Designing Highly Stretchable Resilient Hydrogels

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PART 1: NON-TECHNICAL SUMMARYHydrogels are being used in many applications including in the biomedical field, pharmaceutical formulations, and food materials. Developing highly stretchable and resilient hydrogels could open up additional applications of these materials, for example in artificial muscles, in prosthetic devices towards assisting individuals with restricted mobility, energy harvesting, and enabling power amplification process in soft-robots. Biological elastomeric proteins such as resilin display high stretchability and resilience, but water-containing polymer networks prepared from synthetic monomers typically do not display such properties. High stretchability and resilience in biological elastomeric proteins have been attributed to the balanced combination of entropic and enthalpic elasticity originating from the presence of hydrophilic and hydrophobic segments. In the proposed research, taking inspiration from these biopolymers, hydrogels with hydrophilic and hydrophobic contents will be synthesized using a facile chemical synthesis route. The synthesis strategy and characterization techniques that would be developed and implemented in this project will be applicable to other soft materials. Because of the cross-disciplinary nature of the project, the proposed research will impact many disciplines viz., polymer science, physics, robotics, and biology. In addition to the direct involvement of undergraduate and graduate students in the proposed research activities, the educational and outreach component will also include laboratory demonstrations and active involvement of K-12 students in research activities with the goal of introducing them to polymer science and engineering and encouraging interest in STEM. PART 2: TECHNICAL SUMMARYIn this project, the origin and nature of elasticity in hydrogels consisting of both hydrophilic and hydrophobic components will be investigated. Particularly, the hydrogel considered for this project will be obtained by using free-radical polymerization of acrylic acid (AAc), methacrylamide (MAAm), and polypropylene glycol diacrylate (PPGDA). PPGDA is sparsely soluble in water at room temperature and will act as a hydrophobic block. Multiples of those blocks can associate to form hydrophobic domains in these gels. The resultant hydrogel is highly stretchable, resilient, and can achieve very high retraction velocity when released from a stretched state. It has been hypothesized that during stretching, the hydrophilic blocks display entropic elasticity, whereas dissociation of the hydrophobic domains, unwinding and stretching of the collapsed polypropylene glycol blocks in the aqueous media will have a significant enthalpic penalty. Similar to that observed in protein elastomers, a balance in enthalpic and entropic elasticity will render a hydrogel stiff, stretchable, and resilient. However, a fundamental understanding linking the structure and properties of these hydrogels is not clearly available. In the planned research, such will be addressed by systematically varying the monomer concentrations, hydrophobic content, hydrophobic block molecular weight, and solvent quality or temperature. Variation of these conditions will likely lead to the change in gel structure, which will then be linked to gel properties, particularly stretchability and resilience. A systematic understanding will lead to establishing design principles for synthesizing stretchable and resilient hydrogels from synthetic polymers..This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.