pH-Responsive Hydrogel Bilayer With Reversible, Bidirectional Bending Behavior

Abstract

Smart hydrogels are promising materials for shape-shifting structures regarding their large reversible deformation in response to external stimuli in the absence of mechanical loading. Actuators composed of responsive hydrogels have gained significant attention due to their low power consumption, bio-compatibility, fast response, and accessibility. Among these structures, bidirectional hydrogel-based actuators are more fascinating, especially when they have similar reversible bending in both directions. This paper introduces a new design concept of a hydrogel bilayer made of a poly (HEMA-co-DMAEMA) layer and a poly (HEMA-co-AA) hydrogel layer that swells at low and high pH, respectively. This structure is capable of bending in diverse directions while the pH of the aqueous bath alters. The main characteristic of this structure is having reversible bidirectional bending, which has similar behaviors in both directions, unlike previous hydrogel-elastomer bilayers. Then, we develop an analytical method to solve the swelling-induced bidirectional bending of a pH-sensitive hydrogel bilayer. On the other hand, the finite bending of bilayer structure is studied by the finite element method in several cases to demonstrate the validity and accuracy of the proposed analytical solution. Lastly, the impacts of material composition and geometrical factors are investigated to be used for bilayer actuator design and application.

A responsive supramolecular hydrogel with robust mechanical properties for actuator and strain sensor

“…[1][2][3] Owing to their high similarity to biological tissues, responses to versatile external stimuli, and ease of manufacture, stimulus-responsive hydrogels hold tremendous promise for a wide range of applications, including artificial muscles, soft actuators, intelligent sensors, controlled drug release, and so on. [4][5][6] Currently, a variety of stimulus-responsive types of hydrogels have been developed, which exhibit a transition between swelling and contraction upon stimulation of heat, [7][8][9][10][11] pH, [12][13][14] electric, 15 or light, 16,17 etc. However, conventional stimuli-responsive hydrogel actuators generally suffer from weak actuation force and slow response speed.…”

“…Currently, a variety of stimulus‐responsive types of hydrogels have been developed, which exhibit a transition between swelling and contraction upon stimulation of heat, 7–11 pH, 12–14 electric, 15 or light, 16,17 etc. However, conventional stimuli‐responsive hydrogel actuators generally suffer from weak actuation force and slow response speed 14 . The weak actuating stress is derived from the low elastic modulus and strength of the hydrogel, while strong and tough hydrogels usually respond slowly to external stimuli due to the high chemical/physical cross‐linking density that limits the movement of the chains.…”

“…However, conventional stimuli-responsive hydrogel actuators generally suffer from weak actuation force and slow response speed. 14 The weak actuating stress is derived from the low elastic modulus and strength of the hydrogel, while strong and tough hydrogels usually respond slowly to external stimuli due to the high chemical/physical cross-linking density that limits the movement of the chains.…”

Polyurethane Shape Memory Polymer/pH-Responsive Hydrogel Hybrid for Bi-Function Synergistic Actuations

“…Zhang et al [31] achieved the actuator bending/folding and imitated as the octopus-inspired soft swimmer by the combination the composite hydrogel with poly(dimethylsiloxane) with low coefficient of expansion. Shojaeifard et al [32] had constructed a bi-layer actuator with a different pH-responsive, which can have bidirectional bending and reversible behaviors under the aqueous bath altering. Nevertheless, the programmable design about the original shape of the actuator based on the structure fabrication before response which can promote the hydrogel actuator behavior development.…”