“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/