Responsive Bilayered Hydrogel Actuators Assembled by Supramolecular Recognition
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MRS Advances © 2018 Materials Research Society DOI: 10.1557/adv.2018.222
Responsive Bilayered Hydrogel Actuators Assembled by Supramolecular Recognition Jing Chen, Jingli Yang, Guorong Gao and Jun Fu* Cixi Institute of Biomedical Engineering & Polymer and Composite Division, Ningbo Institute of Materials Technology and Engineering, Chinese Academy of Sciences, Ningbo 315201, China
ABSTRACT
Macroscopic assembling of responsive hydrogels has been used to construct soft actuators that transform their shape upon external stimuli. It remains a challenge to establish a robust assembling interface between gels. Here, we demonstrate a fabrication of bilayered hydrogel actuators assembled by host-guest recognition at the interface. The supramolecular recognition enabled efficient, rapid, and robust macroscopic assembling of hydrogels, which was utilized to create gel bilayers that were actuated upon unbalanced swelling/deswelling.
INTRODUCTION Responsive hydrogels are able to swell or deswell when exposed to different stimuli (e.g., temperature, pH, light, or electric field, etc.). Hydrogel actuators [1] assembled by responsive hydrogels are a family of versatile soft machines [2] that deform or move [3-5], and have potentials for applications [6] to artificial muscles [7], sensors, optical devices [8], walkers [9,10] and 3D micro-fabrication [11], etc. The actuation is originated from the anisotropic structure or different properties of the assembling blocks. Current strategies to prepare responsive hydrogels include the sequential polymerization of monomers, where the second gel layer is locked into the first network at the interface [12], and in-plane patterning of hydrogel films with periodic swelling variations, which led to predictable geometries due to macroscopic twisting and/or folding [13]. However, these strategies usually involve complex fabrication procedures. It remains as a challenge to develop a straightforward strategy to combine already-prepared responsive hydrogels into single devices with a robust interface [14,15] that are insensitive to most stimuli. In this work, supramolecular recognition at interface was employed for macroscopic assembling to fabricate hydrogel bilayers. First, pH responsive polyelectrolyte hydrogels are designed to combine charged monomers and host/guest moieties. Hydrogels with different pKa are able to respond to external pH stimuli differently, which is utilized as components for actuators with asymmetric responsiveness. Second, the host and guest units in the hydrogels would form inclusion complex at the interface as the gels are placed in contact, leading to the formation of Janus hydrogel bilayers. The bilayers are cyclically actuated in water/ethanol solutions. The host-anion gels (H-A), guest-cation gels (G-C) and the macroscopic assembly are illustrated in Figure 1.
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