Akron Ascent Innovation’s new ShearGrip® dry adhesive is an example of a bio-inspired material. The initial research was an attempt to see if spinning thin fibers similar to the structure of gecko toe pads could be a useful route for making large-scale synthetic adhesives.
Why geckos? Lots of animals – and even plants – have evolved different methods to move on surfaces. The gecko is unique because it’s heavy. Gecko feet may even mark the upper-bound on natural methods for dry adhesion. Anything heavier is stuck with claws, suction cups, or elevators.
As with all things, the secret is in the structure. Geckos’ remarkable ability to run up and down surfaces – wet or dry, smooth or rough – comes from their incredibly structured toe pads. The toe pads are covered with ultra-fine hairs, called setae, that are arranged in dense groups. The individual setae are extremely thin – about 5 micrometers wide, or 10-20 times thinner than a human hair. Hair and gecko toe pads are actually made from the same material – keratin – which is a structural protein used in a range of animal features, from rhino horns to antlers, claws, skin, and more. Gecko and other reptilian features fall in the class of beta keratins, which are more rigid. The only difference is a few minor changes in protein structure that cause plate-like structures to form rather the more extended alpha keratin structures. Spiders use a similar trick in minor protein sequence changes to create silk with properties ranging from dragline threads as strong as steel to capture threads that are compliant and can absorb energy.
Some other incredible gecko facts:
Geckos are noisy
The word gecko comes from the chirps and clicks that they make. Their vocalizations are high varied and unique among lizards.
Geckos are extremely diverse
There are over 1,500 species of geckos in the world, and they take a broad range of shapes and sizes. The smallest geckos are less than an inch long and one, the Jaragua sphaero, wasn’t discovered until 2001. The largest known gecko, the kawekaweau, grew up to 24 inches long. It lived in New Zealand, but went extinct in the late 19th century due to new predators.
Gecko vision is bizarre – no eyelids, unique photoreceptors, and multi-point focus
Most species of gecko lack eyelids and need to constantly lick their eyes to keep them moist. The only exception is the family Eublepharidae, which also lacks adhesive toe pads. The most common pet lizard, the leopard gecko, is in this family.
Despite being nocturnal, geckos completely lack rod photoreceptors in their retina. This is surprising because human night vision is entirely due to rods. The absence of rods came about because they evolved from a family of lizards that was active only during the day. Geckos were able to adapt to a new ecological niche by through two unique features: they developed larger cone photoreceptors, and evolved a distinct double-cone photoreceptor.
In addition to their lack of eyelids and unique photoreceptors for night vision, geckos are able to control their focus on multiple points. They can see an image sharply for at least two different depths.
Gecko toe pads are self-cleaning
One reason why nano-structured materials are of interest is that unique surface properties can be introduced from common materials by varying their structure. The adhesion of gecko toe pads is one example, but a second feature of the complex structure is that they are self-cleaning. The hierarchical structure of thin setae with thinner spatulae makes water bead on the surface and prevents dirt or other contaminants from penetrating the structure (Sethi 2008).
After being covered in dirt, geckos can recover over 50% of their original adhesion within 8 steps.
The self-cleaning behavior is seen in a number of plants that show microscopic roughness. The lotus leaf is the most famous example. It has a high roughness on two different length scales, and a hydrophobic wax coating.
Gecko adhesion is complicated
Surface area is the critical factor in adhesion for animals, but it’s not the only thing. Researchers from the University of Massachusetts in Amherst compared the adhesion strength for several different species of gecko and found that the area of the toe pads accounted for about 50% of the increase in adhesion force with size (Gilman 2015). They found that the critical parameter for dry adhesive is the ratio of contact area and compliance. Low compliance is analogous to high stiffness, and indicates that the toe pads of larger geckos stretch less at a given force than smaller geckos.
Gecko adhesion won’t work for humans
It is well established that for a wide range of animals, the size of the adhesive toe pads increases with mass. Researchers from the University of Cambridge in the UK took this finding a step further and showed that as weight increases, a greater percentage of the body needs to be covered by the adhesive toe pads (Labonte 2016). For humans to be effective climbers, this implies that almost half of our body would need to be covered with dry adhesive. By my quick calculation, our hands and feet would each need to be about two feet wide and long in this case.
Gecko toe pads work after death
Materials research is not for the light of heart. Researchers at the University of California in Riverside demonstrated that gecko adhesion is passive, meaning that there is not a need for active muscular exertion to hold on (Stewart and Higham 2014). They showed this by comparing the adhesion strength of geckos before and after death.
References and Further Reading
Autumn et al., “Evidence for van der Waals adhesion in gecko setae,” Nature 2000 405:681-685.
Gilman et al., “Geckos as springs: mechanics explain across-species scaling of adhesion,” PLoS ONE 10(9):e0134604.
Labonte et al., “Extreme positive allometry of animal adhesive pads and the size limits of adhesion-based climbing,” PNAS 2016 113(5):1297-1302.
Sethi et al., “Gecko-inspired carbon nanotube-based self-cleaning adhesives,” Nano Letters 2008 8(3):822-5.
Stewart and Higham, “Passively stuck: death does not affect gecko adhesion strength,” Biology Letters 2014 10:20140701.