-
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conservatiogrowth anutions toinspired huexample, tfrom
spidergonal honein lightweother appliof nature,
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hasictated bymost well-e product.the Swissdog. Uponers and tors, de
Mes-hich led toer, Velcrompany).s drawinghis review
explores the eld of biomimetics as it relates to textiles.
efforts in biomimetic textiles. Finally, it explores
thepotential of use of biomimetic materials and productstowards the
attainment of sustainable textiles.*Author for correspondence
([email protected]).
J. R. Soc. Interface (2011) 8, 761775
on March 21,
2015http://rsif.royalsocietypublishing.org/Downloaded from grave,
from raw material usage to recyclability.Although the science of
biomimetics has gained popu-
larity relatively recently, the idea has been around for
The exploration begins with a general overview, followedby a
historical perspective; it describes some ongoingReceived 7
SeAccepted 17 Jn, drag reduction, dry adhesion, adaptived so on,
than comparable man-made sol-date. Some of these solutions may
havemans to achieve outstanding outcomes. Forhe idea of shing nets
may have originatedwebs; the strength and stiffness of the
hexa-ycomb may have led to its adoption for useight structures in
airplane and in manycations. The term biomimicry, or imitationas
been dened as, copying or adaptation orfrom biology [1]. The term
bionics was rstin 1960 by Steele [2] as, the science of sys-has
some function copied from nature, or
esents characteristics of natural systems orgues. The term
biomimetics introducedt [3] is derived from bios, meaning lifed
mimesis, meaning to imitate [4]. Thisce is based on the belief that
nature followsof least resistance (least expenditure ofhile often
using the most common materialslish a task. Biomimetics, ideally,
should bes of incorporating principles that promoteity much like
nature does from cradle to
for example, designed ships and planes bysh and birds,
respectively [6]. The Wrighdesigned a successful airplane only
after reabirds do not ap their wings continuously; rglide on air
currents [6].
Engineer Carl Culmann in 1866, while vdissecting room of
anatomist Hermann Von Mcovered striking similarity between the
lines(tension and compression lines) in a loaded cand the
anatomical arrangement of bony trathe head of a human femur. In
other words,strengthened the bone precisely in a manner dmodern
engineering [7]. Arguably, one of theknown examples of biomimetics
is a textilAccording to the story, George de Mestral,inventor went
for a walk in the elds with hishis return, he noticed burrs stuck
to his troushis dogs fur. Upon closer inspection of the burtral
discovered their hook-like construction, whis invention of the hook
and loop fasten(http://www.velcro.com/index.php?pageco
There are many more examples of inventiontheir inspiration from
biological systems. Tbillions of years to develop more efcient
solutions, suchas superhydrophobicity, self cleaning, self repair,
energy design new materials and devices. Leonardo da Vinci,RE
Biomimicry in texand potential
Leslie Eadie and
Department of Textile Engineering, ChemistrRaleigh, NC
The natural world around us provides excellehandful of
materials. Throughout the millenhighly sophisticated methods to
solve problemsurfaces, brous structures, structural colouoffer
important lessons for the textile producoverview of the potential
of bioinspired texexamples of pertinent, inherently
sustainablrapidly growing eld and its true potential incan only be
realized through interdisciplinaof nature.
Keywords: biomimetics; bion
1. INTRODUCTION
Animals, plants and insects in nature have evolved overptember
2010anuary 2011 761W
iles: past, presentAn overviewshar K. Ghosh*
nd Science, North Carolina State University,7695, USA
examples of functional systems built with a, nature has evolved
to adapt and developThere are numerous examples of
functionalself-healing, thermal insulation, etc., whichof the
future. This paper provides a generale structures by highlighting a
few speciciological systems. Biomimetic research is adevelopment of
new and sustainable textilesresearch rooted in a holistic
understanding
; biomimicry; textiles; bres
thousands of years. Since the Chinese attempted tomake articial
silk over 3000 years ago [5], there havebeen many examples of
humans learning from nature to
doi:10.1098/rsif.2010.0487Published online 16 February 2011This
journal is q 2011 The Royal Society
-
alities, nature combines these materials in many shapes
beads in a Neolithic burial site at Mehrgarh in theGreater Indus
area indicates the use of cotton bre in
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TEXTILES: A NECESSITY OF LIFE
Almost every animal in nature has some sort of pro-tective
layer, be it bare skin, skin with feathers, hair,fur, scales, shell
or hide. Often it is meant to protectagainst predators and/or the
environment, providecomfort or improve the individuals aesthetic
appeal(attractiveness). Prehistoric humans used leaves, treebarks,
feathers, animal hide, etc., to protect againstthe environment
and/or enhance their aestheticappeal. Humans with highly developed
brain andother anatomical features, during their evolution,found
themselves inadequately protected from a varietyof adverse
environmental conditions. This led to theneed for additional
covering of the skin on parts oftheir bodies; ergo, clothing in
different manifestations.
The practice of fashioning materials into clothing isarguably
older than pottery and perhaps farming [8].The oldest known
fragments of a variety of textiles,found in Nahal Hemar (Dead Sea,
Israel), are fromthe 7th millennium BC (the early Neolithic
period),and those from Catal Huyuk (southern Anatolia,Turkey) from
around 6000 BC [8]. The textiles foundin both cases are mostly ax
(or linen) with occasionaluse of other bres. The sophistication
found in thedesign of the oldest known fragments of woven
fabricfrom the late Neolithic period recovered from a lake-bed in
Switzerland is astonishing and dates back to3000 BC [8].
Over the years, clothing evolved to serve as anexpression of
culture and social status; it also plays apart in attracting or
discouraging a mate. Throughouthuman history, textiles and clothing
have been synon-ymous. The term textiles is derived from the
Latinword texere, meaning to weave; in other words, theterm
textiles referred to woven fabrics only. The processof weaving
involves interlacement of two pre-arrangedorthogonal sets of long
and exible strands oryarns. The terms long and exible, today,
implyyarns that are either assembled from bres or aremanufactured
as continuous exible strands.
Todays textiles include cords and ropes to fabricsmanufactured
through various technologies includingweaving, knitting, non-wovens
and combinations thereof.Textile structures are valued for their
low weight,exibility and extraordinary properties. Whether it is
aprotective turnout coat for the reghter, a parafoil todrop
thousands of pounds of supplies, a set of tyresmounted on the
landing gear of the newest aircraft, theTeon-coated Kevlar airbags
used in landing spacecraftson Mars or the super-absorbent diapers
of your newborn, all of these innovations are bornout of
high-perform-ance materials and technologies as well as
excellentengineering design with textiles. Needless to
say,modern-day textiles are far more than just clothing.Indeed,
more than 75 per cent of the bres used in theUSA in 2008 were in
products other than clothing andhome textiles [9]. In fact, a
broader denition, whichdescribes textiles as exible products made
primarilyof polymeric (natural or man-made) bres, is
moreappropriate today.
Most natural materials are polymers (proteinsand
polysaccharides), polymer composites and someJ. R. Soc. Interface
(2011)the 6th millennium BC [12]. Evidence of the rst useof wool is
a bit murky, but is assumed to be around5000 BC [8]. The earliest
awareness of silk, the onlybre that is found as a long continuous
strand innature, comes from the Shahnshi province of Chinaand dates
back to the Neolithic period [8]. Until theearly twentieth century
and the invention and com-mercialization of regenerated bres
(rayons), thesewere the only available textile bres. The
introductionof a number of key manufactured bres
(polyester,polyethylene, polyacrylonitrile, polypropylene, etc.)
fol-lowed at a relatively rapid pace. Since the introductionof
nylon in the early 1930s, the search for even better(stronger,
tougher, etc.) bres has been on. In the latetwentieth century, a
new generation of polymeric andinorganic high-performance man-made
bres withexceptional properties was introduced [13]. Theseinclude
various meta- and para-aramid (e.g. Nomex,Kevlar are DuPont
registered trademarks), aromaticpolyester (e.g. Vectran is a
Kuraray registered trade-mark), ultra-high molecular weight
polyethylene (e.g.Spectra is a Honeywell registered trademark),
ceramic(Nicalon is a Nippon Carbon Co. registered trademark)bres.
Today, numerous man-made bres are availablein the inventory of a
textile designer, which can meetexacting functional requirements
for use in the homeor in the next space exploration.
3. LESSONS FROM NATURE
For many reasons, textiles provide unique opportunitiesto
emulate nature. The building blocks of every textilestructure at
the lowest level of hierarchy (nano tomicro) are organic bres, and
many of these are natural.In addition, like many natural functional
surfaces, thelarge surface area of brous textiles offers
tremendousopportunities to functionalize them. All of these
attri-butes lend textiles more to biomimetic concepts
thanothers.
3.1. Diverse use of bres
Nature is full of excellent examples of building withbres. A
more obvious example is in the cobwebsand forms, often in a
hierarchical fashion. Textile struc-tures are inherently
hierarchical. Levels of hierarchy,however, vary. At the
macro-level, a simple woven orknit fabric has three levels of
hierarchy: bre to yarn tofabric. However, as pointed out by Vincent
[11], fabricis an assembled structure rather than a material.
The earliest bres used in textiles were ax (orlinen), hemp,
nettle, willow, etc., found in the wild. Ear-liest evidence of the
domestication of bre, ax, comesfrom Iraq and is dated close to 5000
BC [8]. A morerecent discovery of cotton yarn used to string
copperminerals (e.g. Ca and Si). Few metallic elements areused both
as structural (e.g. Zn or Mn in insect mand-ibles) and functional
(e.g. Fe in red blood cells)materials [10,11]. To obtain a very
wide range of function-
-
tinuous bre (lament) available in large quantities andvalued by
humans. It has been used for luxury fabricsand in technical
applications, such as in parachutes, forits neness, low weight,
lustre, softness and strength.People have sought to mimic these
bres for ages [28].The more recent interest in the study of spider
silks isbecause of their unique combination of strength
andtoughness, which make them a model engineeringmaterial. Spider
(Araneae) silks are protein-based biopo-lymer laments with
exceptional mechanical propertiesdespite being spun at almost
ambient temperature andpressure and with water as solvent [29]. It
is an excellentexample of natures use of protein as an adaptable
build-ing material. The superior properties of these silks
areattributed to their semi-crystalline polymer structure [30].
There are over 34 000 species of spiders and mostare capable of
spinning task-specic silk of varyingmechanical properties [31].
Some spiders, specicallythe orb-weaving Araneid and Aloborid
spiders, havethe ability to spin a variety of different silks
depending
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by certain species of spiders. These are made upof short irregular
strands of bres arranged almost ran-domly while the orb-webs made
by other species ofspiders are regular, elegant and elaborate.
Plants and trees provide other superb examples ofbrous
structures. In many cases, the bres arearranged or oriented in a
particular manner to impartdesired mechanical properties to the
structure. A goodexample is the coconut palm (Cocos nucifera,
Linn.)tree. The coir bre derived from its seed husk is well-known
and used in oor coverings, mattress llingsand others. Among other
bres found on a coconutpalm, the layers of brous sheets in the
leaf-sheath(base of the leaf stalk attached to the tree trunk)
withbres in the alternating sheets oriented nearly orthog-onal to
each other appear to be already in a wovenstructure [14] (gure 1).
Interestingly, the leaf-sheathconsists of three distinct types of
multicellular bresmade of mostly cellulose and lignin arranged in
ahighly ordered structure. The mechanical propertiesof these three
types of leaf-sheath bres are vastlydifferent from each other
[14].
Wood and bamboo are excellent examples of naturalbrous
composites with high work of fracture. Woodconsists of parallel
hollow tubular cells reinforced byspirally wound cellulosic brils
embedded in a hemicell-lulose and lignin matrix. The helix angle of
the spiralbrils controls various mechanical properties
includingstiffness and toughness of wood [1619]. Bamboo isone of
the strongest natural brous composites withmany distinguishing
features. It is a hollow cylinderwith almost equidistant nodes.
Bamboo also has a func-tionally graded structure in which bre
distribution inthe cross section in the bamboos culm is
relativelydense in the outer region [20]. The chemical
compositionof bamboo is very similar to wood but its
mechanicalproperties are very different. Wood tracheid andbamboo
bres [21,22] are both hollow tubes (or with alumen) composed of
several concentric layers and eachlayer is reinforced with
helically wound microbrils.The difference in properties originates
from the numberof bre layers and microbrillar orientation angles
[21].
Nature also has an abundance of examples of respon-sive brous
structures. Many plants are able to producepassive actuation of
organs by controlling anisotropicdeformation of cells upon exposure
to moisture. Plantcell walls are made of stiff cellulosic brils
embedded ina moisture-sensitive softer matrix consisting of
hemicellu-loses, pectin and hydrophobic lignin. The absorption
anddesorption ofmoisture by the plant cell wallmatrix
causesanisotropic deformation of the cell wall [23]. The
orien-tation of the cellulosic brils in the cell walls as well
astheir stiffness is crucial in determining the degree as wellas
the direction of the bending actuation [24]. Pinecones are known to
use this hygromorphic behaviour indistributing their seeds. Drying
at ambient humiditycauses a close and tightly packed pine cone to
open upslowly owing to the bilayered structure of the
individualscales [24,25]. The mechanism of pine cone openingrelies
on the humidity sensitive outer layer of the ovulifer-ous scales to
expand or shrink in response to moisture inthe atmosphere, while
the inner layer remains relativelyunresponsive [24]. News reports
(http://news.nationalJ. R. Soc. Interface
(2011)geographic.com/news/2004/10/1013_041
013_smart_clothing_2.html) point to a recent effort at
theUniversity of Bath to develop a bilayer fabric, whichreportedly
opens up its pores in the presence of increasedmoisture (owing to
perspiration in warm weather) andthereby promotes cooling.
From plants to animals, one of the unique uses ofbres in
structural construction is that of the skeletonof glass sponge
Euplectella as reported by Aizenberget al. [26] (gure 2a,b). The
hierarchical structure ismade of lamellar bres of silica
nanospheres at thenanoscale to rectangular lattice formed by rigid
bre-composite beams at the macroscale. The resultingremarkable
truss-like cylindrical, skeletal structuremade of the intrinsically
low strength and brittlematerial, glass, is stable and is able to
withstandtensile and shear stresses caused by currents
whileattached to the ocean oor. Interestingly, the structureis very
similar to that of a triaxial fabric developed byDow in 1969 to
obtain a more isotropic and stablestructure appropriate for space
applications [27].
Formanyyears, traditional silk fromBombyxmori andAntheraea
pernyi moths has been the only natural con-
Figure 1. Photograph of coconut tree leaf-sheath. Inset is
theinner mat of the leaf-sheath. With kind permission fromSpringer
Science Business Media [15, g. 1b,c].
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764 Review. Biomimicry in textiles: an overview L. Eadie and T.
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(b)on their need at a specic time [28]. Orb-weaving spidersuse
specialized abdominal glands to synthesize up toseven different
protein-based silks and glues oftensimultaneously [29,30,32].
Araneids, it is hypothesized,produce the diverse silk properties by
the expression ofdifferent broin genes [30]. Using the seven
differentsilk glands, a typical Araneid orb-weaving spider
pro-duces different silk forms including: (i) the majorampullate,
which is extremely tough and forms the pri-mary dragline as well as
the web-frame, (ii) the minorampullate with high tensile strength
and low elasticityused in web construction, (iii) the agelliform (a
viscidsilk) which is highly extensible and forms the capturespiral,
and (iv) the aciniform, which is the prey wrappingsilk [30,31]. In
short, orb spiders are capable of modulat-ing silk properties
ranging from low modulus highlyextensible elastomers to high
modulus, high tenacityand high toughness bres.
Figure 2. The mineralized skeletal system of Euplectella sp.
showin(b) a fragment of the cage structure showing the square-grid
latt(scale bar, 5 mm). Adapted from Aizenberg et al. [26].
Reprinted
Table 1. Range of properties of different types of spid
material usesstrength(GPa)
cocoon silk (Bombyxmori)
cocoon 0.6
dragline silk (majorampullate)
dragline, frame threads 0.72.3
minor ampullate dragline reinforcement 1agelliform silk capture
spiral within web 0.10.5aciniform envelop prey 360aggregate sticky
silk glue for capture
spiral
Kevlar 2.93.0nylon 0.30.7steel 1.5
J. R. Soc. Interface (2011)There is great deal of similarity
between the pro-cesses used to industrially produce many of
thehigh-performance bres and those used by spiders. Inthe case of
spiders, the synthesis of silk protein(s)takes place in columnar
epithelial cells and is secretedinto a storage gland. The spinning
dope or the silkprotein, in liquid crystal form is drawn down as
itpasses through ducts, from the glands to the spinnerets[29,33].
It has been shown that the silk becomes highlyoriented as it passes
down the ducts [29].
The dragline silk, the main structural web silk, hasbeen the
focus of numerous research investigationsbecause it is among the
strongest known bres of anykind. It is the main component of spider
webs, andalso serves as the spiders lifeline along which theyswing
and move. Typically, dragline silk has very highstrength, high
elongation and excellent toughness asseen in table 1.
g (a) a photograph of the entire skeleton (scale bar, 1 cm)
andice of vertical, horizontal and diagonal struts of the
cylinderwith permission from AAAS.
er silks and other bres (adapted from [3337]).
elongation(%)
modulus(GPa)
energy to break(kJ kg21)
14 6
2239 9.530 130195
5 30300 1 10046 0.6 517
2.54.0 70115 331540 734 600.8 190210 0.76
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Interestingly, one of the strongest bres, Kevlar, apara-aramid, is
stronger than the dragline silk but thesilk is signicantly more
extensible and about vetimes tougher than Kevlar. Spider silk bres
are alsothermally stable up to approximately 2308C. Cunniffet al.
[34] reported two thermal transitions of draglinesilk from the
spider Nephila clavipes, one at 2758C,which is believed to
represent the motion of amorphousregions of the bre, and the other
at 2108C being theglass-transition temperature.
A potential problem with dragline silk is that it con-tracts
signicantly when unrestrained and wetted. Thewetting causes the
length to shrink by more than halfwhile its diameter more than
doubles [3841]. Thisphenomenon is known as supercontraction.
Underrestrained conditions, the supercontraction can gener-ate
stresses in the range of 10140 MPa [38]. Thisnding has implications
for the use of spider silk inapplications where exposure to
moisture is likely. Toget around the problem, incorporation of the
bres ina water-resistant matrix or the development of methodsto
remove the water-sensitive sequence from the poly-mer itself has
been suggested [39]. Interestingly, theminor ampullate silk does
not show supercontraction.
Interestingly, the agelliform silk, used in the cap-ture spiral
of the orb spiders web, is not sticky byitself. To provide
stickiness, the spider uses other silksand glue [32]. The
agelliform silk has also been studiedfor its unique properties.
This silk possesses exceptionalstretch and recovery behaviour and
is signicantlytougher than Kevlar, bone and elastin [36,42].
Thesemechanical properties of agelliform silk are believedto be
derived from the amino acid sequencing andarrangement within the
silk strands, which exhibithelical spring-like congurations [42].
These brespossess considerable strength even though they
exhibitelastomer-like extensibility.
Obviously, emulating the spiders silk and possibly itsproduction
method seem very attractive. The ability toproduce natural protein
bres with tailorable propertiesin a green process to replace the
energy-intensive,often environmentally detrimental and
non-recyclablebres is denitely advantageous.
For reasons delineated above, dragline silk of thegolden-orb
weaver, N. clavipes, has attracted a greatdeal of research effort.
Advances in biotechnologyhave opened up new, potential, pathways to
extract,synthesize and assemble proteins in large scale foreventual
production of silks. These proteins consist ofvarious amino acids
strung together by the organismin an exact sequence to produce
specic characteristics.The amino acid sequences of a number of
different pro-teins in silk bres have been identied [36] and
areknown to form b-pleated sheet crystals. The exactamino acid
sequence for the dragline silk of the araneidspider N. clavipes was
found to be quite similar to thatof silkworm moth silk produced by
B. mori. The slightdifferences noted in the sequence, however,
result in thedrastic property differences observed [28,36].
Spidersdraw bres from a solution containing about 50 percent
protein in liquid crystalline form secreted andstored in a
specialized sac [29,31]. The solution owsthrough a tapered duct and
is drawn down usingJ. R. Soc. Interface (2011)minimal forces as the
bre forms [29]. Vollrath &Knight [43] suggests that the thin
cuticle surroundingthe spiders duct acts as a dialysis system,
which removeswater and sodium ions; the change in the ionic
compo-sition converts the aqueous polymer dope into aninsoluble
protein bre. This mechanism, it is believed,results in the strong
and tough core and coat compositestructure observed in spider-silk
bres [44]. This spinningmechanism of the spider may in fact inuence
the struc-ture formation and the resulting high performance
morethan the sequence of amino acids [29,43].
Various methods to spin articial spider silk havebeen explored.
These include conventional wet spinningof regenerated dragline silk
obtained through forcedsilking [45] and reconstituted B. mori bres
[46,47],solvent spinning of recombinant spider silk protein
ana-logue produced via bacteria and yeast cell culturesdoped with
chemically synthesized articial genes [35],and spinning of silk
monolaments from aqueous sol-ution of recombinant spider silk
protein obtained byinserting the silk-producing genes into
mammaliancells [48]. In general, such manufactured bres
haveproperties close to those of spider silk. The results
gen-erally suggest that it should be possible to manufacturebres
with properties comparable to dragline silk withthe optimization of
the spinning process.
Another recent discovery involves natural brousstructures on
gecko feet, which give them the abilityto stick (dry adhesion) to
and move along verysmooth surfaces, often upside down [49,50]. The
skinon a geckos feet consists of a hierarchical structure ofrows of
setae, and spatulae (gure 3a,b). The footpadof a gecko is covered
with very high density (about5000 mm22) of tiny bres (setae).
Furthermore, eachseta branches into hundreds of spatulae with
dimen-sions of approximately 100 nm (gure 3b) [49]. Thecomplex
structure uses a relatively simple mechanismof adhesion using van
der Waals forces. Simply put,when two surfaces come in intimate
contact with eachother, considerable van der Waals forces can be
gener-ated [51]. Results of direct setal force
measurementsattribute the adhesion to van der Waals forces
ratherthan suction, friction or electrostatic forces. The tinybre
ends (spatulae) allow relatively unconstrainedlocal deformation
which is required to generate intimatecontact with surfaces having
local irregularities. Eachgecko foot-pad seta can resist an average
force of 20mN, resulting in an adhesive force of 10 N for a footpad
area of approximately 100 mm2 [49]. Some geckospecies have adhesion
strength capabilities as high as100 kPa [52]. Although such strong
adhesive forceswould make the movement of the gecko difcult,
thislizard has developed a unique way of walking by curlingits toes
for attachment and peeling during detachmentto eliminate the forces
between its foot and the surface,thereby enabling it to move with
ease [49,50,52].
Since this discovery, many attempts have been madeto construct
the surface structure of gecko feet into aman-made material in
order to achieve dry adhesion.The task seems to be simple in that
it requires fabrica-tion of millions of tiny densely packed
nanobresstanding up on their ends on a substrate much likeocked
fabric surface. Needless to say, it turns out to
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ngRep
766 Review. Biomimicry in textiles: an overview L. Eadie and T.
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more complicated. The rst challenge is to ensurethat the tiny bres
are of sufciently high aspect ratioin order to be able to make
contact with the contactingsurface, which is often microscopically
irregular [53].The need for high aspect ratio leads to the second
chal-lenge in that the ne bres tend to collapse and stick toeach
other leading to matting and entanglement[53,54]. Additionally, if
the spacing between adjacentbres is too small, the intermolecular
forces actingbetween the bres lead to bunching [52].
Theoreticalanalyses as well as experimental data point to theneed
for high modulus bres of high aspect ratio withsmall inter-bre
spacing to achieve good adhesion [54].Synthetic gecko foot bres
have been created using var-ious materials and techniques. These
include, forexample, nanomoulding using silicone [55,56],
polyi-mide [55], polyvinylsiloxane [57] and
polyurethane,photolithography using polyimide [58], carbon
nano-tubes [59] and polyurethane [54]. The reportedadhesion
strength in many of these cases exceededthat of gecko feet. Some of
the most impressive results,arguably, are those obtained when
carbon nanotubesare used as hairs. Independent reports by Ge et
al.[60] and Yurdumakan et al. [61] claim that carbonnanotube-based
gecko tapes can support signicantly
(a)(b)
48 00
Figure 3. (a) Standard electron microscopy (SEM) image
showispatulae on a geckos foot. Adapted from Autumn et al.
[49].[49], copyright q 2000.higher stresses than those supported by
gecko feet.
3.2. Functional surfaces
Natural surfaces offer examples of remarkable diversityof
properties. Much like dry adhesion of gecko feet,examples of lack
of adhesion in nature, in particularon plant surfaces, have
attracted considerable scienticattention. Plants have evolved over
the last 460 Myr toadapt to their natural environment. The surface
struc-tures of plants consist of many different cell types,
cellshapes and cell surface structures, resulting in a hugevariety
of plant surfaces observed today. To create aprotective barrier,
plants have developed a continuousextracellular membrane or
cuticle. The cuticle of mostplants is made of a polymer, cutin, and
soluble lipids.The water repellency and self-cleaning properties
ofmany plant surfaces have been attributed to not onlythe chemical
constituents of the cuticle covering their
J. R. Soc. Interface (2011)surface, but more to the specially
textured topographyof the surface [6264]. In addition to the lipids
that areincorporated into the cuticle of the plant, the
texturedsurface topography is the result of distribution of
smallthree-dimensional crystals of the epicuticular waxes(lipids).
It is these surface structures that provide themodel for
superhydrophobic textiles [65].
While hydrophobicity is present in numerous plantsurfaces, the
superhydrophobic1 (and self-cleaning) be-haviour of lotus (Nelumbo
nucifera) leaves has drawniconic interest. Water drops on lotus
leaves bead upwith a high contact angle and roll off, collecting
dirtalong the way, in a mechanism known as self-cleaning.Plant
leaves, in general, possess textured surfaces withhierarchical
micrometre- and nanometre-sized structu-res [62,66,67] and show
superhydrophobic behaviour.The rst structure is the basic
micro-level mound-likeprotrusions consisting of papillose epidermal
cells. Thesecondary structure consists of nanoscale
branch-likegrowths occurring on the epidermal cells as shown ingure
4a,b [68,69]. This is important for superhy-drophobicity, as is the
low-surface energy epicuticularwax found on lotus leaves. The
micrometre-sized(59 mm diameter) papillae trap air when they
comeinto contact with a water droplet. The roughness of
D 30.0 kV 8 mm1 mm CL: 7.0
rows of setae on the bottom of a geckos foot; (b) SEM image
ofrinted by permission from Macmillan Publishers Ltd: Naturethe
papillae leads to a reduced contact area betweenthe surface and a
liquid drop (or a contaminantparticle) and helps create what has
been called a re-entrant surface [70]. The droplets rest only on
the topof the epidermal cells, and as a consequence, dirtparticles
can be picked up by the liquid and carriedaway as the liquid
droplet rolls off the leaf. Thisoccurs because there are only weak
van der Waalsforces between the leaf surface and the dirt
particles,whereas stronger capillary forces exist between thewater
droplet and the dirt [65]. It is not clear if thesurface geometry
(bumps and hairs) or compositional(lipids) effect plays a more
important role in theobserved superhydrophobicity or the so-called
lotuseffect.
1A surface with a high advancing water contact angle, typically
1508or higher, and a very low contact angle hysteresis (or a very
low slidingangle), typically below 158 is considered
superhydrophobic.
-
(b)
pai
ion
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Superhydrophobic surfaces have important technicalapplications such
as antifogging and self-cleaning coat-ings, microuidics, etc.
Superhydrophobicity in textilesis important in applications such as
outdoor clothing,carpets, architectural fabrics, etc. The
importance ofwater repellency in textiles was recognized long
beforebiomimetics became popular and is highlighted by theexcellent
two-part review by Schuyten et al. [71] pub-lished in 1948.
The approaches to engender the lotus effect on sur-faces in
general and textiles in particular fall into twocategories. The rst
approach involves creating nano-/microscale surface topography
[68,7274] and thesecond approach consists of lowering surface
energyby chemical modication [75,76]. In fact, surface
texturemodication in conjunction with surface chemistrymodication
has been used to create surfaces that cansupport a robust composite
(solidliquidair) interfaceand in turn behave superhydrophobically
and/orsuperoleophobically [77,78].
In a 2006 paper, Gao and McCarthy presented apractical and
simple way of imparting superhydropho-bicity to a textile surface.
The process involved simplesilicone-coating of two polyester
fabrics containingconventional (coarser) and micro- (ner) bres,
respect-ively, using a method described in a patent of 1945
[79].The ner topography of the microbres fabric report-
(a)
Nelumbo frisch OS, 15 kV, 16.5.2008 1900 10 mm
Figure 4. (a) SEM image of the surface of a lotus leaf
showingCopyright q (2009), with permission from Elsevier. (b)
SEMconstituting the surface of a lotus leaf. Reprinted with
permiss(2005).edly produced water repellency superior to that
oflotus leaf [76].
In a recent paper, Choi et al. [80] reported a simpledip-coating
of extremely low-surface energy urodecylpolyhedral oligomeric
silsesquioxane (POSS) moleculeson commercial fabrics to engender
signicant waterrepellency. They dene two critical parameters,namely
bre radius and bre spacing as dominant par-ameters in determining
fabric-wetting behaviour. Theirdata demonstrate biaxial stretching
(to control brespacing) as a means to control the wetting
character-istics of fabrics. The same group earlier
demonstratedsuperoleophobic behaviour of an electrospun brewebof
ploy(methyl methacrylate) blended with urodecylPOSS [70]. Plasma
coating of uropolymers on non-woven fabrics has proven more
benecial for liquidrepellence than in the case of woven fabrics
because ofsurface hairs in non-wovens [75]. Hoefnagels et al.
[81]
J. R. Soc. Interface (2011)report covalent bonding of silica
particles onto cottonbres and subsequent chemical modication
throughdip-coating in polydimethylsiloxane to obtain a
super-hydrophobic surface. Recently, the same group
reportedapplying relatively larger sized (approx. 800 nm)
silicananoparticles onto woven fabrics followed by
surfaceperuorination to achieve superoleophobicity [82].
Similar to the lotus leaf, taro leaf, rice leaf, duck feath-ers,
legs of water striders, butterywings andmanyothersshow notable
superhydrophobic behaviour. Besides beinghydrophobic, duck feathers
(and those of other waterbirds) also exhibit good
thermal-insulating properties.However, when they are excessively
wet, the featherstend to clump together, hold water (thereby
becomingheavy) and do not insulate as effectively. Details of
birdfeather morphology and the hierarchical network formedby barbs
and barbules have been investigated, to under-stand their
hydrophobic behaviour [83,84]. Liu et al. [84]attempted to develop
a durable, potentially self-cleaning,superhydrophobic surface
treatment for soft textiles basedon their understanding of duck
feathers. They assert thatduck feathers exhibit highly ordered and
hierarchicalbranched structures built around a micro-sized
backbone.Branches of various sizes of a duck feather are made up
ofmicro-sized tomenta. These tomenta in turn have nanos-cale
undulates on the surface as shown in gure 5a,b.The water repellency
of the bird feather in general is
2 mm
pillae and epicuticular wax. Reprinted from Koch et al.
[65].mage showing the surface characteristics of a single
papillafrom Sun et al. [68]. Copyright q American Chemical
Societyattributed to the trapped air space in the
multi-scaletexture formed by the barbs, barbules and tomentawith
nano-sized grooves, forming an air cushion at thefeatherwater
interface thereby keeping the feather frombeing wet [83,84].
Liu et al. [84] emulated the microstructures of theduck feather
on cotton and polyester fabrics. Theyapplied chitosan, a naturally
derived polymer, on thefabric surface using an appropriate
precipitationmethod to form nanoscale surface roughness. The
chit-osan was seen to form nanoscale ower-like structureson the
polyester fabric, whereas on cotton, a moreeven coating was
observed. The fabrics were furthermodied with a silicone nish to
achieve lower surfaceenergy. They reported signicant improvement
inwater repellency as a result of the treatments.
Examples of aerodynamic shapes with low drag areabound in
natural iers and swimmers. From birds to
-
diverge, which results in varying water ow patternsaround the
shark in these different regions [90]. Ribletsare known to channel
water through the small valesthey create, which speeds up the ow of
the waterover the surface of the skin, resulting in the reductionof
the turbulent skin friction drag [92]. Laboratoryexperiments on
riblets have shown a reduction of skinfriction drag of almost 10
per cent [89]. The drag-redu-cing potential of riblets is inuenced
by the sharpness of
of d]. R
0.2 mm
Figure 6. Image of denticles and riblets of a great white
sharkscale. Adapted from Bhushan [2] with scale images
originallyobtained by Reif [91].
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animals, nature has optimized shapes and sur-faces to lower aero-
and hydrodynamic drag. Mostanimals have evolved over millions of
years to optimizebody design and skin for efcient locomotion in
waterwith minimal drag. The drag force that acts to holdswimmers
back and slow them down can be brokendown into three different
types: skin friction drag,form drag and wave drag. The form drag
(also knownas pressure drag) depends mostly on the shape of
thebody. Skin friction drag is the force a uid exerts on asurface
in the ow direction and is a result of the no-slip condition at the
boundary layer [85,86]. Flowacross this boundary can be either
laminar (smooth)or turbulent (rough). For increased speed in
thewater, laminar ow is desired [87]. Skin friction dragalteration
in nature follows two basic strategies:(i) maintain a laminar ow
through use of smoothsurfaces and/or (ii) alter body smoothness to
establisha favourable turbulent ow [87]. Wave drag, the
thirdcomponent of drag, occurs only near the surface,where the
pressure surrounding the moving swimmersets up a wave system.
Humans are not efcient swimmers, for their shapesare not well
suited to rapid travel through water. Forhumans, swimming is a
learned trait. Swimming styleis vital to a swimmers speed, but
beyond that, it isimportant to lower the skin friction drag
experiencedby swimmers. Human quest for greater speed has ledto the
examination of examples in nature [8790].The movement of sharks in
water, and in particular,the structure of their skin, has been of
interest.Sharks, on the one hand, are an excellent example ofa
super-predator with the ability to swim at great
(a)
FE_SEM SEI 3.0 kV 100 mm WD 8.6 mm 50
Figure 5. (a) FE-SEM image showing hierarchical structuretomenta
of a duck feather (scale bar, 100 nm) [84speeds and manoeuvre
swiftly in water. Humans, onthe other hand, are not as graceful in
water and thishas led to the examination of sharks speed and
agilityin water.
The skin of most types of sharks is covered by tiny(0.20.5 mm)
hard tooth-like three-dimensional placoidscales, also called dermal
denticles. The denticles havevery ne and equi-spaced ridges and are
aligned alongthe body axis. These tiny riblets of denticles vary
interms of number, size and shape depending on thesharks age and
species (gure 6) [88,92].
These riblets exhibit an overall parallel pattern,facing from
head to tail on the shark skin in an inter-locking fashion. In some
areas of the shark, theseriblets converge while in others they are
found to
J. R. Soc. Interface (2011)(b)
FE_SEM SEI 3.0 kV 100 nm
100 nm
WD 8.4 mm 30.000
uck feather (scale bar, 100 mm) [84]; (b) FE-SEM image
ofeprinted with permission from IOP Publishing.riblet edges, their
optimal height protrusion into thesurrounding sea water and the
spacing between individ-ual riblets [93]. Very thin, vertical
riblets result in thegreatest amount of drag reduction. Sharp,
triangularones create intermediate level of drag reduction.Broad,
tortuous riblets result in the lowest amount ofdrag reduction [59].
Bechert et al. [88] provide a detailedexplanation of the uid
mechanics of drag reductionresulting from shark riblets.
For obvious reasons, the ultimate focus of researchstudies has
been to imitate the surface morphology ofshark skin in applications
such as in swimwear and skindesigns of long-range aircrafts as well
as sea-faringvessels. Indeed, a riblet skin produced by 3M
companyhas been used in Stars and Stripes, the Americas cup
-
through their pelt. The mechanism of low UV reection
3.4. Optical systems
Nature has unique abilities to manipulate light. Mostsurfaces in
nature are not just functional; they oftenproduce brilliant, vivid
and iridescent colours. Colour,of course, is an essential part of
most textiles. Naturalcolours are often produced by a diversity of
photonicstructures that have evolved over millions of years
togenerate effects known as structural colours (in contrastto
colour from pigments). Structural colours result frominterference
or diffraction, or selective reectance ofincident light owing to
the physical nature of a struc-ture. If these submicrometre
structural variationsare periodic with a periodicity of the order
of thewavelength of light, they are often called biologicalphotonic
crystal structures. These biological structuressuggest a new
perspective on ne structure of bres aswell as higher level
assemblies of bres used in textiles.Examples of structural colours
have been reported in alarge number of species, including butteries
[104107],bird [108,109] and beetles [84,110]. There is a vast
bodyof literature on the structural colour in plants and ani-mals.
Srinivasarao [107], Parker [111], Tayeb et al. [112]provide
excellent reviews of mechanisms of structuralcolour.
(a)
afterfeather
Figure 7. (a) Image of a penguin (Pygoscelis papua)
featherincluding afterfeather (scale bar, 5 mm). (b) Optical
micrographof barbs from the afterfeather (scale bar, 500 mm). (c)
Scanningelectron micrograph of barbules (scale bar, 10 mm) [98].
Re-printed from Dawson et al. [98]. Copyright q 1999,
withpermission from Elsevier.
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thought to be related to the thermal behaviour ofbear pelt and has
been a subject of research. Thepolar bear hair is hollow with
foam-like substance inthe middle [101]. Before Koon [102] published
data onpoor bre-optic transmission behaviour of polar bearhairs, it
was proposed that the bear hairs act likebre-optic transmitters
that allow the capture of inci-dent sunlight and the heat is
transferred to the blackskin [103]. Koons data showed that the
polar bearhair is indeed a poor wave guide and that it maysimply
absorb UV light. Stegmaier et al. [101] reportedthe development of
a solar thermal collector, based onthe supposed solar function of
the polar bear fur andskin, with high light-transmission capability
using aspacer fabric with translucence coatings on both
sides.champion racing yacht
(http://www.nasa.gov/centers/langley/news/factsheets/Riblets.html).
Probably, themost well-known commercial application of riblet
sur-face morphology is in Fastskin swimwear technology(Speedo,
Inc.). It was reportedly claimed that a 7.5 percent reduction in
drag would be experienced by the swim-mer as a result of wearing
the suit [94]. However, directmeasurement of drag values at
different speeds found astatistically insignicant drag reduction of
2 per centusing a Speedo Fastskin suit [94]. In another study on
theefcacy of using Fastskin, no evidence of physical or
phys-iological benets of wearing these suits was reported
[95].Besides swimwear, materials mimicking sharks skin havebeen
suggested for applications that include aircraft skinand
interlining of uid-transport pipelines, to name a few.
3.3. Thermal insulation
The thermal-insulating property of duck feathers isattributed to
the trapped air in their nanoscaled andhierarchical structure.
Synthetic alternatives to downhave been attempted, but their
insulating capabilitiesdo not match those of natural feathers [96].
Bonseret al. evaluated the mechanical properties of duck andgeese
down feathers. The inuence of moisture on themechanical properties
of duck feathers has been reportedas nominal [97]. Penguins live in
extremely cold weatherand have the ability to dive deep into the
water withouttrapping any air in their coat to avoid creating
positivebuoyancy. Dawson et al. [98] investigated the coat ofthe
penguin using a heat transfer model to show no con-vective heat
loss with minimum radiative heat loss. Theyattribute the behaviour
entirely to the structure of pen-guin afterfeather (gure 7), a
collection of barbs (about47) attached at the bottom of the
feather. Du et al. [99]used a Monte Carlo simulation method to
examine theheat transfer through the penguin coat and attributedthe
superior insulating properties to the neness of thebarbules in the
feather as the major factor.
Thermal insulation mechanism of polar bear fur hasbeen a subject
of debate for some time. Polar bear pelt,an excellent natural
insulator, allows the animal to sur-vive arctic cold. Polar bears
are known to appear blackwhen illuminated by ultraviolet (UV)
lights despitetheir white appearance to the human eye [100].
Inaddition, it is impossible to view a polar bear throughan
infrared camera because of very low heat lossJ. R. Soc. Interface
(2011)pennaceouspart of feather
barbule
barb
(b)
(c)
-
production of a bre called Morphotex that claims tomimic the
microstructure of Morpho buttery wingsand produce structural
colour. The bre made of eitherpolyester or nylon has more than 60
laminated layers ofnanometre dimension [118]. An exact replica of
theMorpho wing structure was produced using atomiclayer deposition
of Al2O3 on real buttery wing template[115]. Alternative methods of
generating such nenanoscaled structures in textiles need to be
looked at.
ersltiVu
10 mm
Figure 9. SEM image of the entangled brous structurefound on
edelweiss that protects the plant from harmful UVradiation [113].
Reprinted from Kertesz et al. [113]. Copyrightq 2006, with
permission from Elsevier.
770 Review. Biomimicry in textiles: an overview L. Eadie and T.
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Excellent examples of biological optical systems andclues to their
potential applications in textiles can befound in studies involving
anatomical basis of photoniccrystals in nature. Photonic crystals
(also known asphotonic band-gap materials) are periodic
structuresthat have a band gap that forbids the propagation ofa
certain frequency range of light. As a result, photoniccrystals
always reect only that specic band width(colour) of visible light
[113]. Such structures arefound in nature in buttery wings, some
plant species(bracts of edelweiss), marine creatures (e.g.
brittlestar,Ophiocoma wendtii), opals [114], etc.
Butteries probably exhibit the most interesting var-ieties of
optical microstructures and have been studiedextensively. In
general, the buttery wing consists oftwo or more layers of small
scales formed over a mem-brane. Typically, there are two to three
types of scalesof about 200 mm long and 50 mm wide, arranged withan
overlap much like roof shingles [107,115,116](gure 8). The density
of the scales varies from 200 to500 mm22. The various colours
produced are mainlyowing to both pigmentary and structural colour
pro-duction mechanisms. Most of the colours are producedby either
thin lm interference or diffraction [105,107].The membrane of the
wing usually contains the pig-ments melanin or pterins that
accentuate the coloureffects because of structural variations. In
the case ofMorpho butteries, the metallic blue is produced bythe
elaborate structural features on the wings. The
(a) (b)
Figure 8. (a) Image of blue photonic buttery wing. (Online
vcross section through a butterys wing showing discrete muLtd:
Nature [114], copyright q 2003. Image obtained from P.dark melanin
present in the membrane absorbs thelight that is not reected to
make the reected coloursappear bright [107,116].
A bre manufacturer, Kuraray Corp., took inspi-ration from the
ridge formation on the Morphos wingto create a polyester fabric
with low reectivity, butvivid coloration. This fabric was dubbed
Diphorl andwas manufactured using bicomponent polyester bresof
rectangular cross section. The bres were spun fromtwo polyester
components of different thermal proper-ties, which developed twist
(approx. 80120 twistsper inch) upon heat treatment after weaving.
It isclaimed that the structure produces alternating
hori-zontal/vertical alignment of surfaces to cause
repeatedreection and absorption of the incident light in
closeproximity to each other, thereby producing brilliant col-ours
[117]. Teijin Fibres Ltd of Japan began commercial
J. R. Soc. Interface (2011)ion in colour.) (b) Transmission
electron microscopy image of-layers. Reprinted by permission from
Macmillan Publisherskusic, University of Exeter.Advanced photonic
structures are also found inplants. The woolly white lament covered
bracts ofthe edelweiss plant (gure 9) possess special
spectralbehaviour that apparently protects the plant fromharmful UV
exposure at high altitudes. The white la-ments on the bracts are
hollow with ne nanostructureson the surface that can selectively
couple the UV radi-ation in a guided mode along the bre and
dissipate theradiation harmlessly, while the visible part of the
spec-trum is mostly reected or transmitted through thebres
[113].
3.5. Biomimicry and sustainability
Biomimicry, in its strictest interpretation, is the processof
emulating natures ways of nding a solution in-cluding designing and
making with the least
-
separate and recycle. Biomimicry in textiles must alsoconsider
recyclability and aim at reducing the number
ar
rowd m
Review. Biomimicry in textiles: an overview L. Eadie and T. K.
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environmental impact. In fact, biological systemsshould be seen
more as concept generators in termsof transfer of principles and
mechanisms rather thansomething to copy, literally. Modern
technologies havemade it possible to design and manufacture
products/systems that are based on nature. However, the processor
the technology to do so has not always been purelyeco-friendly. It
is primarily because natures implemen-tation of a concept into a
system is far different thanthat developed by humans. In nature,
growth is the pri-mary means of manufacture rather than
fabrication.The hook and loop fastener, Velcro, has been
tradition-ally manufactured using nylon. The key ingredients
arepetroleum derivatives, with the usual environmentalconsequences
of petroleum processing. If biomimicry isto be used as a new
principle in designing textiles, sus-tainability must be part of
it. Biomimetics can help us
10
01980 1990
20
30
40
50
per c
apita
co
nsu
mpt
ion
(kg)
ye
Figure 10. US bre consumption (per capita) and population
gconsumption plus imports less exports of semi-manufactured anline,
natural bres; dashed line, population.rethink our approach to
materials development and pro-cessing and help reduce our
ecological footprint. Thehistory of textiles is full of continuous
search for andinvention of new bre-forming polymers with uniqueand
improved properties. The increase in world popu-lation coupled with
increased standards of living hasdriven per capita consumption of
bres to levels thatmay not be sustainable. As an example, per
capita con-sumption of textile bres in the USA has grown fromabout
25 kg in the early 1980s to about 40 kg in 2008(gure 10). The
increasing demand for bres is alsodriven by their new and
innovative use in new andinnovative products. Ideally, the
increasing demandshould be met largely by using renewable
resourcesand through efcient recycling. Plants and animals innature
hold the key to this route.
The large array of polymeric bres and othermaterials available
to us often lead to blending ormixing of these bres to develop a
new product orimprove an existing one. This makes it immensely
dif-cult, at times, to eventually recycle the product. Use
oflimited variety of materials in nature makes it easier torecycle.
With only two polymers (proteins and
J. R. Soc. Interface (2011)of polymer types we tend to use in a
product. Naturalsystems are inherently energy-efcient and
adaptable.To be sustainable, textile bres and products mustemulate
this feature as well.
The combination of biobres, such as kenaf, hemp,ax, jute,
henequen, pineapple leaf bre and sisal,with polymer matrices from
both non-renewable andrenewable resources to produce composite
materialsthat are competitive with synthetic composites
requiresspecial attention, i.e. biobermatrix interface andnovel
processing.
4. CONCLUSIONpolysaccharides) in use, it is much easier for
nature to
2000 2010200
250
300
350
popu
latio
n (m
illion
)
th over the last three decades (US domestic consumption
(millanufactured products)) [119121]. Dotted line, all bres;
solidWe began this review with the assertion that textilestructures
are similar in a number of ways to plantsand animals found in the
environment. The basic build-ing block of textiles is bres. Nature
also makesextensive use of bres, from nanoscale collagen bresin
tendons to microscale wood bres. In nature, bresare used in diverse
applications, including terminalhairy bres in gecko feet pads to
high-tenacity spidersilk. Furthermore, most natural surfaces are
multi-func-tional. This is also desired in textile products: a
naturalinterface between humans and their environment.
There is ample evidence to suggest that our ancestorslooked to
nature for inspiration to conceive newmaterials and devices long
before the term biomimetic(and similar phrases) was coined. It is
unclear whatinspired prehistoric humans to invent the
processes(i.e. spinning, weaving, etc.) to assemble bres
intoclothing; it may have been an orthogonally interlacedthin and
exible biological structure like the coconutleaf sheath, or the
nest of a weaver bird, or it mayhave been an invention of a
contemporary genius. Thefundamental practice of prehistoric humans
to produce
-
textiles from natural bres has evolved into a vast array
manuscript and many valuable suggestions.
10 Fratzl, P. 2007 Biomimetic materials research: what can
we
772 Review. Biomimicry in textiles: an overview L. Eadie and T.
K. Ghosh
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multi-functionalmaterials through optimization at various scales
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Transfer of a concept from natural to man-madeis not trivial.
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new or unusual combinationsof material functions/properties by
structuring a givenmaterial, rather than by changing its chemical
compo-sition. In fact, textile bre assemblies can readilyprovide an
ideal test-bed for this concept.
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Cannon Professor, Emeritus, North Carolina StateUniversity,
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Biomimicry in textiles: past, present and potential. An
overviewIntroductionTextiles: a necessity of lifeLessons from
natureDiverse use of fibresFunctional surfacesThermal
insulationOptical systemsBiomimicry and sustainability
ConclusionThe authors would like to thank Prof. Subhash Batra,
Charles A. Cannon Professor, Emeritus, North Carolina State
University, Raleigh, USA, for his critical reading of the
manuscript and many valuable suggestions.REFERENCES