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The ability of geckos to climb on surfaces is mainly pegged on their adhesive toe pads. The pads are designed with hair-like structures. These hierarchical structures enhance compliance, intimate contact and promote van der Waals interactions with the surfaces. It has been confirmed that a unique configuration of the gecko toes is key in their locomotion, which happens in two modes namely; gripping and release modes.

During the gripping mode, two diagonally opposite toes are in action and are pulled inwards towards the center of mass of the body. The other two pads are normally detached during the attachment mode, which can be termed as the Y-configuration. The gecko applies both lateral and normal forces to drag the setae array against the surfaces they move on. In the release mode, the setae are configured in a critical angle to enhance the release of the pads. The toe pads scroll upward from the mating surface and reduce the adhesion using the setal shaft, which functions like a lever for perpendicular peeling the spatulae off the substrates.

Gecko-inspired adhesives, which are normally based on micropillars terminated by flaps, have, therefore, received numerous research works in the recent past. However, active control of the adhesives imitating the switchable properties of gecko’s toe pads is still an outstanding challenge. A collaborative research between Professor Antal Jákli at Ñý¼§Ö±²¥ State University and Professor Boxin Zhao at Waterloo Institute of Nanotechnology with experiments carried out by graduate students Hamed Shahsavan and Muhammad Salili (by now both received their PhD) reported the integration of gecko imitating adhesives to cantilevers form liquid crystal network in a bid to design multi-legged gecko grippers with thermally-induced peeling capabilities. Their work is published in Advanced Materials.

The authors developed a multilegged gripper and the pick-and-place mechanism. They made each leg by a hybrid liquid crystal polymer cantilever attached to one edge, and a film terminated fibrillar on the other end. They also fabricated two forms of film-terminated adhesive structures, one fully elastic and the other with viscoelastic top-coat.

In order to achieve adequate preload stress, the authors attached small magnetic patches on the upper surface of the cantilevers. The preload stress was, therefore, provided via the applied magnetic field which was created by an electromagnet placed below the specimen. During the experiment, the release mode was triggered by the bending of the liquid crystal polymers, which initiated the self-peeling of the adhesive patch. The authors realized that for the attachment mode, the applied magnetic force should be adequate in order to impose the required preload stress. 

The researchers observed that the adhesion as well as the bending energy of the liquid crystal polymers was dictated by the temperature on the surface. Higher temperature caused a larger bending, which further induced self-peeling of the liquid crystal network cantilever.

The authors concluded that the liquid crystal polymer cantilevers with cross-linking provided adequate work output to detach film adhesive patches from smooth and flat surfaces. They implemented the self-peeling mechanism of the structures in pick and place handling of smooth 2-dimensional objects. This study demonstrated the possible implementation of the gecko-inspired gripping and release mechanism for developing switchable adhesives, which can be used as grippers.

Reference
Hamed Shahsavan1,2, Seyyed Muhammad Salili2, Antal Jákli2,3, and Boxin Zhao1. Thermally Active Liquid Crystal Network Gripper Mimicking the Self-Peeling of Gecko Toe Pads. Advanced Materials, volume 29 (2017), 1604021.

Significance Statement - Advances In Engeering