The Gecko and the Lotus Plant:

AI and Robotics

Nov 15, 2013 (4 years and 6 months ago)

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The Gecko and the Lotus Plant:

BioMimetics and Nanosurfaces

Jen Ehrlich

Oregon High School, Oregon, WI

jme2@oregon.k12.wi.us

http://teachers.oregon.k12.wi.us/ehrlich/teacher%20pag
e/

a presentation with:

Sue Whitsett

Fond du Lac High School, Fond du Lac, WI

NSTA National Conference

March 2
9
, 2008

Boston, Massachusetts

Van der Waals Forces and Animal Adhesion

Introductory Terms:

Name _________________________

Micro
:

Micrometer or Micron:

Nano:

Nanometer:

Nanotechnology:

Intermolecular Forces

Van der Waals Forces

Tokay Gecko:

Scansors

Setae

Spatula

Jumping Spider:

Scopula

Setae

Setules

ht
tp://whyfiles.org/shorties/152sticky_spider/

Photo courtesy
Ed Nieuwenhuys

http://en.wikipedia.org/wiki/Tokay_gecko

Station 1: Graph Interpreta
tion

1: What do each of the colored regions represent?

2: Describe what happens to the number of setae per 100

m
2

as the mass of the organism
increases.

3: What kind of relationship is this?

4. The following images were taken with a Scanning Electron Microscope. They are close
-
up
pictures of the surface of an animal’s foot. Based on the graph, match each pictu
re with the
correct organism: beetle, fly, spider, lizard.

Station 2: Experiment

Question:

How does the number of setae affect the strength of the attachment force?

Data:

# of “setae”

O

4

8

Tot慬 j慳猠a
E朩

Conclusion:

1. Which

foot held the most mass? Why?

2. In this experiment, magnetic forces were used to demonstrate the van der Waals forces that
geckos and jumping spiders use for attachment. What are van der Waals forces?

Proc Natl Acad Sci U S A. 2003 September 16; 100(19): 10603

10606.

Published online 2003 September 5. doi: 10.10
73/pnas.1534701100.

© 2003, The National Academy of Sciences

3. Below is a series of three pictures of two non
-
polar molecules. Draw electrons (e
-
) in each
molecule to demonstrate how van der Waals forces arise between molecules.

Molecule A

Molecule B

Station 3: Applications

Read these articles and answer the question
s that follow:

“Caught on Tape: Gecko
-
inspired adhesive is superstrong”

“Spiders make best ever Post
-
it notes”

1. What is the name of the force that jumping spiders use to stick to surfaces? ______________

2. How close to the surface do spiders have to

get in order for this force to allow them to stick?

a) a decimeter

b) a centimeter

c) a micrometer

d) a nanometer

3. If all of the spider’s setules are in contact with a surface, a spider can produce an adhesive
force _________ times its own weight, w
hile a gecko could support an adhesive force about
____________ times its own weight.

4. What would be one advantage to using a “Spider Post
-
It Note”?

5. What could be some uses of gecko tape?

6. A gecko and jumping spider would not be able to adhere
to a surface if their feet did not
divide into smaller and smaller hairs. Why?

7. List one reason gecko tape would be better then conventional tape.

8. What is a current problem with the synthetic gecko tape?

9. Should gecko tape be hydrophilic (water

“loving”) or hydrophobic (water “fearing” / water
resistant)? Explain.

2
nd
: Molecule A with a
temporary dipole

-

3
rd
:
Molecule B with an

induced dipole

-

+

1
st
: 2 non
-
polar molecules

Station 4: How Do Geckos Stick?

1. How do we know that geckos do not use sticky secretions as an adhesive?

2. How do we know that geckos do not use suction to stick to surfaces?

3. Define: hydrophobic

4. Define: hydrophilic

5. The fact that geckos stick to trees in very dry regions is proof that they use

(capillary / van der Waals) (circle one) forces to secure themselves to a surface.

6. Explain how the graphs prove that ge
ckos use van der Waals forces as a means of adhesion.

Station 5: Scanning Electron Microscope (SEM) Images

1.

In both the Jumping Spider and Gecko pictures, the picture labeled “b” shows the

organisms’ _________.

2.

Both pictures “c” and “d” for the Jumping
Spider show setules. Why do the two pictures look
different?

3.

Observe pictures “e” and “f” for the Jumping Spider and picture “d” for the Gecko. What is the
advantage to having flat ends?

4.

Below is a square with dimensions 0.25 mm x 0.25 mm. How many set
ae would fit into this
square?

5.

If all of the gecko’s setae were stuck to a surface at the same time, how many pounds would
it be able to support? (hint: 1 kg = 2.2 lbs)

6.

Estimate how many humans this is equivalent to.

7.

How many setae would need to b
e attached to a surface to support a 50 g gecko?

8.

Calculate the percent of setae actually used by the gecko to stick to a wall.

9.

Why do geckos’ feet have so many setae if so few of them are used at one time for
attachment?

10.

If geckos can stick to walls

so well, how do they detach?

Station 6: Spider Student?

The equation for the best
-
fit line on the graph can be written in the format y = mx + b.

In this case, “y”, the vertical axis, is the log of the number of setae per 100

m
2

(N
A
) and “x”, the
horizontal axis, is the log of the mass of the organism (m).

y

=

m

x

+

b

log(N
A
)

=

0.699

log(m)

+

13.8

1. What is the value of the slope of the line? ______________

2. What is the value of the y

intercept? __________
_______

3. Calculate how many setae/

m
2

a 130 lb person would need to stick to a wall.

1
st
: Convert 130 lbs to grams (show work)

Given: 1 kg = 2.2 lbs

1 kg = 1000 g

mass = _______________

2
nd
: Substitute this value into the equation of th
e straight line.

log(N
A
)

=

0.699

log(mass)

+

13.8

log (N
A
) =_____________

N
A

= ____________ setae/100

m
2

(hint: use the 2
nd

log function on your
calculator)

3
rd
: Convert this to setae / 100 cm
2

(hint: 1 X 10
16

m
2

= 1 cm
2
)

N
A

= _____
_______ setae / 100 cm
2

log of setal surface
area in m
2

log of mass of
organism in grams

Van der Waals Forces and Animal Adhesion

Introductory Terms:

Name_____
KEY
___________________

Micro
:

symbol:

definition: 1 millionth or 10
-
6

Micrometer or Micron:

symbol:

m

definition: 1 millionth of a meter

Nano
:

symbol: n

definition: 1 billionth or 10
-
9

Nanometer

one billionth of a meter or 10
-
9

m.

Nanotechnology

the control of shape and size at the nanoscale to design and produce
structures, devices, and systems

Intermolecular Forces

Forces between
molecules (i.e. dispersion, dipole
-
dipole, dipole
-
induced dipole,
hydrogen bonds)

Much weaker than intramolecular forces

Van der Waals Forces

Forces that arise when a temporary dipole in a non
-
polar molecule induces a dipole in
-
polar molec
ule

Tokay Gecko:

a

small, insect
-
eating, noisy reptile from South East Asia

Scansors

lamellae or gill
-
like structures on the bottom of the foot

Setae

hair
-
like bristles that come out of the scansors

Spatula

branches from the setae that end in a fla
t, triangular end

Jumping Spider:

Scopula

protrusion found at the tip of each foot

Setae

hair
-
like bristles that come out of the scopulae

Setules

densely cover the setae and broaden into

a flat, triangular end

http://whyfiles.org/shorties
/152sticky_spider/

Photo courtesy
Ed Nieuwenhuys

http://en.wikipedia.org/wiki/Tokay_gecko

Station 1: Graph Interpre
tation (≈ 5 min)

1: What do each of the colored regions represent?

Each colored region represents the different animals that were tested

2: Describe what happens to the number of setae per 100

m
2

as the mass of the organism
increases.

As the mass of t
he organism increases, the number of setae per 1

m
2

increases

3: What kind of relationship is this?

direct

4. The following images were taken with a Scanning Electron Microscope. They are close
-
up
pictures of the surface of an animal’s foot. Based on t
he graph, match each picture with the
correct organism: beetle, fly, spider, lizard.

lizard

fly

spider

beetle

Station 2: Experiment (
15 min)

Question:

How does the number of setae affect the strength of the attachment force?

Data:

#
of “setae”

2

4

8

16

Mass (g)

100

300

570

900

Conclusion:

1. Which foot held the most mass? Why?

The wooden disc (foot) with 16 setae held the most mass. It has the largest number of
magnetic setae

2. In this experiment, magnetic forces were used to demo
nstrate the van der Waals forces that
geckos and jumping spiders use for attachment. What are van der Waals forces?

Intermolecular forces between non
-
polar molecules caused by temporary dipoles and
induced dipoles

3. Below is a series of three pictures of
two non
-
polar molecules. Draw electrons (e
-
) in each
molecule to demonstrate how der Waals forces arise between molecules.

Proc Natl Acad Sci U S A. 2003 September 16; 100(19): 10603

10606.

Published online 2003 September 5. doi: 10.1073/pnas.1534701100.

© 2003, The National Academy of Sciences

Molecule A

Molecule B

Station 3: Applications (
≈10
-
15 min)

Read these articles and answer the questions that follow:

“Caught on Tape: Gecko
-
inspired adhesive is superstrong”

“Spiders make best ever Post
-
it notes”

1. What is the name of the force that jumping spiders use to stick to surfaces?

van de
r Waals forces

2. How close to the surface do spiders have to get in order for this force to allow them to stick?

a) a decimeter

b) a centimeter

c) a micrometer

d) a nanometer

3. If all of the spider’s setules are in contact with a surface, a spider c
an produce an adhesive
force __
170
____ times its own weight, while a gecko could support an adhesive force about
______
400
___ times its own weight.

4. What would be one advantage to using a “Spider Post
-
It Note”?

they would stick even if they got wet or
greasy

5. What could be some uses of gecko tape?

“gecko glove” could let people dangle from the ceiling

hold tissues together after surgery

help stunt doubles in movie sets

6. A gecko and jumping spider would not be able to adhere to a surface if the
ir feet did not
divide into smaller and smaller hairs. Why?

The larger the surface area of the setae, the greater the adhesive force

7. List one reason gecko tape would be better then conventional tape.

reusable

8. What is a cu
rrent problem with the synthetic gecko tape?

loses its adhesive power after about five applications because it is hydrophilic and attracts
water. The water makes the hairs soggy and causes them to bunch together

e
-

e
-

e
-

e
-

e
-

e
-

e
-

e
-

e
-

e
-

e
-

1
st
: 2 non
-
polar molecules

2
nd
:

Molecule A with a
temporary dipole

e
-

e
-

e
-

e
-

e
-

e
-

e
-

e
-

e
-

e
-

-

3
rd
: Molecule B with an
induced dipole

e
-

e
-

e
-

e
-

e
-

-

e
-

e
-

e
-

e
-

e
-

+

-

9. Should gecko tape be hydrophilic (water
“loving”) or hydrophobic (water “fearing”/water
resistant)?

hydrophobic would be better, but harder to make

Station 4: How Do Geckos Stick? (5
-
10 min)

1. How do we know that geckos do not use sticky secretions as an adhesive?

they do not have glands on
their feet

2. How do we know that geckos do not use suction to stick to surfaces?

can still stick to a surface in a vacuum

3. Define: hydrophobic

“water fearing”; non

polar

4. Define: hydrophilic

“water

loving”; polar

5. The fact that geckos s
tick to trees in very dry regions is proof that they use

(capillary / van der Waals) (circle one) forces to secure themselves to a surface.

6. Explain how the graphs prove that geckos use van der Waals forces as a means of adhesion.

geckos stick equally

well to hydrophobic and to hydrophilic surfaces

Station 5: Scanning Electron Microscope (SEM) Images (10
-
15 min)

1.

In both the Jumping Spider and Gecko pictures, the picture labeled “b” shows the
organisms’ ___
setae
____.

2.

Both pictures “c” and “d” for the
Jumping Spider show setules. Why do the two pictures look
different?

the bottom of the setae has a lot more setules than the top

3.

Observe pictures “e” and “f” for the Jumping Spider and picture “d” for the Gecko. What is
the advantage to having flat ends
?

greater surface area

4.

Below is a square with dimensions 0.25 mm x 0.25 mm. How many setae would fit into this
square?

1 mm
2

= 14,400 setae

0.0625 mm
2

= 900 setae

5.

If all of the gecko’s setae were stuck to a surface at the same time, how many pounds
would
it be able to support? (hint: 1 kg = 2.2 lbs)

130 kg x 2.2 lbs/kg = 286 lbs

6.

Estimate how many humans this is equivalent to.

~2 humans

7.

How many setae would need to be attached to

a

surface to support a 50 g gecko?

x = 2500 setae

6.5 million setae

130 kg

x

0.05 kg

8.

Calculate the percent of setae actually used
by the gecko to stick to a wall.

2500 setae

6.5 million setae

x 100

0.04%

9.

Why do geckos’ feet have so many setae if so few of them are used at one time for
attachment?

they may not all have the correct
orientation

10.

If geckos can stick to walls so well, how do they detach?

their setae need to be at an angle greater than 30
o

Station 6: Spider Student? (10
-
15 min)

The equation for the best
-
fit line on the graph can be written in the format y = mx + b.

In
this case, “y”, the vertical axis, is the log of the number of setae per 100

m
2

(N
A
) and “x”, the
horizontal axis, is the log of the mass of the organism (m).

y

=

m

x

+

b

log(N
A
)

=

0.699

log(m)

+

13.8

1. What is the value of the slope of the

line? ___
0.699
______

2. What is the value of the y

intercept? _____
13.8
________

3. Calculate how many setae/

m
2

a 130 lb person would need to stick to a wall.

1
st
: Convert 130 lbs to grams (show work)

Given: 1 kg = 2.2 lbs

1 kg = 1000 g

kg

1
g

1000
lbs

2.2
kg

1
lbs

130

mass =
59090 g

2
nd
: Substitute this value into the equation of the straight line.

log(N
A
)

=

0.699

log(mass)

+

13.8

log (N
A
) =__17.135_______

N
A

= 1.37 X 10
17

setae/100

m
2

(hint: use the 2
nd

log function on your
calculato
r)

3
rd
: Convert this to setae / 100 cm
2

(hint: 1 X 10
16

m
2

= 1 cm
2
)

N
A

= 1.37 X 10
23

setae / 100 cm
2

log of setal surface
area in m
2

log of mass of
organism in grams

Proc

Natl Acad Sci U S A. 2003 September 16; 100(19): 10603

10606.

Published online 2003 September 5. doi: 10.1073/pnas.1534701100.

© 2003, The National Academy of Sciences

Station 1:

The following graph comes from a scientific study that compared an organism’s number of setae
per 100

m
2

to the organism’s mass.

flies

beetles

lizards

bugs

spiders

Station 2: Experiment

Question:

How does the number of setae affect the strength of the attachment force?

Background:
In this experiment, the wooden discs represent feet, and the washers represent
the setae, or hairs, on the feet. In thi
s experiment, you will determine how the attachment force
of a foot depends on the number of setae on the foot.

Materials:

4
-
gecko feet (wooden discs with staples)

Strong magnets

Mass sets

Chairs/stools

Experimental Set
-
Up:

Procedure:

1)

Set up the experiment as shown above. Start with the foot with 2 setae (staples).

2)

Gradually increase the amount of mass hanging on the wooden foot until the foot
becomes separated from the cookie sheet.

i.e. Hook

100 g on foot; Remove mass; hook 200 g on foot; Remove; hook 300 g from
foot…repeat until foot separates.

3)

Record the maximum amount of mass that the foot could support.

4)

Repeat for each foot.

magnet
s

wooden disc

Pillow

Use ~1.5 inch hole saw to cut wooden feet

Screw eye bolt into hole

Use staple gun to insert “setae”

Station 3:

Science News Online

Week of June

7,

2003; Vol. 16
3, No. 23

Caught on Tape: Gecko
-
inspired adhesive is superstrong

Sorcha McDonagh

As it scurries along the ceiling, a gecko has the sticking power to support not just its own body weight,
but about 400 times as much. Besides that sticking power, the natura
l adhesive on this animal's feet is
clean and reusable, and it works on all surfaces, wet or dry.

Scientists at the University of Manchester in England and the Institute
for Microelectronics Technology in Russia have emulated t
he
animal's adhesive mechanism by creating "gecko tape." It comes
closer to the lizard's sticking power than any other gecko
-
styled

The 1
-
square
-
centimeter prototype patch can bear about 3 kilograms,
almost one
-
third the weight that the s
ame area of gecko sole can
support.

In the July
Nature Materials
, Andre Geim of the University of
Manchester and his colleagues claim that the tape is scalable to
human dimensions: Wearing a "gecko glove," a person could dangle
from the ceiling. In theory
, the tape could hold tissues together after
surgery or support stunt doubles climbing around movie sets.

The gecko tape is modeled on the gecko sole, an intricate fingernail
-
size surface covered with a half
-
million microscopic, hair
-
like structures known

as setae. Each seta's tip branches into even finer hairs
that nestle so closely with every surface the gecko touches that intermolecular attractions called van der
Waals bonds and capillary forces kick in. These bond the gecko's foot to the surface (SN: 8
/31/02, p.
133: Available to subscribers at
http://www.sciencenews.org/20020831/fob5.asp
).

Geim and his team made their synthetic gecko adhesive by
fabricating a tidy array of

microscale hairs out of polyimide, a flexible and wear
-
resistant plastic. When mounted on

a flexible base, the arrangement and density of the h
airs
maximize the number of hairs contacting a surface.

"The smaller the hairs are, and the more of them you have, the
greater the adhesion," notes Ron Fearing, an engineer at the
University of California, Berkeley.

Unlike a gecko's feet, however, the ta
pe begins to lose its adhesive power after about five applications.
Geim blames this shortcoming on polyimide's hydrophilicity, that is, its tendency to attract water. With

COMING UNSTUCK. An array
of tiny plastic pegs (above)
emulates the microstructure of a
gecko's sticky sole. Adhesion
declines when the pegs clump
(below).

Geim/Univ. of Manchester

Geim/Univ. of Manchester

repeated applications, some of the gecko tape's hairs get soggy, bunch together, an
d then clump onto the
tape's base. This happens even when the tape is attached to surfaces that are dry to the touch, because
they carry a layer of water two or three atoms thick.

By using hydrophilic material, Geim departed from the gecko's design. Its s
etae are made of keratin, a
so
-
called hydrophobic protein that repels water.
Geim says hydrophobic materials, which include
silicone and polyester, are more difficult to mold into setae
-
like structures than is polyimide. Even so,
both he and Fearing agree,

it will take water
-
repellant substances to produce a long
-
lasting gecko tape.

Letters:

This article reports that, wearing a gecko
-
inspired glove, "a person could dangle from the ceiling." How
would that person let go?
David D. Jones St. Paul, Minn.

The m
icroscopic hairs on a gecko's feet stick only when the angle at which they meet the surface is just
right. To unstick its feet, a gecko peels them off the surface, changing the angle at which the hairs meet
the surface. The same would go for gecko tape and

thus for gecko gloves. A person wearing gecko
gloves would have to learn to peel his or her hand from the surface
. S. McDonagh

References:

Geim, A.K.,
et al
. 2003. Microfabricated adhesive mimicking gecko foot
-
hair.
Nature Materials

2(July):461
-
463. Av
ailable at
http://www.nature.com/cgi
-
taf/DynaPage.taf?file=/nmat/journal/v2/n7/full/nmat917.html
.

Autumn, K.,
et al
. 2002. Evidence for

van der Waals adhesion in gecko setae.
Proceedings of the
National Academy of Sciences

99(Sept. 17):12252
-
12256. Available at
http://www.pnas.org/cgi/content/full/99/19/12252
.

Autumn, K.,
et

al
. 2000. Adhesive force of a single gecko foot
-
hair.
Nature

405(June 8):681
-
685.
Abstract available at
http://dx.doi.org/10.1038/35015073
.

Cobb, K. 2002. Getting a grip: How gecko toes stick.
Science New
s

162(August 31):133. Available to
subscribers at
http://www.sciencenews.org/20020831/fob5.asp
.

Sitti, M., and R. Fearing. 2002. Nanomolding based fabrication of synthetic gecko foot
-
hairs. IEEE
Conference on Nanotechnology. Aug. 26
-
28. Washington, D.C. Available at
http://robotics.eecs.berkeley.edu/~ronf/PAPERS/nano_02.pdf
.

Weiss, P. 2000. Gecko toes tap intermolecular b
onds.
Science News

158(July 15):47. Available to
subscribers at
http://www.sciencenews.org/20000715/note16.asp
.

Additional information about how geckos stick to walls can be found at
http://www.lclark.edu/~autumn/dept/geckostory.html
.

Sources:

Kellar Autumn

Department of Biology

Lewis & Clark College

Portland, OR 97129
-
7899

Ron Fearing

Department of Electrical Engineering

Uni
versity of California, Berkeley

Berkeley, CA 94720
-
1770

Andre Geim

Department of Astronomy and Physics

University of Manchester

Manchester M13 9PL

England

http://www.sciencenews.org/articles/20030607/fob3.asp

From
Science News
,

Vol. 163, No. 23
,

June

7,

2003, p. 356.

Copyright (c) 2003 Science Service. All rights reserve

Station 3:

Spiders Make Best Ever Post
-
it
Notes

Science Daily

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A scanning electron microscope (SEM) micrograph of the foot of the jumping spider E.
arcuata. In addition to the tarsal claws,
a tuft of hair called a scopula is found at the tip of the
foot, which is what the spider uses to attach itself to surfaces. The long hairs which are
distributed over the entire foot are sensitive to touch. Magnification 200x. (Image courtesy
Institute Of
Physics)

This is the first time anyone has measured exactly how spiders stick to surfaces, and how
strong the adhesion force is. The team used a scanning electron microscope (SEM) to make
images of the foot of a jumping spider, Evarcha arcuata (pictures a
vailable see notes). There is
a tuft of hairs on the bottom of the spider's leg, and each individual hair is covered in more
hairs. These smaller hairs are called setules, and they are what makes the spider stick.

The paper reveals that the force these sp
iders use to stick to surfaces is the van der Waals force,
which acts between individual molecules that are within a nanometre of each other (a
nanometre is about ten thousand times smaller than the width of a human hair). The team used
a technique called
Atomic Force Microscopy (AFM) to measure this force. The flexible contact
tips of the setules are triangular (pictures available see notes), and they have an amazingly high
adhesive force on the underlying surface.

Andrew Martin, from the Institute of Tec
hnical Zoology and Bionics in Germany, said, "We
found out that when all 600,000 tips are in contact with an underlying surface the spider can
produce an adhesive force of 170 times its own weight. That's like Spiderman clinging to the
flat surface of a wi
ndow on a building by his fingertips and toes only, whilst rescuing 170
adults who are hanging on to his back!"

What makes the van der Waals force an interesting form of adhesion is that, unlike many glues,
the surrounding environment does not affect it.
The only thing that affects it is the distance
between the two objects.

"One possible application of our research would be to develop Post
-
it® notes based on the van
der Waals force, which would stick even if they got wet or greasy," said Professor Antoni
a
Kesel, head of the research group in Bremen. "You could also imagine astronauts using
spacesuits that help them stick to the walls of a spacecraft just like a spider on the ceiling."

The total van der Waals force on the spider's feet is very strong, but

it is the sum of many very
small forces on each molecule. The researchers believe the spider lifts its leg so that the setules
are lifted successively, not all at once, and it does not need to be very strong to do this. All you
would have to do to lift a
future kind of Post
-
it® note is peel it off slowly.

The van der Waals force exists because the movement of electrons in atoms and molecules
causes them to become dipolar. A dipolar atom or molecule has a "positive
-
pole" and a
"negative
-
pole". The positive
-
pole of one atom or molecule will be attracted to the negative
-
pole of another. This particular electrostatic attraction is called the van der Waals force, and is
in some ways similar to the magnetic attraction between north and south poles of magnets.

"
We carried out this research to find out how these spiders have evolved to stick to surfaces,
and found that it was all down to a microscopic force between molecules. We now hope that
this basic research will lead the way to new and innovative technology,"

said Professor Kesel.

Note: This story has been adapted from a news release issued by Institute Of Physics.

-
2007 ScienceDaily LLC

Contact:
editor@removeme.sciencedaily.com

Station 4: How do Geckos Stick?

Is it glue?

Some organisms have glands that secrete sticky substances that let them adhere to surfaces. For
example, mussels produce proteins th
at act like threads of glue letting them stick to surfaces such
as rocks and boat hulls. One type of bacteria secretes a polysaccharide that exhibits the largest
adhesive force ever observed by any microorganism. This natural “super
-
glue” might one day be
used as a medical adhesive. Since geckos do not have any glands on their toes, it is clear that
they do not have the ability to secrete any sticky substances.

Is it suction?

In the 1800’s scientists believed that the gecko’s individual setae acted like mi
niature suction
cups. Although some insects and reptiles do use suction as a means of adhesion, experiments
carried out in a vacuum suggested that suction is not involved in the adhesion of geckos.

Is it friction?

Other scientists have proposed that geck
os stick to walls and ceilings using friction. But, since
geckos can stick upside down to surfaces like smooth, polished glass there must be another force
at work.

Is it capillary forces?

Many insects, frogs, and mammals stick with capillary forces due to

a thin layer of water on the
surface. But, since geckos are able to stick to hydrophobic (non
-
polar, water
-
“fearing”) surfaces,
they must use something other than capillary forces.

Is it van der Waals forces?

Van der Waals forces are the weakest of all
the attractive intermolecular forces, but they are also
the most common. An organism that uses van der Waals forces to stick to a surface adheres
equally well to both hydrophobic (non
-
polar, dry, water
-
“fearing”) and hydrophilic (polar, wet,
water
-
“loving
”) substances.

Autumn, Kellar, et. al. “Evidence for van der
Waals adhesion in gecko setae.”

A)
If
geckos used capillary
forces for adhesion, they would stick very well to hydrophilic
surfaces and poorly to hydrophobic surfaces.

B)
Van der Waals Adhesion Prediction:

If geckos used van
der Waals forces for adhesion, they would stick equally well
to both
hydrophobic and hydrophilic surfaces

C)
Semiconductor Wafer Data:
The gecko showed adhesion to
polarizable surfaces that were both hydrophobic and
hydrophilic.

D)
Microelectromechanical

System

(
MEMS) Cantilever Data
:
Gecko setae attached equally well to

polarizable surfaces with
different hydrophobicity.

Scansors/lamellae

f

These images show you close up what the bottom of a jumping spider’s and a gecko’s foot looks like. The pictures were taken w
ith a Scanning Electron
Microscope (SEM) which works by scanning a beam of electrons along the s
urface and creating a 2
-
dimensional image.

a)

Rows of scansors (lamellae) on the gecko’s toe pads

b)

Rows of s
etae cover the scansors

c)

A setae is about 100

m long and 5

m in diameter

d)

Setae branch into 100 to 1000 spatulae that end in flat

200 nm wide.

a

b

c

d

Gecko

Jumping Spider

a

b

Kesel, Antonia B, Andrew Martin, and Tobias Seidl

“Getting a grip on spide
r attachment: an AFM approach to microstructure adhesion in arthropods

Autumn, Kellar and Anne M. Peattie

“Mechanisms of Adhesion in Geckos”

a)

Tarsal claws, scopula, and hairs

b)

Scopulas differentiating into setae

c)

A single setae covere
d with setules

d)

Setule density is lower on upperside of the setae

e)

Setules on the underside broaden to a flat,
triangular

end

f)

The broad triangular ends that are in direct contact with the substrate

e

c

d

Fabulous Foot Facts

For every 1 mm
2

of surface
area, a gecko’s foot contains
about 14,400 setae. If all of
a gecko’s setae we
re stuck
to a surface at the same
time, the gecko would
generate a total adhesive
force of 130 kg. On average,
a 50 gram gecko has a total
of about 6.5 million setae.

In order to stick, setae must
have the proper orientation.
Geckos have lots of extra
se
tae because it is
impossible for all of their
setae to have the correct
orientation at the same time.
This might be especially
difficult to achieve on rough
or dusty surfaces.

So if a gecko can stick so
well, how does it detach?

Geckos are able to detach

themselves from a surface in
around 15 milliseconds. The
only way this occurs is to tilt
the setae at an angle to the
surface of greater than 30
o
.
At this point, the setae can
no longer stick to the
surface, and the gecko is
free to move. This exp
lains
why geckos walk with a “toe
-
peeling” motion. They have
to remove their feet from the
surface just like you would
remove a bandage from your
skin.

Station 5: Scanning Electron Microscope (SEM) Images

a

Proc Natl Acad Sci U S A. 2003 September 16; 100(19): 10603

10606.

Published online 2003 September 5. doi: 10.1073/pnas.1534701100.

C
opyright

© 2003, The National Academy of Sciences

Station 6: Spider Student?

How many setae per 100 cm
2

w
ould a 130 lb person need to stick to a wall?

flies

beetles

lizards

bugs

spiders

y =

m
(
x
)

+ b

log (N
A
) = 0.699 log (m) + 13.8