Bioinspired rotary flight of light-driven composite films

The study addresses the longstanding challenge of achieving flying locomotion with light-driven soft actuators, which typically suffer from slow response, low actuation force, and limited frequency response. Building on the advantages of soft actuators—flexibility, adaptability, safety, and precise remote activation by light—the authors aim to realize controlled flight in a soft, untethered device. They draw inspiration from wind-dispersed vine maple seeds to create a helicopter-like photoactuator capable of ultrafast rotation and flight. The purpose is to demonstrate a new mechanism that converts photothermal energy into rotational flight via synergistic material interactions and aerodynamically favorable structures, enabling applications such as soft robotics, environmental sensing, and miniature devices.

Recent work has developed diverse light-responsive materials and actuators, including polyelectrolyte hydrogels, carbon-based materials, molecular crystals, shape-memory polymers, liquid-crystalline polymers, and low phase-temperature materials. These systems convert light into mechanical work via photothermal and photochemical mechanisms, enabling locomotion modes such as walking, crawling, rolling, jumping, swimming, and limited rotation. However, prior light-driven rotary actuators typically exhibit rotational speeds no more than ~300 rpm, insufficient to generate lift for flight. Advances in material design, photothermal conversion, and structural programming have improved response but flying locomotion remains rare due to constraints in response speed, force output, and frequency. The present work situates itself against this background by introducing a composite graphene/agar/silk fibroin film with microchannel-guided deformation, capable of high-speed rotation and flight under NIR light.

Materials: Graphene nanoplatelets (5 µm lateral size, 6–8 nm thick), agar, and silk fibroin (M=6–10 k) were used to fabricate a composite film; solvents and auxiliaries included N,N'-dimethylformamide (DMF) and SU-8 photoresist. Fabrication of micropatterned template: A silicon wafer was cleaned (piranha), coated with SU-8 2007, soft-baked, exposed through a photomask (14.5 mW/cm² UV), post-baked, developed, rinsed, and hard-baked to form microchannel templates. Film preparation: Agar was dissolved in DMF (1 g in 10 mL, 110 °C, 2 h), cooled to 40 °C, then graphene (60 mg) and silk fibroin (100 mg) were added and agitated for 4 h. The mixture was spin-coated on the patterned silicon template (1500 rpm, 40 s), dried, peeled, cut into strips, and stored at 90% RH. The resulting films had typical dimensions of 10 mm × 2 mm × 60 µm and contained surface microchannels (~50 µm width, 7.5 µm depth) that guide deformation. Characterization: SEM and AFM assessed cross-section and surface morphology (Rq ~163 nm); contact angle ~65.2°; Young’s modulus ~7 MPa; EDX and FTIR confirmed component distribution and hydrogen-bonding interactions; UV-Vis-NIR spectra showed broad absorption (250–1100 nm). Actuation setup: An 808 nm laser (0.6 W/cm² unless stated) at 6 cm distance irradiated the film. High-speed imaging and thermal IR imaging captured deformation, rotation, and temperature rise. Motion analysis quantified take-off dynamics and rotational speed. CFD (ANSYS Fluent) simulated airflow velocity and pressure around a 3D airscrew model to estimate lift. Parameter studies varied graphene content, water content (humidity conditioning), thickness, size, and light intensity to optimize performance. Rehydration and reuse tests measured mass changes, water re-absorption time (~5.5 min at 90% RH), and repeatability over at least seven cycles. Control experiments included vacuum tests (0.075 torr) to isolate aerodynamic lift contributions and material composition controls (films without silk fibroin).

- Under 0.6 W/cm² NIR irradiation, the photoactuator achieved an ultrafast rotational speed of ~7200 rpm and a rapid response/take-off initiation of ~650 ms, with take-off speed ~0.76 m/s. Flight trajectories showed climbing, forward flight, and descent phases, achieving heights up to ~1.3 cm and horizontal distances up to ~6.5 cm. - Mechanism: Photothermal heating (from ~25.5 °C to ~165.4 °C in ~650 ms) vaporizes internal water, forming an off-center hollow protrusion; combined with microchannel-guided twisting, an airscrew-like structure forms. As the protrusion reaches the film edge, vapor escapes between laminae, generating jet propulsion at an off-center location, which induces rotation; rotation plus airscrew geometry yields aerodynamic lift and flight. - Lift estimation: Newtonian analysis with mass 5.3 mg, take-off acceleration a ≈ 17.3 m/s² (from v=0.38 m/s over 22 ms) gave F_lift ≈ 1.44 × 10⁻⁴ N. CFD-derived estimate using ρ=1.2 kg/m³, Ω=754 rad/s, R=5 mm, S=7.85 × 10⁻⁵ m², C_L=0.61 gave F_lift ≈ 4.12 × 10⁻⁴ N, consistent in order of magnitude. - Composition and geometry optimization: Optimal graphene content ~4.6 wt% (ensures photothermal conversion without reducing water uptake); optimal water content ~12 wt%; optimal film thickness ~60 µm; optimal size 10 mm × 2 mm (L×W). Thicker/larger films increased weight and reduced performance; thinner films (<50 µm) bent without protrusion formation. - Light intensity dependence: Rotational speed, flight height, and landing distance increased with intensity; intensities >0.7 W/cm² caused burning. - Reusability and hydration: Post-actuation mass decreased by ~9.5 wt% (water loss), consistent with ~12 wt% water content (TGA). Films reabsorbed water within ~5.5 min at 90% RH, retained structure and function, and were reused for at least 7 cycles. - Motion control: Angle of attack (β) controlled via microchannel alignment; maximum flight height at β ≈ 15°, with decreased lift at higher angles due to flow separation. Rotation direction controlled by jet location (clockwise for lower-left jet, counterclockwise for lower-right). Elevation angle (θ), adjusted by irradiation position and protrusion location, controlled flight direction (left or right rear), height, and range; increasing θ decreased vertical lift component and increased horizontal propulsion component. - Environmental interactions and applications: In wind (3 m/s) from 1.5 m release height, wind-dispersal distance reached ~1.1 m (comparable to vine maple seed at ~1.05 m). With release height 5 m, dispersal distance up to ~10 m at 3 m/s. An intermediate actuator size (~20 mm²) maximized wind dispersal (~2.2 m at 1.5 m height and 3 m/s). Airscrew-like structure prevented tumbling and improved dispersal. - Obstacle crossing: The actuator flew across trenches up to ~65 mm wide and over barriers up to ~11.3 mm high (near maximum flight height ~13 mm). - Control experiments: In vacuum (0.075 torr) the airscrew-like actuator did not become airborne, confirming the necessity of aerodynamic lift. Silk fibroin was essential to maintain integrity under jet propulsion; films without silk showed edge splitting.

The findings demonstrate a new approach to light-driven flight in soft actuators by coupling photothermal heating (graphene) with hygroscopic, adhesive biopolymers (agar/silk fibroin) and microstructured guidance to create an airscrew with jet-induced rotation and aerodynamic lift. Ultrafast rotation (~7200 rpm) overcomes prior limitations of low rotational speeds in photoactuators, enabling sufficient lift for take-off and controlled flight. Quantitative agreement between dynamic estimates and CFD supports rotation-induced aerodynamic lift as the dominant flight driver. Systematic control of angle of attack, jet location, and elevation angle allows tuning of rotation direction, flight height, trajectory, and range. The actuator’s wind-dispersal behavior mimics vine maple seeds and, combined with light-triggered deployment, enables distributed sensing and environmental monitoring. The ability to cross obstacles highlights potential in small-scale robotics for exploration and inspection. Overall, the work addresses the challenge of achieving flying locomotion in photoactuators and expands their applicability.

This work introduces a bioinspired, helicopter-like composite photoactuator that achieves rapid response (~650 ms), ultrafast rotation (~7200 rpm), and controlled rotary flight under NIR light via a synergistic mechanism involving photothermal heating, microchannel-guided twisting, off-center protrusion formation with jet propulsion, and aerodynamic lift. The design enables precise control over rotational direction, flight height, and trajectory by adjusting irradiation intensity and position, and demonstrates applications in wind-dispersal, distributed colorimetric environmental sensing, and obstacle crossing. Future directions include integrating on-board electronics and wireless modules for real-time data transmission, enhancing durability and reuse cycles, optimizing payload capacity, and scaling arrays for large-area environmental monitoring and soft robotic applications.

- Flight relies on ambient air for aerodynamic lift; in vacuum, the actuator cannot become airborne. - Performance is sensitive to water content and humidity; water loss during actuation requires rehydration (~5.5 min at 90% RH) and may limit immediate repeatability. - Light intensity window is constrained; intensities above ~0.7 W/cm² cause burning, while too low intensity limits propulsion. - Geometric constraints exist: films thinner than ~50 µm do not form the protrusion and bend instead; heavier/thicker or larger films reduce rotation and flight performance. - Maximum measured flight height (~13 mm) and barrier/trench crossing capabilities (barrier ~11.3 mm; trench ~65 mm) limit operation in larger-scale environments. - Reuse demonstrated for at least 7 cycles; longer-term durability and potential material fatigue were not fully explored. - Payload capacity and precise quantitative steering under varying environmental disturbances (e.g., turbulent winds) were not assessed in detail.