Common Misconceptions about Gecko Adhesion

1. Aren't gecko toes sticky? No. Gecko toes are not ``sticky'' like tape.

   If you touch a gecko toe it feels soft and smooth, and not sticky at all. If you pressed a gecko toe onto a hard surface it would not stick. The toe will only adhere when the microfibers (setae) are engaged, by dragging or sliding the toe parallel to the surface. (If toes were sticky like tape, it would be difficult for a gecko to walk or run, as it would be too hard to pull its feet up.)

2. Aren't the gecko fibers themselves sticky? No. The material in natural gecko fibers is not sticky.

   Gecko micro fibers (setae) and end plates at the end of stalks (spatulae) are made primarily of beta keratin, a hard protein. (Keratin is similar to fingernails or hair). At the nanoscopic scale, a fiber can make an intimate contact in a very small area. Intermolecular forces provide an adhesive force in the range from 1 to 1000 nanoNewtons.

3. Shouldn't ``gecko tape'' be made of something soft and sticky? No. ``Gecko tape'' made from soft, sticky materials will not be very gecko-like.

   While soft polymers can adhere well to many surfaces, they have several drawbacks compared to gecko-tape made from materials as hard as the keratin in natural gecko fibers. It is interesting to note that while gecko setal arrays are made of a hard material (primarily beta-keratin), arrays of long, angled fibers of keratin have an equivalent stiffness similar to rubber. (By analogy, a soft mattress can be made from a hard material such as steel, by using arrays of coiled steel springs).
   • Soft sticky polymers easily accumulate dirt in the environment and are difficult to clean. By contrast, geckos self-clean dirt from their hard hairs. Think how easily a piece of adhesive tape picks up dirt and loses stickiness.
   • Soft materials tend to wear more quickly. Geckos can use their adhesive toes for months between molts without wearing out.
   • Fibers made from sticky materials will tend to stick to eachother, creating clumps which won't adhere to other surfaces. To reduce clumping, fiber spacing has to be increased, which reduces density. A lower density array with fibers which are stickier has lower overall adhesion.

4. Isn't gecko adhesion very strong? No. Gecko adhesion is not super strong.

   • Geckos need to be able rapidly attach and remove their feet from a surface. (A small gecko can run up a vertical wall at greater than 1 meter per second, and attaching and detaching its feet more than 20 times per second.) A super strong adhesive would slow the animal down. Shown below is a gecko peeling its toes from a surface by hyperextending its toes. Hyperextension would not work well if the toes were very sticky and hard to peel.
     
   • When contacting a smooth clean surface such as glass, the gecko microfiber arrray will have less contact area than a conventional adhesive tape. Since adhesion is proportional to contact area, a conventional adhesive tape will have greater shear (sliding) strength than a microfiber array. Even a smooth soft piece of rubber will adhere very well in shear to smooth glass. Finding that a synthetic soft material sticks better than gecko to a smooth surface such as glass is not surprising. Adhering better than gecko to a rough and dirty surface (as is found in nature) is a real challenge.

5. If a microscopic ``gecko-tape'' patch works, isn't that sufficient? No. There is a big difference between a single adhesive nano-contact and a patch which works over a square centimeter or larger area.

   It is very easy to make a single contact with high adhesion. In our 2002 PNAS paper [Autumn et al 2002], we showed a pulloff force of 290 nanoNewtons from a single polyester spatula. The tip size of the spatula was estimated to have radius of about 350 nanometers. If we packed 200 million spatulae into a square centimeter, and they all made contact, we would have 60 newtons per square centimeter (about 6 atmospheres), a huge adhesive pressure. However, it is very challenging to get 200 million simultaneous contacts to work together. Without all the branching structures of the gecko, or other compliance hierarchy, it will be difficult to obtain simultaneous contact. It is interesting to note that when two rigid surfaces are in contact, typically one gets only 3 points of contact (for similar reasons why a 3 legged stool is stable). So 200 million spatula on a rigid plate would not make enough contacts with a rough surface to get decent adhesion.
   

6. Should a gecko-like adhesive be pressed into a surface to stick? No. A gecko-like adhesive does not need to be pressed into a surface to adhere.

   The animal would not be able to climb rapidly if it needed to press its feet into a surface. (It would push itself off and likely lose its balance). Instead, it drags its feet slightly parallel to the surface to engage the microfibers (seta) and nanotips (spatula). A typical transparent tape or masking tape uses a Pressure Sensitive Adhesive, and requires pressure to engage with a surface. There are many synthetic fibrillar adhesive structures which adhere only when a high pressure is applied. These materials may be useful for many applications, but they are not based on gecko adhesion principles. The ``adhesion coefficient'' is the ratio between maximum sustained load and the applied force to engage the adhesive. The natural gecko has an adhesion coefficient greater than 8. Many fibrillar adhesives have adhesion coefficients much less than 1.

   An example fibrillar adhesive which does not require pressure to engage the fibers is reported in Jan. 2008. Sliding-induced adhesion of stiff polymer, Interface 2008

7 Benchmark Properties for a Gecko-inspired Synthetic Adhesive

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