REVIEW ARTICLES

Application of biomimicry in textiles

S. Das*, M. Bhowmick, S. K. Chattopadhyay and S. Basak
Central Institute for Research on Cotton Technology (ICAR), Adenwala Road, Matunga, Mumbai 400 019, India

doi: 10.18520/v109/i5/893-901

gained popularity relatively recently, the idea has been

Nature has created excellent technologies around us,
and as such, it is the chief mentor to humans on creativ- around for thousands of years3,4.
ity and technology development. Nature uses fibre as a There exist numerous examples of human learning from
building block natural structures like wood, bamboo, nature. Examples of bionics in engineering include the hulls
bone, muscle, etc. all have fibrous structure. Fibre spin- of boats imitating the thick skin of dolphins. Leonardo da
ning and weaving technologies are available in nature Vinci, for example, designed ships and planes by looking at
since time immemorial. Nature has also demonstrated fish and birds respectively. Invention of the radar seems to
sophisticated technologies useful in the development of be related to the fact that some dolphins and bats have been
technical textiles like functional surfaces, camouflage, using sound for communication and object detection for
structural colour, thermal insulation, dry-adhesion, etc. millions of years5. The flawless designs in birds have an
Thus, biomimicry can be an inspiration to develop inno- enormous influence on the development of aviation. Indeed,
vative textiles. This article reviews some of the impor-
the Wright brothers, regarded as the inventors of the
tant technologies of nature relating to textiles.
airplane, used the vultures wing as a model for building the
Keywords: Biomimicry, fibres, spinning, textiles, weaving. wings of their Kitty Hawk plane6. Lifestyle, culture, and
religion of early human civilizations were entwined with
LIFE evolved on Earth about 3.8 billion years ago. A bil- nature. These pre-industrial societies relied on nature to
lion years of evolution resulted in the transformation harvest crops, produce medicine, provide clothing, build
from simple, single-celled prokaryotic cells such as bac- shelter and clean up waste. In contrast, todays society
teria to multi-cellular organisms. Arthropods, plants, fish, depends on industrial manufacture7. Biomimicry will play
etc. evolved after a long process. Animals, plants, insects a great role to achieve this.
and microbes are still evolving to be compatible with Textiles are an indispensable part of human civiliza-
nature. They have been trying to optimize every part of tion. Humans have been using textiles from prehistoric
their body and every action they undertake to survive in age. Although more intelligent than animals, humans
nature, and the process is still ongoing. Evolution is a found themselves inadequately protected from a variety
continuous process1. Over the years life has developed of adverse environmental conditions. The prehistoric
techniques such as structural strength, self-assembly, ma- humans used leaves, tree-barks, feathers, animal hides,
terial recycling, self-cleaning, self-repair, energy conser- etc. to protect themselves against the environment or en-
vation, drag reduction and dry adhesion to survive. These hance their aesthetic appeal. Fabrics were being produced
techniques have inspired humans to achieve outstanding long before the recorded history. Even 20,000 years ago,
outcomes. For example, the idea of weaving may have humans were twisting fibres together to make thread and
originated from the nest of a weaverbird, the strength and string (the oldest preserved string is from Lascaux caves
stiffness of the honeycomb structure may have led to its in France, aged circa 15,000 BC). The first garments made
adoption for use in lightweight structures in aircraft and were probably string skirts, zostras, used to advertise a
many other such applications2. Biomimicry is a word womans fertility8 . Egyptians made linen fabric around
derived from the Greek words bios meaning life and 5500 BC (ref. 9). Initially, humans started using textiles
mimesis meaning to imitate. It is not new, as nature is for protection purposes. Thereafter, textiles became fash-
the greatest teacher for the human race. The word bio- ion, art and design items.
mimicry has been popularized by the scientist and author This review explores the application of biomimetics in
Janine Benyus in her book Biomimicry: Innovation textiles. The exploration begins with a general overview,
Inspired by Nature. Biomimicry has been defined in the followed by a historical perspective it describes some
book as a new science that studies natures models and ongoing and future efforts in biomimetic textiles.
then imitates or takes inspiration from these designs and
processes to solve human problems. Benyus has sug- Learning from nature
gested looking at nature as a model, measure, and men-
tor and emphasized sustainability as an objective of Natural fibre provider of structural integrity
biomimicry. Although the science of biomimetics has
We are bestowed with so many natural fibres. The com-
*For correspondence. (e-mail: sekhar.tex@gmail.com) mon natural fibres from plants are cotton, jute, hemp,
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ramie, sisal, etc. which are cellulosic in nature. Animal- extracellular matrix17. The protective grain layer gets its
based fibres are wool, silk, hair, etc. which are protein- protective nature due to its fine, tough and fibrous struc-
based. Fibres are used for enhancing the strength and ture18. Man learned from nature, even from the earliest
integrity of structures created by nature itself. Many times, the concept that combining materials could be ad-
natural structures are composite in nature, i.e. they are vantageous. The procedures of wattle-and-daub (mud and
made of a combination of two or more materials that straw) and pide (heather incorporated into hard-rammed
results in better properties than those made of a single earth) building construction, still in use today, pre-date
component only10. Fibre is an important, strength- the use of reinforced concrete by the Romans that fore-
providing component of the composite material, because shadowed the pre-tensioned and the post-tensioned rein-
fibre is used as reinforcement material in polymer matrix forced concrete of our own era11. Li et al. 19 copied the
composite so as to enhance the mechanical properties of hollow, multi-layered and spirally wound bastfibre
the polymer. We have adopted the composite material arrangement of bamboo structure and prepared a bio-
concept from nature in many man-made applications. mimetic reinforcing model, which is a double-helical
Wood is one of the best example of natural composite structural model providing the optimum comprehensive
material, consisting of cellulose fibres embedded in lig- mechanical properties.
nin matrix. Wood and bamboo also have fibrous structure We are aware that the prime needs of man are food,
providing strength, for which they are famous. Bamboo clothing, shelter and fuel. The word textile comes from
has a multi-scale, hierarchical and functionally graded the Latin word textilis and the French word texere.
structure. In macro-scale, the structure consists of a hol- Leaves from tree, tree barks, feathers, animal hide, etc.
low tube with micron-scale fibre bundles that are organ- were used by prehistoric humans for protection from
ized into functionally graded structures. In micro-scale, cold, heat, wind, etc. and clothing methods were used to
the individual fibres are perfectly organized into fibre enhance their aesthetic appeal2. From these materials,
bundles in a lignin matrix. Researchers have found that they learned that fibre is the basic unit of any protective
the unidirectional, compact reinforcement of cellulosic gear. The earliest fibres used in textiles were linen, hemp,
fibres in lignin matrix is primarily responsible for the nettle, willow, wool, etc. Linen perhaps was the first
high strength of bamboo. Dry wood is primarily composed textile to be manufactured by the Indians and Egyptians
of cellulose, lignin, hemicelluloses, and minor amounts as early as 2800 BC. It was the Japanese who understood
(510%) of extraneous materials. Cellulose, the major the weaving of linen, gold, silver and silk20.
constituent is approximately 50% of wood by weight.
During the growth of the tree, the cellulose molecules are
arranged in ordered strands called fibrils, which in turn Fibre spinning
are organized into the largest structural elements that
make up the cell wall of wood fibres11. The chemical Nature is the inspiration for spinning continuous strands
structure of bamboo fibres is similar to that of wood. of synthetic fibres. Silk, one of the oldest known natural
Their fibre length varies from 1 to 5 mm (with an average fibre to human civilization, is a continuous protein fibre
of 2.8 mm) and diameter from 14 to 27 m with an aver- produced by the silkworm. There are two main types of
age of 20 m (ref. 12). In most cases, the fibres are ar- silkworm: mulberry silk (Bombyx mori) and wild silk, of
ranged or oriented in a particular manner to impart the which Tussar silk is the most important representative21.
desired mechanical properties of the structure. Fibres can Very high quality silk fibre is also produced by some spi-
be a part of both the primary and secondary plant body. ders belonging to the Arachnida family. There are over
Fibres are primarily responsible for mechanical support 34,000 species of spiders in nature and most of them are
for the tree being both hard and flexible, but when alive, capable of spinning task-specific silk of varying mechanical
they can also serve as a storage medium13. Some other properties22. Some spiders, specifically the orb-weaving
natural composites are bone, teeth, dentin, cartilage, skin, Araneid and Uloborid spiders can produce silk fibres with
mollusc shells, etc. where nature combines hard ceramic very high mechanical properties. Orb-web-spinning spi-
reinforcing phases with natural organic polymer matri- ders produce fibres with mechanical properties un-
ces14. Another example is the bone, a highly complex and matched in the natural world and comparable with the
well organized organ that refers to a family of remarkable very best synthetic fibres23. Spider silk is considered as a
hierarchical structures with different motifs which are all wonder fibre for its unique combination of high strength
constructed of a basic building block, the mineralized and breaking elongation. An earlier study indicated that
collagen fibril 15. Reinforcing fibrous assemblies of pep- the spider silk has strength as high as 1.75 GPa at a
tides and proteins are the basic structure of biology, breaking elongation of over 26%. With toughness more
where they perform a variety of functions16. Skeletal than three times that of aramid, i.e. the industrial fibre
muscle structures consist of hundreds of thousands, and used for making bullet-proof vests and other high-impact
sometimes millions of long and multinucleated fibres applications, spider silk spinning mechanism to be a mys-
organized together in a particular direction by an tery to the fibre scientists and hobbyists24. The man-made

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fibre spinning system is an imitation of the silk-spinning (Figure 1), through which the spin solution can be ex-
system. The history of the development of man-made truded to form a bubble at the apex of each tube29. Sur-
fibre production tells us the story. The first patent for face tension of each bubble is so small that it could be
artificial silk was granted in England in 1855 to a Swiss spun into nanofibres by exerting a small force, either by
chemist named Audemars, who dissolved the fibrous the spiders body weight or tension created by the rear legs.
inner bark of a mulberry tree, chemically modifying it to To mimic the spinning process of the spider, the bubble-
produce cellulose. He formed threads by dipping needles electrospinning process was developed to produce nano-
into this solution and drawing them out but it never fibres. In this process polymer jets are ejected from the
occurred to him to emulate the silkworm by extruding the bubble formed from the highly charged aerated polymer so-
cellulosic liquid through a small hole. After almost eight lution. The charges get accumulated on the bubble surface
decades, in September 1931, the American chemist Wal- in the presence of an electric field. Once the electric field
lace Carothers reported on research carried out in the exceeds a critical value needed to overcome the surface
laboratories of the DuPont Company on giant molecules tension, a fluid jet is ejected from the apex of the conical
called polymers, particularly a polymer he referred to bubble. Subsequently, the jet solidifies into a nanofibre30.
simply as 66, a number derived from its molecular
structure. So, the Nylon was born. Nylon was the first
Weaving
commercially successful fibre to be mass produced using
the silkworm method of fibre production, i.e. melting the
Weaving is a process by which threads or continuous
fibre and passing it at high pressure through a small
strands of any substance are crossed and interlaced so as
orifice and then solidifying it25. Thereafter, several man-
to be arranged into a perfectly expanded form, and thus
made fibres have been invented, e.g. polyester, polyeth-
be used for covering human or other bodies. The techno-
ylene, polyacrylonitrile, polypropylene, Kevlar, etc.
logy of weaving was invented many years ago. Fabrics
Today, numerous man-made fibres are available in the
were being produced around 20,000 years ago31. The
inventory of a textile designer, which can meet exacting
baya weaver (Ploceus philippinus) is a weaverbird found
functional requirements for use at home or in space explo-
across South and Southeast Asia. The male weaverbird
ration. This is all possible because nature has made silk
constructs its nest using fibrous materials. They weave
first and shown us how to make a long continuous fibre.
the leaves and other nesting materials to produce a
Kevlar fibres are made from lyotropic liquid crystalline
strong nest. The baya weaver might be the possible inspi-
polymer26. In nature, spiders and silkworms spin continu-
ration for human weavers. The weaverbird nest is usually
ous fibres by liquid crystalline protein which is passed
15 cm long and 12 cm high and is often suspended from a
through their spinnerets. These fibres have high strength
branch. The weaverbirds weave the outer shell of the
and toughness, which have attracted tremendous interests
nest progressively, stage by stage. These include con-
of researchers in various disciplines for a long time to
struction of the initial attachment, ring, roof, egg cham-
learn the mechanism of silk spinning to produce artificial
ber, antechamber, entrance and entrance tube. The initial
high-performance fibres resembling spider silk fibre27.
attachment is constructed by first holding the initial
Many attempts have been made to produce artificial spi-
strip under its foot against a twig, then looping it back
der silk by mimicking the spinning process of the spider.
and alternately reversing the winding of the strip between
But so far a comprehensive understanding of the molecu-
the twig and the strip itself, which is similar to nipper-
lar processes which occur during spinning of protein
ing a knot, a type of fastening used today to lash
fibres and the investigation of how the spinning condi-
tions affect the properties of the final material are lack-
ing. It is still a mystery how native silk fibres are
produced with a minimal force by the spiders; unlike the
man-made fibres formed from spin solution, which
requires very high pressure or a large drawing force.
By mimicking the spinning process of the spider, Spi-
nox Ltd (Oxford University) has developed a biomimetic
rig into which protein dope is fed and from which spun
fibre is drawn. The biomimetic process at present typi-
cally suffers from severe die swell problems and fibres
obtained by this process are more brittle and less stronger
than spider silk. To combat the die swell problems, Spi-
nox now wants to model the spinning process in the spi-
der and compare it with the biomimetic ring to achieve
internal draw down 28. Spiders have silk-producing spin- Figure 1. (Left) An electron microscope image of finger-like spinnerets
nerets consisting of a great number of nanoscale tubes of a spider. (Right) Biomimetic rig developed by Spinox.

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REVIEW ARTICLES
two parallel ropes together. The initial attachment is then small (0.20.5 mm), very fine and regularly spaced (30
developed into a roughly vertical ring that provides the 100 m) longitudinal ridges, similar to riblets 34.
basic support for the whole nest (Figure 2). Fast swimming produces vertical vortices or spirals
On the basic support, the male weaverbird weaves its of water, keeping it closer to the sharks body, thereby
nest using a certain basic set of movements. The bird uses reducing the surface drag. These micro-scale ridges influ-
its beak to first seize a strip of nest material near one of ence fluid flow in the transverse direction, thereby limit-
its ends, then with a vibratory motion it pokes the end of ing the degree of momentum transfer. The ratio of scale
the strip into the bulk of the nest. Once the strip sticks in height to tip-to-tip spacing has a critical role in reducing
the nest, it releases its grip, moves its head to the other the longitudinal and transverse drags. A variety of shark
side of the nest mass and grabs the end of the strip again skin mimicking engineered materials find a variety of
and performs a similar action from the opposite direction applications, for example, riblets are fine, rib-like surface
of the nest. In this way it stitches its nest using its beak. geometries with sharp surface ridges that can be aligned
The woven design of the entrance tube consists of inter- either parallel or perpendicular to the flow direction and
lacement of two sets of yarns at 45 angle, which pro- might reduce drag35.
vides the best resistance to the shear stress generated In the beginning of mid-1980s, vinyl-film saw-tooth
when the bird hangs from one side of the entrance at the riblets were applied to the hulls of racing boats. They
bottom of the tube during nest-building. Weaverbird uses have also been used on hulls of ships. They find applica-
stitching, knotting and weaving actions during the build- tions in aircraft industries for reducing drag. Another
ing of its nest. In this way, its actions are similar to the large, commercially used riblet technology is to reduce
weaving and knotting process adopted by human32,33. drag in liquid flow through pipes36. Probably, the most
Thus observing the similarity in the weaving of present- successful commercial application of riblet surface mor-
day plain fabric, it can be inferred that the concepts may phology is in Fastskin swimwear technology (Speedo,
have developed by mimicking the construction mecha- Inc.) developed in 2004. A drag reduction by several
nism adopted by the weaverbird. per cent was observed compared to other race suits in a
static test. This mimicking of micro features of shark
scales is used for designing swim suits with new fibres
Shark skin effect
and weaving techniques37,38. The shark skin also impedes
bacterial growth, thereby acting as antibacterial fouling
Shark is one of the fastest swimmers in water. For swim-
surface inhibiting the growth of microorganisms on such
ming at great speed it is important to lower the frictional
grooved surface. Mimicking shark skin, a product called
drag of the skin of the shark against water. So nature has
Sharklet was manufactured by Sharklet Technologies. It
provided it with such a technology and it has evolved
is a sheet of plastic with a microscopic texture that
over millions of years. The sharks skin protects it against
impedes the growth of bacteria. It is being manufactured
biofouling and reduces the drag experienced as it swims
for use in hospitals, restaurants, and other places where the
through water. The skin of shark is rough and covered by
potential spread of bacterial infections creates a hazard.
minute placoid scales, also called dermal denticles. Under
Coating surfaces with Sharklet is seen to greatly reduce
a microscope, it can be observed that the shark skin is
the growth of bacterial colonies, due solely to the nano-
covered with small V-shaped bumps made from the same
scale structure of the product. The topography of Sharklet,
material as its teeth (Figure 3). The scales are regularly
aligned along the axis of the body; they are particularly

Figure 2. (Left) Weave structure of a weaverbird nest. (Right) plain Figure 3. (Left) SEM of shark skin and (Right) Fastskin Fsii (FS2)
weave structure. swim suit, mimicking shark skin.

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having ridge and ravine like qualities creates mechanical clinging ability. Geckos, in particular, have developed the
stress on the settling bacterium, a phenomenon known as most complex adhesive structures capable of smart adhe-
mechanotransduction. The theory is that nanoforce gradi- sion with the ability to cling to different smooth and
ents caused by variations in topographical features will rough surfaces, and also detach at will. Their feet contain
induce stress gradients within the lateral plane of the millions of very fine hairs which can create dry adhesion
membrane (plasma membrane) of a settling cell or micro- to smooth and rough surfaces. These animals make use of
organism during initial contact39. about three million microscale hairs (about 14,000 mm2)
that branch off into hundreds of nanoscale spatulae. This
Hook-and-loop fastener hierarchical surface construction provides the gecko the
adaptability to create a large real area of contact with
Hook-and-loop fasteners are generally made of two strips, surfaces. van der Waals forces are the primary mecha-
one with loops that hook onto the other strip. When nism utilized to adhere to surfaces, and capillary forces
the two components are pressed together, the hooks catch are a secondary effect that can further increase the adhe-
in the loops and the two pieces fasten or bind temporarily. sion force41. The foot of a Tokay gecko (Gekko gecko)
The hook-and-loop fasteners have been used for just has about 5000 setae mm2 and can produce 10N adhesive
about every conceivable application where a temporary force with approximately 100 mm2 of pad area42. Despite
bond is required. It is especially popular in clothing such strong adhesive forces which would hinder the
where it replaces buttons or zippers, as a shoe-fastener, in movement of the gecko, this lizard has developed a
hand bags, etc. The hook-and-loop fastener was invented unique technique of walking by curling its toes for
by Swiss engineer, Georges de Mestral in 1941 (ref. 40). attachment and peeling during detachment to eliminate
There is a story behind this invention. One day when he the forces between its foot and the surface, thereby ena-
was returning from a hunting trip with his dog in the bling it to move with ease. Scientists have been inspired
Alps, he observed the burrs (seeds) of burdock that kept by the clinging ability of geckos and many attempts have
sticking to his clothes and his dogs fur. He examined the been made to construct the surface structure like gecko
seeds under a microscope, and noticed that they contained feet with man-made materials in order to achieve dry
hundreds of hooks which could fasten with loops, such as adhesion. Synthetic gecko foot fibres have been created
clothing, animal fur or hair (Figure 4). He was inspired by nanomoulding using silicone, polyimide, polyvinyl
by this and invented hooks-and-loop fastener. Mestral siloxane and polyurethane and carbon nanotubes43,44. A
saw the possibility of binding two materials reversibly in team of polymer scientists and a biologist at the Univer-
a simple fashion, if he could figure out on how to dupli- sity of Massachusetts Amherst have developed artificial
cate the hooks and loops. This inspiration from nature or Geckskin, Crosby has reported that Our Geckskin device
the copying of natures mechanisms is viewed by some is about 16 inches square, about the size of an index card,
like Steven Vogelor Werner Nachtigall as a key example. and can hold a maximum force of about 700 pounds
while adhering to a smooth surface such as glass45.
Dry adhesion gecko-feet
Lotus effect
The gecko has a unique clinging ability; it can create dry
adhesion using its amazing feet. Several creatures,
including insects and spiders, have also developed unique Lotus effect observed in nature: Evolution has opti-
mized the wettability of different animal and plant sur-
faces for different purposes. The wetting nature of different
natural surfaces ranges from hydrophilic to super hydro-
phobic. Some of the natural surfaces are so hydrophobic
that water droplets can roll over them without wetting the
surfaces. The classic example of this kind of surface is
the lotus leaf surface and the phenomenon it is called
lotus leaf effect (Figure 5). Other examples of such sur-
faces are rose petals, duck feathers and butterfly wings.
The super-hydrophobicity of their surfaces generates self-
cleaning properties, i.e. when the water droplet rolls over
the surface, it takes away all the dirt on the surface, i.e. it
self-cleaning. To investigate the reason for the lotus leaf
Figure 4. (Left) Arctium lappa, the capitula surrounded by an involu- effect, the surface of lotus leaf was observed under elec-
cre made out of many bracts, each curving to form a hook, allowing tron microscope. Though to the naked eye, lotus leaf is
them to be carried long distances on the fur of animals. (Right) Velcro,
the brand name for fabric hook-and-loop fasteners, which is a bio-
clean and smooth, on a nanoscale it is not so. On the con-
mimic material of A. lappa. trary, it is rough due to papillose epidermal cells that
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form the papillae or microasperities. In addition to the applied to surfaces, and if the chemistry of the surface is
microscale roughness, the surface of the papillae is also hydrophobic, a real self-cleaning effect can be achieved.
rough. The nanoscale roughness is created by three- The effective surface contact area of dirt particles is ex-
dimensional epicuticular waxes, which are long-chain tremely minimized by the surface structure and thus, ad-
hydrocarbons that are hydrophobic. So the lotus leaf hesion is very low. When a drop of water rolls over such
surface basically consists of systematically arranged a surface, dirt particles are removed. Because of the
three-dimensional nipple-like structures made of nano- roughness of the surface and the low contact area, the ad-
sized wax crystal forms, which are no greater than a few hesion energy of the particle to the solid surface is very
nanometres in size, but are water-repellent46. This super- low48.
hydrophobicity of a surface is dependent on two impor-
tant factors. First, a low surface energy and chemical Lotus effect mimicry in textiles: Natural lotus effect
composition of the solid surface and secondly, a high de- phenomenon is useful as far the application in textile
gree of surface roughness. This rough structure on the materials is concerned. If this can be imitated in textile
surface of the lotus leaves causes a reduced contact area materials, then it can produce a whole range of products
with water. The water penetration is prevented by the like umbrella, rainwear, carpets, upholstery, protective
presence of the hydrophobic nano-sized wax crystals47. clothing, sportswear, automotive interior fabrics, etc. and
As a result, the water forms droplets and rolls over the even self-cleaning garments. In this regard, the first
surface. patent was filed on hydrophobic textiles in 1945; an alkyl
silane was used in hydrophobic textile materials49.
Characteristics of hydrophobicity and lotus effect: The Hydrophobic properties of a surface can be achieved by
primary parameter that characterizes wetting of a surface the use of nonpolar hydrophobic agents such as paraffin
is the static contact angle, which is defined as the angle wax, silicones, silanes and fluorinated polymers50. How-
that a liquid makes with a solid. The contact angle ever paraffin wax, silicones and silanes only make the
depends on several factors, such as the surface energy, textile surface waterproof, which is uncomfortable in the
surface roughness and its cleanliness. If the value of the case of apparels. A variety of fluorine-based polymers are
static contact angle is 0 90, the liquid wets the popular for this purpose because of their high water and
surface, whereas if the liquid does not wet the surface, oil resistance, organic solvent resistance and lubricity.
the value of the contact angle will be 90 180. Because of these advantages, fluoropolymers have been
Surfaces with high surface energy, formed by polar mole- used in the textile industry since the 1960s (ref. 51). The lo-
cules, tend to be hydrophilic, whereas those with low en- tus effect phenomenon was first studied by Dettre and
ergy and built of non-polar molecules tend to be Johnson in 1964 using glass beads coated with paraffin or
hydrophobic. Surfaces with a contact angle less than 10 PTFE telomer. They created a microscopic rough surface to
are called super hydrophilic, while those with a contact an- generate the hydrophobic surfaces and developed a theo-
gle between 150 and 180 are called super hydrophobic. retical model52. Recent approaches to this kind of finishing
Wettability of a surface mainly depends on two factors: (i) include achieving nanomicroscale surface topography by
the surface free energy, and (ii) the surface roughness. The nanoparticles attached to the fibres that increases surface
self-cleaning property of a surface depends on its smooth- roughness. Silicate and fluorocarbon nanoparticles are
ness extremely smooth surfaces show a reduced soiling used in commercial level for this purpose. It has been re-
behaviour, because the particles have only low mechanical ported that the super hydrophobicity can be achieved on
hold and can be removed either by air or liquid. When cotton fabric using a homogeneous silicacopper hybrid
overlapping structures with dimensions of a few micro- nano-composite47. Joshi et al. 53 used nanosilica and nano-
metres and superposed structure of 50100 nm are clay along with a surface tension lowering agent to get a
lotus effect on cotton fabric. Lauryl acrylate has been
used as hydrophobic monomer and malic anhydride as re-
actor to produce durable, nonfluro hydrophobic finish on
cotton fabric54. Ramaratnam et al.54 reported the prepara-
tion of ultrahydrophobic polyester textile surface using a
mixture of polystyrene grafted layer with silver nanopar-
ticles. Here grafting lowered the surface energy of the
substrate and nanoparticles increased roughness of the
surface55. Plasma etching at different atmospheres also
provides hydrophobic rough surface. In this regard,
Twardowski and Makowski56 reported that super hydro-
phobicity can be achieved on polyester fabric using the
Figure 5. Lotus effect on the surface of a lotus leaf (left) and hydro- argon plasma etching method. Researchers have prepared
phobic surface (right). hydrophobic etched polypropylene by argon atmosphere
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plasma treatment in the presence of polytetrafluoro ethyl- battlefield was introduced. Beginning with the British
ene gas. The hydrophobicity of electrospun PVA fabrics Armed Forces, various other militaries changed the
can be achieved using SF6 plasma treatment. The applica- colours of their clothing predominantly to ones that
tion areas of these fabrics are biomaterial, filtration and blended in, more with the terrain, such as khaki or olive
medical devices57. Recently, hydrophobic textiles have drab. That was the reason why olive green shaded hues
been made by etching titanium dioxide-coated layer of became significant in military clothing. Camouflage
the fabric with CF4 plasma treatment58. fabrics are used for hiding soldiers and military equip-
ment, and now are one of the main components of
warfare. The main functional requirement for such fabrics
Camouflage for military use includes not only the physical aspects
like resistance to various environmental conditions,
Nature is a deadly battlefield of hide-and-seek between water, wind, fire, heat and specific battlefield threats, but
prey and predator. Both try to conceal their identity or also the camouflaging requirements62,63. The major
visibility from each other so that they can survive. Some design requirement of a camouflaged fabric is to obtain a
animals have developed special skills to hide in the envi- colorimetric match to its anticipated surrounding. This
ronment they live by having special colours, texture and match needs to cover both the visible and other colours of
patterning on their bodies that help them to conceal their the spectrum, as is used in silicon-based surveillance sen-
presence. This phenomenon of blending with the envi- sors, such as image intensifiers, low-light TV, and both
ronment is called camouflage. So, camouflage plays a near and infrared (IR) devices. Modern camouflaged
vital role in the struggle of surviving of living beings. garments should be able to provide protection not only in
There are many ways to camouflage. They vary from the visible range, but also in a wide spectral range, in-
species to species. The most common techniques are: (i) cluding UV, near IR, far IR, radar and acoustic ranges.
crypsis, where the animal blends with the background; The camouflaging technique uses chosen shapes and col-
(ii) disruptive coloration; (iii) self-decoration with mate- ours to produce perfect harmony with the surrounding.
rials such as twigs, sand, or pieces of shell from their liv- The modern military forces use combat uniforms, that not
ing environment; (iv) changing skin pattern and colour, only break-up the outline of the soldier during the day-
and (v) Mimesis. The most common camouflage tech- time, but also provide a distinctive appearance that makes
nique, however, is by changing the skin colour. The skin it difficult to detect them with light amplification devices,
colour, texture and patterning are important for conceal- such as night-vision devices64. Nanotechnology has made
ment. But most of the camouflage techniques get nullified it possible to develop military clothes that can change
by movement of the species. Hence, active camouflage pattern and colour with the change in environment.
is more effective. Some animals achieve active camou- Chameleonic camouflage allows the soldier to become a
flage by both colour change and counter illumination. mirror of his surroundings65. Currently, conventional
One of the examples of such type of camouflage is that of colour and pattern type of camouflage is used by the
the coleoid cephalopods (octopus, squid, cuttlefish). They infantry in reconnaissance and infiltration operations. The
can easily hide themselves in colourful coral reefs, tem- modern dismounted soldier may be carrying any or all of
perate rock reefs, kelp forests, sand or mud plains, sea- a night-vision sight and/or goggles, thermal-imaging
grass beds and other environments by rapidly adapting sight, personal role radio and combat net radio, laser
their body pattern. Although most examples of animal rangefinder, laser designator and laser weapon pointer,
camouflage involve body colouration or patterning, deco- noise cancelling unit, IR and visible beacons, electronic
rator crabs in the brachyuran superfamily Majoidea (ma- countermeasures, global positioning and/or blue force
joids) are a large and diverse group of crabs, best known tracking and camouflaging should be effective against all
for a distinctive form of decoration camouflage. They of these66. To hide in near-IR light, low-emissivity paints
attach materials from the environment to specialized are used in fabrics that emulate the near-IR reflection of
hooked setae on their body5961. Humans have tried to use vegetation, rocks, sand and soil of the intended environ-
this kind of camouflage from prehistoric times. ment. The MAYA suit imitates its intended environ-
Human civilizations have adopted camouflage tech- ment by specially designed shapes, shades and colours.
niques, mostly for hunting or military purpose. But The texture of the suit also disrupts the revealing con-
camouflage has also influenced other aspects of society, tours of a human body.
for example, arts, popular culture and design. Throughout The MAYA suit has multispectral properties and pro-
the 18th and the 19th centuries, due to the prevalence of vides protection from visual detection, day and night vi-
non-accurate weapons on the battlefield, military clothes sion devices, and thermal sensors and cameras. It also has
included bright and high-contrast colour arrangements to two-side camouflage for different battlefields67. An object
enable distinction between different units. However, with can be effectively concealed from electromagnetic radia-
the growing use of accurate weapons, since 1880s, adop- tion detection by placing a reflecting or absorbing surface
tion of some form of camouflaging the soldiers in the on it. Conventional radar absorbing materials (RAMs)
CURRENT SCIENCE, VOL. 109, NO. 5, 10 SEPTEMBER 2015 899
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