Light-driven dandelion-inspired microfliers

Autonomous unmanned aerial vehicles (UAVs), especially micro aerial vehicles (MAVs), have gained significant attention due to their diverse applications. However, scaling down MAVs presents challenges in propulsion and actuation efficiency due to limitations in motor power density and increased viscous effects. Bio-inspired designs, mimicking insect flight, have shown promise, but are often limited by battery weight and flight duration. Wireless power supplies and light-powered systems offer solutions, but may introduce limitations such as disturbed flight balance. Wind-dispersed plant seeds, like dandelion seeds, offer an alternative, low-energy approach to flight, characterized by efficient and passive flight. While previous research has explored dandelion-inspired devices, these often lack controllability. This research aims to develop a dandelion-inspired microflier with robust controllability using a novel light-driven actuation system.

The aerodynamics of insect flight and the development of insect-scale MAVs have been extensively studied, utilizing piezoelectric artificial muscles and dielectric elastomer actuators. However, limitations in battery technology restrict untethered flight duration. Wireless radio frequency power supplies and light-powered systems have been explored to overcome this, but integration of electronics adds complexity. Natural wind-dispersed plant seeds provide a model for energy-efficient flight, with dandelions demonstrating particularly effective long-distance dispersal. Previous research has explored dandelion-inspired battery-free wireless sensing devices, but controllability remained a significant challenge. This work aims to address these limitations by developing a light-driven microflier inspired by dandelion seed dispersal.

The study designed and fabricated an untethered dandelion-inspired microflier using an ultralight and super-sensitive light-driven bimorph soft actuator. This actuator consists of electrostatically laminated polyimide (PI) and low-density polyethylene (LDPE) layers with embedded surface-functionalized ultrasmall gold nanorods (AuNRs). The large mismatch in coefficients of thermal expansion (CTEs) between PI and LDPE, combined with the AuNRs' photothermal conversion, enables reversible actuation upon light irradiation. The microflier's structure comprises a tubular-shaped bimorph soft actuator film, one end of which is cut into thin strips and attached with approximately 40 fiberglass strands forming the "pappus." The fabrication process involved layer-by-layer self-assembly coating of the PI film, electrostatic lamination with a negatively charged LDPE film containing AuNRs, and attachment of the fiberglass strands. COMSOL simulations were used to model the actuator's deformation and stress distribution. Experiments involved measuring the microflier's falling velocity under various light intensities and assessing its mid-air flight performance using an infrared light source and a tunnel to generate an updraft. Flow visualization experiments using particle image velocimetry (PIV) were conducted to examine the airflow behavior around the microflier. The effects of varying light intensity, pappus length, and release height on flight performance were also investigated.

The researchers successfully fabricated an ultralight bimorph soft actuator with high sensitivity and responsiveness to light. The actuator demonstrated a rapid bending response (0° to 180° in 0.33 s) and recovery (within 1.62 s) upon NIR light irradiation (808 nm, 150 mW cm⁻²). The maximum bending angle was dependent on light intensity, with a 30° angle achievable at a low intensity of 10 mW cm⁻². The actuator exhibited an actuation stress of up to 0.91 MPa and withstood over 1000 cycles without fatigue. The dandelion-inspired microflier demonstrated light-controlled falling velocity. With the "pappus" closed, the terminal falling velocity was 0.98 m s⁻¹, while with the "pappus" open (under 50 mW cm⁻² light irradiation), it decreased to -0.41 m s⁻¹. The falling velocity could be continuously adjusted by varying light intensity. The microflier achieved sustained light-fueled mid-air flight above a light source, with a sustained flight time of ~8.9 s and a maximum height of ~350 mm. The flight height was influenced by release height, light power, and pappus length. Flow visualization revealed a separated vortex ring above the microflier, similar to that observed in natural dandelion seeds, contributing to lift generation. Unexpectedly, the microfliers exhibited autorotation, with clockwise or counterclockwise rotation depending on the subtle twisting of the bimorph actuator strips created during fabrication. This rotation could be controlled by manipulating the shape programmability of the actuator. The three-dimensional symmetrical structure and light-induced updraft were crucial for the successful mid-air flight. Experiments with a simulated solar light source also demonstrated successful, though shorter, mid-air flight.

The findings demonstrate a successful bio-inspired approach to developing light-driven microfliers. The unique combination of an ultralight, highly sensitive bimorph soft actuator and a dandelion-like structure allows for precise control over flight characteristics. The observed separated vortex ring confirms the mechanism of lift generation, mimicking the natural process in dandelion seeds. The unexpected autorotation capability highlights the potential for programmable flight control. The ability to achieve mid-air flight with both infrared and simulated solar light sources demonstrates the potential for practical applications in various environments. The study's success suggests a new avenue for developing energy-efficient, untethered microfliers for applications such as environmental monitoring and wireless communication.

This research successfully demonstrated light-driven dandelion-inspired microfliers with controllable falling velocity and sustained mid-air flight. The use of an ultralight, highly sensitive bimorph soft actuator enabled precise control of flight behavior through light irradiation. The observed autorotation adds a novel aspect to the microflier's capabilities. Future work should focus on enhancing photoactuation efficiency, increasing lift force, miniaturizing integrated electronics, and incorporating sensors for advanced flight control and environmental monitoring. This platform could provide insights for the development of advanced artificial microfliers and untethered spacecraft.

The current design shows a reduced difference in terminal velocity between the open and closed states with heavier payloads (above 24 mg). The system lacks in-device control of the microflier's state, limiting flight control capabilities. The absence of integrated multifunctional sensors and a microcontroller restricts real-time environmental monitoring and flight attitude feedback.