Manipulating Mechanical Properties with Electricity: Electroplastic Elastomer Hydrogels

Abstract

Materials and Methods Hydrogel PreparationAll reagents were purchased from Sigma-Aldrich, unless otherwise noted, and were used as received. Poly(dimethylsiloxane) (PDMS) was commercially purchased locally, under the brand name GE Silicone II Kitchen & Bath. COOH-functionalized multi-walled carbon nanotubes (COOH-MWNTs, diameter: 8-15 nm, length: 10-50 μm, 2.56 % (w/w) functional content) were purchased from cheaptubes.com. Typical hydrogel preparationSodium acrylate (4.8 g, 51 mmol) and sodium (4-styrene sulfonate) (3.2 g, 14.3 mmol) were combined with 36 mL of deionized water and gently heated (< 40 C) until all solids were dissolved. Poly(ethylene glycol) diacrylate, (PEG-DA, M n = 575, 400 μL, 0.78 mmol) was added and the mixture was purged with N 2 for 5 min. Ammonium persulfate (APS, 72 mg, 0.47 mol%) was added as a radical initiator for copolymerization. Note: adjustments in PEG-DA stoichiometry relative to the other monomers produced hydrogels that were qualitatively stiffer (increased PEG-DA) or softer (decreased PEG-DA).Depending on the dimensions of the sample being prepared, 2 to 8 mL of the reaction mixture was pipetted into a mold. For electrochemical experiments the mold for the sample was created by temporarily affixing, using PDMS adhesive, a square glass cell to a Teflon base bearing a freshly polished glassy carbon electrode (GCE). The mold/sample combination was then heated at 85 °C for 1.5 h. After cooling to RT, the hydrogel was doped by simple submersion in either a solution of 2.0 M FeCl 2 /0.5 M citric acid or 2.0 M FeCl 3 /0.5 M citric acid for a period of 20-48 h (Supplementary Fig. S1). A 1:3 ratio by volume of doping solution to pre-polymer was used.

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

“…Iron concentration was then determined by UV−vis spectroscopy. 1 Qualitatively, the Fe 3+ -gels produced by direct doping were red in color and dramatically stiffer than the OR-gel. The gels also exhibited some deswelling (79% → 44% H 2 O, w/w) due to the combination of increasing ionic strength within the gel and the volume minimizing introduction of cross-links.…”

“…This washing replaces the need for the citric acid codopant that we employed previously. 1 The introduction of iron in the +3 state to gels at pHs > 2 results in nonuniform doping as the outer edges of the sample become cross-linked and inhibit iron penetration. To facilitate doping efficiency, the iron-doping levels were controlled by timed exposure to a high concentration solution of FeCl 3 (2 M).…”

“…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.…”

Stepped Moduli in Layered Composites

“…[17] Electrical potential provides another simple way to manipulate mechanical properties. Oxidative and reductive potentials change the cross-linking capacity of ions within a gel-like matrix, [18] and electrical current induces phase transition (solid to liquid) by Joule heating. [19] Magnetorheological fluids and elastomers exhibit tunable elastic stiffness in response to an external magnetic field.…”

An Investigation on Shape Morphing by Modulus Variation: Forward Approach

“…The focus on this study is primarily to demonstrate the expansion of the spatial domain of a deformed structure due to modulus change. One of the most state-of-the-art smart materials are the Electroplastic elastomer hydrogels (EPEHs) that can reversibly switch between hard and soft states with the application of an electrical potential [17,18]. Unlike conventional intelligent materials, these materials do not exhibit any time response issue, heat-transfer induced phase transformation or heat energy dissipation.…”