Bioinspired rotary flight of light-driven composite films

The development of soft actuators, inspired by biological systems, has gained significant traction due to their flexibility, adaptability, and safety. Light-driven actuators are particularly attractive for remote and precise actuation, finding applications in soft robotics, grippers, and biomimetic devices. However, existing light-driven actuators face challenges in achieving flying locomotion due to slow response, limited force output, and low frequency response. This research addresses this limitation by presenting a bio-inspired design that overcomes these limitations.

Numerous advanced light-responsive materials have been developed for soft actuators, including polyelectrolyte hydrogels, carbon-based materials, crystals, shape-memory polymers, liquid-crystalline polymers, and low phase temperature materials. These materials convert light stimulation into structural changes or macroscopic property alterations, enabling various locomotion modes such as walking, crawling, rolling, jumping, and swimming. While progress has been made in improving the response of these actuators, achieving flight remains challenging due to insufficient speed, force, and frequency.

The rotary flying photoactuator was fabricated using a composite film of graphene nanoplatelets, agar, and silk fibroin. Graphene provides lightweight structure and photothermal properties, while agar and silk fibroin offer hygroscopic and adhesive properties crucial for actuation and film integrity. The film, with microchannels patterned via template-assisted spin coating, has dimensions of 10 mm × 2 mm × 60 µm. Material characterization included scanning electron microscopy (SEM), atomic force microscopy (AFM), energy-dispersive X-ray (EDX) analysis, and Fourier transform infrared spectroscopy (FTIR), confirming the presence and interaction of the components and demonstrating the photothermal absorption properties of the composite. The actuation mechanism was investigated using high-speed cameras, thermal imaging, and computational fluid dynamics (CFD) simulations.

The photoactuator demonstrates remarkable flying capabilities. Upon NIR light irradiation, it achieves ultrafast rotation (~7200 rpm) and a take-off speed of ~0.76 m/s within ~650 ms. The flight trajectory shows distinct climbing, forward flight, and descent phases, indicating the involvement of aerodynamics. The actuator can reach a height of 1.3 cm and a distance of 6.5 cm. The actuation mechanism involves a synergistic interplay between photothermal heating of graphene, causing water vaporization, and the hygroscopic properties of the agar/silk fibroin, leading to protrusion formation and an airscrew-like structure. This structure, combined with jet propulsion from escaping water vapor and aerodynamic lift, enables the rotary flight. The lift force was calculated using both Newtonian mechanics and CFD simulations, showing consistent results. The actuator exhibits reusability for at least 7 cycles due to the ability of the agar/silk fibroin to reabsorb moisture. The rotational speed, flight height, and distance are tunable via light intensity, angle of attack (controlled by microchannel alignment), and elevation angle (controlled by irradiation position). The actuator successfully mimics the wind-dispersal behavior of vine maple seeds and is capable of traversing obstacles, such as trenches (up to 65 mm wide) and barriers (up to 11.3 mm high). Finally, a proof-of-concept demonstration of its potential as a collective environmental monitoring sensor is presented, showcasing simultaneous temperature, humidity, and particulate matter measurements using integrated colorimetric sensors.

The findings demonstrate a significant advance in light-driven actuators, achieving flying locomotion with unprecedented speed and control. The bio-inspired design, combining photothermal actuation with aerodynamic principles, overcomes the limitations of previous light-driven actuators. The ability to precisely control the actuator's flight parameters provides significant potential for various applications, particularly in soft robotics and miniature devices. The successful demonstration of collective environmental monitoring highlights the practicality of the actuator for distributed sensing.

This study successfully demonstrated a bioinspired rotary flying photoactuator with exceptional performance. The synergistic material design and the integration of aerodynamic principles led to ultrafast rotation, controlled flight, and obstacle-crossing capabilities. The potential applications of this actuator in soft robotics, environmental monitoring, and other miniature device applications are promising, opening new avenues for future research. Future work could focus on further miniaturization, integration of advanced sensors and wireless communication, and exploring applications in diverse fields.

The current design is limited in the size of obstacles it can overcome, the range and duration of flight, and the payload it can carry. The performance is also dependent on the ambient humidity levels, requiring further investigations into designing actuators for diverse environmental conditions. The long-term stability and durability of the composite material under repetitive actuation also warrant further investigation.