Gecko Adhesion Frequently Asked Questions
Gecko adhesive has unique properties not matched by conventional adhesive tapes
Geckos adhere to just about any surface, wet or dry, smooth or rough, hard or soft.
Gecko adhesive is unique in that it is self-cleaning during repeated use.
Gecko adhesion can be mechanically switched on and off. Sliding against
a surface uncurls the seta to engage the adhesive.
By relaxing sliding tension, the adhesive can be released.
Gecko adhesion principles work with hard materials, and are primarily dependent
on the geometry of the fibers. Thus gecko-inspired synthetic adhesives can be made
from a wide range of materials which can resist extreme environments such as
high temperatures. In the future, gecko inspired synthetic adhesives
can be made non-toxic, biocompatible, or biodegradable.
K. Autumn has identified
7 key properties of gecko adhesion
Romanian translation of this web page
(by Web Geek Science)
Common Misconceptions about Gecko Adhesion
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.)
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.
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.
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
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.
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
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,
7 Benchmark Properties for a Gecko-inspired Synthetic Adhesive
Kellar Autumn (MRS Bulletin, June 2007) has coined
the acronym GSA (``Gecko-inspired Synthetic Adhesive'') to describe structures
which show gecko-like adhesive properties, and to distinguish these materials
from the common PSA (``pressure sensitive adhesives'') used in
transparent and masking tape.
Autumn (MRS Bulletin 2007) has identified seven benchmark functional properties of the gecko adhesive system:
1 anisotropic attachment,
2 high pulloff to preload ratio,
3 low detachment force,
4 material independence / van der Waals adhesion,
6 anti-self matting, and
7 non-sticky default state.
There is one startup,
working towards manufacture of GSAs.