Bio-inspired reversible crosslinking, using chelating polymers and metal ion binding, for use as soft actuation and selective growth

Ionoprinting, a technique used to pattern hydrogels with metal cations, has been used to create hydrogel actuation. The technique prints metal cations into the surface and near surface of a hydrogel creating localised ionic crosslinking and contraction, resulting in rapid structural morphing of the hydrogel. A synthetic acrylate based hydrogel has been developed that contains 20 mol% phosphate functionalised side groups; the phosphate functional group is found in many protein based underwater adhesives utilising polypeptide metal cation bonding to promote “wet” adhesion. The crosslinking strength of varies metal cations with this phosphate containing hydrogel have been determined, with vanadium(III) and (IV), iron(III), manganese(II) and calcium(II) cations all significantly contracting the hydrogel. The magnitude of the ionic crosslinking is ion concentration dependent, with an increase in concentration causing increasing crosslinking and decreasing swelling, until system saturation is reach. The global swelling of the hydrogel can be controlled by solvent immersion. It was observed that the polarity of the solvent had a direct linear relationship to the magnitude of the swelling, i.e. the greater the polarity of the solvent the greater the swelling. In our study, Ionoprinting has been used to create 3D polygons and origami shapes from flat hydrogels; we have created a morphing trianglar based pyramid, a cube, an octahedron and an “umbrella”. The printed 3D shapes can be unfolded in one of two ways; either the immersion of the hydrogel into a less polar solvent, retaining the ionoprinted lines, or immersion in a solution of chelating agent thus removing the ionoprinted lines. The unfolded polygon can be refolded again by placement into a more polar solvent or through re-ionoprinting again, respectively. Ionoprinting offers a simple and effective way of creating morphing shape-changing hydrogels, a process that is both reversible and reprogrammable.