Materials that are strong, ultralightweight, and tough are in demand for a range of applications, requiring architectures and components carefully designed from the micrometer down to the nanometer scale. Nacre, a structure found in many molluscan shells, and bone are frequently used as examples for how nature achieves this through hybrid organic-inorganic composites. Unfortunately, it has proven extremely difficult to transcribe nacre-like clever designs into synthetic materials, partly because their intricate structures need to be replicated at several length scales. We demonstrate how the physics of ice formation can be used to develop sophisticated porous and layered-hybrid materials, including artificial bone, ceramic-metal composites, and porous scaffolds for osseous tissue regeneration with strengths up to four times higher than those of materials currently used for implantation.

Freeze‐casting, the templating of porous structures by the solidification of a solvent, have seen a great deal of efforts during the last few years. Of particular interest are the unique structure and properties exhibited by porous freeze‐casted ceramics, which opened new opportunities in the field of cellular ceramics. The objective of this review is to provide a first understanding of the process as of today, with particular attention being paid on the underlying principles of the structure formation mechanisms and the influence of processing parameters on the structure. This analysis highlights the current limits of both the understanding and the control of the process. A few perspectives are given, with regards of the current achievements, interests and identified issues.

The formation of regular patterns is a common feature of many solidification processes involving cast materials. We describe here how regular patterns can be obtained in porous alumina by controlling the freezing of ceramic slurries followed by subsequent ice sublimation and sintering, leading to multilayered porous alumina structures with homogeneous and well-defined architecture. We discuss the relationships between the experimental results, the physics of ice and the interaction between inert particles and the solidification front during directional freezing. The anisotropic interface kinetics of ice leads to numerous specific morphologies features in the structure. The structures obtained here could have numerous applications including ceramic filters, biomaterials, and could be the basis for dense multilayered composites after infiltration with a selected second phase.

Although extensive efforts have been put into the development of porous scaffolds for bone regeneration, with encouraging results, all porous materials have a common limitation: the inherent lack of strength associated with porosity. Hence, the development of porous hydroxyapatite scaffolds has been hindered to non-load bearing applications. We report here how freeze casting can be applied to synthesize porous scaffolds exhibiting unusually high compressive strength, e.g. up to 145 MPa for 47% porosity and 65 MPa for 56% porosity. The materials are characterized by well-defined pore connectivity along with directional and completely open porosity. Various parameters affecting the porosity and compressive strength have been investigated, including initial slurry concentration, freezing rate, and sintering conditions. The implications and potential application as bone substitute are discussed. These results might open the way for hydroxyapatite-based materials designed for load-bearing applications. The biological response of these materials is yet to be tested.

Ceramics exhibit among the highest stiffness and strength of all known material classes 1 .Because of the strong and directional bonding between constitutive atoms, they present a high

This review describes the mechanisms responsible for lowtemperature degradation (LTD) of zirconia ceramics and its detrimental consequences for biomedical devices. Special emphasis is given to the critical issue of zirconia degradation actually observed for hip prostheses. Experimental methods to accurately measure and predict LTD in a given zirconia ceramic are presented. Different solutions to inhibit LTD or at least reduce its kinetics are reviewed, with the objective of highlighting alternative options for the generation of new zirconia-based biomedical ceramic devices. 1 Annu. Rev. Mater. Res. 2007.37:1-32. Downloaded from arjournals.annualreviews.org by 195.68.31.231 on 08/06/07. For personal use only. Zirconia: zirconium dioxide (ZrO 2 ), generally used in the form of a polycrystalline technical ceramic, stabilized with another oxide Y-TZP: yttria-stabilized tetragonal zirconia polycrystal Femoral head: component of a hip prosthesis in the form of a ball affixed to a metallic femoral stem and bearing against the acetabular cup LTD: low-temperature degradation 2 Chevalier · Gremillard · Deville Annu. Rev. Mater. Res. 2007.37:1-32. Downloaded from arjournals.annualreviews.org by 195.68.31.231 on 08/06/07. For personal use only. Tetragonal-to-Monoclinic Transformation in ZirconiaThe t-m transformation in zirconia is martensitic in nature. A martensitic transformation is a "change in crystal structure . . . that is athermal, diffusionless and involves the simultaneous, cooperative movement of atoms over distances less than an atomic www.annualreviews.org • Low-Temperature Degradation of Zirconia Implants 4 Chevalier · Gremillard · Deville Annu. Rev. Mater. Res. 2007.37:1-32. Downloaded from arjournals.annualreviews.org by 195.68.31.231 on 08/06/07. For personal use only. www.annualreviews.org • Low-Temperature Degradation of Zirconia Implants 5 Annu. Rev. Mater. Res. 2007.37:1-32. Downloaded from arjournals.annualreviews.org by 195.68.31.231 on 08/06/07. For personal use only. 6 Chevalier · Gremillard · Deville Annu. Rev. Mater. Res. 2007.37:1-32. Downloaded from arjournals.annualreviews.org by 195.68.31.231 on 08/06/07. For personal use only. www.annualreviews.org • Low-Temperature Degradation of Zirconia Implants 7 Annu. Rev. Mater. Res. 2007.37:1-32. Downloaded from arjournals.annualreviews.org by 195.68.31.231 on 08/06/07. For personal use only. 8 Chevalier · Gremillard · Deville Annu. Rev. Mater. Res. 2007.37:1-32. Downloaded from arjournals.annualreviews.org by 195.68.31.231 on 08/06/07. For personal use only. www.annualreviews.org • Low-Temperature Degradation of Zirconia Implants 9 Annu. Rev. Mater. Res. 2007.37:1-32. Downloaded from arjournals.annualreviews.org by 195.68.31.231 on 08/06/07. For personal use only. 10 Chevalier · Gremillard · Deville Annu. Rev. Mater. Res. 2007.37:1-32. Downloaded from arjournals.annualreviews.org by 195.68.31.231 on 08/06/07. For personal use only. 12 Chevalier · Gremillard · Deville Annu. Rev. Mater. Res. 2007.37:1-32. Downloaded from arjournals.annualreviews.org by 1...

Abstract

The freezing of concentrated colloidal suspensions is a complex physical process involving a large number of parameters. These parameters provide unique tools to manipulate the architecture of freeze-cast materials at multiple length scales in a single processing step. However, we are still far from developing predictive models to describe the growth of ice crystals in concentrated particle slurries. In order to exert reliable control over the microstructural formation of freeze-cast materials, it is necessary to reach a deeper understanding of the basic relationships between the experimental conditions and the microstructure of the growing solid. In this work, we explore the role of several processing variables (e.g., composition of the suspension, freezing rate, and patterning of the freezing surface) that could affect the formulation strategies for the architectural manipulation of freeze-cast materials. We also demonstrate, using freeze-cast lamellar structures, how reducing the lamellar thickness by less than half increases compressive strength by more than one order of magnitude.