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

Abstract

Novel multi-stage adaptive morphing of a hydrogel cube has been achieved by combining multi-metal ionoprinting and redox chemistry of iron. A demonstration of the two-stage deployment has been shown for (1) the selective opening and closing of the cube’s lid, where the hinge point has been ionoprinted with iron, and (2) the full unfolding and folding of the cube into its cruciform net, with remaining hinges ionoprinted with vanadium. The selective unfolding and folding is achieved by alternating the oxidation state of iron between +2 and +3. This is achieved using redox chemistry selective for iron. This approach could be applied, in principle, to more degrees of staging by adding additional redox responsive ionoprinted cations and appropriate selection of reducing agents.

4D sequential actuation: combining ionoprinting and redox chemistry in hydrogels

“…In this study, we present a hydrogel actuator that can perform controlled sequential actuation through the combination of ionoprinting with selective redox reactions. Sequential actuation is demonstrated through the application of ionoprinted vanadium and iron hinge lines, where the iron hinge is triggered to selectively unfold through the selective reduction of Fe 3+ by a mild reducing agent, ascorbic acid, in the similar manner as demonstrated by the authors' previous proof-of-concept work on the same cruciform-cube geometry [21]. Here we present the first study to show controlled sequential actuation of hydrogels, with control over the initial and final angle created and the rate and order of unfolding.…”

“…These regional cations cause localized ionic crosslinking, deswelling and stiffening of the hydrogel resulting in the creation of a foldline along the ionoprinted hinge. Functionality of side groups that have been previously used within ionoprinted hydrogels include acrylic acid, catechol and phosphate [18][19][20][21]. Ionoprinting has been successfully used to print cations of copper, iron, zinc, vanadium, calcium, aluminum and titanium [18][19][20][21][22].…”

“…Functionality of side groups that have been previously used within ionoprinted hydrogels include acrylic acid, catechol and phosphate [18][19][20][21]. Ionoprinting has been successfully used to print cations of copper, iron, zinc, vanadium, calcium, aluminum and titanium [18][19][20][21][22].…”

“…The ionoprinting of iron has been explored in the open literature, with iron being oxidized to iron(III) cations (Fe 3+ ) [21][22][23][24]. The presence of adjacent, relatively stable oxidation states of iron (+2/+3), that can switch between 'hard' and 'soft' cations, has resulted in their use in polymeric systems for shape-memory, self-healing and sol-gel transition [25][26][27][28][29].…”

Materials with Electroprogrammable Stiffness

“…To date, cations of copper, aluminum, titanium, iron, vanadium, calcium, and zinc have been successfully ionoprinted in different types of hydrogels containing functional catechol, phosphate, and hydroxyl side groups. [ 142–148 ] For example, inspired by the composition of mussel secretions critical to wet adhesion, a dopamine methylacrylamide (DMA) hydrogel was prepared with network‐bound catechol, which was capable of forming strong complexes with metal ions of various types, including aluminum, titanium, copper, iron, and zinc. [ 144,149 ] By immersing the gel in solutions of varying pH, Lee and coworkers could control the stoichiometry of the ionoprinted catechol–metal ion complex, which affected local crosslinking densities at ionoprinted sites and enabled reversible actuation.…”