One of the most promising innovations in medical adhesives draws its innovation from none other than the gecko.  Oftentimes, scientists look to nature for inspiration to solve difficult problems because nature has probably already solved them! Geckos are no exception.  They are amazing creatures; able to run along both vertical and inverted surfaces with tremendous ease without the use of suction (vacuum) or chemical (glue).


What is the secret to the geckos incredible ability? How are they able to cling to virtually any surface?

The answer: Setae


Setae are gecko foot hairs. A single foot hair, seta, is around 110 micrometers in length and 4.2 micrometers in diameter.  Setae are oriented in the same direction and uniformly distributed into arrays.  The seta themselves branch to form their own nanoarray of hundreds of spatula structures, allowing them to easily contact a surface.  A spatula is a single stalk with a triangular-shaped tip and is about .2 micrometers long and wide [1,8].


These setae allow the gecko to cling to any surface, both hydrophobic and hydrophilic, rough or smooth.  To give you an idea of how many spatula there are on a gecko, a Tokay gecko has four feet with five toes each.  Each toe has about 20 rows of lamellae.  These lamellae have a ton of setal arrays with thousands of setae, approximately 200,000 setae per toe.  Then finally, each seta consists of hundreds to thousands of spatula [1,8].
































Each seta is capable of producing an average force of about 200 microNewtons in shear and 40 microNewtons in adhesion.  This means that the approximately 6.5 million setae on one gecko could lift about 290 lbs (133 kg) [1].


Another aspect of gecko foot hairs that make them so appealing to the medical industry is that they are self-cleaning. This self-cleaning property is especially important in relation to adhesives since dirt particles reduce the adhesiveness of the gecko foot hairs.  While it is not completely understood how this is done, some models suggest that the setae clean themselves due to an energetic disequilibrium between the adhesive forces that attract a dirt particle to the surface and between the particle and the setae.  Another possibility is something known as particle rolling [1].

The Gecko

The mechanism for the gecko’s adhesion has long been the subject of investigation.  Various theories have circulated in the past 175 years including glue, suction, interlocking friction, static electricity, and capillary forces.  However it was not until recently that the relatively weak van der Waals forces was discovered as the dominate mechanism for adhesion [2].



Van der Waals force is the attractive or repulsive force between molecules and not from covalent bonding or electrostatic interaction.  It is also sometimes known as the intermolecular forces that exist between atoms or nonpolar molecules.  This includes forces between two permanent dipoles, a permanent and an induced dipole, and two induced dipoles (London’s forces).  Van der Waals forces are extremely weak.  They are typically around just 1% of the strength of a covalent bond.  However the sheer number of spatulae on a gecko provides sufficient van der Waals forces between them and the surface for the gecko to be able to stick to anything [2].


Attachment and detachment of the setae is important as well.  It was found that both are controlled through mechanical means.  When unloaded, the setae are curved towards the gecko’s body so the nanoarrays are misaligned with the surface.  When the toes contact a surface, the setae bend from the resting position.  This flattens the stalks so that the tips point away from the gecko.  Then the spatulae are not uniformly flush with the surface, maximizing the surface area of contact.  In short, the setae must go through three phase to attach: orientation, preload, and drag.  Detachment is accomplished by simply increasing the angle that the setal shafts make with the surface, decreasing the surface area of contact [8].





















Attachment Mechanism

This website was created by Gregory Chun for BME 240 at the University of California, Irvine

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Last Updated June 10, 2009

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