A two-scale model for simultaneous sintering and crystallization of glass–ceramic scaffolds for tissue engineering R. Huang a , J. Pan a, * , A.R. Boccaccini b , Q.Z. Chen b a Department of Engineering, University of Leicester, Leicester LE1 7RH, UK b Department of Materials, Imperial College London, Prince Consort Road, London SW7 2BP, UK Received 23 August 2007; received in revised form 22 November 2007; accepted 1 February 2008 Available online 20 February 2008 Abstract Bioglass Ò -based glass–ceramic foams have been developed recently as highly porous, mechanically competent, bioactive and degrad- able scaffolds for bone tissue engineering. However, the development of the material so far has been based on a trial-and-error approach, and the existing materials are far from being optimized. In this paper, a mechanism-based model is presented for sintering deformation of Bioglass Ò foams. The porous foams consist of struts which, in turn, consist of Bioglass Ò particles. A corresponding two-scale model is developed based on existing viscous sintering models. Crystallization plays a key role in the sintering deformation of Bioglass Ò foams and is taken into account in the model. Qualitative comparison between the model predictions and experimental observations is pre- sented, showing that the model is able to capture the complicated interplay between crystallization and viscous flow during the sintering process. Ó 2008 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Bioactive glass; Sintering; Crystallization; Model 1. Introduction Bioactive glasses are promising materials for tissue engi- neering scaffolds which promote the regeneration ability of host bone tissue [1–3]. Bioactive glasses have some major advantages over other bioceramics, such as excellent osteo- conductivity and bioactivity, ability to deliver cells, and controllable biodegradability. However, the mechanical properties of porous scaffolds made from bioactive glasses are unsatisfactory in the case of scaffolds fabricated by dry- powder processing with porogen addition [4] or by the sol– gel and gel-casting techniques [5]. Recently, Chen et al. [6] reported for the first time the fabrication of highly porous, mechanically competent, bioactive and biodegradable scaf- folds using a foam replication technique and 45S5 Bio- glass Ò powder. The foam replication technique consists of the following steps: (a) coating of a sacrificial polyure- thane foam with bioactive glass particles by slurry dipping; (b) drying of the coated foam (green body); (c) removal of the polymer template by slow burning; and (d) sintering of the glass network to the desired density. Experimental observations have confirmed that significant crystallization occurs at the temperatures required to densify the foams by viscous sintering (>900 °C) [6]. If the sintering temperature is <900 °C, neither crystallization nor densification will occur. If the sintering temperature is too high, the crystal- lization can turn the bioactive glasses (in particular the composition 45S5 Bioglass Ò [7]) into inert materials [8]. Therefore, an optimum sintering condition exists at which the foam can densify to give the required mechanical prop- erties and to produce fine crystals of Na 2 Ca 2 Si 3 O 9 which do not totally suppress the glass bioactivity, e.g., by forma- tion of an apatite layer on the Bioglass Ò surface in contact with physiological fluids [6]. All the previous investigations into the fabrication of Bioglass Ò foams by the foam replica technique have been of an experimental nature. It has been difficult to obtain 1742-7061/$ - see front matter Ó 2008 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.actbio.2008.02.004 * Corresponding author. Tel.: +44 116 2231092. E-mail address: jp165@le.ac.uk (J. Pan). Available online at www.sciencedirect.com Acta Biomaterialia 4 (2008) 1095–1103 www.elsevier.com/locate/actabiomat