ASC Supplement 2013: Taking a lead from nature

26 April 2013 12:55  [Source: ICB]

Bioinspiration derived from the humble gecko lizard has lead to new adhesive technology

There are many natural examples of adhesive systems with very attractive properties. One of the most often cited is the gecko, whose feet, without any stickiness, can be applied and removed with ease yet adhere with tremendous strength.

 

Geckskin - made of a soft elastomer - mimics the structure and adhesive strength of the gecko's feet

Copyright: RexFeature

Researchers have studied these feet and attempted to mimic their behaviour, but have been unsuccessful until recently. The trick to bioinspiration - according to Al Crosby, a professor in the Department of Polymer Science and Engineering at the University of Massachusetts Amherst - is to proceed with caution.

By doing so, he, along with doctoral students Michael Bartlett and Dan King, postdoctoral researcher Andrew Croll and biologist Duncan Irschick, was able to develop a 16 square inch (0.01 square meter) reversible, high-capacity adhesive device that works via dry adhesion and can hold 700 lbs (318 kg) on a smooth wall. Named Geckskin, the material was selected as one of the top five science breakthroughs of 2012 by CNN Money.

INVESTIGATION, NOT IMITATION
Interestingly, the researchers did not start out trying to mimic the exact structure of gecko feet. In fact, they began by creating a mathematical model that led to a theory for the interrelationships between the geometry and material properties that must be maintained in order to have high capacity adhesion that is reversible. "We specifically did not think about the structure of the gecko feet at this point. The idea was to capture the performance of gecko feet without considering its specifics," Crosby notes.

What they found was that both a large contact area and a high level of stiffness are required. These results then provided insight into how gecko feet work. They have tendons that are connected to both the bone and skin, and these tendons are very stiff tissue.

In addition, because they are connected to the bone and skin, the interface of the foot pad with the external surface is maximised. Notably, the little hairs on the feet of the gecko - the setae - which have been assumed by most other investigators to be critical to the adhesive performance, are not critically important.

The information was used to develop Geckskin. Its adhesive pad is made of a soft elastomer, such as polydimethylsiloxane, woven into layers of stiff fabric that serve as the "tendon" and "skin". This design allows the pad to drape over the surface, thus providing a large interface.

One interesting fact about this study is that even though Crosby and his colleagues conducted the study with a broad perspective, they ended up replicating many features of gecko feet. In addition, the results are contrary to much of existing theory, which Crosby believes opens up numerous new possibilities for how adhesion can be achieved.

The team is currently studying various species of geckos and their different foot structures and adhesive capabilities, as well as investigating the potential connection between habitat and the evolution of foot morphology. "While these methods are not the traditional ones employed in adhesion development efforts, we believe they will provide insight into improved adhesion design," Crosby asserts.

He will also continue to use caution when pursuing the mimetic design of adhesion systems. "Bioinspiration and biomimetic design are powerful tools, but they can easily lead one down the wrong path. When man first attempted to imitate flight, he focused on the feathers he saw on birds. It wasn't until the structural aspects of avian wings were considered that flight became possible. It is very important to consider all of the aspects involved - the engineering, physics and chemistry, as well as well as the biology - and recognise the constraints within which the natural system developed."


Author: Cynthia Challener



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