A multilayer MXene-based film bends rapidly and reversibly under humidity, light, and solvent exposure, offering stable multi-stimulus actuation for soft robotics applications.
(Nanowerk Spotlight) Nature moves with an ease that machines cannot match (yet). A leaf curls in response to moisture. Muscles contract in milliseconds. A butterfly adjusts the tilt of its wings with no circuitry, no motors, and no delay. Reproducing that kind of seamless, adaptive motion through materials that bend, twist, or flex not by command but by design remains a central challenge in soft robotics and materials science.
The aim is to create systems that behave more like living ones. Robots that walk without rigid joints. Sensors that deform in real time. Machines that respond to light, heat, or water like skin or muscle. But synthetic materials do not move like that. Not yet.
The obstacle is not a lack of responsive compounds. Many materials swell in humidity, expand with heat, or contract under light. The problem is that they typically do only one of those things, and often do it slowly, unpredictably, or just once. They crack. They separate at the interface. They lose function after a few cycles. Most cannot handle more than one type of input. Creating a soft actuator that is fast, strong, reversible, and sensitive to multiple environmental cues remains one of the defining limitations in the field.
Developed by a team at Chongqing Normal University in China, the material bends sharply in response to humidity, light, and solvents without degrading after repeated use. Built from MXene nanosheets, carbon nanotubes, and polyvinyl alcohol, the actuator uses structural mismatch and interfacial chemistry to convert environmental changes into motion. It achieves large-scale deformation in fractions of a second. It does not rely on motors or circuits. It moves because of what it is made of, and that is what sets it apart.
The device is structured as a bilayer film. One layer consists of MXene combined with polyvinyl alcohol, and the other is made from carbon nanotubes also mixed with the same polymer (MP/CP bilayer film). The two layers respond differently to the same external conditions, and this difference creates a mismatch in expansion or contraction across the film. The polymer acts as a molecular adhesive, forming hydrogen bonds with both nanomaterials and stabilizing their interface. Together, the components form a flexible structure that can bend when exposed to humidity, infrared light, or polar solvents.
Schematic illustration of the fabrication process for the MP/CP bilayer film. (Image: Reprinted with permission by Wiley-VCH Verlag) (click on image to enlarge)
Under controlled testing, the actuator reached a bending angle of 516 degrees within 0.5 seconds at 90 percent relative humidity. It returned to its original shape within one second. When exposed to near-infrared light at an intensity of 200 milliwatts per square centimeter, the bending increased to 1128 degrees, with a rapid response rate of 141 degrees per second. These response times and magnitudes place the actuator among the fastest of its kind based on carbon or two-dimensional nanomaterials.
The mechanism driving this motion is strain mismatch, a difference in how much each layer changes dimension under external stimuli. In the presence of water vapor, the MXene layer swells as it absorbs moisture, while the carbon nanotube layer remains relatively unchanged. This uneven expansion causes the entire structure to curl. In the presence of light, the MXene layer heats and releases water, shrinking in thickness. Meanwhile, the nanotube layer expands slightly under heat. This reversal in the direction of strain causes the structure to bend in the opposite direction. In both cases, the motion is driven by internal physical changes rather than external force or control.
Photothermal response is especially effective due to the high light absorption and heat conversion capacity of MXene. Under light exposure, water molecules desorb from the MXene layers, producing contraction. At the same time, the nanotube layer expands due to its thermal coefficient. The actuator can complete this cycle repeatedly. After one hundred actuation cycles under infrared light, the film retained its bending angle and speed without measurable degradation. Structural analysis showed no delamination or fracture at the interface between the two layers.
The actuator also responds to chemical solvent vapors. When exposed to ethanol or acetone, it bent over 200 degrees. These solvents are polar, meaning they interact with the MXene and PVA network by entering the structure and increasing internal spacing. Nonpolar solvents like hexane had no effect. This selectivity highlights how the actuator’s performance is based not only on material properties but also on the molecular interactions between stimuli and structure. The film’s bending angle increased with higher solvent concentration and showed no performance loss after 100 stimulus cycles.
Mechanical strength is another key requirement for soft robotics, and the bilayer film performed well under tension. It reached a tensile strength of 32.5 megapascals, nearly three times that of comparable unmodified films. It could be folded, rolled, and shaped without breaking.
Tests showed that its electrical resistance remained stable after a month in ambient air, while unmodified MXene composites saw their resistance increase more than twentyfold under the same conditions. This improvement is attributed to the PVA matrix, which protects MXene from oxidation and enhances interfacial bonding.
To demonstrate practical applications, the team designed several proof-of-concept devices. A humidity- and light-responsive curtain opened or closed depending on environmental conditions. An infrared-controlled switch used the actuator to close a circuit and light an LED without any external mechanical parts. A soft walking robot moved across a surface through sequential bending cycles, with friction-controlled directionality. A butterfly-shaped device flapped its wings under alternating humidity and light exposure, mimicking the motion of biological flight.
Each demonstration used the same film geometry and relied solely on external environmental cues to trigger motion.
These applications highlight the advantage of integrating multiple functions into a single material. Rather than coupling separate sensors, motors, and actuators, the bilayer film combines sensing and movement in a single structural unit. Its ability to respond rapidly and predictably to different environmental signals makes it suitable for soft robots, wearable electronics, and adaptive surfaces that interact directly with their surroundings.
The study shows that precise control over interfacial chemistry can unlock new performance levels in soft actuators. By combining MXene and carbon nanotubes within a hydrogen-bonded polymer network, the researchers created a material that transforms physical stimuli into mechanical motion across multiple modes. It is strong, fast, and stable under repeated use.
As soft robotic systems become more complex and autonomous, materials like this bilayer film may form the basis of future machines that respond to their environment not through programming, but through structure itself.
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