Stimuli-Responsive Iron-Cross-Linked Hydrogels That Undergo Redox-Driven Switching between Hard and Soft States

Abstract

A unique class of stimuli-responsive hydrogels, termed electroplastic elastomers (EPEs), whose mechanical properties can be reversibly tuned between hard and soft states with the application of an electric potential, is described. Electrochemically reversible cross-links formed within a permanent, covalently crosslinked polymeric hydrogel network are switched between strongly binding Fe 3+ and weak to nonbinding Fe 2+ , as determined by potentiometric titration. With the incorporation of graphene oxide (GO) into the EPE, a significant enhancement in modulus and toughness was observed, allowing for the preparation of thinner EPE samples, 80−100 μm in thickness, which could be reversibly cycled between soft (Young's modulus: ∼0.38 MPa) and hard (∼2.3 MPa) states over 30 min. Further characterization of EPE samples by magnetic susceptibility measurements suggests the formation of multinuclear iron clusters within the gel. ■ INTRODUCTIONWe report herein the synthesis and extensive characterization of a unique class of stimuli-responsive hydrogels, which we term electroplastic elastomers (EPEs). These iron-cross-linked hydrogels can be transitioned through a continuum from soft to hard by the application of an electrochemical stimulus that reversibly switches the iron between +2 and +3 oxidation states. A portion of this work has been previously communicated. 1 As has been established in the past few decades, stimuliresponsive hydrogels are attractive candidates for a broad range of applications such as tissue engineering, 2 drug and protein delivery, 3,4 sensors, 5 actuators, 6 and energy storage. 7 A variety of stimuli, including light, 8 temperature, 9 pH, ionic strength, electric field, 10 magnetic field, 11 enzyme catalysis, 12 and redox reactions, 13 have been utilized to elicit changes in one or more properties of these materials, with typical responses being degradation, reversible swelling/deswelling, actuation, sol−gel transition, 14 change in wavelength of diffracted light, changes in mechanical properties, 15 and self-healing. 16 Among these many stimuli, we have chosen to focus our attention on electricity because the addition or removal of reagents or heat, which is particularly suitable for some applications, is not always desirable or achievable.In our responsive EPE hydrogels we harness the unique ability of metal ions to act as reversible cross-links in polymeric systems. There are numerous examples of the exploitation of this characteristic in previously reported stimuli-responsive metal-containing polymers and supramolecular materials. 7,17−20 Of particular relevance to our work are those systems that rely on redox-driven changes in coordination around the metal center. Iron, due to the accessibility and stability of the +2 and +3 oxidation states, is one of the most widely exploited metals.Tong and co-workers have, for example, reported on the reversible sol−gel transitioning of poly(acrylic acid) (p(AA)) using the Fe 2+ /Fe 3+ redox couple as controlled by light, air oxidation, 21 or an applied p...

Multiple Shape Memory, Self-Healable, and Supertough PAA-GO-Fe3+Hydrogel

“…In the present work, the A4GO3.3 hydrogel after immersed in FeCl 3 /HCl for 2 h was further immersed in pure water for 1 h, and the strength increases from 1.5 MPa (curve a in Figure ) to nearly 2 MPa (curve b in Figure ). The Fe 3+ reinforced PAA hydrogels can also be softened again, Meyer and co‐workers reported a Fe 3+ cross‐linked PAA‐GO hydrogel with redox‐driving state change between hard and soft triggered by electric potential . In the present study, the strength of the F‐A4GO3.3 hydrogel after immersed in pure water for 1 h is found to be greatly decreased to 0.7 MPa by immersing in 1 m of HCl for only 0.25 h (curve c in Figure ).…”

“…Figure A displays the extraction time dependence of the Fe 3+ concentration extracted by HCl from the F‐A4GO3.3 hydrogel previously treated in FeCl 3 /HCl. The Fe 3+ concentration increases at first 6 h and then levels off up to 24 h. This fact together with the curve d in Figure suggests that the most proportion of the Fe 3+ ions in the hydrogel would be dissolved out by HCl, resulting in the strength of the as‐prepared hydrogel without Fe 3+ . Based on this concept, we determined the Fe 3+ content in the F‐A4GO3.3 hydrogel after FeCl 3 /HCl immersion by extraction in 1 m HCl for 24 h. Figure B manifests that the Fe 3+ content in the hydrogel increases with increasing immersion time, in consistent with that in Figure B.…”

“…In order to decelerate the fast cross‐linking of Fe 3+ with the carboxyl groups and to avoid the hard shell, we introduced HCl into FeCl 3 solution, because the bonding of Fe 3+ to PAA is decreased at low pH . This is due to the competition binding of Fe 3+ ions and H + ions to the COO − groups at PAA, which shifts to COOH formation at high H + concentration.…”

Electroresponsive Supramolecular Graphene Oxide Hydrogels for Active Bacteria Adsorption and Removal

“…[13][14][15] Electro-responsive hydrogels, as one of the attractive smart hydrogels, can change their physical properties, such as mechanical strength, volume, conductivity, and optical transparency in the presence of electric fields. [16][17][18][19][20][21][22][23][24] Such electrically controlled switch may lead to distinct bacteria adsorption/desorption behaviors of the hydrogels. Therefore, it is possible to use such electro-responsive hydrogels as novel bacteria removal materials.…”

Novel Multi-Stage Three-Dimensional Deployment Employing Ionoprinting of Hydrogel Actuators

“…The easily accessible oxidation states of iron have been used by different research groups to induce variable strength ionic crosslinking for shape-memory, self-healing, and sol-gel transition [5,[7][8][9][10]. Iron(III) cations can been described as "hard" cations binding strongly to oxygen atoms in negatively charged ligands over others; while iron(II) cations are "soft" cations which prefer to bind to neutrally charged ligands [11].…”