Ice-templated Hybrid Materials

Ulrike Wegst, Assistant Professor, Drexel University

Monday, February 7, 2011

This seminar is part of the Jones Seminars on Science, Technology, and Society series

Initially developed for the manufacture of highly dense ceramics, freeze-casting, a process that uses the solidification of a liquid carrier such as water for templating, has in recent years been discovered as a route to create highly porous hybrid materials with complex, hierarchical architectures. Freeze-casting is highly attractive for the manufacture of materials for applications that range from scaffolds for tissue engineering to structures for energy generation because it offers several advantages over other techniques. One advantage is that all classes of materials — polymers, ceramics, metals and their composites — can be shaped with it; another is that materials can be processed with benign, biocompatible liquid carriers; a third is that the resulting hierarchical microstructures can be carefully controlled by both the physical and chemical properties of the components used and the processing parameters such as the cooling rate; finally advantage can be taken of component self-assembly during solidification. The amount, type, size and geometry of the particles and the type of liquid carrier determine the slurry's viscosity and amount of sedimentation as well as the slurry's thermal properties and freezing behavior. In combination with the freezing front velocity and additives, they also determine pore connectivity and morphometry. As a result, the thickness and spacing of the cell walls and the size and the number of the material bridges between them can be controlled, as can be the cell wall's bulk and surface properties, and thus the materials interaction with a second phase. This is important for the manufacture of composites by infiltration or for the optimization of the interaction between scaffold and native tissue in biomedical applications. Material composition and processing parameters further determine the anisotropy ratio between the strong and stiff freezing direction and the relatively weaker direction perpendicular to it. As a result, the freeze-casting process is ideally suited for the custom-designed manufacture of complex, hybrid materials with hierarchical structures. By freeze-casting it is possible to emulate in synthetic materials multi-level hierarchical composite structures, which are thought to be the origin of the mechanical property amplification which is frequently observed in biological materials. Systematic studies are under way to correlate the composition, structure and properties of freeze-cast materials with processing parameters.

About the Speaker

Dr. Ulrike G. K. Wegst, Anne Stevens Assistant Professor in the Department of Materials Science and Engineering of Drexel University, studied Physics and Materials Science at the Georg-August-Universität Göttingen, Germany and at the University of Cambridge, UK. She received her PhD from the University of Cambridge for her analysis of the Mechanical Performance of Natural Materials. Until 2000 she worked as a Research Associate in the Engineering Design Centre of the Cambridge University Engineering Department on the development of a software-based methodology for the environmentally-conscious selection of materials and processes, since then implemented in the CES Eco-Selector software. From 2000 to 2001 Ulrike Wegst was a Visiting Scientist at the Institut National Polytechnique de Grenoble, France, where she started her work on the qualitative and quantitative characterization of biological materials using synchrotron-generated X-rays. From 2001 to 2007 she was a staff scientist at the Max Planck Institute for Metals Research, Stuttgart, Germany. Since 2005, she is a Faculty Guest Scientist at the Lawrence Berkeley National Laboratory. The research of Dr. Wegst's focuses on three main topics: biological materials, biomimetics and multifunctional hybrid materials. Important in all is a systematic understanding of the relationship between structure, properties and function. To achieve this, mechanical property measurements at a number of length scales of the material's hierarchical structure, ranging from macroscopical to in situ testing in SEM and FIB, are combined with microstructural characterization by electron microscopy and X-ray tomography, and modeling. The lessons learned are captured in the software-based Biomimetic Design Guide tool to enable the systematic transfer of biological principles of function and efficiency to technology. Dr. Wegst applies biomimetic principles in the design of novel hybrid materials and their manufacture by freeze-casting ('ice-templating') for applications that range from biomaterials for spinal cord repair and bone regeneration to multifunctional materials for energy generation and those in which a particular combination of structural, mechanical, optical, thermal and electrical properties is required.