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].