Abstract
Ionoprinting has proven itself as a technique capable of enabling repeated post-synthesis programming of hydrogels into a variety of different shapes, achieved through a variety of different actuation pathways. To date, the technique of ionoprinting has been limited to conventional brittle hydrogels, with reversible actuation requiring a change in submersion solution. In this study, ionoprinting has been combined for the first time with a tougher interpenetrating network polymer (IPN) hydrogel with dual pH and temperature responsiveness. This new methodology eliminates the brittle material failure typically occurring during shape change programming and actuation in hydrogels, thus allowing for the realisation of more highly strained and complex shape formation than previously demonstrated. Critically, the temperature responsiveness of this system enables actuation between an unfolded (2D) and a folded (3D) shape through an external stimuli; enabling reversible actuation without a change in submersion solution. Here, the reversible thermally induced actuation is demonstrated for the first time through the formation of complex multi-folded architectures, including an origami crane bird and Miura folds, from flat hydrogel sheets. The robustness of the IPN hydrogel is demonstrated through multiple reprogramming cycles and repeated actuation of a single hydrogel sheet formed into 3D shapes (hexagon, helix and zig-zag). These advancements vastly improve the applicability of ionoprinting extending its application into areas of soft robotics, biomedical engineering and enviro intelligent sensors.
Highlights
Hydrogels are three-dimensional polymer networks which are able to absorb large amounts of water [1]
The process of ionoprinting employs an oxidative bias generated by an electric field to produce metal cations, these are utilised for ionic crosslinking within a hydrogel [19]
This study has shown that the process of ionoprinting can be used as a post-synthesis technique for the programming of a homogenous hydrogel into a variety of complex shape changes
Summary
Hydrogels are three-dimensional polymer networks which are able to absorb large amounts of water [1]. A number of approaches have been shown to significantly increase the toughness of these materials, the most widely demonstrated has been the utilisation of multiple polymer networks [3,4]. Such additional polymer networks may involve covalent crosslinking The dynamic nature of the ionic crosslinks allows them to be formed after manufacturing through the introduction of metal cations and destroyed on demand by removal of the metal cations through a competing coordinating species This dynamic nature allows for post-synthesis shape change programming and subsequent removal of the cations, returning the hydrogel to its originally synthesised state, allowing for new re-programming of further shape changes. The shape changes demonstrated to date have been very limited and simplistic, which is believed to be due to the inability to create complex reversible shape changes derived from brittle hydrogels (due to material failure during ionoprinting and actuating conditions)
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