TLT 02-09 Cover Story

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COVER STORYDrs. R. Arvind Singh, Eui-Sung Yoon and Robert L. Jackson

Key concepts:

Alreadypopularinmaterialsscienceandengineering,biomimeticsnowisemergingintribology.

VelcrowasinspiredfromthestructureofthehookingdevicesfoundonthefruitpartoftheplantGalium aparine.

TheoutersurfaceoftheCathayPacicAirbus340,modeledafterthegroovedscalesofsharkskin,reducesdragandimprovesfuelefciency.

Bi mimetics: The


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he study and simulation o biological systems with de-sired properties is popularly known as biomimetics.

Such an approach involves the transormation o the

underlying principles discovered in nature into man-

made technology.

Biomimetics, already popular in the elds o materials sci-

ence and engineering, is being applied in diverse areas rang-

ing rom micro/nano-electronics to structural engineering

and now is emerging in tribology. In this article, well present

examples o bio-inspired research works closely related to

the eld o tribology. In addition, examples o bio-inspired

inventions also are given, with the view that it would create

interest and appreciation in the readers.

VeLcRo

Invented by George de Mestral and patented in 1955, Velcro

is perhaps the earliest, most successul and popular o all

the man-made inventions based on natures architecture. The

design o the Velcro astener was inspired rom the structure

o the hooking devices ound on the ruit part o the plant,

Galium aparine (common names: Goosegrass, Spring Cleav-

ers, Stickywilly)1.

When these ruits come in contact with oraging wildlie,

they hook themselves onto the animals hairs and get inter-

locked. The ruit is then transported by the animals somedistance away rom the plant.

A Velcro astener has two co-opted suraces1, in which

one surace is covered by sti nylon hooks and the other by

sot adaptive loops. When these two parts are pressed togeth-

er, they get securely attached. In this hook-like rictional sys-

tem, separation o the two parts requires a orce in the orm

o strong jerks. The present-day Velcro astener orms the

basis o a multimillion dollar industry. As it can withstand

hundreds o cycles o attachment/detachment operations,

Velcro has good durability and is thereore a very useul and

reusable invention.

tIRe tReAD eFFect

Another o mans creations analogous to a principle ound in

nature, namely the distinctive patterns on a tree rogs oot,

is the design o treads on automobile tires. Tree rogs such as

Amolops sp. possess large disc-like pads at the tip o their toes

that assist them in attaching to suraces such as leaves. The

pads consist o fat-topped cuboidal columnar cells, which

are separated rom each other by canal-like spaces1. During

climbing, water gets squeezed out rom the contact through

the channels between the oot and the suraces on which

they climb, making a perect van der Waals contact.

Automobile tires have treads on them. While driving on

wet roads, water fows out through the channels ound be-tween the treads, giving rise to intimate contact between the

treads and the road, thereby creating sucient grip during

motion. This eect is known as the Tire Tread Eect.

GecKo eFFect

Creatures such as beetles, fies, spiders and lizards have the

ability to attach themselves to suraces without alling o,

even when the suraces are vertically inclined. The pres-

ence o micro/nanostructuressmall hairs called setae on

their attachment padsenables these creatures to attach and

detach easily over any surace. As the mass o the creature

increases, the radii o the terminal attachment structuresdecrease while the density o the structures increase2. The

gecko is the largest animal that has this kind o dry attach-

ment system and, thereore, is the main interest or scientic

research.

In unctional terms, the tiny hairs ound on gecko eet3

are able to conorm to the shape o surace irregularities to

which the gecko is adhering. By mimicking the shape and

geometry o gecko setae, synthetic adhesives have been made

rom polymeric materials. An example is Gecko Tape4,

which can be used or several detachment-attachment cycles

beore the degradation o its adhesive property. This tape has

science of imitating nature

Tribologists are learning from some of the natural worlds most

advanced engineers, including the lotus plant, gecko, tree frog,

shark, pangolin and a host of others.

T

W W W . S T L E . O R G T R I B O L O G Y & L U B R I C A T I O N T E C H N O L O G Y F E B R U A R Y 2 0 0 9 4 1


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arrays o fexible polyimide pillars abricated using electron-

beam lithography and dry etching in oxygen.

To create a gecko adhesive, the pillars must be suciently

fexible and placed on a sot, fexible substrate so that indi-

vidual tips can act in unison and attach to uneven suraces

all at the same time. To demonstrate the eectiveness o the

gecko tape as a dry adhesive, a Spider-man toy was attached

to a glass plate through the microabricated gecko tape4.

Polyurethane elastomer microber array with fat spatulate

tips was abricated by molding a master template using deep

reactive ion etching, and the notching eect also can actas eective adhesive suraces5. These suraces, when tested

against a smooth glass hemisphere or their adhesive prop-

erty, show three times higher adhesion than fat polymeric

suraces.

Recently, scientists produced a super-adhesive surace

called Geckel6 that is based on the suraces o geckos and

mussels. Using electron-beam lithography, they rst created

arrays o polymeric nanopillars and subsequently coated

them with a polymer modeled on an amino acid, which is one

o the building blocks o the glue protein in mussels. The

result (gecko plus mussel) was named Geckel. This adhe-

sive material has been tested to stick through 1,000 contact/release cycles and remains highly adhesive even underwa-

ter. By studying the structure o gecko setae, researchers also

have designed high riction suraces7 showing an increase in

riction coecient by more than our times.

sHARK sKIn eFFect

Friction between a solid surace and a fuid also can be con-

sidered a tribological phenomenon. Inspiration rom aquatic

animals surace material and texture would benet the de-

sign o suraces that could increase eciency in cases such as

underwater navigation.

One such example is the Shark Skin Eect. Sharks skin

has grooved scales on its entire body. The scales are directed

almost parallel to the longitudinal body axis o the shark.

The presence o this non-smooth surace texture on the

shark skin eectively reduces drag by 5%-10%. Swimsuits

with biomimetically designed suraces that mimic the non-

smooth surace texture on the shark skin have proved to be

aster than conventional suits, as they reduce drag along key

areas o the body.

Surace texture such as those on sharks skin also helps to

reduce the riction between a solid surace and air. A trans-

parent plastic lm with similar microscopic texture (ribs par-

allel to the direction o fow) reduces aircrat drag by about

8% and is eective in saving uel by about 1.5%8. The com-

mercial aircrat Cathay Pacic Airbus 340 already has been

tted with ribbed structures on its body surace8.

Another interesting example is snake scales. Studying the

rictional suraces o snake scales would benet when de-

signing suraces with anisotropic rictional characteristics.

Snakes have riction-modiying structures consisting o or-dered double-ridge microbrillar array1. The double-ridge

microbrillar geometry provides signicant reduction in

adhesive orces, thereby creating ideal conditions or sliding

in orward direction with minimum adhesion. Meanwhile,

the highly asymmetric prole o the microbrillar ending in-

duces rictional anisotropy, as it acts as a locking mechanism

prohibiting backward motion. Snake skin also has micropo-

res that deliver an anti-adhesive lipid mixture, which urther

acilitates easy motion owing to boundary lubrication1.

4 2 F E B R U A R Y 2 0 0 9 T R I B O L O G Y & L U B R I C A T I O N T E C H N O L O G Y W W W . S T L E . O R G

Figure 1|(a)Hookingdevicesfoundonthefruitpartoftheplant,Galium

aparine,(b)amagniedimageofthehookingdevice1,(c)thetwoco-opted

surfacesofaVelcrofastener1,(d)atreefrog,(e)schematicofat-topped

cuboidalcolumnarcellsseparatedbycanal-likespacesintreefrogs

(Amolops sp.)1,(f)aclose-upviewoftreadsonacartire.

Figure 2|(a)-(d)Terminalelementsfoundontheattachmentpadsof

variousinsectsandgecko2.Asthesize(mass)ofthecreatureincreases,

theradiusoftheterminalattachmentstructuresdecreases,whilethe

densityofthestructuresincreases,(e)Gecko,(f)SEMimageofgeckosetae.3


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LotUs eFFect

A number o plants have water-repellent leaves, which ex-

hibit superhydrophobic property. Lotus (Nelumbo nucifera)

is the most popular example (water contact angle ~ 162 de-

grees9). The unique ability o Lotus lea surace to avoid wet-

ting, popularly known as the Lotus Eect, is mainly due to

the presence o microscale protuberances covered with waxy

nanocrystals on their surace. Colocasia (Colocasia esculenta)

is another example o a plant whose leaves are superhydro-

phobic in nature (water contact angle ~ 164 degrees 9), due

to the presence o micron-sized protuberances and wax crys-tals.

Both the protuberances and the wax crystals make the

suraces o Lotus and Colocasia leaves superhydrophobic

in nature, which means water droplets easily roll over lea

suraces taking contaminants and dust particles with them.

This phenomenon is popularly known as the Sel-Cleaning

Eect. The sel-cleaning property is highly important or

water plants. In their habitats, these plants observe the pres-

ence o ree water, which supports pathogenic organisms.

These plants protect themselves rom water-borne inections

by hindering any adhesion o water necessary or the germi-

nation o pathogens9. Another reason or water-repellency isthe act that CO2 diuses 10% times slower in water than in

air. The presence o water-repellent surace ensures that su-

cient intake o CO2 is observed or photosynthesis9.

By studying the surace morphologies o water-repellent

leaves, tribologists design and create hydrophobic suraces

to reduce adhesion between suraces at small scales, which

arise due to water condensation. Scientists and engineers are

aware o the act that surace orces such as adhesion and

riction signicantly oppose easy motion between tiny ele-

ments in miniaturized devices such as micro/nanoelectrome-

chanical systems (MEMS/NEMS).

Minimizing the surace orces such as adhesion and ric-

tion, and also the occurrence o wear in miniaturized devic-

es, is a real challenge, as the size o these devices is extremelysmall (usually their sizes are smaller than insects such as dust

mites). Among the various attractive orces that contribute to

adhesion at small-scales, the capillary orce that arises due

to the condensation o water rom the environment is the

strongest. Hence, there arises a need to modiy suraces at

nano/microscale in order to achieve increased hydrophobic-

ity that would drastically reduce adhesion due to capillary

orce, which in turn would also reduce riction.

Miniaturized devices such as MEMS/NEMS traditionally

are made rom silicon. The higher interacial energy (hydro-

philic nature) o silicon makes it a poor tribological material.


Figure 3|(a)Shark,(b)ribletsandgroovesfoundonsharkskin 8,(c)a

swimming-clothwithsimilarsurfaceasasharkskincouldreduce

frictionalresistance,(d)anaircraftcoatedwithaplasticlmthathas

similarmicroscopictextureasfoundonasharkskin8.

Figure 4|(a)Lotus(Nelumbo nucifera),(b)protuberancesonthesurfaceof

aLotusleaf11,(c)amicro-electro-mechanicalsystemthathassixgear

chains,(d)thesameMEMSdeviceincomparisonwiththesizeofadustmite,

(e)nano-patternsthatmimictheprotuberancesofaLotusleaf10,(f)

Lotus-like(fresh)surface11,(g)Colocasia-like(fresh)surface12,(h)

Colocasia-like(dry)surface12.


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Thereore, silicon suraces need to be treated chemically/to-

pographically to enhance their tribological perormance.

Inspired by the intriguing surace structures/protuber-ances o the Lotus lea that give rise to a superhydrophobic

surace, nanoscale patterns that mimic the protuberances re-

cently have been abricated on thin polymeric lms coated on

silicon waers, using a capillarity-directed sot lithographic

technique10. These nanopatterned suraces are hydrophobic

in nature (water contact angle ~ 99 degrees) when compared

to a bare silicon waer (water contact angle ~ 22 degrees).

Investigation on adhesion and riction properties at the

nanoscale has revealed that these suraces exhibit superior

tribological properties when compared to bare silicon fat

suraces. With the increased hydrophobicity and the re-

duced real area o contact projected by them, the nano-pat-terned suraces exhibit adhesion and riction values that are

lower by an order o magnitude than those o the bare silicon

fat suraces. Based on the published results, these nano-pat-

terned polymeric suraces with remarkable nanotribological

properties have been recognized as prospective suraces or

tribological application in MEMS/NEMS.

Eective tribological suraces also have been created by

the direct replication o natural leaves o water-repellent

plants11,12. Indeed, this could be the rst biomimetic approach

o creating eective tribological suraces by the direct repli-

cation o natural suraces. Suraces o Lotus and Colocasia

leaves were replicated using the real suraces o the leaves as

natural templates. The Colocasia lea surace was replicated

both in its resh and dried conditions, as it exhibits a signi-

cant change in its morphology upon drying, unlike the Lotus

whose protuberances only shrink in size when drying.

In the case o the Colocasia lea, the bumpy protuber-

ances in its resh condition become depressions upon drying,

leaving behind a bump at the center and a ridge that sur-

rounds each bump. It was ound that the replicated suraces:

Lotus-like surace11, Colocasia-like (resh) surace12 and Co-

locasia-like (dry) surace12 are hydrophobic in nature (water

contact angles > 90 degrees). Results rom microriction tests

reveal that the replicated suraces exhibit riction coecients

almost our times lower than that o the bare silicon fat sur-

aces and, thereore, could be applied in MEMS application.

WeAR-ResIstAnt sURFAces

Man has begun to learn rom nature to design materials and

textures that have superior wear-resistance characteristics.

For example, a sandsh, living in the Sahara desert movesrapidly over the desert sand, and the scales on its body 13

have excellent sand erosion wear resistance. Erosion experi-

ments conducted using sand on the sandshs scales, glass

and sot steel or 10 hours showed that the wear trace on

the sandshs skin was the smallest, which suggests that its

wear resistance is comparatively much higher than glass and

sot steel13. The biomaterial comprising the sandshs scales

and their surace texture together contribute toward its high

wear resistance.

Mollusk (conch) shells experience water-sand erosion

on sandy beaches. Their antiwear mechanism arises rom

the combination o their biotissues and their unique shape,which together prevent abrasion. The biomaterial o mollusk

shells is a bioceramic composite that has a complex micro-

structure, the result o a billion years o evolution.

Understanding the ormation and the microstructure o


Figure 5|(a)AsandshthatlivesintheSaharadesert,(b)scalesonthe

sandshsbody13,(c)molluskshells(conchs)thatareusuallyfoundon

sandybeaches,(d)bivalveshells,(e)apangolinwithalayerofscaleson

itsbody,(f)aphotographofapangolinscale14.

Figure 6|Schematicoftypicaljointwithcompressiveloaddeformingthe

cartilage.


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these bioceramic composites will

help in the design o ceramics with

superior mechanical properties such

as toughness. Current methods used

to abricate ceramics still are unable

to control parameters such as crystal

density, orientation and morphologi-

cal uniormity to the degree o perec-

tion nature has achieved in mollusk

shells. Studies on the microstructure

o bivalve shells are expected to pro-

vide guidelines in developing biomi-

metic composite materials with bet-

ter tribological properties.

Another interesting example

is pangolin scales. A pangolin is a

soil-burrowing animal with a layer

o scales covering its body. As the

pangolin burrows into soil, its scales

are subjected to wear. A study onthe chemical constitution o pango-

lin scales14 revealed the presence o

18 amino acids. The protein in the

scales mainly consists o -keratin

and -keratin. The specic elonga-

tion o pangolin scales is about 15%

due to the presence o the proteins.

Thereore, the plasticity o the pangolin scales is low, result-

ing in anti-abrasive characteristics14. Studies on biomateri-

als that exhibit remarkable riction and wear characteristics

would provide insights toward enhancing the perormance

o soil-engaging engineering components such as those inagricultural machinery and earth moving machinery. A more

detailed overview on the experimental biomimetic research

works can be ound in re. [15].

tHeoRetIcAL MoDeLInG

For biological suraces to be ully utilized in new technolo-

gies, the undamental physical mechanisms causing their

unique behaviors must be understood. By creating theoretical

models and then correlating the models to the experimental

observations o biomimetic suraces, these mechanisms can

be mathematically characterized and conrmed. In addition,

the models can be used to optimize the suraces or a specicapplication or goal.

Researchers have used existing tribological models and

theories to consider biomimetic suraces, even though they

are very dierent rom conventional tribological suraces.

Since the undamental mechanics governing the suraces

are the same, these theories usually perorm very well. For

instance, researchers have developed models o adhesion o

a geckos eet against a rough surace using classical elastic

beam theory and rough surace contact. This model was then

used to design an optimal surace array based on the gecko

eet adhesion mechanism. In 2006 Tian and others ound

that during attachment and detach-

ment the gecko can control the

adhesion by several orders o mag-

nitude by changing the motion o

the oot. This mechanism was also

optimized or practical use in new

adhesive tapes.

In contrast to previous works

which sought to model the highly

adhesive nature o gecko eet, others

have modeled the highly hydropho-

bic nature o some plant lea suraces

(such as the previously discussed lo-

tus lea). Nosonovsky and Bhushan

at Ohio State University modeled

the wetting properties o suraces

with dierent surace proles and

structures that mimicked those o

plants to obtain theoretically hydro-

phobic suraces. These models arebased on undamental models used

to describe the meniscus and surace

tension o water particles such as the

well-known Kelvin equation. The

model can then be used to optimize

surace design.

The design o lubricated bearings

also can benet greatly rom our knowledge o the mecha-

nisms at work in biological joints. Biological joints are very

dierent than conventional industrial bearings. Friction,

wear and lubrication o suraces greatly aect the reliability

and eciency o the various joints in the human anatomy.When these joints ail, such as with severe osteoarthritis, the

joints may require surgical repair or replacement.

However, the materials (metals, ceramics and polymers)

used in most articial joints are much dierent than the ac-

tual biomaterials in a human joint and are more closely re-

lated to industrial bearings. A typical joint16 usually has a

soter layer o cartilage and other materials separating the

hard bone. As discussed by Drs. Eddy Tysoe and Nic Spencer

in the April 2006 issue o TLT, natural joints use sot material

much more liberally than hard material, and perhaps it is to

their advantage. More recent approaches use soter materials

that are more similar to the original tissue such as hydrogelsand actual living cells to replace joint suraces.

A large number o works have numerically modeled the

elastohydrodynamic lubrication o natural joints. Even then,

the undamental mechanical models used are essentially the

same ones used to consider industrial hydrodynamic bear-

ings and EHL in rolling element bearings. Most works suc-

cessully use the traditional theory o elasticity and Reynolds

equation to consider hydrodynamic lubrication. Some mod-

els also incorporate the use o contact mechanics to calcu-

late the riction, wear and stresses in the joints. Biomimetic

modeling o bone also has been used to obtain materials with


Figure 7|Schematicofpossibleself-adaptingsurface

structurestocausecontrolleddeformation.


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unique properties or hip implants.

Recently, numerical modeling has been

used to design prototype suraces designed to

be sot and deormable like biological joints.

These biomimetic sel-adapting suraces

change their surace proles at the nano- and

microscale to improve perormance. Using a

built-in, closed-loop control system, the sur-

aces deorm to provide additional load sup-

port as needed. The control system is created

by the coupled elastic and hydrodynamic

mechanisms without computerized or elec-

tronics control. The mechanism is similar to

that seen in gas oil bearings.

Theoretical modeling has been used when

studying the locomotion o animals such as

water insects and snakes. Some insects and

animals use surace tension to travel over

water (water-walking) rather than through

it. By using undamental fuid mechanics,researchers were able to characterize this

motion into dierent regimes based on the

mechanism used to generate propulsion17.

They ound that small creatures are usually

supported by surace tension while larger creatures tend to

use momentum and water slapping. Perhaps these same

mechanisms could be harnessed to create bearings that use

surace tension to carry load.

Based on this survey, it appears that traditional mechani-

cal models can be used to characterize and explain biologi-

cal mechanisms. The diculty is obtaining accurate material

properties and models that describe the oten unusual bio-logical materials encountered. Thereore these same models

also can be used to optimize biomimetic suraces beore pro-

totypes are abricated.

Dr. R. Arvind Singh is a visiting scientist at the Nano-Bio

Research Center, Korea Institute of Science and Technol-

ogy (KIST), Seoul, South Korea, [email protected].

Dr. Eui-Sung Yoon is the director of the Nano-Bio Research Cen-

ter, Nano-Science Research Division, Korea Institute of Science

and Technology (KIST), Seoul, South Korea,[email protected].

Dr. Robert L. Jackson is an assistant professor in mechanical en-

gineering at Auburn University in Alabama, robert.jackson@

eng.auburn.edu.

ReFeRences

This article references work from more than 70 technical papers.

For the complete list, contact any of the three authors.

1. Scherge, M. and Gorb, S.N. (2001), Biological Micro- and

Nanotribology, Springer-Verlag, Berlin Heidelberg.

2. Arzt, E., Gorb, S., and Spolenak, R. (2003), From Microto Nano Contacts in Biological Attachment Devices, Proceed-

ings of National Academy of Sciences (PNAS), 100 (19), p.

10603.

3. Naik, R.R. and Stone, M.O., Integrating Biomimetics,

Materials Today, 18, Sept. 2005.

4. Geim, A.K., Dubonos, S.V., Grigorieva, I.V., Novoselov,

K.S., Zhukov, A.A., and Shapoval, S.-Y. (2003), Microabri-

cated Adhesive Mimicking Gecko Foot-Hair, Nature Materi-

als, 2, p. 461.

5. Kim, S. and Sitti, M. (2006), Biologically Inspired Poly-

mer Microbers with Spatulate Tips as Repeatable Fibrillar

Adhesives,Applied Physics Letters, 89, p. 261911-1.

6. Lee, H., Lee, B.P., and Messersmith, P.B. (2007), A Re-

versible Wet/Dry Adhesive Inspired by Mussels and Geckos,

Nature Letters, 448, p. 338.

7. Bhushan, B. and Sayer, R.A. (2007), Surace Character-

ization and Friction o a Bio-Inspired Reversible Adhesive

Tape, Microsystems Technology, 13, p. 71.


Figure 8|Characterizationofanimalwaterwalkingintermsofnon-dimensionalquantities.

Dr. R. Arvind Singh Dr. Eui-Sung Yoon Dr. Robert L. Jackson


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8. Ball, P. (1999), Shark Skin and Other Solutions, Nature,

400, p. 507.

9. Neinhuis, C. and Barthlott, W. (1997), Characterization

and Distribution o Water-Repellent, Sel-Cleaning Plant

Suraces,Annals of Botany, 79, p. 667.

10. Yoon, E.-S., Singh, R.A., Kong, H., Kim, B., Kim, D.-H.,

Jeong, H.E., and Suh, K.Y. Tribological Properties o Bio-Mi-

metic Nano-Patterned Polymeric Suraces on Silicon Waer,

Tribology Letters, 21 (1), p. 31-37.

11. Singh, R.A., Yoon, E.-S., Kim, H.J., Kong, H., Park, S.,

Jeong, H.E., and Suh, K.Y. (2007), Enhanced Tribological

Properties o Lotus Lea-Like Suraces Fabricated by Capil-

lary Force Lithography, Surface Engineering, 23 (3), p. 161.

12. Singh, R.A., Yoon, E.-S., Kim, H.J., Kim, J., Jeong, H.E.,

and Suh, K.Y. (2007), Replication o Suraces o Natural

Leaves or Enhanced Micro-Scale Tribological Property, Ma-terials Science & Engineering C: Biomimetic and Supramolecu-

lar Systems, 27, p. 875.

13. Rechenberg, I. (2003), Tribological Characteristics o

Sandsh, in Nature as Engineer and Teacher: Learning or

Technology rom Biological Systems, Shanghai, Oct. 8-11.

14. Tong, J., Ma, Y.-H., Ren, L.-Q., and Li, J.-Q. (2000), Tri-

bological Characteristics o Pangolin Scales in Dry Sliding,

Journal of Materials Science Letters, 19, p. 569.

15. Singh, R.A. and Yoon, E.-S. (2008), Biomimetics in Tri-

bologyRecent Developments,Journal of the Korean Physi-

cal Society, 52 (3), p. 661.

16. Mansour, J.M. (2003), Chapter 5: Biomechanics o Car-

tilage, in Kinesiology: The Mechanics and Pathomechanics

o Human Movement, C.A. Oatis, Ed. Philadelphia: Lippin-

cott Williams and Wilkins, p. 66.

17. Bush, J.W.M. and Hu, D.L. (2006), Walking on Water:

Biolocomotion at the Interace, Annual Review of Fluid Me-

chanics,38

, p. 339.


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TLT 02-09 Cover Story