Photochemically responsive polymer films enable tunable gliding flights

Miniaturized passive fliers using smart materials face challenges in precise shape-morphing control for aerodynamics and contactless modulation of gliding modes. Current active flight modes for centimeter-scale robots require high power density and bandwidth, typically necessitating piezoelectric or dielectric materials reliant on electrical energy transfer via cables or batteries. While stimuli-responsive materials offer potential for miniaturization and wireless control, their power output limitations and response instability during flight hinder their application in hovering robots. Researchers are exploring alternative flight options, such as passive gliding mechanisms inspired by nature. Many seeds utilize wind-assisted passive flight for dispersal, categorized into parachute-like, gliding, and autorotating structures. While studies on artificial dandelions using smart materials show light-controllable take-off and landing, their fragility limits load-carrying capacity. Gliding mechanisms offer more robustness and higher loading capacity, but lack the ability to modify aerial gliding performance. This study investigates whether light can directly reconfigure wing geometry and dynamically control gliding performance in a single-piece polymer, offering facile tuning of gliding modalities and trajectory steering.

The study reviews existing research on stimuli-deformable materials, soft robotics, miniature robots capable of various locomotion modes (walking, swimming, jumping), and the challenges of creating hovering flying robots. It discusses the limitations of existing materials and approaches for creating flying microrobots, highlighting the energy constraints and control challenges. The research also reviews bio-inspired designs for passive fliers, focusing on the aerodynamic principles of natural seeds (maple samara, Javan cucumber, dandelion) and existing attempts to create artificial equivalents. It emphasizes previous research on artificial dandelions, noting their light-controllable properties but limitations in payload capacity, and highlights research on bionic rotary passive fliers and light-vapor-powered active robots. It notes the existing limitations in modifying aerial gliding performance in current glider-inspired research, and contrasts the complex electromechanical integration of previous approaches with the proposed simpler optical shape-morphing polymer solution.

The researchers fabricated a photo-deformable maple samara by assembling a soft actuator film (azo-LCN), a natural seed root, and an additive mass chord. The azo-LCN film, made via photo-polymerization, crosslinks azobenzene photoswitches undergoing *trans*-*cis* isomerization under UV irradiation and reverting to the *trans* state under visible light. The film's photochemical deformation and kinetics were characterized using UV-Vis spectroscopy. The artificial seed's gliding performance was assessed through free-fall experiments and wind tunnel tests. A vertical wind tunnel with electronic wind speed control was used to study the impact of light on rotary behavior, with the glider's height and spinning rate measured. Free-fall experiments both indoors and outdoors were conducted to demonstrate light-tuned wind dispersal. Indoor experiments used a crosswind flow, while outdoor experiments utilized a 14m high building and natural wind. The study also investigated the scalability of the design, creating miniature gliders (3cm to 0.3cm) and extending the azo-LCN platform to other gliding modes (Javan cucumber seed, parachute, dandelion seed). Materials used include 4-Methoxybenzoic acid 4-(6-acryloyloxyhexyloxy)phenyl ester, 4[4[6-Acryloxyhex-1-yl)oxyphenyl]carboxybenzonitrile, diacrylate crosslinker, azobenzene, and photoinitiator. Fabrication involved creating liquid crystal cells, polymerizing the LC mixture, and assembling the artificial seeds. The wind tunnel setup involved a custom cardboard tube, fan, power supply, and Arduino control for regulating wind speed. The artificial seeds were assembled by trimming azo-LCN films, gluing natural seed roots and LCN strips, and adding reflective tape or pH indicator strips for tracking purposes.

The artificial maple samara exhibited autorotational behavior similar to its natural counterpart, showing comparable aerodynamic efficiency under various loading conditions. Photochemical actuation reversibly altered the artificial seed's descent velocity and spinning rate, with UV light decreasing both and visible light increasing them. In wind tunnel experiments, UV illumination caused the glider to rise in height and decrease its spinning rate, demonstrating light-controlled altitude adjustment. Outdoor experiments showed that UV-illuminated artificial seeds traveled significantly farther than unilluminated ones, demonstrating controlled dispersion in real-world conditions. The design was successfully miniaturized to sizes ranging from 3cm to 0.3cm, maintaining light-tunable gliding performance. The azo-LCN platform was successfully adapted to mimic the gliding behavior of Javan cucumber seeds, parachutes, and dandelion seeds, demonstrating the adaptability of the approach to different gliding mechanisms. In Javan cucumber seed mimics, UV light induced lateral bending, modifying the gliding trajectory. Parachutes demonstrated that UV illumination increased terminal velocity. Artificial dandelion seeds showed UV-induced closure and visible light-induced opening, enabling light-controlled take-off and landing.

The study successfully demonstrates the feasibility of using light to control the flight dynamics of passive microfliers. The use of light for actuation offers significant advantages over electrical methods, including long-range control and elimination of onboard electronics, leading to lighter structures. While the frequency of photo-responsive actuation is too slow for hovering, it is suitable for modulating the gliding behavior of passive fliers. The first-order kinetics of photoisomerization allows for precise control of shape-morphing, which directly affects the aerodynamic properties and gliding performance. The robustness of the photoresponse guarantees reliable and repeatable control. The study's findings provide a new design paradigm for controlling swarms of passive fliers, enabling contactless manipulation of their trajectories and enhancing the potential for environmental monitoring and other applications. The integration of indicators allows for environmental monitoring during flight. Future research could explore the use of lightweight porous materials to further improve the gliding performance and expand applications.

This research presents a novel approach to controlling microflier flight dynamics using photochemically responsive polymer films. The study successfully demonstrated reversible and bistable shape-morphing for light-tuned gliding performance in a bio-inspired design, scaled down across orders of magnitude, and extended to various gliding modes. The contactless nature of light-based control and the scalability of the design pave the way for more advanced applications, such as controlling swarms of microfliers for environmental monitoring and other tasks. Future work could focus on optimizing material properties, exploring more complex flight behaviors, and integrating more advanced sensing and control systems.

While the study demonstrates the feasibility of light-based control, the effectiveness of light control might be limited in environments with low light intensity or presence of obstacles affecting light transmission. The current study focuses on a limited number of seed designs, although the approach showed potential for broader adaptation to different passive flight mechanisms, further investigation with more natural seeds is needed. The outdoor experiments were conducted with a specific wind speed, and further research is needed to evaluate the performance under a wider range of wind conditions. The study focused on the use of UV and visible light; further investigation of other wavelengths could optimize control.