The discoveries about gecko's feet led to the idea that these structures and mechanisms might be exploited in a new family of adhesives, and research groups from around the world are now investigating this concept. And thanks to the development of nano science and technology, people are now able to create biomimetic adhesive inspired by gecko's setae using
nanostructures. Indeed, interest and new discoveries in gecko-type adhesives are booming, as illustrated by the growing number of papers published on this topic. is not geckolike since it requires a very large preload of 50N to yield 3N and 0.3 atm of adhesion, yielding a value of \mu^'=0.06 • The synthetic setae of Northern and Turner(2005) perform significantly better with a \mu^'=0.125, but still below the benchmark of real gecko setae • Ali Dhinojwala and others from University of Akron and Renesselar Polytechnic Institute published their description of a carbon-nanotube carpet that generated adhesive force even greater than that of gecko setae in 2005. However, this product only works at a nanometer scale rather than the centimeter scale of real gecko toes.-->
Effective design Effective design of geckolike adhesives will require deep understanding of the principles underlying the properties observed in the natural system. These properties, principles, and related parameters of the gecko adhesive system are shown in the following table. This table also gives us an insight into how scientists translate those good properties of gecko's setae (as shown in the first column) into the parameters they can actually control and design (as shown in the third column). • JKR refers to the Johnson, Kendall, Roberts model of adhesion In summary, the key parameters in the design of synthetic gecko adhesive include: • Pattern and periodicity of the synthetic setae • Hierarchical structure • Length, diameter, angle and stiffness of the shafts • Size, shape and stiffness of the spatulas (end of the satae) • Flexibility of the substrate There is a growing list of benchmark properties that can be used to evaluate the effectiveness of synthetic setae, and the adhesion coefficient, which is defined as: \mu^'=F_\text{adhesion}/F_\text{preload} where F_\text{preload} is the applied preload force, and F_\text{adhesion} is the generated adhesion force. The adhesion coefficient of real gecko setae is typically 8~16.
Materials In the first developments of synthetic setae,
polymers like
polyimide,
polypropylene and
polydimethylsiloxane (PDMS) are frequently used since they are flexible and easily fabricated. Later, as nanotechnology rapidly developed,
Carbon Nanotubes (CNTs) are preferred by most research groups and used in most recent projects. CNTs have much larger possible length-to-diameter ratio than polymers, and they exhibit both extraordinary strength and flexibility, as well as good electrical properties. It is these novel properties that make synthetic setae more effective.
Fabrication techniques A number of
MEMS/
NEMS fabrication techniques are applied to the fabrication of synthetic setae, which include
photolithography/
electron beam lithography,
plasma etching,
deep reactive ion etching (DRIE),
chemical vapor deposition (CVD), and micro-molding, etc.
Examples In this section, several typical examples will be given to show the design and fabrication process of synthetic setae. We can also gain an insight into the development of this biomimetic technology over the past few years from these examples.
Gecko tape This example is one of the first developments of synthetic setae, which arose from a collaboration between the
Manchester Centre for Mesoscience and Nanotechnology, and the Institute for Microelectronics Technology in Russia. Work started in 2001 and 2 years later results were published in
Nature Materials. The group prepared flexible fibers of polyimide as the synthetic setae structures on the surface of a 5
μm thick film of the same material using electron beam lithography and
dry etching in an oxygen plasma. The fibres were 2 μm long, with a diameter of around 500 nm and a periodicity of 1.6 μm, and covered an area of roughly 1 cm2 (see figure on the left). Initially, the team used a
silicon wafer as a substrate but found that the tape's adhesive power increased by almost 1,000 times if they used a soft bonding substrate such as Scotch tape – This is because the flexible substrate yields a much higher ratio of the number of setae in contact with the surface over the total number of setae. The result of this "gecko tape" was tested by attaching a sample to the hand of a 15 cm high plastic Spider-Man figure weighing 40 g, which enabled it to stick to a glass ceiling, as is shown in the figure. The tape, which had a contact area of around 0.5 cm2 with the glass, was able to carry a load of more than 100 g. However, the adhesion coefficient was only 0.06, which is low compared with real geckos (8~16).
Synthetic gecko foot hair As nanoscience and nanotechnology develop, more projects involve the application of nanotechnology, notably the use of
carbon nanotubes (CNTs). In 2005, researchers from the
University of Akron and
Rensselaer Polytechnic Institute, both in the US, created synthetic setae structures by depositing multiwalled CNTs by chemical vapour deposition onto quartz and silicon substrates. The nanotubes were typically 10–20 nm in diameter and around 65 μm long. The group then encapsulated the vertically aligned nanotubes in PMMA polymer before exposing the top 25 μm of the tubes by etching away some of the polymer. The nanotubes tended to form entangled bundles about 50 nm in diameter because of the solvent drying process used after etching (as is shown in the figure on the right). The results were tested with a
scanning probe microscope, and it showed that the minimum force per unit area as 1.6±0.5×10−2 nN/nm2, which is far larger than the figure the team estimated for the typical adhesive force of a gecko's setae, which was 10−4 nN/nm2. Later experiments with the same structures on
Scotch tape revealed that this material could support a
shear stress of 36 N/cm2, nearly four times higher than a gecko foot. This was the first time synthetic setae exhibited better properties than those of natural gecko foot. Moreover, this new material can adhere to a wider variety of materials, including glass and Teflon. This new material has some problems, though. When pulled parallel to a surface, the tape releases, not because the CNTs lose adhesion from the surface but because they break, and the tape cannot be reused in this case. Moreover, unlike gecko's setae, this material only works for small area (approx. 1 cm2). The researchers are currently working on a number of ways to strengthen the nanotubes and are also aiming to make the tape reusable thousands of times, rather than the dozens of times it can now be used.
Geckel and coated with a mussel-mimetic polymer, a synthetic form of
the amino acid that occurs naturally in
mussels (left). . Unlike true gecko glue, the material depends on van der Waals forces for its adhesive properties and on the chemical interaction of the surface with the
hydroxyl groups in the mussel protein. The material improves wet adhesion 15-fold compared with uncoated pillar arrays. The so-called "geckel" tape adheres through 1,000 contact and release cycles, sticking strongly in both wet and dry environments. So far, the material has been tested on
silicon nitride,
titanium oxide and gold, all of which are used in the electronics industry. However, for it to be used in bandages and medical tape, a key potential application, it must be able to adhere to human skin. The researchers tested other mussel-inspired synthetic proteins that have similar chemical groups and found that they adhere to living tissue.--> Geckel is an
adhesive that can attach to both wet and dry surfaces. Its strength "comes from coating fibrous silicone, similar in structure to a gecko's foot, with a polymer that mimics the 'glue' used by mussels." The team drew inspiration from
geckos, who can support hundreds of times their own body weight. Geckos rely on billions of hair-like structures, known as
setae to adhere. Researchers combined this ability with the sticking power of mussels. Tests showed that "the material could be stuck and unstuck more than 1,000 times, even when used under water", retaining 85 percent of their adhesive strength. Phillip Messersmith, lead researcher on the team that developed the product, believes that the adhesive could have many medical applications, for example tapes that could replace
sutures to close a wound and a water resistant adhesive for bandages and drug-delivery patches. Rather than relying on photolithography or other micro-fabrication strategies, the researchers employed
electrospinning to produce small diameter fibers based on the principle of contact splitting exploited by geckos. The product has reported shear strength greater than 80 pounds per square inch, with clean removal and reusability on many surfaces, and the ability to laminate the material to various face stocks in one or two sided constructions. The approach is claimed to be more scalable than other strategies to produce synthetic setae and has been used to produce products for consumer markets under the brand name Pinless. ==Applications==