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 depend on breakthroughs in the core component: soft actuators. Traditional soft actuators either struggle to be manufactured at the microscale, lack design flexibility, or rely on wired power supplies that limit their mobility. A recent study published in Nano Letters, however, reports the development of an MXene–cellulose nanofiber (CNF) composite actuator fabricated via aerosol jet printing (AJP), which simultaneously addresses three major challenges—scalability, customizability, 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 be able to rapidly convert electrical or optical energy into mechanical motion, while also being made of soft, safe materials that can adapt to complex environments. The two core materials selected by the research team happen to meet these requirements perfectly:

• MXene (transition metal carbides): As a two-dimensional nanomaterial, MXene boasts a “dual thermal-conversion buff”—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-control functionality. Moreover, MXene’s high 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—research shows that incorporating CNF can increase the bending angle of actuators by nearly 30% while 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 enables rapid absorption of water-based inks, thereby 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 capability of two-dimensional materials but also exhibits the flexible adaptability inherent to natural materials; moreover, it can be precisely shaped via printing technologies, laying the foundation for subsequent functional breakthroughs.

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, such as high-temperature annealing and vacuum drying, resulting in lengthy processing times and high costs. However, the emergence of aerosol jet printing (AJP) has fundamentally transformed this situation:

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 is carried out at room temperature, requires no post-processing, and allows the actuator to be used immediately after printing, thereby significantly simplifying 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, enables flexible control over film thickness, conductivity, and actuation performance. Through process optimization, the research team successfully addressed challenges such as the “coffee-ring effect” and ink clogging during printing, and can even perform selective single- or double-sided printing on PC films, thereby enabling 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. Moreover, 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 breakthroughs: “multi-stimulus compatibility, miniaturization, and long service life,” with performance that is nothing short of remarkable:

1. Dual stimulus response: seamless switching between wired and wireless

The actuator can respond simultaneously 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, driving bending of the PC substrate with a maximum bending angle of 131° and a curvature of 2.86 cm⁻¹.

• Wireless mode: Eliminating 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 a curvature rate of 1.21 cm⁻¹—making it ideally suited for battery-free microrobot applications.

In both modes, the actuator temperature can be maintained below 120°C, thereby preventing thermal softening and failure of the PC substrate while ensuring operational safety.

2. Miniaturization + Long Lifespan: Durable and Compact

The research team has successfully fabricated an actuator with a minimum footprint of just 2.5 mm², yet it still maintains a high curvature of 0.87 cm⁻¹, thereby meeting the actuation requirements of microrobots. More importantly, the device exhibits exceptional durability: under low-temperature conditions (below 120°C), after 1,000 actuation cycles, it fully recovers its original shape without any delamination of the thin film or degradation of 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 mechanical toughness of the film, and the flexibility of the PC substrate ensures that the film does not fracture under repeated bending.

3. Targeted + Complex Actuation: Flexible Adaptation to Multiple Scenarios

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

• Single-sided printing: the actuator continuously bends toward the print surface, enabling directional oscillation;

• Double-sided printing: By printing MXene-CNF on opposite 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 undergo opening and closing motions upon light stimulation.

This design flexibility enables the actuator to adapt to the diverse motion requirements of soft robots—ranging from the sinuous crawling of snake-like robots and the self-closing of self-folding boxes to the precise grasping of microgrippers—all with ease.

Application Scenarios

• Micro-robotics: Its sub-millimeter size and wireless control capabilities enable it to serve as the “legs” or “wings” of micro-robots, allowing precise locomotion within confined spaces—such as human blood vessels and 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, where they can provide soft actuation via weak electrical or optical stimulation—for example, helping patients with hand dysfunction perform grasping movements.

• Minimally invasive medical devices: Their soft materials and precise actuation control help prevent damage to human tissues, making them suitable for use as miniature manipulation units in endoscopic surgery to achieve precise lesion resection or tissue sampling.

Illustrated 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) The Duke University logo is directly written onto an MXene-CNF–based coating. (c, d) Scanning electron microscope images of 1-mm-wide MXene-CNF films printed on a silicon wafer. (e) A single-line printing pattern demonstrates the ability to draw both 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. Print-process control for MXene-CNF inks. (a) MXene-CNF electrodes printed onto non-porous glass under identical printing conditions, compared with polycarbonate film in optical microscopy images. (b) Bar graph showing the relative conductivity of thin films on different substrates (sample size: six devices; error bars represent standard deviation). (c) Increasing the printing speed results in thinner films and improved visual quality. (d) Conductivity and thickness of MXene-CNF–based thin films printed on glass under varying atomizing nozzle flow rates and (e) numbers of print passes (at an atomizing nozzle flow rate of 25 SCCM). (f) Speed and resistance exhibit a linear relationship, enabling precise resistance control 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) Thermal imaging shows a top-down view of a representative microheater (100 μm × 3 mm) on a glass substrate 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 duration at constant power. (e) Application of a 10-V pulse (approximately 263 mW) for 30 seconds over three cycles, 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, with two printed 500 µm × 8 mm AgNW electrodes connected to the resistor; the entire substrate is mounted on a glass slide. (b) Top-down thermographic image captured while clamping the multi-stimulus actuator at both ends and maintaining camera focus. (c) Relationship between the maximum bending angle achieved and the applied power, for three device samples in which two-thirds (8 mm) of the device is suspended in air and one-third (4 mm) rests on the glass substrate. (d) Cyclic durability of the device without a substrate. (e) Time required to reach the maximum bending angle for a representative device mounted on glass, under applied voltages of 2 V (approximately 36 mW), 2.5 V (approximately 65 mW), and 3 V (approximately 111 mW). (f) Photographs showing the maximum bending angle 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 3 mm × 5 mm printed MXene–CNF actuator placed on a computer, showing the thermal-cycling durability under bending at maximum deflection (n = 3 devices; error bars represent the standard error interval), with an incident near-infrared power density of approximately 10 mW/mm². (c) The bending angles achieved by a 3 mm × 10 mm actuator printed at speeds 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 the standard error interval), under an incident near-infrared power density of approximately 10 mW/mm². (d), (e), and (f) show the thermo-mechanical response curves of a 4 mm × 20 mm actuator with two 4 mm × 5 mm coatings at different power densities, as observed on (d) the same side, (e) the opposite side, and (f) the contralateral side, respectively.

The core value of this research lies not only in the development of a high-performance actuator, but also in the establishment of a “scalable + customized” manufacturing approach: leveraging AJP technology to enable both mass production of standardized actuators and flexible customization 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 also 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 scale, 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 is today’s seemingly “small” breakthrough in printing technology.

Technological advancement often stems from the refinement of core components, and the emergence of MXene-CNF actuators has undoubtedly injected strong momentum into the soft robotics industry. Let us look forward to the early commercialization of this technology, unlocking new possibilities for smarter living!

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