Programmable stimuli-responsive zwitterionic hydrogels for soft robotic applications

Abstract

As a prominent class of actuators, stimuli-responsive hydrogels have attracted significant interest in the soft robotics community for biomedical applications. Key features of hydrogels, including high water content and similar physicochemical properties to that of tissues, make them excellent materials to be used in a wide range of biomedical applications such as drug delivery, and tissue engineering. Particularly, hydrogels with stimuli-responsiveness, self-healing, shape-morphing, low cytotoxicity, and tunable physiochemical properties can be used as functional building blocks in biomedical devices and robots, enabling minimally invasive medical procedures. Introducing programmability to the shape-morphing of hydrogels opens up new opportunities, especially, in the fabrication of remotely controllable biomedical robots. In this work, we synthesized responsive hydrogel nanocomposites and bilayers with preprogrammed shape transformations that enable desirable robotic functionalities. For this, we used zwitterionic/acrylate chemistries that impart self-healing, stimuli-responsiveness, and biocompatibility to our hydrogel system. Introducing heterogenous physiochemical properties, at the microscale, and employing a multilayering approach, at the macroscale, rendered differential swelling to the hydrogels, which were then employed as a programming strategy to facilitate 2D-to-3D shape-morphing of the hydrogel upon exposure to environmental cues. As a proof-of-concept, we demonstrated tethered and untethered soft robotic functionalities, such as actuation, magnetic locomotion, and targeted transport of soft and light cargo in confined and flooded media. Our future direction includes developing novel bio-inks from this hydrogel system for extrusion additive manufacturing given their excellent tunability of mechanical properties coupled with the shear thinning rheology of the hydrogel. We believe that the proposed hydrogel formulation will expand the portfolio of functional materials for fabricating miniaturized soft actuators for biomedical applications.