Optimization of Bioglass ® Scaffold Fabrication Process Qizhi Chen, ‡,§,† Dirk Mohn, ¶ and Wendelin J. Stark ¶ ‡ Department of Materials Engineering, Monash University, Clayton, Victoria, 3800, Australia § Division of Biological Engineering, Monash University, Clayton, Victoria, 3800, Australia ¶ Department of Chemistry & Applied Biosciences, ETH Zurich, 8093, Zurich, Switzerland The production of mechanically reliable scaffolds from bioceramics for use in bone tissue engineering remains challenging. This paper describes the establishment of optimal processing param- eters of Bioglass ® scaffolds using the replication/slurry-dip- coating technique, based on theoretical design and experimental investigation. The foams fabricated under the optimized condi- tions, i.e., 5–20 lm particles and sintering at 1000°C–1100°C for 1–2 h, showed reproducible mechanical properties that could be predicted by Gibson and Ashby’s theory. Excessively small (nano-sized) or overly large (>30 lm) particles both resulted in poor quality scaffolds with unsatisfactory mechani- cal performance, due to a high population of microcracks in struts and poor fusion between particles during sintering, respectively. In conclusion, a mechanically reliable scaffold can be achieved using Bioglass ® and the replication/slurry-dip-coat- ing technique, provided that the particle size of the Bioglass powder is within the range of 5–20 lm and an appropriate sintering program (1000°C–1100°C, 1–2 h) is used. I. Introduction E NGINEERING bone tissue, which is hard and functions to support the body, requires a strong scaffold material. Currently, bioceramic materials are the most popular choice as biomaterials for applications involving bone tissue engi- neering. 1–3 In an organ, cells and their extracellular matrices (ECM) are usually organized into three-dimensional (3-D) tissues. Therefore, tissue engineering typically requires a highly porous 3-D matrix (scaffold) to accommodate cells and to guide their growth and differentiation into tissue in three dimensions. Numerous techniques have been developed for production of these types of porous scaffolds using ceramic materials, including loose-particle packing, 4 phase separation/ freeze-drying, 5 foam-replica techniques, 6–8 3-D printing, 9 and sol–gel casting. 3,4,10–12 For bone tissue engineering, the foam- replica technique, combined with a slurry-dip coating method, produces a porous structure that most resembles cancellous bone (also called spongy bone). 13 The commercially available bioceramic, Bioglass ® , has been fabricated into highly open, connected, and porous scaffolds using this replication tech- nique. 14 However, the poor mechanical reliability of porous ceramic scaffolds remains a serious limitation. Chen et al. 13 addressed this issue by developing ZrO 2 scaf- folds using the replication technique combined with an elec- trospray coating process. However, there are good reasons for the redevelopment of the mechanically reliable Bioglass ® scaffolds using the replication/slurry-dip coating technique. Firstly, the electrospray coating method can only produce scaffolds smaller than 5 mm in size, due to the limited pene- tration of sprayed particles into polymer foam. In contrast, the slurry-dip coating process, when combined with the repli- cation technique, can produce ceramic scaffolds of any size, with virtually no limitations. Secondly, a bone tissue scaffold needs to be degradable, as this would avoid the detrimental effects of a persisting foreign substance and would allow its gradual replacement with new bone. Unfortunately, mechani- cal strength and biodegradability, which are the two essential requirements for bone tissue scaffolds, are antagonistic. In general, mechanically strong materials (e.g., ZrO 2 , crystal- line hydroxylapatite, and related calcium phosphates and crystalline polymers) are virtually bioinert, while biodegrad- able materials (e.g., amorphous calcium phosphates and amorphous polymers) tend to be structurally fragile. Clinical investigation has shown that implanted hydroxyl- apatite and calcium phosphates are inert, remaining within the body for as long as 6–7 years post implantation. 15 Among a number of bioceramics, Na 2 O-containing bioactive glasses (e.g., 45S5 Bioglass ® ) are the only ones that could meet the requirements for both mechanical strength and bio- degradability through the formation of Na 2 O-containing crystalline phase, Na 2 Ca 2 Si 3 O 9 . 14,16,17 This mechanically capable crystalline phase can transform to a biodegradable, amorphous calcium phosphate at body temperature and in an in vitro biological environment. This transformation cou- ples the above two irreconcilable properties into one scaffold, and the temporal profile of the transition can be tailored to satisfy the requirement of bone tissue engineering. 14 This is the primary reason why 45S5 Bioglass ® was chosen as the biomaterial used in the present study. The particle size of a glass powder used in the slurry-dip coating process has a critical influence on the sintering kinet- ics of scaffolds and thus on the quality of the final prod- ucts 13 . Therefore, the primary objective of this work was to explore the optimal replication/slurry-dip-coating procedure that would produce mechanically reliable Bioglass ® scaffolds, with a focus on the influence of particle size on the quality of the final scaffold products. II. Experimental Procedures (1) Materials Nano-sized and micro-sized 45S5 Bioglass powders were used in this work (Fig. 1). Flame spray synthesis was used to pre- pare the bioactive glass nanoparticles. 18–20 Briefly, equivalent amounts of calcium and sodium 2-ethylhexanoate were mixed with hexamethyldisiloxane and tributylphosphate and diluted with xylene. The solution was pumped (10 mL/min) through a capillary (diameter 0.4 mm), dispersed with oxygen (10 L/min) and ignited with a methane (1.13 L/min) and oxygen (2.4 L/min) flame. The as-formed bioactive glass par- ticles were collected using a baghouse filter and subsequently sieved with a 250 lm mesh sieve to separate the agglomerates. S. Bose—contributing editor Manuscript No. 29311. Received February 11, 2011; approved June 23, 2011. † Author to whom correspondence should be addressed. e-mail: qizhi.chen@monash. edu 4184 J. Am. Ceram. Soc., 94 [12] 4184–4190 (2011) DOI: 10.1111/j.1551-2916.2011.04766.x © 2011 The American Ceramic Society J ournal