Polydopamine-Modified MXene-Integrated Poly(N-isopropylacrylamide) to Construct Ultrafast Photoresponsive Bilayer Hydrogel Actuators with Smart Adhesion

Abstract

In nature, living organisms, such as octopuses, cabrito, and frogs, have already evolved admirable adhesive abilities for better movement and predation in response to the surroundings. Inspired by biological structures, researchers have made enormous efforts in developing actuators that can respond to external stimuli, while such adhesive property is very desired, yet there is still limited research in responsive hydrogel actuators. Here, a bilayer actuator with high stretchability and robust interface bonding is presented, which has a smart adhesion and thermoreception function. The system consists of an adhesive passive layer copolymerized of amphoteric ([2-(methacryloyloxy) ethyl] dimethyl-(3-sulfopropyl), SBMA) and acrylic acid (AA), and an active layer hydrogel composed of poly­(N-isopropylacrylamide) (PNIPAm) containing polydopamine-modified MXene (P-MXene) and calcium chloride (CaCl2). The coordination of carboxylate and Ca2+ at the interface of the two layers enhances the interfacial bonding from 14 to 30 N m–1, which facilitates withstanding large strain and preventing stratification. The resulting hydrogel actuator can bend approximately 360° in a mere 10 s, exhibiting excellent photothermal effect, a large angle bending deformation, and ultrafast photoresponsive ability. As a proof of concept, the photothermal actuators are programmed to present various shapes and grab objects. Importantly, the hydrogel actuator exhibits remarkable adhesion capabilities toward diverse substrates, with a maximum peel force of up to 280 N m–1. Relying on their own adhesion and the photoresponse properties, these flexible adhesion actuators show outstanding gripping capability, enabling them to grip and release objects of different shapes and weights. More interestingly, the hydrogel exhibits a smart adjustable adhesion capability at different temperatures, which enables it as a gripper to recognize temperature signals through real-time different feedback actions based on its own adhesion. This study presents innovative insights into biomimetic hydrogel actuators, providing new opportunities for developing intelligent soft robots with multiple functions.

Design and mechanism of photothermal soft actuators and their applications

“…Wang and colleagues prepared a hydrogel bilayer actuator by combining an active layer of PNIPAM with a PDA-modified P-MXene and calcium chloride (CaCl 2 ) composite hydrogel, and a passive layer of acrylic acid (AA) and amphoteric [2-(methacryloyloxy)ethyl]dimethyl-(3-sulfopropyl) (SBMA). 168 A strong interfacial bond formed by coordination of carboxylate and Ca 2+ . The remarkable adhesion capability of the PAA/SBMA layer to PNIPAM (280 N m −1 ) and high light response to bending enable a smart adhesion actuator to be designed for the mechanical gripper.…”

Novel Acid-Driven Bioinspired Self-Resettable Bilayer Hydrogel Actuator Mimicking Natural Muscles

“…In recent years, hydrogel actuators, − which can deform by the control of external stimuli, have been widely reported as candidates for mimicking natural muscles. These hydrogel actuators usually have a bilayer structure, and the volume of at least one layer can be modulated by external stimuli, such as pH, − light, − or temperature. − Some motions of living creatures, such as walking, gating, clamping, etc., have been successfully mimicked by hydrogel actuators. However, the actuation of most reported hydrogel actuators is an alternated switch between two thermodynamic equilibrium states achieved by sequentially imposing equivalent opposite external stimuli, such as alternately adding the same amount of acid and alkaline, heating and cooling, etc.…”