The First Photocontrollable Transport Adhesive Bioinspired from the Gecko

Since the 1960s, life sciences and engineering sciences have been working together to make technological artifacts as efficient as natural organisms and phenomena. Bioinspired objects and materials, known as biomimetics, probe and attempt to mimic nature’s intelligence, including forms and systems that can be seen at a nanoscopic level. Thus, development of the first photocontrollable transport adhesive was inspired by the laws that govern the gecko’s ability to move around on vertical walls. Like most biomimetic devices, creating this adhesive has been made possible through the collaboration of various specialists in nanotechnology development.

A research group at Kiel University, in Germany, developed a multilayer adhesive that mimics the adhesion system of gecko digits to various surfaces. The multidisciplinary team includes chemists and polymer specialists Emre Kizilkan, Jan Strueben, Anne Staubitz, and zoologist Stanislav N. Gorb. Their study, entitled “Bioinspired photocontrollable microstructured transport device,” was published on January 18, 2017, in the Sciences Robotics journal.

Quantum Behavior of Gecko Digits

The ability of the gecko foot to achieve dry adhesion, i.e. without using a sticky substance, and to reposition on smooth, rough, dry or wet surfaces—while supporting its weight with only one digit—continues to inspire researchers. Some try to reproduce its macroscopic anatomy; others, the quantum physics phenomenon known as Van der Waals forces, which occurs at the nanoscopic level of the spatulae in each digit. For example, the motor skills of Stanford University’s Stickybot III robot reproduced the lizard’s foot shape and deployment movement, while engineers reproduced Van der Waals forces at the foot tips using electronics or a heat source. In 2012, a research group at the Massachusetts Institute of Technologies created the Geckskin adhesive, which can only bond to surfaces under considerable friction and pressure (heat source), limiting its applications to target materials that are resistant to such forces. However, the Kiel University team succeeded in creating a material that can stick on and peel off smooth surfaces through simple exposure to ultraviolet light.

Nanoscopic structure of the gecko spatula digits

Adhesive Structure

The adhesive is a three-layer tape. The upper layer is textured with mushroom-shaped setae, the flat side of which sticks to the target material surface upon contact with the adhesive. The setae are embedded in a polydimethylsiloxane layer, which in turn is linked to an azobenzene liquid crystal layer. The bottom layer is also made with polydimethylsiloxane. The top material allows the adhesive to stick to a surface, but only when the setae are properly aligned, i.e. in the absence of UV light. Indeed, the adhesive is activated when the azobenzene crystals are exposed to UV light. Consequently, exposure to UVs causes a change in the molecular structure of the crystals.

This behavior acts on the upper layer, causing orientational changes in the setae and displacement of the parts glued to the glass surface, which releases the adhesive. By varying the UV intensity, it is possible to vary the degree of adherence. The interaction between the setae and the azobenzene crystals is caused by Van der Waals forces. Roughly speaking, it is the result of a low intensity electrical interaction at short distances between atoms and molecules or between molecules and crystals. In this case, a repellent interaction takes place with the orientational changes of the substrate multipoles—the polydimethylsiloxane and azobenzene layers.

Potential Applications

The team members tested their adhesive by suspending glass spheres, glass slides, and Eppendorf tubes. They believe that it will eventually be possible to lift objects as heavy as a human being. Since the contact between the adhesive and the object leaves no residue, this technology can be used to transport objects in work environments with very strict hygiene standards, such as pharmaceutical and electronic laboratories.