Synthetic setae

 ( Nanotechnology ) 

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:

$\backslash mu^\text{'}=F\_\backslash text\{adhesion\}/F\_\backslash text\{preload\}$

where $F\_\backslash text\{preload\}$ is the applied preload force, and $F\_\backslash text\{adhesion\}$ is the generated adhesion force. The adhesion coefficient of real gecko setae is typically 8~16.

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. 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).

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 experimentsGe, L., Sethi, S., Ci, L., Ajayan, P.M. and Dhinojwala, A. (2007), "Carbon nanotube-based synthetic gecko tapes", Proc. Natl. Acad. Sci. USA, Vol. 104, pp. 10792–5. 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.

The resulting adhesive, named 'geckel', was described to be an array of gecko-mimetic, 400 nm wide silicone pillars, fabricated by electron-beam lithography 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 , 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 Surgical suture to close a wound and a water resistant adhesive for bandages and drug-delivery patches.

In 2006, researchers at BAE Systems Advanced Technology Centre at Bristol, UK, announced that they had produced samples of "synthetic gecko" – arrays of mushroom-shaped hairs of polyimide – by photolithography, with diameters up to 100 μm. These were shown to stick to almost any surface, including those covered in dirt, and a pull-off of 3,000 kg/m^2 was measured. More recently, the company has used the same technique to create patterned silicon moulds to produce the material and has replaced the polyimide with polydimethylsiloxane (PDMS). This latest material exhibited a strength of 220 kPa. Photo-lithography has the benefit of being widely used, well understood and scalable up to very large areas cheaply and easily, which is not the case with some of the other methods used to fabricate prototype materials.

In 2019, researchers from Akron Ascent Innovations, LLC, a company spun-out from University of Akron technology, announced the commercial availability of " ShearGrip" brand dry adhesives. 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.

As progress continues in , research has begun to focus on developing robust climbers. Various robots have been developed that climb flat vertical surfaces using suction, magnets, and arrays of small spines, to attach their feet to the surface.

More recently, robots have been developed that utilize synthetic adhesive materials for climbing smooth surfaces such as glass.

These crawler and climbing robots can be used in the military context to examine the surfaces of aircraft for defects and are starting to replace manual inspection methods. Today's crawlers use vacuum pumps and heavy-duty suction pads which could be replaced by this material.

Stickybot is an embodiment of the hypotheses about the requirements for mobility on vertical surfaces using dry adhesion. The main point is that we need controllable adhesion. The essential ingredients are:

• hierarchical compliance for conforming at centimeter, millimeter and micrometer scales,
• anisotropic dry adhesive materials and structures so that we can control adhesion by controlling shear,
• distributed active force control that works with compliance and anisotropy to achieve stability.

• Prof. Kellar Autumn's Autumn Lab at Lewis & Clark College
• RiSE Project Web