In today’s era of rapid technological advancement, soft robotics is transitioning from the laboratory to real-world applications—ranging from minimally invasive surgical instruments in medicine and flexible actuation units for wearable devices to precise motion control in microrobots—all of which hinge on breakthroughs in the core component: soft actuators. Traditional soft actuators either face challenges in miniaturization, lack design flexibility, or rely on wired power supplies that constrain mobility. A recent study published in Nano Letters, however, reports an MXene–cellulose nanofiber (CNF) composite actuator fabricated via aerosol jet printing (AJP), which simultaneously addresses three major pain points—scalability, customization, and multi‑stimulus responsiveness—thereby paving the way for the next generation of soft robotics.

MXene+CNF

The core requirements for soft actuators are “efficient energy conversion + flexible adaptability”—they must not only rapidly convert electrical or optical energy into mechanical motion, but also be made of soft, safe materials that can accommodate complex environments. The two core materials selected by the research team happen to meet these demands perfectly:

• MXene (transition metal carbides): As a two-dimensional nanomaterial, it possesses an intrinsic “dual thermal‑conversion boost”—it can efficiently convert electrical energy into heat (electrothermal effect) and rapidly absorb near‑infrared light to generate heat (photothermal effect), making it pivotal for achieving both wired and wireless dual‑mode control. Moreover, MXene’s high electrical conductivity and excellent stability ensure the reliability of the actuation process.

• CNF (cellulose nanofiber): A nanomaterial derived from natural cellulose that is not only environmentally friendly and biocompatible but also significantly enhances actuation performance. Studies have shown that incorporating CNF can increase the bending angle of actuators by nearly 30%, while simultaneously improving the flexibility and printability of MXene films.

• Substrate Selection: Polycarbonate (PC) Film: A lightweight, flexible, and porous PC film is an ideal substrate. Its porous structure rapidly absorbs aqueous inks, preventing ink bleeding during printing, while its excellent thermal stability ensures that it does not deform or degrade during actuation at temperatures below 120°C.

The “MXene-CNF/PC” system, formed by the integration of these three components, not only retains the high‑efficiency energy‑conversion capabilities of two‑dimensional materials but also exhibits the flexible adaptability inherent to natural materials. Moreover, it can be precisely shaped via printing techniques, laying the groundwork for subsequent functional advancements.

Aerosol jet printing technology

Traditional soft actuator fabrication typically relies on methods such as electrospinning and solution casting, which either suffer from low resolution—making it difficult to achieve micron-scale structures—or require complex post‑processing steps like high‑temperature annealing and vacuum drying, resulting in lengthy production times and high costs. The advent of aerosol jet printing (AJP) technology has fundamentally transformed this landscape:

Core advantages of the printing process:

• AJP technology functions like a “precision nanoscale printer”—it atomizes MXene–CNF aqueous ink into tiny droplets and, under the synergistic action of nitrogen carrier and sheath gases, precisely deposits them onto flexible substrates such as PC films. The entire process operates at room temperature with no post‑processing required; once printing is complete, the actuators are ready for immediate use, significantly streamlining the fabrication workflow.

• It achieves sub-millimeter resolution (with a minimum feature size of 10 μm) and, by tuning parameters such as carrier gas flow, sheath gas flow, and printing speed, allows flexible control over the film’s thickness, electrical conductivity, and actuation performance. By optimizing the fabrication process, the research team successfully addressed challenges like the “coffee-ring effect” and ink clogging during printing, and can even perform selective printing on one or both sides of a polycarbonate substrate, paving the way for complex actuation modes.

Ingenious design for substrate compatibility:

The study found that the porous structure of the PC film is key to enhancing print quality: compared with glass substrates, the PC film rapidly absorbs aqueous inks, preventing liquid accumulation that leads to film non-uniformity, and resulting in a sevenfold increase in the electrical conductivity of the printed MXene–CNF film. Meanwhile, the hydrophilic–hydrophobic properties of the PC film can be tuned through simple surface treatments, further optimizing the interfacial adhesion between the actuator and the substrate and reducing energy losses during the actuation process.

Core Advantages

The core advantage of this novel actuator lies in its triple breakthrough—“multi‑stimulus compatibility, miniaturization, and long service life”—with performance that is nothing short of remarkable:

1. Dual Stimuli-Responsive: Seamless Switching Between Wired and Wireless Modes

The actuator can simultaneously respond to electrical stimulation and near-infrared (NIR) light stimulation:

• Wired mode: By passing a weak electric current (as low as 36 mW), the MXene–CNF film generates temperature changes via Joule heating, inducing bending of the PC substrate; the maximum bending angle reaches 131°, and the curvature reaches 2.86 cm⁻¹.

• Wireless mode: Without the need for wired connections, remote actuation can be achieved by irradiating with near-infrared light (at a power density as low as 10 mW/mm²), enabling bending angles exceeding 90° and curvature rates up to 1.21 cm⁻¹, making it ideally suited for battery-free microrobot applications.

In both modes, the actuator’s temperature can be kept below 120°C, preventing the PC substrate from softening and failing due to high temperatures while ensuring operational safety.

2. Miniaturization + Long Lifespan: Durable and Compact

The research team successfully fabricated an actuator with a minimum footprint of just 2.5 mm², which still maintains a high curvature of 0.87 cm⁻¹, thereby meeting the actuation requirements of microrobots. More importantly, it exhibits exceptional durability: under low-temperature conditions (below 120°C), after 1,000 actuation cycles, it fully recovers its original shape, with no delamination of the thin film or degradation in performance.

This long‑life performance stems from the synergistic effects of the materials: the high stability of MXene prevents failure of the conductive network, the fibrous structure of CNF enhances the film’s mechanical toughness, and the flexibility of the PC substrate ensures that it remains intact under repeated bending.

3. Targeted + Complex Actuation: Flexibly Adaptable to Multiple Scenarios

Through selective printing design, the actuator can achieve a variety of motion patterns:

• Single-sided printing: The actuator always bends toward the print side, enabling directional oscillation;

• Double-sided printing: By patterning MXene–CNF on both sides of the PC film in distinct regions, complex deformations such as sinusoidal bending and folding can be achieved—similar to the “sleeping flower” structure demonstrated in the paper—which can perform opening and closing motions upon light stimulation.

This design flexibility enables the actuator to adapt to the diverse motion requirements of soft robots—whether it’s the sinuous crawling of a snake‑like robot, the self‑folding closure of a box, or the precise grasping of a micro‑gripper—all with ease.

Application scenarios

• In the field of microrobots: Its sub-millimeter size and wireless control capabilities enable it to serve as the “legs” or “wings” of miniature robots, allowing precise navigation within confined spaces—such as human blood vessels or industrial pipelines—to perform tasks like inspection, maintenance, or drug delivery.

• Wearable devices: Their flexible materials and low-power characteristics enable integration into smart wristbands, rehabilitation gloves, and other devices, providing soft actuation via mild electrical or optical stimulation—for example, helping patients with hand dysfunction perform grasping motions.

• Minimally invasive medical devices: With their soft materials and precise actuation control, these devices can minimize damage to human tissues and serve as miniature manipulation units in endoscopic procedures, enabling precise lesion resection or tissue sampling.

Image and Text Guide

Figure 1. Aerosol-jet printing of MXene–CNF films. (a) When printing MXene–CNF films at room temperature, five key process parameters must be carefully controlled to regulate film quality and dimensions: nozzle flow rate, carrier gas flow rate, printing speed, ink temperature, and ultrasonic current. (b) Direct writing of an MXene–CNF–based coating using the Duke University logo. (c, d) Scanning electron microscope images of 1‑mm‑wide printed MXene–CNF films on a silicon wafer. (e) A single‑line printing pattern demonstrates the ability to draw straight lines and curved single lines. (f) Multiple adjacent printed lines can be used to fabricate complex shapes featuring 90° angles, curved features, and sharp corners.

Figure 2. Process control for MXene–CNF printing. (a) Under identical printing conditions, an MXene–CNF electrode is printed onto a non-porous glass substrate; (b) optical microscopy images comparing this with a polycarbonate film; (c) bar graph showing the relative electrical conductivity of thin films on different substrates (sample size: six devices; error bars represent standard deviation); (d) increasing the printing speed results in thinner films and improved visual quality; (e) electrical conductivity and thickness of MXene–CNF–based films printed on glass under varying atomic nozzle flow rates; (f) electrical conductivity and thickness of MXene–CNF–based films printed on glass at different numbers of passes (with the atomic nozzle flow rate set to 25 SCCM); (g) a linear relationship between printing speed and resistance enables precise control of resistance when printing MXene–CNF thin films on polycarbonate substrates.

Figure 3. Thermal performance of printed MXene–CNF microheaters. (a) Schematic side view of the experimental setup used to measure the electrically induced temperature response and relaxation time of the printed MXene–CNF microheater. (b) Thermographic images showing a representative microheater (100 µm × 3 mm) on glass, viewed from above, under different applied power conditions. The microheater has a resistance of 400 Ω, corresponding to a sheet resistance of approximately 13.3 Ω/sq. (c) The peak electrode temperature of the MXene–CNF microheater exhibits a linear relationship with the applied electrical power. (d) Time‑dependent temperature profiles demonstrating stability over a 5‑minute period at constant power. (e) Voltage application in three cycles, each consisting of a 10‑V pulse (approximately 263 mW) for 30 seconds, illustrating the consistent heating performance of the microheater (100 µm × 3 mm).

Figure 4. Electrical actuation of the printed multi‑stimulus actuator: (a) Side‑view schematic of the multi‑stimulus actuator, showing a 3 mm × 4 mm MXene–CNF resistor printed on a 3 mm × 12 mm PC substrate, connected to two printed AgNW electrodes measuring 500 µm × 8 mm; the entire assembly is mounted on a glass slide. (b) Top‑down thermal imaging taken while clamping the actuator at both ends and keeping the camera in focus. (c) Relationship between the maximum bending angle achieved and the applied power, for three device samples, with two-thirds (8 mm) of the device suspended in air and one-third (4 mm) resting on the glass substrate. (d) Cyclic durability of the device without the substrate. (e) Time required to reach the maximum bending angle for a representative device placed on glass, under applied voltages of 2 V (~36 mW), 2.5 V (~65 mW), and 3 V (~111 mW). (f) Photographs of the maximum bending angles attained when applying voltages ranging from 0 to 3 V to a representative multi‑stimulus actuator mounted on glass.

Figure 5. Wireless control of printed soft actuators via infrared radiation. (a) Side-view schematic illustrating the experimental setup for remote actuation. (b) A printed MXene–CNF actuator measuring 3 mm × 5 mm placed on a computer, along with its thermal‑cycling durability under bending, based on maximum deflection height (n = 3 devices; error bars represent standard error), at an incident near‑infrared power density of approximately 10 mW/mm². (c) The bending angle of a 3 mm × 10 mm actuator as a function of printing speed ranging from 0.5 to 3 mm/s, demonstrating the influence of printing speed—and consequently the thickness of the MXene–CNF film—on the final actuation performance (n = 3 devices; error bars represent standard error), at an incident near‑infrared power density of approximately 10 mW/mm². (d), (e), and (f) show the thermo‑mechanical response of a 4 mm × 20 mm actuator with two 4 mm × 5 mm coatings at different power densities, depicted respectively on the same side (d), the opposite side (e), and the far side (f).

The core value of this research lies not only in the development of a high‑performance actuator, but also in the establishment of a “scale‑up plus customization” manufacturing approach: leveraging AJP technology to enable both mass production of standardized actuators and flexible tailoring of size, shape, and actuation mode to suit specific application scenarios. Looking ahead, the research team plans to replace the substrate with higher‑temperature‑resistant materials to further expand the actuator’s operational temperature range, while optimizing the ink formulation to enhance its stability in humid environments, such as human bodily fluids.

As MXene–CNF actuator technology continues to mature, soft robotics will finally break free from the constraints of “large size, wired operation, and single functionality,” rapidly advancing toward miniaturization, wireless operation, and multifunctionality. Perhaps in the near future, we will see millimeter-scale robots “cruising” through blood vessels, and flexible rehabilitation devices helping patients regain motor function—yet the starting point for all this lies in today’s seemingly “small” breakthrough in printing technology.

Technological progress often stems from refining core components, and the advent of MXene‑CNF actuators has undoubtedly injected powerful momentum into the soft robotics industry. Let’s look forward to the early commercialization of this technology, unlocking new possibilities for smarter, more intelligent living!

Original article link: Nano Lett. 2025, 25, 15501−15508