Chemically Fueled Transient Porous Hydrogels as Autonomous Soft Actuators

Abstract

Biomimetic materials take inspiration from biological systems to develop transformativereconfigurable synthetic materials. One example of a biological system is the contraction of muscle fibers driven by biochemical reactions. Actin filaments are crucial to cellular functions, facilitating movement, division, and structural integrity through ATP-driven polymerization and depolymerization. This ability to self-assemble and disassemble in response to biochemical signals provides a model for creating materials that mimic the sophisticated control found in biological systems.

This thesis describes the use of a chemical reaction network to rapidly and autonomouslyreconfigure the size of a porous polymer hydrogels within tens of minutes. This hydrogel, composed of acrylic acid and acrylamide monomers, exhibits exceptional microporosity and an ability to expand approximately 150 times its original size within 15 seconds when exposed to water. The chemical reaction network utilizes a carbodiimide molecule, which transiently converts carboxylic acid moieties to anhydride bonds, causing transient shrinking of the hydrogel. The hydrogel spontaneously swelled due to hydrolysis of the anhydride bonds. We enhance the swelling rate of the porous hydrogels by adding polymer beads loaded with formaldehyde and sulfite buffer to the reaction vessel, which generate a time delayed pH increase that accelerates the hydrolysis of the anhydride bonds.

This multi-reaction network forms porous hydrogels that contract and reswell autonomously in less than 10 minutes,compared to about 1 hour without the delayed pH change. This approach not only enables a faster reconfiguration of the porous gel induced by EDC through the hydrolysis of the anhydride due to a transient pH change, but also integrates this external stimulus into a single-step, autonomous process. This work also deepens our understanding of natural and synthetic actuation systems and how to couple different reactions to enhance their response. By harnessing the principles of bioinspired actuation, our work bridges the gap between the orchestrated movements of biological systems and the engineered behavior of synthetic materials, infusing our constructs with a level of adaptability and responsiveness that mirrors the natural world.