Ice-templated porous alumina structures

International audienceThe 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

Scaling strength distributions in quasi-brittle materials from micro- to macro-scales: A computational approach to modeling Nature-inspired structural ceramics

International audienceThis paper presents an approach to predict the strength distribution of quasi-brittle materials across multiple length-scales, with emphasis on Nature-inspired ceramic structures. It permits the computation of the failure probability of any structure under any mechanical load, solely based on considerations of the microstructure and its failure properties by naturally incorporating the statistical and size-dependent aspects of failure. We overcome the intrinsic limitations of single periodic unit-based approaches by computing the successive failures of the material components and associated stress redistributions on arbitrary numbers of periodic units. For large size samples, the microscopic cells are replaced by a homogenized continuum with equivalent stochastic and damaged constitutive behavior. After establishing the predictive capabilities of the method, and illustrating its potential relevance to several engineering problems, we employ it in the study of the shape and scaling of strength distributions across differing length-scales for a particular quasi-brittle system. We find that the strength distributions display a Weibull form for samples of size approaching the periodic unit; however, these distributions become closer to normal with further increase in sample size before finally reverting to a Weibull form for macroscopic sized samples. In terms of scaling, we find that the weakest link scaling applies only to microscopic, and not macroscopic scale, samples. These findings are discussed in relation to failure patterns computed at different size-scales

Freezing as a Path to Build Complex Composites

International audienceOne-Sentence Summary: Freezing is harnessed to create composites that replicate the intricate structure of nacre and to synthesize porous bone substitutes with high strength. Abstract: Materials that are strong, ultra-light weight and tough are in demand for a range of applications, requiring architectures and components carefully designed from the micrometer down to nanometer scales. 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 currently used for implantation

Sintering and robocasting of beta-tricalcium phosphate scaffoldsfor orthopaedic applications

{beta}-tricalcium phosphate ({beta}-TCP) scaffolds with designed, three-dimensional (3-D) geometry and mesoscale porosity have been fabricated by direct-write assembly (robocasting) techniques. Concentrated {beta}-TCP inks with suitable viscoelastic properties were developed to enable the fabrication of the complex 3-D structures. A comprehensive study of the sintering behavior of TCP as a function of the calcium content in the starting powder was also carried out, and the optimal heat treatment for fabricating scaffolds with dense {beta}-TCP rods has been determined. Such analysis provides clues to controlling the microstructure of the fabricated structures and, therefore, enabling the fabrication by robocasting of TCP scaffolds with tailored performance for bone tissue engineering applications

Fabrication andin vitro characterization of three-dimensional organic/inorganic scaffolds by robocasting

International audienceA key issue for the fabrication of scaffolds for tissue engineering is the development of processing techniques flexible enough to produce materials with a wide spectrum of solubility (bioresorption rates) and mechanical properties matching those of calcified tissues. These techniques must also have the capability of generating adequate porosity to further serve as a framework for cell penetration, new bone formation, and subsequent remodeling. In this study, we show how hybrid organic/inorganic scaffolds with controlled microstructures can be built using robotic assisted deposition at room temperature. Polylactide or polycaprolactone scaffolds with pore sizes ranging between 200–500 μm and hydroxyapatite contents up to 70 wt % were fabricated. Compressive tests revealed an anisotropic behavior of the scaffolds, strongly dependent on their chemical composition. The inclusion of an inorganic component increased their stiffness but they were not brittle and could be easily machined even for ceramic contents up to 70 wt %. The mechanical properties of hybrid scaffolds did not degrade significantly after 20 days in simulated body fluid. However, the stiffness of pure polylactide scaffolds increased drastically due to polymer densification. Scaffolds containing bioactive glasses were also printed. After 20 days in simulated body fluid they developed an apatite layer on their surface