Nature-Inspired Adhesion Technology: Geckos

Adhesive Technology Inspired by Nature: Geckos Throughout history, traces of adhesive technologies can be found. With the help of biologically derived resins, hunting tools were made and developed, daily life was depicted, and simple treatment methods were applied. In joining with adhesives, wood resins and mussels (Figure 1); in the field of surface technology, the reptile family known as geckos has pioneered important research. Attachment pads found on the body, feet and legs of many animals such as insects, spiders and lizards have the ability to adhere to and detach from various surfaces. For this reason, they move easily on vertical walls and ceilings (2). Among this group of animals, geckos are the heaviest in mass and, among climbing animals with intelligent adhesion ability, have the most developed and complex foot structure. These organisms have approximately three billion nano-scale spatulae extending from approximately three million micro-scale hairs (setae) at a density of about 14,000 per mm² on their feet (3). The setae found on gecko feet create numerous weak attractions between molecules on two surfaces, allowing them to remain suspended. Creating and breaking the bonds made by each hair with the surface is, unlike any adhesive or tape, easy and quick. For this reason, we can say that the natural properties of this organism have inspired surface technologies. Scientists recreated gecko-like adhesion using silicone, plastic, carbon nanotubes and other materials, but encountered a scaling problem. In theory, with adhesive strength quite high, 3-6 million setae could lift approximately 130 kilograms of mass. However, in reality, a gecko can only carry a 2-kilogram load with its front feet. According to Stanford University researchers, the scaling problem stems from the fact that loads are not distributed evenly across wide adhesive areas. For example, on gecko toe pads, only some of the setae are in close contact with the surface, and those not in contact do not share the load equally among themselves. Researchers suggest that the reason for this may be that gecko skin, like a rubber band, becomes stiffer when more force is applied to it. When a lizard climbs a wall, the setae stretch unevenly and some experience more force than others. Therefore, while some setae maximize their adhesiveness, others are underutilized or do not adhere to the surface at all. For this reason, researchers focused on distributing forces more evenly and developed an adhesion system based on the principle seen in geckos. Scientists began creating artificial gecko adhesives by using silicon micro-wedges that mimicked the reptile's setae. They assembled these into 24 scale-sized tiles, each containing hundreds of thousands of micro-slits. The tiles were then attached to an octagonal plate by being connected to springs with tendon-like cords. Unlike gecko skin, the springs apply equal force to the tiles after being stretched beyond a certain threshold, distributing loads evenly across the tiles. This development work enabled the patch mounted on the surface to provide similar adhesive strength for sizes from a square millimeter to the human hand. Even if a single tile is peeled off, the weight it carries is transferred to the tiles with the lightest loads that do not exceed the spring threshold, so the overall system remains adhesive. The 7 parameters by which polypropylene-based artificial gecko adhesives are related to the natural adhesion properties that this organism has as an inspiration source are as follows: 1. Directional Bonding: Setae do not adhere by being pressed against the surface; instead, a shear behavior parallel to the surface is required for the fibers to grip the opposite side. 2. High Pre-Load/Shear Ratio: A pre-load of less than 0.1 Newton is sufficient to bind the fibers together, and after the pre-load is removed, it can support a shear load of 4 Newtons each. 3. Low Detachment Force: The polypropylene-based gecko adhesive is directional and exhibits adhesion in the direction parallel to the surface. However, it can be easily removed with a force of less than 0.001 Newton. 4. Non-Aggregation: Polypropylene microfibers do not stick to each other and do not clump even in long loading/unloading cycles. 5. Non-Adhesive at Rest: Polypropylene, the polymer used in the synthetic adhesive, is nearly as hard as the gecko's keratin structure and the patch surface is not adhesive at rest. 6. Topography Independence: It can adhere to challenging surfaces. 7. Self-Cleaning: Research is ongoing. Stanford University researchers developed a robotic system called FarmHand (Farm Hand) in the Biomimetics and Dexterous Manipulation Laboratory. Materials successfully transported in tests include raw eggs, grape bunches, plates, liquid containers, basketballs and grinding motors. Using gecko adhesive in this multi-fingered, anthropomorphic gripper is challenging, but requires special attention to the tendons controlling FarmHand's fingers and the design of the finger pads beneath the adhesive. Like the gecko's toe pads, the gecko adhesive provides strong grip through microscopic fins. In robotic design, when these fins are in full contact with the surface, they adhere through Van der Waals force, a weak intermolecular force arising from subtle differences in the positions of electrons outside molecules. As a result, adhesives have strong gripping ability and the actual force required to achieve this is quite minimal. Additionally, they leave no residue and do not give an adhesive feel. In the early 2000s, when research in this field began and intensified, it was assumed that the binding mechanism of setae on gecko feet was Van der Waals force, and in artificial applications, this system was heavily based on examples such as FarmHand and StickyBot mentioned above (Figure 6). In 2014, Canadian researchers turned to the theory that the mechanism enabling geckos to walk on surfaces due to the unique fibrillar properties of their foot pads was contact electrification (CE). In addition to adhesion force, by measuring electrical charges occurring between gecko setae and the surface, measurements showed that the optional adhesion property on the surface operates through the electrical double layer (EDL) mechanism rather than capillary Van der Waals force at the contact interface. Accordingly, in the neutral state (Figure 7-a), gecko foot setae are placed on a polymer thin film spread over a copper plate. When the foot contacts the polymer film (Figure 7-b), electrical charges separate between the setae and the thin film. The EDL created at the contact interface induces certain electrical charges on a copper plate at the back that is grounded with an electrometer. Electrical charges separated by contact (Figure 7-c) penetrate first into the polymer thin film (di) and then into the seta depth (dg).   TEFLON AF's (amorphous fluoropolymer) weaker ability to generate both Van der Waals and capillary forces compared to PDMS (polydimethyl siloxane), the development of increased adhesion forces observed by TEFLON AF, once again confirms the definitive contribution of CE-driven electrostatic interactions to gecko adhesion (8).

References

* https://www.shutterstock.com/image-photo/madagascar-day-gecko-phelsuma-madagascariensis-140581852 (1) Petrone, L., Kumar, A., Sutanto, C. et al. Mussel adhesion is dictated by time-regulated secretion and molecular conformation of mussel adhesive proteins. Nat Commun 6, 8737 (2015). https://doi.org/10.1038/ncomms9737 (2) https://www.science.org/content/article/gecko-inspired-adhesives-allow-people-climb-walls (3) Bhushan B. (2016) Gecko Effect. In: Bhushan B. (eds) Encyclopedia of Nanotechnology. Springer, Dordrecht. https://doi.org/10.1007/978-94-017-9780-1_378 (4) https://plastics-themag.com/With-this-polymer-we-will-soon-be-able-to-make-geckos-feet-in-the-kitchen (5) https://en.wikipedia.org/wiki/Synthetic_setae (6) Kellar Autumn, Anne M. Peattie, Mechanisms of Adhesion in Geckos, Integrative and Comparative Biology, Volume 42, Issue 6, December 2002, Pages 1081–1090, https://doi.org/10.1093/icb/42.6.1081 (7) https://news.mit.edu/2009/stickybot-092509 (8) Izadi H, Stewart KME, Penlidis A. 2014 Role of contact electrification and electrostatic interactions in gecko adhesion. J. R. Soc. Interface 11 : 20140371. http://dx.doi.org/10.1098/rsif.2014.0371     A. Tuğçe Onur Materials and Process Senior Engineer Adhesive Engineer, EAE Turkish Aerospace Industries Inc.