Geckoes inspire light-activated sticky pads

The apparent gravity-defying ability of geckoes to walk across ceilings and up and down walls has inspired a new material that might one day allow robots to do the same thing.

A research group led by Emre Kizilkan, a biologist at Kiel University in Germany, took cues from the lizard to develop a multi-layered substance capable of rapidly switching between adhesive and non-adhesive states, controlled by ultraviolet light. 

Geckoes can walk upside-down on horizontal surfaces because their feet comprise a hierarchy of spatula-shaped nanostructures.

Shear force derived from leg movements switches these tiny fibres from non-sticky to sticky and back again at the twitch of a muscle.

Kizilkan’s new material replicates this crucial binary state. Described in Science Robotics, it has been dubbed a “bioinspired photocontrollable microstructured transport device”, or BIPMTD. 

“The stimulus enabling surfaces to switch from an adhesive to a non-adhesive state or vice versa can be mechanical, magnetic, or thermal,” the researchers write in their paper

. “Besides these, light is a stimulus that can be controlled very quickly and precisely […] a very attractive stimulus for developing bioinspired photoresponsive reversible adhesive systems.”

Like a gecko’s foot, the BIPMTD also has a multi-layered structure: the sticky top layer is made of mushroom-shaped adhesive microstructures embedded in polydimethylsiloxane. 

These are controlled by a second layer of liquid crystals containing azobenzene – a chemical compound that responds to light – which is sealed from beneath by another layer of polydimethylsiloxane. 

When applied, UV light alters the molecular size of the azobenzene compound, thus changing the shape or curve of the material and causing it to peel away from a surface or object, mushroom by tiny mushroom. 

When the UV light disappears, the liquid crystals recover their original form, reinstating stickiness. 

During testing, BIPMTD was used to pick up, move and let go of a series of objects including a glass slide, a sphere, and a plastic Eppendorf tube – all objects considerably larger than the gecko-foot analogue. 

The new material can also be tuned to respond to different levels of UV intensity, potentially increasing its range of applications. 

Robots that walk upside down next to the bedroom light fitting might not be particularly useful, but thanks to the team at Kiel they are now at least possible.