Novel Multi-Stage Three-Dimensional Deployment Employing Ionoprinting of Hydrogel Actuators
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Novel Multi-Stage Three-Dimensional Deployment Employing Ionoprinting of Hydrogel Actuators Anna B. Baker1, Duncan F. Wass2 and Richard S. Trask1 1 Aerospace Engineering, Queen's Building, University Walk, Clifton, Bristol, BS8 1TR, UK 2 School of Chemistry, Cantock's Close, Clifton, Bristol, BS8 1TS, UK 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. INTRODUCTION Hydrogels are three-dimensional hydrophilic polymer networks that can be highly swollen in water to form a gel that has a water content approaching 99%; they are also able to respond to a range of stimuli including pH, temperature, light and ionic strength [1]. Their use as actuation materials and structures for biomedical and soft-robotics applications has been widely studied recently; with the desired actuation pathway often being created by inhomogeneity to the material or structure either during construction or synthesis, or as presented here, programmed into the material post construction or synthesis [2–5]. Ionoprinting is a technique used to pattern metal cations into the surface and near surface of a hydrogel using an electric potential. The localized deposition of metal cations creates regional ionic crosslinking resulting in volumetric changes in the hydrogel, giving rise to a controlled variation in mechanical performance of the system. This patterning technique introduces localized stress within the hydrogel which can be used to create folding patterns for the actuation and morphing of different geometries [6]. 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 [5,7–10]. 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]. The two oxidation states of iron are separated by reduction potential of 0.771V [12]. Hierarchical multi-step folding of hydrogel bilayers has been achieved by G. Stoychev et al. by use of an active hydrogel layer and passive polymer layer [13]. The hierarchical multi-step folding behavior is created and controlled by the diffusion front and the shape of the bilayer, resulting in a s
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