Strong, Ultrafast, Reprogrammable Hydrogel Actuators with Muscle-Mimetic Aligned Fibrous Structures

Abstract

Hydrogel actuators displaying programmable shape transformations promise to be core components in future biomedical and soft robotic devices. However, current hydrogel actuators have shortcomings, including poor mechanical properties, slow response, and lack of shape reprogrammability, which limit their practical applications . Existing molecular designs offer limited efficiency in synergistically addressing these issues in a single hydrogel system. Herein, we propose a strategy to develop hydrogel actuators with muscle-mimetic aligned microfibrillar morphology, combining thermoinduced microphase separation and mechanical alignment. The key to our design is the introduction of metal–phenolic complexes, which not only induce irreversible sol–gel transition via the concentrated coordinate ions above lower critical solution temperature (LCST) but also fix the alignment of bundle network due to dynamic network rearrangement. Our design concept is observed to simultaneously achieve excellent mechanical properties (tensile strength ≈ 1.27 MPa, toughness ≈ 2.0 MJ m–3) and ultrafast actuation (40.1% thermal contraction as short as 1 s), which is a long-lasting challenge in the field. In addition, the dynamic hydrogels can be reprogrammed into spiral, helical, and biomimetic actuators. This work opens new opportunities to realize real-world applications for smart hydrogels as soft machines by fundamentally breaking the current property limit.

Influence of theα-Methyl Group on Elastic-To-Glassy Transition of Supramolecular Hydrogels with Hydrogen-Bond Associations

“…Supramolecular hydrogels with dynamic associations as physical cross-links are a class of soft materials with fascinating viscoelastic behaviors, when compared to hydrogels cross-linked by permanent covalent bonds. − So far, numerous tough hydrogels have been developed by incorporating physical associations (e.g., van der Waals interactions, hydrogen bonds, ionic bonds, and coordination bonds). , These supramolecular hydrogels possess balanced properties, including high mechanical toughness, self-recovery, and good processability, which greatly broaden the applications of hydrogel materials in biomedical and engineering fields. − Hydrogen bonds, as a prevalent non-covalent interaction, exist widely in living systems and play a fundamental role in life activities. − A typical hydrogen bond is formed as X–H···Y, in which X and Y are the proton donor and acceptor, respectively . Generally, a singular hydrogen bond is recognized as a relatively weak interaction with the dissociation energy about 5–17 kJ/mol. , However, the cooperativity of hydrogen bonds can significantly enhance the strength to be robust associations, accompanying with the increase in dissociation energy .…”

Hydrogel-based strong and fast actuators by electroosmotic turgor pressure

“…Most hydrogel actuators suffer from low speeds due to their diffusion-based actuation mechanisms. Many studies have attempted to improve these actuation speeds by increasing diffusion rates or using different actuation mechanisms; these include the molecular engineering of stimuli-responsive hydrogels (12)(13)(14)(15), the incorporation of active materials into hydrogel matrices (16), use of the elastic potential energy of the hydrogel network (17), and the pneumatic or hydraulic actuation of the hydrogel cover structure (18). Despite substantial improvements in speed, actuation forces have been limited to <1 N (19) because hydrogels are intrinsically soft.…”

Hierarchically Structured Hydrogel Actuator for Microplastic Pollutant Detection and Removal

“…Recently, active stimulus-responsive actuating materials, which could change length, volume, or shape under the variation of external pH, temperature, light, humidity, electrical, or magnetic fields, have been specially designed to execute complicated human-unattainable tasks (e.g., thermal-expansion polymers, hydrogel materials, photoresponsive liquid crystal networks, magnetic composites, and dielectric elastomer). − Most of them could generate a huge actuating deformation, nevertheless, they are limited by single-direction actuation, complicated fabrication, and insufficient actuating force and thereby show inferior capabilities to operate under a fluid environment. Micro- and nanomotors are newly emerging tools that can work as the environmental microcleaners or sensors. − However, it is extremely tough to control the underwater movement of such nanolevel-sized machines by means of the unpredictable chemical self-propelled reaction.…”