A three-dimensional polymer network may absorb a large quantity of water and form a hydrogel. Familiar examples include jello and superabsorbent diapers. Hydrogels are under intense development for biomedical applications such as scaffolds in tissue engineering and carriers for drug delivery. Most existing hydrogels, however, are weak, brittle, and not very stretchable. They also dry out as water evaporates. These issues have severely limited the scope of applications of hydrogels. This project will develop stretchable, tough, and water-retaining hydrogels. Hydrogels of enhanced mechanical properties and environmental stability will open new applications. Examples include stretchable, transparent, ionic conductors; fire-retarding blankets; swellable seals; and environmentally responsive actuators and sensors. Large-scale use of hydrogels is attractive for a number of reasons. Many kinds of polymers can form hydrogels; this diversity enables suitable polymers to be selected to achieve specific functions. Hydrogels consist mostly of water, and can be formed by naturally occurring polymers; many hydrogels are inexpensive and environmentally friendly. The mechanical behavior of hydrogels will also serve as a vehicle to bridge the gap between research and education: Fresh insights that arise from this project will be incorporated into graduate courses; the project will offer research opportunities to high-school students, and will use Internet sites such as iMechanica and YouTube to bring recent developments in the mechanics of materials to readers worldwide.
Recent findings highlight a significant opportunity: hydrogels of enhanced properties may achieve much broader applications, well beyond those envisaged so far. This opportunity poses new scientific questions concerning the development of stretchable, tough, water-retaining hydrogels. The project will draw upon concepts of nonlinear fracture mechanics, as well as polymer science, to investigate the mechanisms that control multiple mechanical properties of hydrogels, including stretchability, stiffness, strength and toughness. The project will examine approaches that enhance the mechanical properties and environmental stability of hydrogels, by introducing energy dissipation mechanisms to toughen hydrogels, by embedding fibers to stiffen and strengthen hydrogels, by modifying weak bonds to enable healing after deformation, and by adding hygroscopic components to alter the chemical potential of water inside the gels. Energy-dissipation mechanisms of two broad types will be investigated: recoverable dissociation of weak crosslinks such as ionic bonds and crystallites, and frictional sliding between hydrogels and embedded fibers.
