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Harnessing Biology for Bioinspired Structural Materials with Intrinsic and Extrinsic Freeze Casting


Bioinspiration (also known as bioinspired design) utilizes engineering and science to take lessons from nature and employ them to advanced materials and techniques to create designs that can bring benefit to society. One such bioinspired process is that of freeze casting. This process mimics the templating fabrication method used by many biological composites where a template of a biopolymer will form and direct the deposition of biomineral. Similarly, freeze casting uses a template of growing ice crystals to form and direct the formation of a porous ceramic scaffold. The true advantage of the freeze casting process is the ability to alter the final material’s microstructure with relatively simple alterations to the processing conditions and parameters. Here these alterations and methods of controlling the final material structure are suggested to be grouped as either intrinsic control methods, those working within the freeze casting process that affect the chemistry and/or freezing dynamics, or extrinsic control methods, those working on the freeze casting process though outside forces. This dissertation describes examples of each intrinsic and extrinsic control of the freeze casting process. Intrinsic control was demonstrated through the use of chemical additives (ethanol, isopropanol, n-propanol and n-butanol) that induced enlarged structures through the formation of clathrate hydrates and hydrophobic hydration. These structures were shown to be the cause of enlarged porosity in the final scaffolds. Bioinspired, two-phase polymer-ceramic composites created from these scaffolds were shown to have increased strength over both the infiltrating polymeric phase and the uninfiltrated scaffolds due to a resistance to the primary ceramic failure mode of buckling. Extrinsic control was demonstrated through the use of sacrificial 3D printed templates that created bone-like materials with porosity at multiple length scales. These materials were shown to be biocompatible and cell viable for use as biomedical implants. Finally, while control of these structures is impressive, it was shown through statistical analyses that there is significant variability in the final microstructures and mechanical properties of materials created through freeze casting. Therefore, while the technique holds great promise, additional research is required to mass produce similar materials for biomedical and commercial applications.

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