Photocurable Emulsion-Templated Organohydrogels with Elastomer-in-Hydrogel Architecture for pH-Responsive Soft Actuation

Abstract

Soft actuators are stimuli-responsive materials capable of reversibly changing their shape/volume owing to external stimuli such as pH, temperature, and light. Hydrogels based on methacrylated chitosan (CsMa) are interesting pH-responsive biomaterials that exhibit high water uptake, pKa around 6.0, biocompatibility, and permeability to (bio)molecules. However, their poor mechanical properties often limit their application as soft actuators. In contrast, methacrylated poly(glycerol sebacate) (PGSeMa) elastomers are mechanically robust and stable across a broad range of conditions but nonresponsive to pH changes. Therefore, these hydrogels and elastomers present complementary properties that allow their integration into an elastomer-in-hydrogel architecture for the development of pH-responsive soft actuators. Herein, we present a one-step preparation of an organohydrogel via photo-cross-linking of an oil-in-water emulsion of PGSeMa-in-CsMa stabilized by the cross-linkable surfactant Pluronic F-127 dimethacrylate (F127Ma). The resulting organohydrogels display better mechanical properties, with a compressive modulus from 60 to 220 kPa, than hydrogels based on CsMa or other methacrylated biomacromolecules (<60 kPa), while maintaining a high degree of swelling (>200%) and preserving intrinsic chitosan pH responsivity. This strategy enables the use of the CsMa-based organohydrogel to access a bilayer soft actuator with suitable interfacial adhesion between the layers through one-step photo-cross-linking of the lighter PGSeMa solution over the denser organohydrogel emulsion precursor. These organohydrogels and bilayer actuators display great potential to be used as soft actuators within a physiologically relevant pH range (4.5–7.5). Therefore, this one-step preparation of organohydrogel with elastomer-in-hydrogel architecture is a versatile strategy to improve the mechanical properties of methacrylated biomacromolecule-based hydrogels without compromising their stimuli responsiveness, providing a direct protocol for the fabrication of bilayer soft actuators from renewable resources.