(Nanowerk Spotlight) Building robots that can effortlessly mimic the movements of insects on water has been a persistent challenge in robotics. The ability to move autonomously and efficiently in environments like water surfaces – without needing external control or bulky power sources – holds immense potential in fields such as medicine and environmental monitoring. These environments demand more than just mobility; they require precise control, adaptability, and efficiency. But designing small-scale robots capable of navigating these spaces has proven difficult.
Robots powered by external magnetic fields or chemical propulsion often lack the fine control needed for real-world applications, consuming excessive fuel or relying on cumbersome external components. This limitation has made such technologies impractical for many of the critical tasks scientists envision.
Nature, however, offers an elegant solution. Certain aquatic insects, like water striders, glide across the water using surface tension and small bursts of propulsion. By secreting chemicals that reduce water tension behind them and adjusting their posture, these insects achieve both movement and control.
Researchers are trying to replicate this with synthetic systems, but balancing propulsion and control in a small, untethered robot has remained elusive. However, recent advances in bioinspired design have brought us closer than ever before.
A new study published in Advanced Functional Materials (“Self-Propelled Morphing Matter for Small-Scale Swimming Soft Robots”) demonstrates how combining chemical propulsion with shape-morphing materials can create autonomous, small-scale robots capable of complex and efficient movement across water surfaces.
Bioinspired design of a self-propelled shape-morphing swimming robot. a) A water treader swims by secreting a biosurfactant to generate Marangoni propulsive force while adjusting their posture to steer and change trajectory. b) A bioinspired soft robot integrates surface tension motors with photochemicalmorphing structure to mimic the swimming mechanism of water treaders by releasing chemical fuel for propulsion, whilemorphing their structure for steering respectively. (Image: reprinted from DOI:10.1002/adfm.202413129 CC BY) (click on image to enlarge)
In this breakthrough, the researchers integrated two powerful technologies: a Marangoni motor, which generates propulsion by releasing a controlled chemical fuel, and a light-responsive material called liquid crystal networks (LCNs), which allows the robot to change shape in response to light. Together, these technologies mimic the natural mechanics of insect movement, enabling robots to self-propel and steer without the need for external power or intervention. This advancement addresses many of the shortcomings that have plagued previous designs, marking a significant step toward developing robots that can operate independently in challenging environments.
The key innovation in this design is the combination of propulsion and steering into one streamlined system. The Marangoni motor, inspired by aquatic insects’ surfactant-secreting mechanisms, propels the robot forward by releasing a small amount of chemical fuel that alters the surface tension of the water. The motor itself is composed of structural proteins – derived from squid sucker rings – known for their strength and durability. When these proteins are combined with a volatile chemical fuel, they create a propulsion force that propels the robot forward. Crucially, the system is self-regulating, meaning it autonomously manages the release of fuel to ensure continuous, steady motion without constant external input.
In parallel, the robot’s body incorporates LCNs—materials that can bend and deform when exposed to specific wavelengths of light. These LCNs contain azobenzene molecules that undergo structural changes when illuminated, shifting between extended and bent shapes. By controlling which parts of the robot bend and when, the researchers have effectively designed a shape-morphing chassis that allows the robot to alter its movement. This gives the robot five distinct modes of locomotion, ranging from straight-line movement to precise turns.
By bending one of its legs upward, the robot reduces its contact with the water on that side, causing it to rotate. Conversely, by bending the leg downward, it increases drag, enabling sharper, more controlled turns. This combination of propulsion and shape-changing mechanisms allows the robot to perform complex movements that can be adjusted based on its environment and the tasks at hand.
This innovative locomotion strategy mirrors how semiaquatic insects move across water. The combination of chemical propulsion and controlled shape changes gives the robot the ability to glide, steer, and adjust its trajectory with remarkable precision. Unlike traditional robotic systems that rely on bulky components to control direction or propulsion, this design achieves both in a single, streamlined system.
Once the robot achieves its desired shape and trajectory, it retains that shape for a period of time, further enhancing energy efficiency by reducing the need for continuous actuation. The use of light-responsive materials as the primary control mechanism eliminates the need for wires or external devices, making the robot truly autonomous.
The advantages of this integrated approach are numerous. First, it offers true autonomy. Unlike previous designs that required magnetic fields, acoustic signals, or continuous fuel input, this robot can both propel and steer itself using only its internal systems. This autonomy opens up new possibilities for applications in environments where external control is impractical – such as inside the human body for medical interventions or in remote locations for environmental monitoring.
The light-responsive LCNs allow for precise control over the robot’s movement, making it ideal for navigating tight or complex spaces, such as delivering drugs to specific locations within the body or collecting data from sensitive ecosystems.
Moreover, the modular nature of the robot’s design means that its components can be adapted for different environments and tasks. The chemical propulsion system, which uses a protein-based motor, is highly versatile and can be applied to various substrates. This flexibility makes it possible to customize the robot for specific applications, enhancing its usefulness in a wide range of fields. For example, in industrial processes that require monitoring or intervention in confined spaces, these robots could be deployed without requiring major modifications.
Despite the promising potential of these robots, there are still some challenges to address before they can be used in widespread real-world applications. One major area for improvement is the efficiency of the propulsion system. While the protein motor provides enough power to move the robot, further optimization is needed to reduce fuel consumption and extend the operational lifetime of the robot. This will be particularly important in scenarios where the robot must operate for extended periods without human intervention, such as in environmental monitoring or remote medical applications.
Additionally, while the photochemical actuators are effective at controlling the robot’s movement, their durability and responsiveness in more extreme environments need to be improved. Ensuring that these actuators remain functional over long periods, especially in varying conditions like water temperature or salinity, will be crucial for expanding the robot’s utility. Researchers are already exploring ways to refine the design of these actuators to make them more robust and responsive, which will be essential for their successful deployment in real-world scenarios.
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