Fabrication of a biomimetic hydrogel actuator with rhythmic deformation driven by a pH oscillator

A diversified and biocompatible rhythmic deformation (RD) system is successfully fabricated by coupling a heterogeneous hydrogel with a pH oscillator. By tailoring the geometry of the building blocks, a heterogeneous hydrogel actuator with diversity could be easily constructed through interfacial adhesion. Moreover, the RD behaviour can be regulated by the system temperature and actuator shape.

Catechol-functionalized hydrogels: biomimetic design, adhesion mechanism, and biomedical applications

Hydrogels are a unique class of polymeric materials that possess an interconnected porous network across various length scales from nano- to macroscopic dimensions and exhibit remarkable structure-derived properties, including high surface area, an accommodating matrix, inherent flexibility, controllable mechanical strength, and excellent biocompatibility. Strong and robust adhesion between hydrogels and substrates is highly desirable for their integration into and subsequent performance in biomedical devices and systems. However, the adhesive behavior of hydrogels is severely weakened by the large amount of water that interacts with the adhesive groups reducing the interfacial interactions. The challenges of developing tough hydrogel-solid interfaces and robust bonding in wet conditions are analogous to the adhesion problems solved by marine organisms. Inspired by mussel adhesion, a variety of catechol-functionalized adhesive hydrogels have been developed, opening a door for the design of multi-functional platforms. This review is structured to give a comprehensive overview of adhesive hydrogels starting with the fundamental challenges of underwater adhesion, followed by synthetic approaches and fabrication techniques, as well as characterization methods, and finally their practical applications in tissue repair and regeneration, antifouling and antimicrobial applications, drug delivery, and cell encapsulation and delivery. Insights on these topics will provide rational guidelines for using nature's blueprints to develop hydrogel materials with advanced functionalities and uncompromised adhesive properties.

One-Pot and One-Step Fabrication of Salt-Responsive Bilayer Hydrogels with 2D and 3D Shape Transformations

Bilayer hydrogels are one of the most promising materials for use as soft actuators, artificial muscles, and soft robotic elements. Therefore, the development of new and simple methods for the fabrication of such hydrogels is of particular importance for both academic research and industrial applications. Herein, a facile, one-pot, and one-step methodology was used to prepare bilayer hydrogels. Specifically, several common monomers, including N-isopropyl acrylamide, acrylamide, and N-(2-hydroxyethyl)acrylamide, as well as two salt-responsive zwitterionic monomers, 3-(1-(4-vinylbenzyl)-1H-imidazol-3-ium-3-yl)propane-1-sulfonate (VBIPS) and dimethyl-(4-vinylphenyl)ammonium propane sulfonate (DVBAPS), were chosen and employed with different combinations and ratios to understand the formation and structural tunability of the bilayer hydrogels. The results indicated that a salt-responsive zwitterionic-enriched copolymer, which could precipitate from water, plays a dominant role in the formation of the bilayer structure and that the ratio between the common monomer and the zwitterionic monomer had a significant effect on the structure. Due to the salt-responsive properties of polyVBIPS and polyDVBAPS, the resultant bilayer hydrogels exhibited excellent bidirectional bending properties in response to the salt solution. With the optimal monomer pair and ratio determined, the bend of the hydrogel could be reversed from ∼-360 to ∼266° in response to a switch between water and a 1.0 M NaCl solution. Additionally, this method was further used to fabricate small-scaled patterns with structural and compositional distinction in two-dimensional hydrogel sheets. These two-dimensional hydrogel sheets exhibited complex and reversible three-dimensional shape transformations due to the different bending behaviors of the patterned hydrogel stripes under the action of an external stimulus. This work provides greater insight into the mechanism of the one-step, one-pot method fabrication of bilayer hydrogels, demonstrates the ability of this method for the preparation of small-scale patterns in hydrogel sheets to endow the complex with a three-dimensional shape transformation capability, and hopefully opens up a new pathway for the design and fabrication of smart hydrogels.