Synchronous Ultraviolet Polymerization Strategy to Improve the Interfacial Toughness of Bilayer Hydrogel Actuators

Abstract

Bilayer hydrogel actuators are of great interest in mechanical valves, soft robots, and bionic devices benefiting from their flexibility and adaptability to actuate in different environments. They respond rapidly to external stimuli through differential deformation of the internal structure to achieve the actuation effect. However, the bilayer hydrogel is prone to delamination due to the low interfacial toughness of the two gel layers, thus they exhibit poor actuating performances. In this work, a synchronous ultraviolet (UV) polymerization strategy was proposed to enhance the interfacial toughness of bilayer hydrogel actuators. Based on the synchronous UV polymerization strategy, a gelatin/poly­(N-hydroxyethyl acrylamide)–poly­(N-isopropyl acrylamide-co-N-hydroxyethyl acrylamide) [gelatin/PHEAA–P­(NIPAM-co-HEAA)] bilayer hydrogel actuator with gelatin/PHEAA functional layer and P­(NIPAM-co-HEAA) actuating layer was prepared. The obtained bilayer hydrogel showed a maximum interfacial toughness of 508.11 ± 45.62 J/m2, which was attributed to the covalent bonding and topological entanglement of polymer chains at the gel–gel interface induced by the permeation–polymerization step. In addition, the copolymerization of NIPAM with the hydrophilic monomer N-hydroxyethyl acrylamide (HEAA) increased the lower critical solution temperature of the bilayer hydrogel actuator, which allowed the actuator to exhibit stable actuating ability at 90 °C and to be used as a bionic gripper for high-temperature pickup. Overall, a synchronous UV polymerization strategy was presented. It simplified the fabrication of bilayer hydrogel actuators and enhanced the interaction between bilayer hydrogels by forming strong covalent bonding and local topological entanglement structure at the hydrogel interface, which provided a new idea for preparing bilayer hydrogel actuators with high interfacial toughness.

Stimuli-Responsive Polymer Networks: Application, Design, and Computational Exploration

“…When thinking of stimuli-responsive behaviors in any material system, temperature is certainly one of the most commonly studied and encountered in literature . Temperature is used as a stimulus in responsive networks in a variety of applications, for instance, temperature sensing, ,, liquid absorbance, − polymer electrolytes, , soft robotics, ,− biomedical applications, − , and others. − A multitude of properties can be found in these materials that make them suitable for these various applications. Temperature is often used to impart or impact these properties, such as self-healing, degradability, swellability, anisotropy, muscle-like movements, and shape memory and programmability.…”

“…For soft robotics applications, which can overlap with biomedical applications in the form of artificial muscles, the primary objective is to actuate motion. Additionally, the enhancements of generally desirable material properties are constantly being pursued. ,− Temperature is also a critical stimulus in these applications, due to its capability of controlling the actuation of muscle-like movements and contractions. Hydrogels with bilayered structures represent a useful strategy for obtaining actuated motions under stimulus, utilizing temperature-driven changes in water miscibility as mentioned above .…”

“…Additionally, the enhancements of generally desirable material properties are constantly being pursued. ,− Temperature is also a critical stimulus in these applications, due to its capability of controlling the actuation of muscle-like movements and contractions. Hydrogels with bilayered structures represent a useful strategy for obtaining actuated motions under stimulus, utilizing temperature-driven changes in water miscibility as mentioned above . Indeed, changing water capacity in response to temperature variations is one of the most common approaches to achieving actuated motions for various hydrogel-based materials. , In addition, shape memory and programmability are other desirable properties, which can be achieved through temperature-responsive dynamic chemistries such as transesterification …”

Bilayer Hydrogel Actuators with High Mechanical Properties and Programmable Actuation via the Synergy of Double-Network and Synchronized Ultraviolet Polymerization Strategies

“…Herein, we designed a high-strength UCST-type MC-AHA bilayer hydrogel driver by combining the synchronized UV polymerization strategy with the double-network method [ 29 ]. Compared to single-layer MC hydrogels (tensile stress and strain: 1205.94 KPa and 231.88%; compressive stress at MC hydrogel strain: 683.81 KPa) and AHA hydrogels (tensile stress and strain: 1188.96 KPa and 232.57%; compressive stress at AHA strain: 896.14 KPa), the MC-AHA bilayer hydrogel had a tensile stress and strain of 1161.21 KPa and 222.07% and a compressive stress of 759.87 KPa at the MC-AHA strain.…”