Shape-change programming of zwitterionic hydrogels via chemical gradients directed by surface energy

Abstract

Introducing anisotropic swelling to stimuli-responsive hydrogels promises a host of opportunities for their shape-change programming and utility in soft robotic applications. Here, we report a straightforward yet robust method of creating differential swelling within pH-responsive self-healing hydrogels to program their shape-change and mode of deformation. Our strategy relies on a large disparity between the polarity of constituent zwitterionic and non-zwitterionic monomers, which facilitates their deterministic separation and migration toward confining surfaces of different surface energies. Using this method, we obtained hydrogels with a large gradient of chemical formulation across the thickness, capable of controlled and rapid bending upon exposure to a change in environmental pH. We localized such chemical gradients across the hydrogel thickness in-plane by patterning confining substrates with areas of high and low surface energy. Thanks to this strategy and the self-healing properties of our hydrogels, we achieved deformation of two-dimensional hydrogel films to complex three-dimensional structures. The proposed shape-change programming strategy offers a simple yet robust method to produce programmable actuators that are useful in soft aquatic robotic applications.