BIOMIMETICS: ADAPTING PERFORMANCE AND FUNCTION OF NATURAL MATERIALS FOR BIOBASED COMPOSITES

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STATE OF THE ART PAPER
BIOMIMETICS:ADAPTING PERFORMANCE AND FUNCTION
OF NATURAL MATERIALS FOR BIOBASED COMPOSITES
Scott Renneckar*{
Associate Professor
Department of Sustainable Biomaterials
Virginia Tech
Blacksburg,VA 24061
E-mail:srenneck@vt.edu
(Received August 2012)
Abstract.Natural materials may serve as an excellent template for the design of high-performance
manufactured materials.Because natural materials are fabricated at standard temperature and pressure
frombenign chemicals,adapting or mimicking nature’s design methods and structures offers the potential
to enhance performance,lower energy requirements,decrease toxic chemical accumulation,and create
materials with life cycles that better correspond with the environment.As the forest products industry has
matured,adapting naturally occurring designs has meant potential innovation for commodity markets,
decreasing manufacturing inputs (energy,carbon,or other materials),and enhancing product per-
formance.This study reviews nature-inspired pathways for biocomposites to 1) enhance the specific
mechanical properties,2) assemble materials fromaqueous systems,3) create hybrid inorganic–biopolymer
composites,and 4) develop functional hydrophobic coatings and photonic colored films.To achieve
controlled architectures found in natural materials,innovative production technologies that allow for timed
processes and self-assembly must be applied to biobased composite manufacturing.
Keywords:Biomimetics,biomimicry,biocomposites,foams,polymer composites,biomineralization,
iridescence.
INTRODUCTION
Scientists have recognized that motifs found in
natural materials offer the organism a level
of functionality and performance that is very
difficult to reproduce in synthetic materials
(Bhushan 2009).The study of the structure and
composition of natural materials,at varying
length scales,can serve as a guide to enhance
the properties of manufactured materials (Bond
et al 1995).After unlocking these natural
“secrets” that offer functionality,the archi-
tectures usually can be duplicated in the labora-
tory,achieving similar performance from the
resulting material,eg synthetic analogs to the
gecko foot.The field of biomimetics,loosely
defined as adapting the design process from
naturally occurring structures and mechanisms
(eg texture of a lotus leaf for superhydro-
phobicity or the structure of spider dragline silk
for strong fibers),can significantly impact the
manufacturing of composite materials (Reed
et al 2009).This approach is paradigmchanging,
shifting away from iterations of tweaking cur-
rent materials and manufacturing processes for
slight improvements.Biomimetics as a design
tool focus on the structure or process that was
optimized across millennia for the needed per-
formance of the organism.Because this perfor-
mance was critical to the organism’s survival,
failed designs disappeared.Determining the
mechanisms of how biopolymer materials have
been able to meet performance with unique
micro- and nanoarchitectures,augmented by
incorporation of minerals in some cases,provides
* Corresponding author
{ SWST member
Wood and Fiber Science,45(1),2013,pp.1-12
#
2013 by the Society of Wood Science and Technology
a pathway toward rational composite design
based on optimized models.
COMPOSITE DESIGN
Assembling Lightweight Structured
Polymer–Polymer Materials as in Nature
Natural materials possess excellent mechanical
properties relative to their mass.Hence,one
physical property that differentiates natural mate-
rials from synthetic materials is density.Natural
materials are light compared with the majority
of materials from the industrial revolution
(bricks,engineering steel,and glass).Glass and
metals,which make up a large percentage of
society’s materials,have densities between 2
and 20 g/cm
3
.Soft tissues found in natural mate-
rials (ie spider silk,collagen,keratin,wood)
have a fraction of the density of these materials,
typically about 0.3 to 1.5 g/cm
3
.Natural mate-
rials such as plants are light because they are
derived fromlight reactants such as carbon diox-
ide,water,and gaseous nitrogen compared with
silicon,iron,and aluminum.Also,many natural
materials such as wood,bone,and even beak are
extremely light because of their cellular nature.
The toucan beak is more than 1/3 the length of
the bird,however the foamed keratin structure
only weighs 1/20 of the bird’s mass (Seki et al
2005).Lighter beaks and bones require less
energy input from the organism for locomotion,
just as weight decrease of a vehicle enhances
fuel efficiency.
Ordered structures within the cellular walls of
natural materials contribute to the high perfor-
mance of these naturally occurring foams (Fratzl
and Weinkamer 2007).Typically,synthetic poly-
mer foams (ie polystyrene foam) do not achieve
the same ordered structure found in natural
materials.For example,cellulose microfibers
are oriented within the secondary cell wall of
woody tissue.To mimic in-plane orientation of
the cell wall,Svagan et al (2007,2008) were
able to use a simple method involving lyophili-
zation based on an aqueous suspension of
cellulose microfibers and starch to form highly
ordered foams.This structure is similar to the
cell wall with fibrils ordered within the plane of
the ligament (wall of the cellular foam),albeit
without controlled axial orientation as in wood
cells.The method was used with material
combinations of nanoscale clays and cellulose
(Gawryla et al 2008).As the suspension of nano-
particles freezes,ice crystals begin to exclude
solute (or dispersed particles) resulting in the
orientation of these materials between ice crys-
tals (Gawryla et al 2009).These studies show
effective ways to build cellular structures in
processes that can produce bulk-scale materials
with intimate control of structure.In addition to
decreasing weight,the high percentages of void
volume (99.5% in some cases [Sehaqui et al
2010]) provide the potential for excellent ther-
mal insulation properties.The key to the bio-
mimetic design of cellular materials is to have
multiple levels of order from the microscopic
cellular nature to the nanoscale organization of
nanoparticles,as demonstrated by the lyophi-
lization method (Svagan et al 2010).Another
technique uses templating of microspheres to
form foams with ligaments with ordered nano-
particles (Zhang and Cooper 2005).In a simpli-
fied model,the expanding cell lumen has a
cytoplasmic membrane with a secondary wall
templated on the interior of the primary wall.
It is actually common practice to template sin-
tered spheres and remove the template to cre-
ate low-density lightweight materials.This
method provides a greater degree of control
of the microscale structure within the foam.
Blaker et al (2010) used an approach that com-
bined both lyophilization and templating;bac-
terial cellulose nanowhiskers were mixed with
polylactic acid and subsequently templated on
the surface of ice microspheres.After solvent
removal,the samples were lyophilized and
the 3-D structure with controlled pore size
was isolated.
These foams used biobased nanoparticles that
contain inherent organization at multiple-length
scales.These levels of organization are assem-
bled from the molecular level up to the micro-
scale level.Correspondingly,this scale of order
2
WOOD AND FIBER SCIENCE,JANUARY 2013,V.45(1)
covers the detail related to polymer structure.
Structural polymers within organisms are
derived from building block intermediates such
as carbohydrates and amino acids that repeat in
a way that allows for specific agglomeration,
folding,and assembly.These natural polymers
are simple in composition because they contain
few elements—plant-based polymers only have
carbon,hydrogen,and oxygen in the majority of
their structures,whereas protein-based materials
also have nitrogen,sulfur,and occasionally sele-
nium.However,although simple in composi-
tion,there are many different ways to assemble
these atoms,providing a complex array of pos-
sible primary structures.For example,starch,
cellulose,and pullulan are polysaccharides
derived from the building block glucose.Their
linkage types are different for these glucans cre-
ating a difference in structure.Nature has
narrowed down the linkages that offer the
highest degree of structural reinforcement and
the linkages that lead to easy breakage to access
glucose as an energy reserve.Hence,the molec-
ular structure (bonding patterns of the atoms)
within the polymer dictates the organization that
can be used for structural polymers,such as
cellulose,or energy reserve polymers,such as
starch.Cellulose molecular structure dictates
assembly into fibrils with its cellulose chains
aligned along the particle axis for full axial rein-
forcement.For cellulose,individual chains are
assembled in sheets as they leave the terminal
complex synthase.These sheets are stacked
together to form ordered fibrils with defined
symmetry (Doblin et al 2002;Saxena and
Brown 2005).Tendon,a structured collagen
material,has a similar hierarchical structure in
which three tropocollagen polymer chains are
assembled into larger microfibril structures.
These fibrils are assembled into larger ordered
fibers at different stages and are oriented along
the axis of the tendon (Baer et al 1992).This
structure is significant because it allows for
self-reinforcement,orientation,and prevents
crack propagation (Brown et al 2012).
Natural polymers themselves do not have spe-
cial properties except inherent stiffness because
of ring structures (ie pyranose and phenol
propanol) and chemical functional groups (ie
amines,carboxylic acids,and hydroxyls).In
fact,reprocessing these materials will degrade
the majority of the performance characteristics
found in the natural material—recombinant spi-
der silk does not give tenacity equal to drag-line
silk (Lazaris et al 2002),keratin does not have
the same strain to failure as a feather (Athamneh
et al 2008),and the ultimate tensile breaking
strength of rayon fiber (cellulose) is much less
than that of a cellulose wood fiber (Eichhorn
et al 2001).These examples reveal that perfor-
mance is not dictated solely by composition.
Instead,additional levels of order are required
for enhanced performance in materials.In these
materials,structure is created in cellular envi-
ronments that involve highly organized events
in which intermolecular interactions play a key
role in assembly of secondary and tertiary struc-
tures of the polymers.
One manufacturing process for developing com-
posites with multiple levels of order is to begin
with natural polymers already structured and
only partially modify the structure during manu-
facturing.A simple route is to only partially
dissolve semicrystalline materials;the crystal-
line cores are not disrupted,but surface chains
are modified creating moldable materials.This
technique was used to create all-cellulose com-
posites from bacterial cellulose that had tensile
strengths of 411 MPa and stiffness of 18 GPa
(Soykeabkaew et al 2009).Also,filter paper
showed a 4-fold enhancement of strength to
200 MPa when the surfaces of the fibers
were selectively dissolved to form the “matrix”
material while retaining the crystalline cores
(Nishino et al 2004;Nishino and Arimoto
2007).These values are similar to what is found
for glass-reinforced composite materials.Over-
all,retaining the supramolecular structure of
the biopolymers enhances performance as the
modulus/strength is increased relative to pro-
cessed samples that did not retain their native
crystalline cores.Another similar route is to het-
erogeneously modify the cellulose microfibril
surface making heat-moldable samples while
Renneckar—BIOMIMETICS:ADAPTING NATURAL MATERIALS FOR BIOBASED COMPOSITES
3
still retaining the unmodified crystalline core
(Matsumura et al 2000).
Order can be induced into solid fibers and films
by having the polymer solution in an organized
state during processing.Drag-line silk from spi-
ders has levels of structure that provide the fiber
a specific strength and stiffness stronger than
steel fiber (Omenetto and Kaplan 2010).In the
case of spider silk,there is liquid crystalline
protein solution that orients as the filament is
spun from the spinneret (Viney 1997).Molecu-
lar order is induced along the fiber that includes
alternating tertiary structures of proteins (alpha
helices and beta sheets) creating a series of
structures with different mechanical properties
that allows for enhanced toughness.Certain
cellulose derivatives and isolated cellulose
nanoparticles can also undergo liquid crystal
formation in solution,which can be further pro-
cessed into ordered solid materials (Dave and
Glasser 1993).Lignin can help nucleate this
phenomenon for cellulose ester solutions (Rials
and Glasser 1989).Whereas most of these poly-
mer solutions require toxic solvents,spider silk
processing shows that water-soluble proteins
can be oriented and spun,subsequently becom-
ing insoluble after formation.This case demon-
strates that there is room for improvement for
spinning biopolymers fromless environmentally
burdensome solvents (Viney and Bell 2004).
However,Iwamoto et al (2011) have shown that
nanocellulose aqueous suspensions of oxidized
fibrils can be wet-spun into fibers with orienta-
tion induced by increasing spinning rate.These
fibers can reach a tensile modulus of almost
24 GPa and breaking strength of 320 MPa.
In addition to spider silk,most natural polymers
are assembled in aqueous solutions and are
insoluble after assembly.By exploiting inter-
molecular interactions and timed events,it is
possible to build organized films of polymers or
nanoparticles from aqueous solutions or disper-
sions that can begin to resemble natural tissues.
Tang et al (2003) reported mimicking the sea-
shell material,nacre,through the layer-by-layer
(LbL) assembly process.Intermolecular interac-
tions were exploited in this process with highly
controlled deposition of thin layers of polymers
(single digit nanometers) through sequential
adsorption of oppositely charged polymers and/
or nanoparticles (Decher 1997) (Fig 1).The tech-
nique has been adapted to cellulose nanocrystals
(Podsiadlo et al 2005),lignin (Pillai and
Renneckar 2009),hemicelluloses (Elazzouzi-
Hafraoui et al 2008),and proteins (Lvov et al
1995),among other materials.The technique
Figure 1.Principle of layer-by-layer modification of wood pulp fiber surfaces with solutions of polyelectrolytes.Because
of surface charges,oppositely charged polyelectrolytes or nanoparticles adsorb on surfaces of substrate when treated in
sequential order.Cycles can continue creating multiple layers.
4
WOOD AND FIBER SCIENCE,JANUARY 2013,V.45(1)
allows almost any surface to be used as a
substrate—with examples such as glass,wood
(Renneckar and Zhou 2009),cotton fiber (Dong
and Hinestroza 2009),wood fiber (Lin et al
2008),cellulose acetate films (Mamedov and
Kotov 2000),and even stable air bubbles
(Winterhalter and Sonnen 2006).For wood and
fiber surfaces,the deposited films served as an
adhesive layer (Zhou et al 2010) and have been
used to create functional materials such as elec-
trically conductive paper (Agarwal et al 2006).
As a result,wood and fiber can be modified using
simple systems along with controlled layers of
wood-derived polymers or other additives that
modify properties.
In addition to the LbL method,assembly in aque-
ous environments can be driven by the surface-
active nature of the molecule,in which the
molecule is partially polar and partially nonpolar.
This method of assembly is the principle behind
the cell membrane formation creating a regulated
environment for cell processes to occur.As pre-
viously mentioned,the cell wall of wood is
templated within a cellular environment,but
there is further organization and blending of
materials during tissue formation.Within the
cell wall of wood,cellulose microfibrils are
deposited in the presence of a collection of
hetero-polysaccharides commonly referred to as
hemicelluloses.These materials are often part-
ially acetylated and absorb in thin layers on
cellulose.Attempting to mimic a cellulose–
hemicellulose layered structure,Gradwell et al
(2004) measured the assembly of model cell
wall polysaccharides on cellulose surfaces.
Across a few nanometers,the nature of the cel-
lulose surface was transformed with substituted
polysaccharide that could interact with another
matrix material.To achieve modification,the
adsorbing polymer must be partially hydropho-
bic (Kaya et al 2009).In other words,the poly-
saccharide assembly in aqueous environments is
not driven by hydrogen bonding (Fig 2).In fact,
Kaya et al (2009) created model systems that
began to mimic the interfacial area with wood-
like polymers.When materials were deposited
in thin layers,bulk properties of the materials
began to change and the system was driven
toward interfacial interactions with different
phases forced to interact in narrow domains.
Liu et al (2005) showed howthis loss of the bulk
phase caused two distinct nonmiscible polymer
blends to behave as a miscible blend (in techni-
cal terms,this is a shift in glass transition of
polymers) using folded melt blends of polymers.
In wood,there are distinct domains of cellulose,
but across only a few nanometers,there is a
close association with the hemicelluloses and
lignin (Terashima et al 2009) (Fig 3).
A third approach to creating polymer–polymer
composite materials similar to natural materials
is to mimic the mechanisms involved in adding
a polymer to an existing template.For woody
tissues,a lignin matrix seals the polysaccharide
scaffolding that includes the cellulose microfi-
bril and hemicelluloses.The process occurs
Figure 2.Surface modification of cellulose with a water-soluble polysaccharide modified with hydrophobic cinnamate
groups.Left image shows monolayer surface coverage based on concentration;right image shows atomic force microscope
image of modified cellulose surface.Reprinted with permission fromKaya et al (2009) (
#
2009 American Chemical Society).
Renneckar—BIOMIMETICS:ADAPTING NATURAL MATERIALS FOR BIOBASED COMPOSITES
5
external to the cytoplasm of the cell;phenolic
monomers undergooxidativecouplingviaenzyme-
catalyzed pathways within the polysaccharide
scaffold.Model systems incorporating this
strategy encompass enzyme-catalyzed poly-
merization of phenolic monomers within the
presence of polysaccharides.Early work on the
subject showed how the substrate impacted
the resulting polymer structure (Siegel 1957).
That work was followed up recently showing
how the substrate controlled the superstructure
of the lignin-like polymers (Micic et al 2004).
Nanoscale cellulose particles have provided a
route toward dispersible fibrils on the same
size scale as native cellulose.Typically,it is
difficult to achieve good dispersibility of
nanoparticles in polymer matrices without
mechanical input.Following nature’s method
of composite formation,Li et al (2010) used
oxidized cellulose microfibrils dispersed in an
aqueous systemand polymerized phenol mono-
mers via horseradish peroxidase and hydrogen
peroxide around the cellulose fibrils.The
resulting composite material possessed evenly
dispersed nanoparticles within the phenolic
polymer.Unlike the control material without
cellulose,the composite material was no longer
soluble in simple organic solvents,similar to
the behavior of woody tissue.This approach
was also used with functional carbon nanotubes
to formnanocomposite materials that contained
excellent dispersion of nanoparticles within the
matrix material (Peng et al 2009).Other studies
used templates such as bacterial cellulose
hydrogels inside dialysis membranes to control
the diffusion of monomers (Touzel et al 2003).
Cellulose nanoparticle suspensions can be
concentrated into gels that have abundant
particle–particle interactions.Once this gel is cre-
ated,polymer matrices can be dissolved into a
variety of solvents and the solution is exchanged
into the nanoparticle gels.Solvent exchange tech-
niques with hydrophobic polymers,such as poly-
styrene or polybutadiene,have allowed the
creation of a number of unique nanocomposites
with a percolating fibril network (Capadona et al
2007).For some of these systems,the mechanical
properties are reversible by more than an order of
magnitude (20-800 MPa) when immersed in
water.The particle–particle hydrogen bonding
within the fibril network is switched off by expo-
sure to water.This process mimics the structural
concept of the property-changing sea cucumber,
which can quickly change the modulus of its skin
when threatened (Capadona et al 2008).In sum-
mary,these studies showroutes toward nanocom-
posite creation to get the full potential of the
reinforcement.The processes usually occur near
room temperature without the need for high-
energy processing or toxic solvents in many cases.
Assembling Hard Materials as in Nature:
Mineralization of Biopolymers
Hard natural materials such as bone,tooth,and
shell actually have a bulk density much lower
than synthetic ceramics,as well as enhanced
Figure 3.Hypothetical cross-sectional layout of wood polymers in secondary cell wall based on measurements from
Terashima et al (2009).
6
WOOD AND FIBER SCIENCE,JANUARY 2013,V.45(1)
fracture resistance.Hard materials in nature,
such as tooth enamel,bone,ivory,seashell,dia-
toms,and antler have calcium,sodium,silicon,
and phosphorous compounds.These elements
are arranged in specific patterns with other ele-
ments such as oxygen,carbon,and hydrogen
generating crystalline planes of atomic symme-
try.Calcium carbonate and hydroxyapatite are
typical minerals within natural ceramics that are
related to seashell and bone,respectively.The
process of mineralization can augment material
properties to enhance performance.For collagen
materials,a difference in function between bone
and tendon is notable;a difference in perfor-
mance between synthetic hydroxyapatite ceram-
ics and bone is notable as well.The latter does
not have the same degree of fracture toughness
(TenHuisen et al 1995),indicating it is not the
presence of a particular component but the
placement within the structure that matters.Inti-
mate mixing of the components is required to
achieve desired performance (Rho et al 1998)
(Fig 4).If isolated collagen and hydroxyapatite
particles were combined,the performance of the
mixture would be poorer than that of bone
because the structure would be significantly dif-
ferent (Tampieri et al 2003).An analogy is
found in the cell wall formation in wood.Iso-
lated lignin itself is characterized as being
highly brittle.However,lignification occurs at
cell wall maturation,synergistically reinforcing
the polysaccharide scaffolding between the
lamella of the secondary cell wall.A parallel
system is found for protein–mineral materials
in which the nanoscale inorganic component
such as hydroxyapatite is nucleated and assem-
bled at specific sites within the collagen extra-
cellular matrix (Weiner and Traub 1992).These
systems ensure the maximum extent of interfa-
cial contact among domains forming hybrid
properties that are different from the individual
bulk materials.
Specific chemical groups nucleate mineraliza-
tion on biopolymer structures.Ionic functional
groups along the collagen fibril assist in the
bone mineralization by nucleating growth of
hydroxyapatite crystals from the surrounding
calcium and phosphate-rich fluids.Translated
to biomimetic composites,ionic functional
groups introduced to the surface of synthetic
fibers can induce mineralization into electrolyte
solutions.A solution that mimics the composi-
tion of blood serum,simulated body fluid (SBF),
was developed by Tas (2000).This area of work
has been well studied to engineer tissue scaf-
folds for bone repair options.Modified scaffolds
with COOH,PO
4
H
2
,OH,and SO
3
H,have all
been used to induce mineralization.On model
surfaces,anionic groups tend to mineralize at
the fastest rate,and this is related to their ability
to bind calcium (Liu et al 2006).For cellulose-
based fibers and scaffold,there are a number
of methods to introduce these functional groups
because the surface hydroxyls are readily mod-
ified by chemical means.Wan et al (2007)
Figure 4.Controlled mineralization of collagen fibrils
with hydroxyapatite in which minerals nucleate at specific
zones.Reprinted from Rho et al (1998) with permission
fromElsevier.
Renneckar—BIOMIMETICS:ADAPTING NATURAL MATERIALS FOR BIOBASED COMPOSITES
7
phosphorylated bacterial cellulose hydrogels and
found drastic improvement in hydroxyapatite
growth relative to unmodified cellulose without
the anionic phosphorate esters.
Also,modification by polymer adsorption offers
a noncovalent modification route toward func-
tionalization.Based on modification of model
cellulose surfaces regenerated fromtrimethylsilyl
cellulose (Liu et al 2011) with carboxymethylcel-
lulose (CMC) and calciumchloride,Zimmermann
et al (2011) modified bacterial cellulose with
this method to induce hydroxyapatite formation.
A similar route of CMC adsorption was used to
pretreat electrospun fibers for mineralization
with SBF solution enhancing the carboxylic acid
content by an order of magnitude (Rodriguez
et al 2011).Detailed micrographs showed the
hydroxyapatite minerals nucleated from the fiber
surface after modification with this anionic
polymer.Additives do not always need to be
polymeric to help induce mineralization on
cellulose-based substrates.Rhee and Tanaka
(2000) added citric acid to SBF solutions to
nucleate hydroxyapatite growth on the surface
of cellulose cloth.
The drawback of biomineralization is the slow
kinetics of the process,which decreases the abil-
ity of mineralized structures to be manufactured
in a timely manner.Typically,it takes between 7
and 14 da to mineralize samples depending on
the level of modification required.However,
there are a number of different electrolyte solu-
tions used for the mineralization process.
Enhancing the concentration of ions intensifies
the rate of mineralization;the different solutions
are referred to in terms of the concentration of
ions found in SBF (1.5￿,2￿,6￿,etc).By
enhancing concentrations of compounds in the
fluid,mineralized scaffolds can be produced in
24-72 h.Hofman et al (2006) successfully used a
double soak procedure to get substantial miner-
alization of woven lyocell (regenerated cellulose
with 4% carboxylic acid content) after 3 da of
treatment.However,the caveat is that higher
levels of concentration may cause minerals to
precipitate without the surfaces actually nucleat-
ing the growth.
Bone,a lightweight structural composite,illus-
trates two design principles that can be adapted
for biobased composites arising from the micro-
structural architecture and the degree of miner-
alization.There are two types of bone,soft
trabecular bone and hard cortical bone.Differ-
ences arise in the microstructure.Trabecular
bone is highly cellular with porosity greater
than 50%.Light yet stiff composite panels have
demand in furniture markets,and efficiently
designed hybrid biobased composites may have
a future role in addressing this need while con-
serving total amount of fiber in a panel.Also,
the cellular nature is important for composite
materials because the thermal insulating proper-
ties change,providing a unique approach for the
design of panel-based products for green build-
ing products.The other design principle is that
the stiffness of bone is tuned with the degree of
mineralization (analogous to the density/stiffness
relationship of wood) (Currey 1979).Hence,as
the composite becomes less cellular and more
mineralized,the mechanical properties of the
material increase.These principles would allow
a hybrid biobased composite to be tuned for
application by controlling the degree of miner-
alization.Being able to tune mechanical proper-
ties,along with density,provides the ability for
enhanced product design.
FUNCTIONALITY THROUGH CONTROLLED
STRUCTURES
The scales of a butterfly wing have brilliant
colors without the use of chromatic dyes.The
brilliant colors arise from the microstructure of
the scales.In one sense,the wing scales are
similar to a stack of ceramic roofing tiles;there
is a defined repeating structure and spacing such
that incoming light is reflected at different
planes.The light reflected at different levels
causes interference with other incoming light,
providing the iridescent appearance.Based on
the properties and spacing of the tiles,color is
controlled across multiple wavelengths of visible
light.Brilliant color without paint provides a
powerful and stable way to create colored sur-
faces that will not erode with time because of
8
WOOD AND FIBER SCIENCE,JANUARY 2013,V.45(1)
chemical changes to dyes.With this design prin-
ciple,cellulose nanoparticles were used to create
defined layers resulting in interference patterns
with color (Cranston and Gray 2006).In the sim-
plest method,a suspension of nanoparticles is
spin-coated (a method of drop deposition on a
rotating surface with centripetal forces spreading
the suspension in an even layer),and concentra-
tion of the suspension is used to control the inter-
ference patterns.Based on filmthickness,color is
tuned by the deconstructive interference patterns
of light reflected from the interfaces of the film/
substrate and film/air.Another method is to dip-
coat a surface using the LbL method described
previously with an ultrathin iridescent film cre-
ated on the surface and the color controlled by the
number of layers (Wagberg et al 2008).A third
method uses solution casting of cellulose
nanoparticles;during evaporation,the particles
form a chiral nematic liquid crystalline suspen-
sion with stacking of layers of nanocrystals that
have a certain rotation per layer within the suspen-
sion.Because the spacing of these layers impacts
the overall chiral nematic pitch,additives can be
used to control and tune pitch difference,provid-
ing the ability to tune color from the visible to
the near IRregions.Shopsowitz et al (2010) dem-
onstrated that ordered nanoparticle gels can be
locked into place using silicate glass precursors
(TEOS) as additives followed by acidic cross-
linking.After heating the samples higher than
the thermal decomposition temperature of cellu-
lose inthe presenceof oxygen,a mesoporous glass
film is formed with iridescent color.The unique
approach is that depending on the viewing angle,
the films can change colors.
In addition to color,structure is also very impor-
tant at the surface for wetting properties.Both
the leaf of the lotus plant and the foot of the
water strider insect have structured roughness
that helps to trap air and prevent wetting of
water (Bhushan 2009).The result is that a water
droplet placed on a lotus leaf has a contact angle
>150
￿
with very little hysteresis after rewetting.
Acontact angle of this magnitude causes a water
droplet to appear as a sphere sitting on the sur-
face.Commercial paint systems are available to
make surfaces with these attributes,and this sub-
ject is one of the most developed areas of
biomimicry.Clearly,this area of research is appro-
priate for wood and fiber science because water
has a large-scale impact on durability of wood
products and many materials are used in outdoor
exposure.A recent development is freeze-dried
nanocellulose particles sprayed (air-brushed) on
surfaces and then chemically modified with fluo-
rine chemistries (Mertaniemi et al 2012).During
the process,nanocellulose agglomerates into sub-
micron particles with an appropriate surface
roughness and the hydroxyl functionality allows
facile modification with fluorine compounds to
render the surface superhydrophobic.Another
method using cellulose fibers is oxygen plasma
etching of paper,which creates a surface with
inherent roughness by selective removal of acces-
sible areas.Sequentially,a fluorine plasma is used
to treat the remaining cellulose at the surface,
which results in a contact angle on paper that is
>160
￿
(Fig 5) (Balu et al 2008).This method of
modification allows for bulk material modification
as a roll-to-roll process.Li et al (2008) treated
filter paper and cotton fabric with a potassium
methyl siliconate creating superhydrophobic sur-
faces based on the time of immersion.The
resulting polymethylsilsesquioxane coating is
a fluorine-free alternative method for creating
hydrophobic surfaces.
FUTURE OUTLOOK
Organisms in nature rely on polymers and
polymer–mineral hybrids to create a wide variety
of properties from a few elements.Mechanical
Figure 5.Water droplet on modified cellulose surface
exhibiting superhydrophobicity.Reprinted with permission
fromBalu et al (2008) (
#
2008 American Chemical Society).
Renneckar—BIOMIMETICS:ADAPTING NATURAL MATERIALS FOR BIOBASED COMPOSITES
9
performance and function were developed from
controlled structure and architecture within the
material.Because natural materials impart a
broad range of useful functionalities for the
organism,there are still a variety of design prin-
ciples for manufactured materials that can be fur-
ther developed to enhance performance while
decreasing energy consumption and mitigating
environmental impact during fabrication and dis-
posal.For example,the antifouling structure of
shark skin can be used as a bacterial-resistant sur-
face without the use of biocides,and the adhesive-
free bonding system of the gecko foot would
make pressure-sensitive adhesives obsolete,mak-
ing recycling of “stickies” easier.Stronger wet-
spun cellulosic fibers fromaqueous solutions with
better matrix–fiber interfaces would allow devel-
opment of biobased-reinforced composites for
structural applications,and further development
of hybrid polymer–ceramic composites through
biomimetic processes would decrease the energy
consumption required for many current ceramics.
New developments in manufacturing paradigms
for biomimetic materials are required because
self-assembly and biomineralization occur in
batch processes across extended periods of time.
However,the overall design principle of con-
trolled structure will probably continue to be the
key element that drives innovation in sustainable
biomaterials manufacturing in the near future.
Coupling design principles to new manufacturing
methods,such as 3-D printing or microreactor
systems,may provide routes to incorporate timed
processes and assembly into biobased composites
with functional architectures.
ACKNOWLEDGMENTS
This work was supported in part by the U.S.
Department of Agriculture NIFA NRI com-
petitive grants program (Grant Number 2010-
65504-20429).
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