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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

762 Review. Biomimicry in textiles: an overview L. Eadie and T. K. Ghosh

on March 21, 2015http://rsif.royalsocietypublishing.org/Downloaded from 2. 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

Review. Biomimicry in textiles: an overview L. Eadie and T. K. Ghosh 763

on March 21, 2015http://rsif.royalsocietypublishing.org/Downloaded from formed 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].


on March 21, 2015http://rsif.royalsocietypublishing.org/Downloaded from (a)

(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


on March 21, 2015http://rsif.royalsocietypublishing.org/Downloaded from 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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on March 21, 2015http://rsif.royalsocietypublishing.org/Downloaded from be lot 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)

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on March 21, 2015http://rsif.royalsocietypublishing.org/Downloaded from 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].


on March 21, 2015http://rsif.royalsocietypublishing.org/Downloaded from ocean 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.


on March 21, 2015http://rsif.royalsocietypublishing.org/Downloaded from is 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.


on March 21, 2015http://rsif.royalsocietypublishing.org/Downloaded from 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


on March 21, 2015http://rsif.royalsocietypublishing.org/Downloaded from 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


on March 21, 2015http://rsif.royalsocietypublishing.org/Downloaded from REFERENCES

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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

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Biomimicry in Textiles