A collaborative team of researchers from the University of California, Berkeley, the Georgia Institute of Technology and Ajou University in South Korea has developed a revolutionary insect-scale robot that incorporates engineered self-morphing fans that mimic the agile movements of Rhagovelia bugs.
The unique fan-like propellers of Rhagovelia water striders – which allow them to glide across fast-moving streams – open and close passively, like a paintbrush, ten times faster than the blink of an eye. These millimetre-sized semiaquatic insects use the specialised fan-like structures on their propulsion legs that enable rapid turns and bursts of speed.
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Until now, it was believed that these fans were powered solely by muscle action. However, the researchers found that Rhagovelia’s flat, ribbon-shaped fans can instead passively morph using surface tension and elastic forces, without relying on muscle energy.
‘Observing for the first time an isolated fan passively expanding almost instantaneously upon contact with a water droplet was entirely unexpected,’ said Victor Ortega-Jimenez an integrative biologist now at the University of California, Berkeley.
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This remarkable combination of collapsibility during leg recovery and rigidity during propulsion allows the bugs to execute sharp turns in just 50 milliseconds and move at speeds up to 120 body lengths per second, rivalling the rapid aerial manoeuvres of flying flies.
Creating an insect-size robot inspired by ripple bugs was a major challenge, particularly because the microstructural design of the fan remained a mystery. The breakthrough came when Dongjin Kim,a postdoctoral researcher at Ajou University,and Professor Je-Sung from Ajou University captured high-resolution images of the fan using a scanning electron microscope.
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‘We initially designed various types of cylindrical-shaped fans, which we generally think is what hair looks like. However, the functional duality of the fan – rigidity for thrust generation and flexible for collapsibility – could not be achieved with cylindrical structures. After numerous attempts, we overcame this challenge by designing a flat-ribbon-shaped fan. We strongly suspected that biological fans might share a similar morphology, and eventually discovered that the Rhagovelia fan indeed possess a flat-ribbon micro architecture, which had not been previously reported. This discovery further validated the design principle behind our artificial flat-ribbon fan,’ said Kim.
With these insights, they were able to decode the structural basis and function of this natural propulsion system and recreate it in a robotic form. The result was the engineering of a one-milligram elastocapillary fan that deploys itself, which was integrated into an insect-size robot. This microrobot is capable of enhanced thrust, braking and manoeuvrability, validated through experiments involving both live insects and robotic prototypes.
‘Our robotic fans self-morph using nothing but water surface forces and flexible geometry – just like their biological counterparts. It is a form of mechanical embedded intelligence refined by nature through millions of years of evolution. In small-scale robotics, these kinds of efficient and unique mechanisms would be a key enabling technology for overcoming limits in miniaturisation of conventional robots,’ said Professor Je-sung Koh.
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The study not only establishes a direct link between fan microstructure and aquatic locomotion control, but also lays the foundation for future design of compact, semi-aquatic robots that can explore water surfaces in challenging, fast-flowing environments.
The ripple bug’s fan structure, which rapidly collapses and reopens as it enters and exits water, has revealed an unprecedented biomechanical duality — high flexibility for rapid deployment and high rigidity for thrust. This duality addresses longstanding limitations in small-scale aquatic robotics, such as inefficient stroke recovery and limited manoeuvring capacity.
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‘When designing small-scale robots, it’s important to account for the specific environment in which they will operate — in this case, the water’s surface. By leveraging the unique properties of that environment, a robot’s performance and efficiency can be greatly enhanced. The Rhagobot, for instance, can travel quickly along a flowing stream thanks to its intelligent fan structure, which is powered by surface tension and the drag forces from the water surface,’ said Jesung Koh.
Finally, these discoveries can have wide-ranging implications for bioinspired robotics, particularly in the development of environmental monitoring systems, search-and-rescue microrobots and devices capable of navigating perturbed water-air interfaces with insect-like dexterity.
The research has been published in Science.