(a) Photo of a water strider insect. (b) Optical microscope image of a superhydrophobic leg of a water strider. Scanning electron microscope (SEM) images of a water strider’s leg showing (c) microsetae and (d) fine grooved structure of a seta. (e) Schematic illustration of a light-driven water-walking soft robot inspired by water striders that can move in any direction and deliver a load, as shown by the football, to a specific location (or goal).
(a) Photo of a water strider insect. (b) Optical microscope image of a superhydrophobic leg of a water strider. Scanning electron microscope (SEM) images of a water strider’s leg showing (c) microsetae and (d) fine grooved structure of a seta. (e) Schematic illustration of a light-driven water-walking soft robot inspired by water striders that can move in any direction and deliver a load, as shown by the football, to a specific location (or goal).

Inspired by the water strider insect, which glides across the surface of rivers and ponds, researchers have designed a light-activated soft robot that can propel itself across water [Yang et al., Nano Today 43 (2022) 101419, https://doi.org/10.1016/j.nantod.2022.101419].

The insect uses its middle legs as ‘paddles’ to generate vortices in water to propel itself across the surface, while a superhydrophobic – or highly water-repelling – covering of tiny hairs (or microsetae) with a nanostructured, grooved surface on its legs and feet prevents it from sinking into the water and reduces drag.

The water-walking soft robots created by the team from the School of Materials Science and Engineering at Tianjin University have a small ‘body’ of Cu foam coated with AgNO3 aggregates and a conventional surface modifier (PFDT), which creates a superhydrophobic surface. This superhydrophobic body is attached to soft-actuation ‘legs’ for propulsion, fabricated from polymerizable miniaturized Au nanorods (or MiniGNR nanomonomer) embedded in a liquid crystal network (LCN).

The Au nanorods efficiently absorb light energy and transform it into localized heat through a non-radiative relaxation process called localized surface plasmon resonance. The localized heating effect drives the deformation of the LCN leg, which returns to its original shape when cooled. Exposure to near-infrared radiation, therefore, deforms and bends the LCN-based leg, which returns to its original shape as soon as the light source is removed. Subjecting the actuator to a cyclic light source repeatedly bends and straightens the leg, propelling the device across the water surface. The superhydrophobic body reduces drag and stops the robot from sinking into the water.

“One of the key challenges to be solved [was] how to improve the photothermal conversion efficiency and robustness of the GNRs-LCN soft actuators,” explain Ling Wang and Wei Feng.

The solution, the researchers suggest, lies in the use of MiniGNRs that, because of their diameters of < 10 nm, offer a better photothermal conversion efficiency than larger nanorods. The polymerizable nanomonomer, moreover, is more compatible with the LCN matrix.

The team were able to create single-legged soft robots that move in one direction over the surface of water and three-legged devices capable of movement in any direction. Each leg on the multi-directional robot responds to a slightly different wavelength of light, enabling control over the direction of motion. The tri-legged robot can also carry a small load and deliver it to a specific location.

The clever devices could pave the way for untethered, light-controlled multi-functional directional soft robots for aquatic conditions for environmental monitoring, ocean exploration or offshore industrial applications.

“We are now trying to create a fully autonomous soft robot that harvests available energy such as sunlight, [as well as] amphibian soft robots,” say Wang and Feng.