This paper reports a novel way of producing macroporous ceramics with aligned microporous walls by unidirectionally freezing foamed aqueous ceramic suspensions. The stability of the air bubbles introduced by magnetic stirring was improved remarkably using polyvinyl alcohol (PVA) as both a binder and emulsifying agent. Furthermore, the unidirectional freeze casting method could not only trap the air bubbles in the solidified sample successively but also induce the preferential growth of ice dendrites. This endowed the samples with uniform macropores surrounded by aligned microporous alumina walls after freeze drying and sintering. The sample produced with a PVA content of 8 wt% showed a high compressive strength of 4.6 ± 1.6 MPa at a high porosity of 91 vol%.
Highly porous calcium phosphate (CaP) ceramics were fabricated by freezing foamed aqueous CaP suspensions with various CaP contents (15, 20, and 25 vol %) containing polyvinyl alcohol (PVA) as the binder and emulsifying agent. All the samples fabricated showed uniformly dispersed macropores, which were created by air bubbles introduced in the suspensions. In addition, aligned micropores were formed in the sintered CaP walls as a replica of the preferentially grown ice dendrites during freezing when CaP contents of 15 and 20 vol % were used. The overall porosity decreased from 83 to 73 vol % with increasing initial CaP content from 15 to 25 vol %, whereas the compressive strength and elastic modulus increased significantly from 1.2 « 0.2 to 4.7 « 0.9 MPa and from 20 « 11 to 163 « 42 MPa, respectively. The samples showed good biocompatibility, as assessed by the in vitro cell test using a pre-osteoblast cell line.
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