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“One night while on vacation in Hawaii, he was lying on his bed when he saw a large spider on the ceiling. He was starting to think it might be dangerous when help arrived. A Gecko walked across the ceiling and devoured the spider.”
(Forbes 81-82).
The Gecko’s Foot
John BarskiChemistry 508
MCE Thesis
The habitat of the gecko
• More than 850 species (50 species live in Australia)• Native to Australia, New Zealand, India, and other
tropical locations• Live in deserts, forests, and jungles• Hide in tree bark, under rocks, and in holes in the
ground• Some geckos are very rare (Geunther’s leaf-tail
gecko)• Others are very common (Leopard gecko)• Smallest gecko is the Minute Gecko
Geunther’s leaf-tail gecko (rare)
Leopard gecko (common)
Minute Gecko (smallest lizard)
Physical Attributes
• Large eyes which can see in dark (nocturnal reptile)
• No eye lids
• Geckos shed their skin like snakes (and sometimes, they eat the skin – dermatophagy)
• Tails can re-grow if they get cut off (tail autotomy)
Physical Attributes
• Hatch from eggs (geckos lay 2-3 eggs)
• Eats moths, spiders, and other insects
• 3.5cm – 35cm in length and weighs ~70 grams
• Makes a barking noise (no other lizard has a voice)
• Foot pads are not sticky, even though it can climb walls and walk on ceilings
• Feet are covered in lamellar pads which have many setae
Each seta has a high number of spatulae
How do other creatures climb walls and walk on ceilings?
FLIES SPIDERS
Spatula size comparison
Gecko Setae and Spatula
Setae are SCLEROPROTEIN or fibrous protein, which is a tertiary structure protein as opposed to a globular protein.
Scleroproteins are only found in animals and are usually structural or storage proteins (i.e. the protein is inert).
Scleroproteins are water-insoluble and are used to form connective tissue, tendons, fingernails, hair, bone, and muscle.
Keratin is a scleroprotein.
Keratins are the main constituent of structures that grow from the skin: the α-keratins in the hair (including wool), horns, nails, claws and hooves of mammals and the harder β-keratins in the scales and claws of reptiles, their shells (chelonians, such as tortoise, turtle, terrapin), and in the feathers, beaks, and claws of birds
2 forms of KERATIN
ALPHA BETA Hydrogen bonds are Hydrogen bonds are
Intra-molecular Inter-molecular
How does the gecko climb walls and ceilings?
• Not gravity
• Not magnetism
• Not mechanical (claws do not hang onto walls)
• Not electrostatic
• No wet adhesive on the feet (no glands)
• Not a muscular interaction
Two Competing Theories
• Van der Waals attractive force• Capillary action
Supporting Evidence
• Geckos have millions of tiny hairs (setae) on feet• Setae are hairs that have hairs (spatula) on them• Increased surface area of the hairs on the foot helps
geckos climb• Humidity assists the adhesive force
The attraction is purely physical
• There exists an attraction between the hairs on the foot of the gecko and the surface of the wall
• This effect is more dramatic in humid environments, but minimized in saturated or dry environments.
Pull-off force with varying humidity
Intermolecular Interactions
• Ionic bonding• Electron transfer • Bonding between metal/nonmetal elements by
strong electrostatic forces
• Hydrogen bonding• Electrostatic attraction between two dipolar
molecules
• Weak attractive forces• Ion-dipole forces• Dipole-dipole forces (Permanent dipole)• Polarization• Induced dipole• London dispersion forces (van der Waals)
Van der Waals Forces
Neutral molecules actually experience slight charge concentrations which fluctuate quickly (10-16 seconds).
The charged end of one molecule can then induce a dipole in a neighboring molecule resulting in a lattice of charged particles.Johannes Diderik van der Waals - 1869
When does PULL become PUSH?
When two molecules approach each other, one molecule can induce a dipole moment in the other and they are attracted to each other.
If the two molecules get too close, their valence electrons begin to interact and repel each other, which is known as Pauli repulsion.
Pauli Exclusion Principle – No two electrons can possess the same set of quantum numbers.
Lennard-Jones PotentialIn
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Lennard-Jones Potential
• As two neutral molecules approach each other, they are initially attracted, but then are repelled.
• In this equation, ε represents well depth, σ represents hard-sphere diameter of the molecule, and r represents distance between molecules.
• The potential is 0 when σ = r. This is known as the Equilibrium Distance.
Artificial Gecko Setae
These pillars are artificial. The closer the pillars are together, the bigger their attractive force.
Notice that the pillars begin to stick together if they are too long.
Artificial vs. Natural
Applications of “Gecko” Materials
• “No Slip” gloves for handling glassware• Jar lid removal devices• Pet combs (or glove) for removing animal hair from fabric• “Gecko Tape” for a variety of uses• Substitute for VELCRO®
• “Gecko Suit” for climbing walls or space walks• “Geckobots” for entering small spaces with a camera
How big would a “Gecko Glove” need to be to hold a 70 kg person?
Based on the following data, each “Gecko Glove” would need to be approximately 4 sq. in., or half the size of the palm of your hand! You will need 4 of them.
Gecko palm area = 227mm²Setae density = 5300 setae/mm²Force of attraction of one seta = 20μN/setaForce of gravity = (9.8 m/s²)*(mass)1 Newton = 1 kg·m/s²Mass of one person = 70 kg
Thank You!
1. http://nobelprize.org/nobel_prizes/physics/laureates/1910/waals-bio.html
2. http://www.chemguide.co.uk/atoms/bonding/vdw.html#top
3. http://polymer.bu.edu/Wasser/robert/work/node8.html
4. http://en.wikipedia.org/wiki/Lennard-Jones_potential
5. http://www.elmhurst.edu/~chm/vchembook/566secprotein.html
6. http://en.wikipedia.org/wiki/Image:KeratinF9.png
7. http://en.wikipedia.org/wiki/Lennard_Jones_potential
8. http://en.wikipedia.org/wiki/John_Lennard-Jones
9. http://www.pubmedcentral.nih.gov/articlerender.fcgi?tool=
pubmed&pubmedid=16260737
10. http://www.newscientisttech.com/channel/tech/mg19025526.500.html
11.http://www.chemistrycoach.com/quantum.htm
12.http://recently.rainweb.net/hive/515/