, while biphasic mixtures of HAp/β- tricalcium phosphate (β-Ca 3 (PO 4 ) 2 , TCP) were produced at pH 5.5 and (pH 7.5 at elevated temperature). The novel rapid drying was up to 200 times faster than conventional drying, only needing 1 h with no prior ageing step, and favoured the formation of smaller/finer nanopowders, while producing pure HAp or phase mixtures vir- tually identical to those obtained from the slow conventional drying method, despite the absence of a slow ageing process. The products of this novel rapid process were actually shown to have smaller crystallite sizes and larger SSA, which should result in increased bioactivity. © 2016 Elsevier B.V. All rights reserved. Keywords: Hydroxyapatite β-TCP Nanoparticles Nano-synthesis Biocompatibility Sol-gel 1. Introduction Hydroxyapatite (Ca 10 (PO 4 ) 6 (OH) 2 ; HAp) is the major constituent [1] (70–90 wt%) of biological apatite in natural bone [2], and it is therefore a useful bioceramic. The calcium phosphate ions are naturally metabolised during resorption, and do not induce abnormal calcium or phosphate levels in human organs [3]. Due to its chemical and struc- tural properties being so similar to the bone mineral [4], along with a good osteoconductivity and biocompatibility, HAp is widely used as a biomaterial in the form of restorative, grafting, and coating materials [1]. HAp also has useful properties for the separation of proteins with ac- tivity in chromatography [5]. Human bones, and many HAp-based bioceramics, are actually a mixture of ~75% HAp and 25% beta- tricalcium phosphate (TCP, β-Ca 3 (PO 4 ) 2 ), which also demonstrates su- perior bio-resorbability to pure HAp [6,7]. HAp can be synthesised by several methods: sol-gel approaches [1, 4], wet-chemical synthesis [8–11], mechanochemical synthesis [12], combustion synthesis [13], electrochemical deposition [14], hydrother- mal synthesis [15], multiple emulsion technique [16], high gravity methods [17], etc. Mechanochemical reactions occur by applying a strong mechanical energy that destroys the original materials and avails the atoms for the formation of different structures [18]. The electrochemical approach, often used to deposit HAp layers onto the surfaces of implant materials, is widely used as coating method [19]. Hydrothermal synthe- sis is a technique that involves reactions at elevated temperature and pressure of aqueous solutions/suspensions to directly crystallise ceram- ic materials [20]. Of the many available methods for HAp synthesis [5,21–25], the pre- cipitation from suitable calcium (Ca) and phosphorous (P) precursor salt solutions is the most widely used, this being a convenient low cost method for obtaining HAp powder [26]. However, the precipitation from solution suffers several drawbacks, such as the necessity for high (non-acidic) pH to avoid the formation of Ca-deficient HAp, and rela- tively high calcination temperatures for the formation of crystalline HAp [1]. Also, the reaction time required for completing the formation of HAp is relatively long, and usually a slow ageing step is required for the precipitated sol-gel [27]. The low temperature required for the for- mation of HAp crystals, and a high degree of homogeneity, are the main advantages of the sol–gel process in comparison with conventional solid state methods for HAp powder synthesis [4]. Looking at the extensive literature on sol-gel synthesis and ageing of HAp powders, it is obvious that many protocols have been employed, and factors such as total time of synthesis investigated, to study their effect on the final HAp product, to achieve the optimum single or diphasic calcium phosphate end prod- uct for a predesigned function. In a typical aqueous sol-gel synthesis of HAp, the gel is aged at room temperature and then slowly dried at low temperatures below 100 °C, over a total period of a week or more. Lui et al. [28] obtained a calcium Materials Science and Engineering C 70 (2017) 796–804 ⁎ Corresponding authors. E-mail addresses: isabelmsalvado@ua.pt (I.M.M. Salvado), rpullar@ua.pt (R.C. Pullar). http://dx.doi.org/10.1016/j.msec.2016.09.054 0928-4931 © 2016 Elsevier B.V. All rights reserved.http://dx.doi.org/10.1016/j.msec.2016.09.054 0928-4931/© 2016 Elsevier B.V. All rights reserved. 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