Design Strategies for Dynamic Self-assembled Protein Materials
Carter, Nathan Andrew
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Structures in nature exhibit unique and complex architectures whose order propagates from nano- (10-9 m) to macro-scales (mm to m). These structures give rise to a rich diversity of adaptive function that allows for life in all environments on Earth. This complex functionality has driven research into bio-inspired materials where scientists investigate the complex relationship between sequence, structure and function of these materials. A good illustrative example of the effect that hierarchical structure can have is a brick wall. Bricks are laid so that the layer on top is shifted in either direction by half of a brick. This alternating pattern is what gives the wall its strength. If a crack occurs in the mortar, it will only propagate until it hits a boundary (a neighboring brick). Designing nanostructures can have similar effects on materials we use every day. Some of the most prevalent are adhesives that mimic the structures on gecko feet, which allow them to stick to any surface. This work presents bottom-up design strategies for self-assembling protein materials whose hierarchical structure may prove useful in a variety of applications in soft-robotics and energy storage. Proteins are a useful class of molecules, because they contain a level of structural complexity beyond that of synthetic materials. They are an inherently 'green' material feedstock; made in a lab using microbes like E. coli. Additionally, with the ease and availability of genetic engineering techniques we can easily modify the structure. This is especially true for the class of proteins, repeat proteins, which are the focus of this manuscript. Repeat proteins comprise small repeated sequences which are structurally independent from each other and can be strung together to create open, extended architectures. Here we explore the self-assembly emergent properties of the consensus tetratricopeptide repeat (CTPR18) . We show that this protein assembles into highly ordered 1D and 2D arrays that are shape tunable based the molecular environment (solvents, charge, etc). These nanomaterials may prove useful as molecular recognition scaffolds. We further explore the hierarchical self-assembled films of CTPR18. These films form highly oriented lamellar structures that seemingly propagate the entire length of the films. These lamellae directly affect the materials mechanical properties. Accordingly, by changing the film casting conditions, we can impart a structural gradient in the film, which proves useful in tuning the water-induced bending motion of these films. Herein, we show the ability to change the speed and directionality of actuation by simply changing the underlying film morphology. Lastly, we show that these films are electroresponsive as well, owing this function to ion transport through the inherently charged character of CTPR18. These dual responsive materials may prove useful in soft robotics. Additionally we are beginning investigations into the usefulness of CTPR18 films as alternate materials for ion-transport materials like those used in lithium polymer (more commonly LiPo) and sodium-ion batteries.
- Doctoral Dissertations