Modular Assembly And Complex Shape Changing Of Stimuli-Responsive Hydrogels

Stimuli-responsive shape changing hydrogels can change their shapes under appropriate environmental stimuli. They represent one class of important hydrogels which can be used widely in the development and design of smart or bio-mimetic devices. However, they typically have monotonous shape changing behavior, exhibiting simple shrinking/swelling, which greatly limits the applications. Therefore, it is a crucial research direction to develop stimuli-responsive hydrogels with complex shape changing behaviorsIn this dissertation, A novel "modular assembly" approach was investigated to integrate hydrogel "building blocks" into hydrogel systems with various shapes and complex shape changings, utilizing host-guest interaction on the macroscopic interface. This approach bears similarity with Lego toys which rely on mechanical locking to create complex 3D geometries in a modular fashion.Two different types of host hydrogels with β-cyclodextrin (β-CD) host units in their structures were synthesized. The first type of host hydrogels (responsive host hydrogels, RH hydrogels) provided pH/ionic strength responsiveness or thermo-responsive isotropic volume change behavior. The second (non-responsive host hydrogels, NRH hydrogels) without responsiveness was also prepared. In addition, a guest hydrogel (non-responsive guest hydrogels, NRG hydrogels) was synthesized as a non-responsive hydrogel functionalized with guest groups (ferrocene, Fc) in its structure.Using these hydrogel building blocks, we assembled multi-block systems based on the host-guest interaction between the interface of host and guest hydrogels. Herein, the guest hydrogel blocks function as "double side adhesives" to hold the host hydrogel blocks together. Under an appropriate external stimulus, the shrinking/swelling of stimuli-responsive blocks are constrained by the non-responsive hydrogels. As a result, the entire system showed bendings or other complex 3-D shape changing behavior. These shape changings can be controlled accurately, through appropriate mechanical designs. This approach permits assembling of mechanically pre-deformed hydrogel blocks, further extending the capability in ways that are difficult with known metheds own sophisticated shape changing capability. Additionally, the system can be disassembled back into the original building blocks under oxidizing condition, because the β-CD-Fc inclusion can dynamically associate/dissociate under reduced and oxidized states respectively. The disassembled host hydrogel blocks can be used repeatedly and assembed into new systems with different shape changing behaviors under the same stimulus.In addition to use guest hydrogel blocks as "double side adhesives", we aimed to explore the use of solution of polymer containing guest groups as "glue". Here, we prepared linear copolymers with Adamantyl pendant groups. With its aqueous solution as supramolecular guest "glue", we can assemble host hydrogel blocks into diverse hydrogel systems directly. Compared with the guest hydrogel "double sides adhesive", the "glue" has three advantages:firstly, host hydrogel blocks can be assembled seamlessly, because the thickness of the bonding interface is almost negligible. Secondly, the interface was transparent because the "glue" is colorless and can be distributed uniformly on the surface of the host hydrogels. Finally, we can select specific local surface of the hydrogel blocks freely to assemble.We designed and assembled several functional systems using the "glue" approach including a responsive tunable convex lens and a responsive buckling surface system. Venus flytrap is a miraculous plant, which can close its leaves suddenly to catch an insect. Inspired by that, we designed a hydrogel system that when triggered externally, can change shape slowly initially, yet can undergo a sudden large shape change beyond a critical point, much like the action of the Venus flytrap. More importantly, the system can reversibly snap back to the original shape. This is even beyond the capability of Venus flytrap and any known synthetic material system.