Redox-Responsive Artificial Molecular Muscles: Reversible Radical-Based Self-Assembly for Actuating Hydrogels

Abstract

Interest in the design and development of artificial molecular muscles has inspired scientists to pursue new stimuli-responsive systems capable of exhibiting a physical and mechanical change in a material in response to one or more external environmental cues. Over the past few decades, many different types of stimuli have been investigated as a means to actuate materials. In particular, materials that respond to reduction and oxidation of their constituent molecular components have shown great promise on account of their ability to be activated either chemically or electrochemically. Here, we introduce a novel redox-responsive mechanism of actuation in hydrogels by describing a systematic investigation into the radical-based self-assembly of a series of unimolecular viologen-based oligomeric links, present at only 5 mol % of the polymer linkers in a three-dimensional network. The actuation process results in an overall reversible contraction of a family of hydrogels, down to 35% of their original volume in the first 25 min and ultimately to 9% after a few hours, even while remaining submerged in water. The mechanism of contraction starts with a decrease in electrostatic repulsion upon chemical reduction, leading to a loss of counterions and intramolecular self-assembly of the main-chain viologen subunits. The overall mode of actuation takes place relatively quickly in comparison to hydrogels of similar size, and the rate of contraction is accelerated as higher molecular weight oligoviologen links are implemented. The contraction process ultimately leads to a 2-fold increase in elasticity of the material, and upon exposure to oxygen and water, the hydrogels quickly oxidize and regain their original size and mechanical properties, thus resulting in a reversible actuation process that is capable of lifting objects which are 5–6 times heavier than the contracted hydrogel itself.

Functional, Redox-Responsive Poly(phenylene sulfide)-Based Gels

“…As exemplified by the distinctly different swelling changes between 16a and 16d, the ability to tune volume contraction by cross-linking constitutes a valuable feature for realization of actuation technologies. Overall, the oxidation-induced contraction exhibited by the PPSHS gels offers behavior complementary to a recently reported viologen-based redox-triggered gel system, 31,32 which is contracted in the reduced form. We discovered that repeated cycles of oxidation and thiol conjugate addition ultimately results in degelation for all but the most highly crosslinked gel 12d (reactions i)-vi), Figure 3).…”

Trends in polymeric shape memory hydrogels and hydrogel actuators

“…126,127 Particularly poly(amido amine)-based redox-responsive hydrogels have, due to their property of being biocompatible and highly versatile, gained significant attention. Greene et al 128 introduced a redox-responsive mechanism of actuation in hydrogels by describing a systematic investigation into the radical-based self-assembly of a series of unimolecular viologen-based oligomeric links. Moreover, an artificial molecular muscle at work was demonstrated ( Fig.…”

Hydrogel‐Based Artificial Muscles: Overview and Recent Progress

“…In addition, the generation of hydroxyl ion and the resultant increment of the pH value through electrochemical reaction around cathode can stimulate the deformation of pH‐responsive hydrogels . In other cases, the deformation of a redox‐responsive hydrogel can be triggered electrochemically …”

“…The most fundamental application is a device that pushes or lift an object . Actuators that generate simple contraction and expansion motion also can provide such work, but bending motion or directionally amplified motion is needed for high efficiency in most cases.…”