advanced sustainability research

Advanced Sustainability - Ss09Porfessor Matias del Campo

Deborah Kaiser

Structural Integrity in nature

Reverence for the beauty and usefulness of the naturally occurringmaterials around us has been felt and expressed ever since man learned touse them for improving the quality of life and standard of living. Variousstages, in the growth of our civilization are therefore aptly named afterstone, iron and bronze. It is customary to refer to the current millennium asthe age of materials.

This is no less a measure due to the rapid strides that the field of materialsscience and technology has made over the last fifty years or so. In ourpersistent attempts to improve the performance and versatility of a givenmaterial or combination of materials as in composites, we have learnt toobserve and analyse the way nature has so successfully developedmaterials in living organisms and systems.

Nature is yielding some of its long held secrets only now because of theincreased sophistication and capabilities of the instruments of investigationthat are at our disposal. Nature uses very few materials to create abewildering variety of life forms. Same material is used in many differentways to meet vastly different needs as exemplified by collagen.

Natural materials are mostly constituted from organic, inorganic crystalsand amorphous phases. The organic phase generally occupies a very smallfraction of the total volume and has functions ranging from toughening thetissue to synthesising highly functional minerals.

The inorganic components can be single crystals or aggregates of themarranged in wellordered arrays to give a hierarchy of length scales. Theinterfaces between the soft organic matter and the relatively hard inorganicmaterial is of paramount importance in determining the properties of thecomposite and nature has devised strategies for assuring integrity of theinterfaces under demanding conditions of stress.

Natural materials are self-generating, hierarchical, multifunctional,nonlinear, composite, adaptive, self-repairing and biodegradable. Bones inanimals illustrate some of these charac- reduces in density and weight. Thisphenomenon is indicative of the ability of bone to adapt itself to the changingdemands of the sustained stress levels to which it is subjected (Lakes1993). In this respect bone is a smart material even if the response time islonger.

Biogenic inorganic crystals exhibit stunningly different properties from theircorresponding synthetic counter parts. This has led to the questioning ofthe assumption that biogenic and inorganic crystals are intrinsically thesame (Berman et al 1993).

For example, silica in the sponge f.SiO2/3_H2Og monorhaphis is found inthe form of a spicule of up to 3_0m length. In the cross section,it iscomposed of several concentric layers varying in thickness from 10 _min the centre to 3_m at the periphery. The layers are deposited on an axialorganic fibre. Under three-point bend testing, it exhibits strength of 593MPaas against a value of 155MPa for synthetic silica. The work to fracture is also30 times higher in the spicule (Levi et al 1989). Intercalation of acidicmacromolecules into the crystal lattice appears to be common.

Similarly, spider dragline silk is far superior to steel of comparabledimensions and the energy to fracture on equivalent weight basis is 100times higher (Vincent 2001).

The shell of the mollusc abalone, made of essentially calcium carbonate(see figure 2a), has 3000 times greater fracture resistance than the singlecrystal of calcite (Jackson et al 1988; Currey 1977).

Such examples together with increasing pressure towards the conservationof the environment have led materials scientists and engineers to carefullystudy natural systems, their design and their methods for the synthesis ofconstituent materials.

In the present research compendium, some examples are provided of theway nature builds tissues and synthesises materials and review theattempts being made to use such strategies for learning to design systemsand develop novel materials.

Biomimicry - Intro

Definition and scope

Biomimetics is the field of scientific endeavour, which attempts to designsystems and synthesise materials through biomimicry. Biomeaning life andmimesis meaning imitation are derived from Greek.

Perceptions regarding the scope of biomimetics appear to vary very widelydepending upon the specialized discipline of the investigator. Japaneseelectronic companies are supporting biomimetic research with a view tolearning the way biological systems process information.

Recent interest of Japanese in biomimetic research seems to arise from thehealth and welfare problems of an ageing society leading to studies on thedevelopment of human supportive robots (ATIP 1999).

Biomedical engineers consider biomimetics as a means of conductingtissue engineering and trace the origins of biomimetics to ancient timeswhen Mayan, Roman and Chinese civilizations had learnt to use dentalimplants made of natural materials.

Material scientists view biomimetics as a tool for learning to synthesisematerials under ambient conditions and with least pollution to theenvironment.

Chemists have always wondered at the ease with which ammonia isproduced in biological nitrogen fixation, methanol is produced in biologicaloxidation of methane and oxygen is generated in photosynthesis (Shilov1996). They hope to learn the synthesis of polymers that can perform theroles of enzymes in such processes. Biologists study biomimetics not onlyfor an understanding of the biological processes but also to trace theevolution of various classes of organisms.

Biochemists have interest in the field due to the complexities associatedwith the interaction of biopolymers with ions of metals leading to themineralization in living organisms.

Even geologists have an interest in biomimetics because ofbiomineralization: the formation of extra- or intra-cellular inorganiccompounds through the mediation of the living organism.

Engineers and architects attempt to explore the relationship betweenstructure and function in natural systems with a view to achieve analogoussynthetic design and manufacture.

On the whole, the field of biomimetics addresses more than one issue.Those engaged in this field of research activity try to mimic natural methodsof manufacture of chemicals in order to create new ones (imitate barnaclesfor producing a natural glue), learn new principles from phenomenaobserved in nature (flight of birds and insects, swimming of fish and aquaticanimals), reproduce mechanisms found in nature and copy the principles ofsynthesising materials under ambient conditions and with easily availableraw materials.

Figure 1. (a) The crossed lamellar structure of anabalone shell. (b) Co-aligned aragonite rods inmollusc shell.

Designs from nature

Designs found in nature are the result of millions of years of competition forsurvival. The models that failed are fossils. Those that survived are thesuccess stories (Benyus 1997). Consequently they are optimised foreconomy of energy consumption and use of space Mattheick (1994).

Nature makes economic use of materials by optimising the design of theentire structure or system to meet multiple needs. For example, feathersbesides helping the bird fly insulate it from the environment.

The many ways in which nature tries to design a system to suit a function isbest illustrated with respect to fish. Fish reduce the drag as they swim boththrough chemical and structural devices. Some of them releasesubstances, which make their skin slippery. In others, the body is designedto aid avoidance of turbulent floor around it during swimming. In some fishgill slits are formed and located on the body such that smooth flow of wateraround the fish is ensured.

In sharks, skin scales possess tiny ridges that run parallel to the longitudinalbody axis and reduce the drag (Dickinson 1999). In porpoises, the skin isdesigned and made to absorb pressure fluctuations thus preventingturbulence.

The cuttlefish has a buoyancy tank with a number of chambers, which arefilled with nitrogen or water to facilitate energy efficient movement from onedepth to another. The structure of the buoyancy tank is such as to resistexternal pressures of the order of 7 atm (Birchall & Thomas 1983).

Attempts to adopt designs based on the study of plants and animals have along history. Oft quoted examples are the attempts of Leonardo da Vinci todesign an aircraft based on his study of birds, the design of the CrystalPalace, London, and of the Eiffel Tower, Paris (Meadows 1999).

The Palace design was based on the observation of the structure, unusualsize and great strength of the leaves of a water lily called Victoria amazonica.Joseph Paxton, a gardener by profession, was fascinated by the intricateribs and cross ribs at the back of the leaf of this lily and built a greenhousethe roof of which incorporated a similar scaffold.

He later entered the design in a competition organised for architects calledto design an exhibition hall for arts and industrial goods. When completedthe Crystal Palace was 108 ft in height and covered an area of 18 acres. Itstood the test of time from 1851 to 1936 before a fire destroyed it.

Similarly, the origins of the design of the Eiffel Tower are traced to theinspiration derived from the structure of the head of the femur in the thigh.Hermann Von Meyer, professor of anatomy at Zurich observed that the headof the femur has many fingers of bone arranged in curving lines.

The Swedish engineer Karl Cullman recognised the engineering importanceof the arrangement and noted that they coincide with the lines of stressexperienced by the bone. He showed that the femur and its structure is thebest way of transferring the off-centre forces of the hip to the long bones ofthe leg.

Gustaff Eiffel, the French structural engineer implemented such principles inbuilding the now well-known landmark of Paris.

Among the more recent examples of designs drawing inspiration fromnature may be mentioned the invention of the fabric fastener Velcrorand thepaints based on the behaviour of water on the lotus.

George de Mestral, a Belgian, noticed that the cocklebur from burdock plantstuck tenaciously to the fur of his rather big dog when it ran through grassduring their walks. On microscopic examination, he discovered that theburrs had tiny hooks. He translated the natural design into commercialpractice by combining a part with hooks and a surface with a felt to createthe now common fabric fastener (Meadows 1999).

Makers of a newly patented paint have derived their inspiration from the age-old observation that water does not stick tolotus leaves. It has been foundthat the lotus leaf has tiny wax-coated protuberances on its surface. Thesenew paints clean themselves after every rain (Dickinson 1999).

Many more such examples can be cited from daily life. Today the defencefunding agencies of the advanced countries are spending considerablesums of money to evolve designs of robots based on living things like fish,geckos, aquatic birds with webbed feet, bees, butterflies and a host ofothers.

Biomineralization

Nature employs more than 60 inorganic materials together with organicmatter to create myriads of organisms.

In the bones of vertebrates, the most common mineral phase is carbonatedapatite and the organic part is collagen fibril.

Members of phylum Echinodermata have calcite that contains magnesiumand is distributed in protein matrix. The shells of molluscs (snails, slugs,clams, oysters, cuttlefish, squid etc.) contain aragonite.

Biomimetic approaches based on an understanding of the biomineralizationprocess are aimed at synthesising nanoparticles, polymer mineralcomposites and templated crystals.

The structures that arise are highly organised from molecular (1–100Å) tomacro-scales through nanometric (10–100nm) and mesoscopic(1–100_m) domains. These are hierarchical in nature and meet thefunctional requirements.

The properties of natural materials arise in a large measure due to theordered spatial structures that exist over many length scales. We find abewildering variety of microstructures arising from biomineralization. Thegrain size, shape, boundaries and the crystallographic texture vary greatly.

At least seven different types of microstructures have been observed(Chateigner et al 2000;Vincent 2001). These are: (i) Prismatic both simpleand spherulitic; (ii) nacreous both columnar and sheet; (iii) gross-lamellar,both simple and complex; (iv) homogeneous.

The simple prismatic microstructure consists of mutually adjacent prismswhile the spherulitic prismatic structure consists of coarse first-orderprisms with second-order prisms originating from spherulitic sectors at thesurface. In the columnar nacreous structure columns of uniform-sizedtablets are formed.

The sheet nacreous structure has a typical brick wall pattern. The crossedlamellar structure consists of parallel laths or rods with two non-horizontaldip directions of their elongated sub-units in adjacent lamellae. Thisstructure is layered at five distinct length scales and can be considered to bea ceramic “plywood” (Kuhn-Spearing et al 1996). This is the most commonamongst molluscs and often it is the only type of microstructure in a givenshell occupying both its inner and outer layers.

Microstructure resulting from mineralization

The anisotropy associated with this microstructure has the ability to deflectcracks and offer resistance to their propagation resulting in theenhancement of toughness of the shell.

Many of the tissues in living organisms are bio-composites consisting ofthe soft organic material and the hard mineral. Calvert (1992) has classifiedbiocomposites into four types on the basis the interplay between ofinteractions at the mineralmatrix interface.

In type I composites (Chiton teeth, algae) the matrix is inert and does notpossess specific nucleation sites. Growth of the crystals is spatiallyrestricted. Consequently, there is no control of the matrix on crystal size,orientation and morphology and it acts as a support encouragingheterogeneous nucleation of the inorganic phase.

In type II composites (Avian egg shells, limpet teeth) the matrix offersspecific sites for nucleation and controls crystal orientation and mayencourage a polymorph preferentially. Size and shape of thecrystal are not still subject to control by the matrix.

In type III composites, the matrix inhibits nucleation of the crystals. Thestructure, size and morphology of the inorganic phase are also matrixcontrolled. Amorphous inorganic phases form.

In type IV composites, site directed and regiospecific nucleation occurswith regulation of the growth, structure, morphology and orientation of theinorganic crystals. Bone, mollusc shell are examples of this type.

The most striking feature of the biocomposites is that the organic matrixoccupies barely 3 to 5% of the volume but imparts considerableimprovement in the mechanical properties of the mineral. Thus, nacre,which is the lustrous inner layer in the mollusk shell, has 500 to 3000 timesgreater toughness than chalk, which constitutes 95% of its bulk.

Detailed studies have shown that propagation of cracks is impeded invarious ways thus contributing to the enhancement in fracture toughness.

It is not possible to achieve similar fracture toughness in syntheticcomposites. One of the reasons for our inability to fabricate compositessimilar to the bio-composites lies in the glues that are employed. Syntheticcomposites use either epoxy or silicon adhesives. The former are stiff whilethe later are elastic.

Bio-composites

The energy involved in breaking both is inadequate to impart necessaryfracture toughness. Nature finds a unique way of improving the fracturetoughness as has been shown recently in studies conducted on abaloneshells.

The natural proteins in the shell provide a modular fibre in which differentdomains are held together with intermediate strength bonds as compared tothe bonding within the molecule. On increasing the stress and beforemolecule's backbone can break, the modules unfold and begin to yield.Such events occur repeatedly in long molecules that are compactedinto number of domains. The work required for fracture is thus enhancedconsiderably (Smith et al 1999).

Tissues in living organisms have specific roles to perform. Thesecomposite tissues are specifically designed to meet well-defined needs.The example of bamboo illustrates this concept very well .

Bamboo has the ability to withstand high velocity winds. On a macroscopicscale the nodes, which occur periodically along its length, impart stabilityand rigidity to the plant.

The cleavage and tensile strengths across the fiber direction are also raisedconsiderably in the presence of the nodes. An analysis of the stressesexperienced by the bamboo during bending indicates that the maximumstresses are generated at the outer part of the culm.

The microstructure of the bamboo indicates that the density of distributionof the vascular bundles, which act as the reinforcing component, is thehighest in the outer green layer.

The f-me structure of the bundle also changes as one approaches the innerlayers. Further the winding of the vast fibers in the vascular bundles iscomplex and consist of several alternate thick and thin layers.

The micro fibrils in each layer are distributed in a helical way with differentelevation angles for the thick and thin layers when measured with respect tothe fibre axis. These angles change gradually to avoid discontinuitiesbetween different layers.

Bamboo is thus a functionally graded material and a hierarchically designedcomposite (Amada 1995).

Structure– junction relationships in bio-composites

In contrast, synthetic fibre reinforced composites have far simpler andhomogeneous distribution of the fibre in the matrix. When attempts weremade to copy the bamboo structure, it was found that the strength of thesynthetic composite could be doubled for the same ratio of graphite andepoxy resin composites. Industrial production is still difficult due to theintricacies of the winding machines that will be required for the distributionof the filament (Zhou 1994).

Observations and studies made on trees, teeth and bone reveal thefollowing aspects of their design.

(a) Biological structural members optimize themselves to maintain uniformstress across the cross section

(b) Both trees and bones add material in the overloaded areas tocompensate for the stress increase

(c) Bone also has the ability to reabsorb material from under-loaded regions

(d) They are self-annealing and self maintaining as in the case of sea-urchin.

Biomineralization

Plant surfaces and their multifunctional properties

Approximately 460 million years ago, the first plants moved from theiraqueous environment to the drier atmosphere on land, and they needed aprotective outer coverage. The key innovation was the plant cuticle, acontinuous extracellular membrane, which covers the primary above-ground organs of flowers, stems, leaves, fruits and seeds of all lower (e.g.,ferns) and higher land plants (e.g., flowering plants).

Only the roots of plants, some mosses and secondary plant tissues, suchas wood and bark, can forgo this protective layer.

The development of the cuticle as hydrophobic outer coverage was one ofthe key innovations that enabled plants to leave their primarily aquatichabitat and to overcome the physical and physiological problemsconnected to an ambient environment, such as desiccation.

Plants settled into nearly all conceivable habitats. In their specificenvironments, the cuticle serves as the crucial multifunctional layerrepresenting one of the largest interfaces between biosphere andatmosphere, covering more than 1.2 _ 109 km2 in total.

Cuticles stabilize the plant tissue and have several protective properties.One of the most important properties is the transpiration barrier. Cuticlesreduce the loss of water to the same, or even higher degree, as a syntheticpolymer of comparable thickness would.

This property is based on their hydrophobic material, made basically of apolymer called cutin and integrated and superimposed lipids called''waxes''.

The cuticle and their waxes play an important role in cellular structuring andsurface wettability, e.g., by folding of the cuticle, or by formation of threedimensional wax crystals on the plant surface.

Cell forms, sizes and their fine structures have a great influence on severalfunctional approaches of the plant boundary layer.

The multifunctional properties of the plant cuticle,summarized in the figure on the right upper corner, are the reduction of waterloss and leaching of ions from the inside of the cells to the environment,the reduction or increase of surface wetting and reduction ofparticle adhesion, as for example, pathogen adhesion.

The plant cuticle also plays an important role for insect and microorganisminteraction, as in attachment or sliding of insects.

It protects the plants against harmful radiation, e.g., it can increase thereflection of visible light for temperature control and can induce turbulent airflow to increase mass and heat transfer from the plant surface to theenvironment.

Additionally, it is a stabilization element for the cells. The diversity of plantsurface structures arises from the variability of cells shapes and theirsurfacestructures, and by the formation of multi-cellular surfacestructures.

Some of these structures, such as special morphological types of cells likehairs or epicuticular waxes (introduced later), are characteristic for a specialgroup of plants, thus they are useful features for grouping of plants insystematic orders (taxons).

Based on the large variations of surface chemistry and structures, plantsurfaces provide a huge variety of functions. Some surfaces, like the leavesof the Lotus plant (Nelumbo nucifera), are extremely water repellent(superhydrophobic).

However, others, such as the air-roots of epiphytic orchids, some lichensand mosses, show opposite behavior; these are constructed for the mostefficient water absorption (superhydrophilic) through their surfaces.

Such wetting phenomena are based on physicochemical factors, and theseare not restricted to the living organism, but transferable into technicalsurfaces, with a biomimetic approach.

Several new methods for surface functionalization have been developed,and special interest has been given to techniques for the development ofsuperhydrophobic surfaces after the model plant Lotus and the feet ofseveral arthropods and some vertebrates and their remarkable ability toreversibly attach to varying surfaces.

The plant epidermis and its functional approaches The epidermis of plants isthe outermost layer of primary tissues.

Wettability of surfaces and Biomimetics

Function and diversity of plant surface structures

Diversity of structure, morphology and wetting of plant surfaces

Fig. 2b. Because pectin is not always formed as a layer, visible inFig. 1 Schematic survey of the most prominent functions of theplant boundary layer on a hydrophobic micro-structuredsurface.

A) Transport barrier: limitation of uncontrolled waterloss/leaching from interior and foliar uptake; B) surfacewettability; C) anti-adhesive, self-cleaning properties: reductionof contamination, pathogen attack and reduction ofattachment/locomotion of insects; D) signaling: cues forhost–pathogen/ insect recognition and epidermal celldevelopment; E) optical properties: protection against harmfulradiation; F) mechanical properties: resistance againstmechanical stress and maintenance of physiological integrity; G)reduction of surface temperature by increasing turbulent air flowover the boundary air layer.

Stratification of the outer part ofepidermic cell

Several morphological modifications of the epidermis cells are known; thus,the model presented in Fig. 2a shows only the basic, layered stratification.

Starting with the outside of the epidermis cell, we find as the outermostlayer a thin extracellular membrane, called cuticle. The plant cuticle is acompositematerial mainly built up of a cutin network and hydrophobicwaxes.

Waxes are the main transport barrier, preventing water loss and leaching ofmolecules from inside of the living cells. This barrier function reduces alsothe uptake of molecules from the environment, which might become acrucial factor when the uptake of, e.g., nutrients or fungicides in agriculturalsystems is desired.

The cuticle is present on all primary tissues of the aboveground organs oflower (e.g., ferns) and higher land plants. Tissues and species without acuticle have no transpiration barrier; thus, only few exceptional species ortissues can exist without this protective layer (some examples were given inparagraph 1.1).

In terms of functional aspects, which were summarized in Fig. 1, the mostimportant part of the epidermis cell is the cuticle. Hence the cutin and itsintegrated (intracuticular) and overlying (epicuticular) waxes will beintroduced in more detail in the following paragraphs.

The next layer, shown in Fig. 2a, is the pectin layer. It connects the cuticle tothe much thicker underlying cellulose wall and the finer cellulose fibrilswhich are shown in the more detailed scheme of Holloway in transmissionelectron microscopy, Holloway did not include it into his scheme, but headds polysaccharides, which are also integrated into the cellulose wall.

The last shown layer, below the cell wall, is the plasma membrane. Thismembrane separates the living part of the water containing cell from theouter nonliving part of the epidermis.

Wetting is the fundamental process of liquid interaction at solid–gaseousinterfaces. It describes how a liquid comes into contact with a solid surface.

Wetting is important in many everyday situations, for example, liquidpaintings on walls, printing of texts and for the transport of fluids (water, oil,blood and many others) through pipe systems, and it is the basis of severalcleaning procedures.

Surfaces and wetting

However, there are many situations where it is desirable to minimizewetting, because adhering water droplets on window glass and carwindshields reduce the view and leave residuals after evaporation.

Rainwear should stay dry even during heavy showers, and also movementin water costs extra energy because of friction force at the interfaces.

At the two surfaces in relative motion, condensation water vapor from theenvironment forms meniscus forces responsible for the adhering, frictionand wear.

Bio-films induce apparent defects in technical materials, and also theiracidic excretions are damaging to buildings and technical materials.

The plant cuticle with its integrated and exposed waxes is in general ahydrophobic material, but structural and chemical modifications inducevariations in surface wetting, ranging from superhydrophilic tosuperhydrophobic.

The sculptures of the cells, the presence of hairs and the fine structure of thesurfaces, e.g., folding of the cuticle or existing epicuticular waxes, have astrong influence on surface wettability.

Why are the leaves of most flowers hydrophilic? There is a simpleexplanation for this phenomenon.

Flowers are developed to attract pollinators, in most cases small insects,and these should be able to walk on the surfaces, but coverage with threedimensional waxes forms a slippery surface for most insects, and wouldlead to less pollination.

Additionally, it is important to notice that hydrophobic leaves might becomehydrophilic by the accumulation of environmental, hydrophiliccontaminations like spores, bacteria, dirt particles and chemical aerosolson their surfaces.

Superhydrophilic surfaces are characterized by the spreading of water onthe surface. Superhydrophilicity is based on different morphologicalstructures.

Plant surfaces and their wetting behavior

Superhydrophilic plant surfaces.


However, wetting is also influenced by the chemistry of the surface, e.g.,secretion of hydrophilic compounds by epidermal glands. Optimizedorgans for the uptake of water are the roots of the plants. Most roots arecharacterized by papillae and hairy cells, but also porous surface structureshave been described for the air roots of epiphytic plants.

Superhydrophilicity is also a great advantage for lower plants (mosses),which have no roots for water uptake and lack vessels for water transport. Inthose species, superhydrophilicity of the surface is the basis for the uptakeof water and nutrients from the environment.

Water absorption via the leaf surface is not limited to lower plants. In higherplants, all specimens of the Bromelia family, e.g. pineapple (Ananascomosus) have water absorbing, multicellular absorptive trichomes on theepidermis cells.

The way for the absorbed water goes over the absorbing hairs through totheir centre.

There, hydraulic epidermis cells allow the uptake of water by opening theepidermis. In dry periods, these cuticle covered cells prevent the loss ofwater from inside the cells.

Some genera in this group form funnels by arranging their leaves in form ofa rosette. In these funnels, water can be stored after a rain shower andorganic litter can accumulate and decay, so hairs at the funnel surface alsoabsorb nutrients dissolved in the water

Another reason why plants profit from dry surfaces is that the growth ofmost pathogen microorganisms, including bacteria and fungi, is limited bywater shortage.

Additionally, the lensing effects of water droplets on leaf surfaces canincrease incident sunlight by over 20-fold directly beneath individualdroplets, which may have important implications for processes such asphotosynthesis and transpiration for a large variety of plant species.

Many terrestrial plants and animals are water repellent due to theirhydrophobic surface components in connection with microscopicroughness.

Superhydrophobic plant surfaces and the Lotus leaf

The morphological characteristics of superhydrophobic leaves were ahierarchical surface structure of convex to papillose epidermal cells and avery dense arrangement of three-dimensional epicuticular waxes.

The hierarchical (double structured) surface is characteristic for the Lotusleaf (Nelumbo nucifera), as stated earlier, but it also exists on many othersuperhydrophobic leaves.

On Lotus leaves the composite or hierarchical surface structure is built up ofconvex cells and a much smaller superimposed layer of hydrophobic three-dimensional wax tubules. Wetting of such surfaces is minimized, becauseair is trapped in the cavities of the convex cell sculptures and the hierarchicalroughness enlarges the water–air interface while the solid–water interfaceis reduced.

Water on such a surface gains very little energy through adsorption andforms a spherical droplet, and boththe contact area and the adhesion to the surface are dramatically reduced.

The leaves of the Lotus plant afford an impressive demonstration of self-cleaning14. This self-cleaning effect was found to be a result of the intrinsichierarchical surface structure built by randomly oriented small hydrophobicwaxtubules on the top of convex cell papillae.

This self-cleaning results in smart protection against particle accumulationand is also a protection against plant pathogens like fungi and bacteria.

In 2000 the trademark Lotus-Effect_ was registered to label self-cleaningproducts based on the model of Lotus.

In nature, the self-cleaning is not restricted to plant surfaces. Insects,especially those with large wings which cannot be cleaned by their legs,have water repellent wing surfaces and exhibit self-cleaning ability. Here notonly the removal of particles is of interest, but also the maintenance of flightcapability, which may be lost due to a load of weight on the wings.

Besides the superhydrophobic leaf structures described above, a secondmethod of water repellence has been developed in plants. Hairy leafsurfaces, such as those on the leaves of the lady's mantle can veryefficiently repel water.

Superhydrophobic hairy surfaces.


On such surfaces, a deposited drop bends the fibers (hairs), but thestiffness of the hairs prevents contact with the substrate, and promotes afakir state of the water droplet.

Superhydrophobic hairy surface structures are also known from animals,as for example water beetles and the water spider. These hairy systems mayalso be extremely useful for underwater systems because they minimize thewetted area of immersed surfaces and therefore may greatly reduce drag,as well as the rate of biofouling.

Bionics contains a wide spectrum of research fields such as, for example,lightweight constructions, fluid dynamics, robotics, micro- and nano-electromechanical systems (MEMS, NEMS), and sensors. The dimensionsof interest reach from the molecular level up to the function of complexorganisms.

Biological surfaces are evolutionarily optimized interfaces and provide alarge diversity of structures and functions. The first prominent example of asuccessful transfer of biological surface structures is the drag reducingsurface structure of shark skin and the artificial surfaces (rippled foils)developed after this model.

The second one was the description of superhydrophobic and self-cleaningplant surfaces by Barthlott and Neinhuis.

Today, well described examples are the feet of the gecko, which areperfectly adapted for reversible attachment on surfaces, and theselfadhesive surface structure of beetle feet.

Another bio-inspired attachment system is the hook and loop fastener,which plants use for the dispersal of their seeds by attaching the fruits toanimals. Recently the structure of shark skin has been used as a model forthe development of swimming dresses with reduced surface drag whendiving into water.

Self-repairing processes in plants sealing fissures serve as conceptgenerators for the development of biomimetic coatings for membranes ofpneumatic structures.

Technical applications of biomimetic surfaces


This chapter intends to clarify the role mechanics has assumed inunderstanding the correlation of structure to properties and functionality inbiological materials, and the biologically inspired materials that can bedeveloped from this knowledge.

The following serves to elucidate on this role: the helical structure of fibroustissue, the multi-scale structure of wood, and the biologically inspiredoptimal structure of functionally graded materials.

One of the most ubiquitous of biological materials is fibrous tissue, which isan aggregate of cells characterized by a helical fibrous structure.

The mechanics of fibrous tissues has become of interest recently becausethe peculiar mechanical characteristics of tissues within an organism aretailored to their specific functional needs, both by the properties of theconstituent "building blocks" and their structural organization.

Thus, they tend to have a hierarchical structure and very often they havehelical fibers in one or more levels within their hierarchy. Specific examplesare also presented of some of the "building blocks" found in the plantand animal kingdoms.

In the examples cited, specific properties are related to structure andcomposition. Although there is a huge range of biological fibrous structures,all come from a small number of building blocks in terms of fibers andmatrix materials (or ground substances).

The main fibrous building blocks are broadly polypeptides in mammalianstructures (e.g., collagen and elastin) and polysaccharides in plantstructures and insects (e.g., chitin and cellulose).

The matrix materials are globular proteins or glycoproteins and water inuncalcified soft tissues. In hard tissues, hydroxyapitite and lignin are themost common matrix/filler materials.

The small variation in the basic components of biological materials impliesthat the large variations in observed properties are the direct result ofstructural variations.

Fibrous Tissues

Mechanics of Helical Structures

Helical fibers can be tailored to suit the mechanical environment by nature ofrelative fiber movements caused by realignment of fibers within the helix.

These movements can be controlled by the inter-fiber force, which is relatedto the helix angle, and the nature of solid or viscous friction between thefibers, or by the shear modulus of a solid matrix.

This leads to a very versatile range of mechanical properties governingflexibility, damage tolerance, energy absorption, and linear force-pressureactuation that are dictated by the nature of the inter-fiber forces.

Inter-fiber forces arise because the asymmetrical nature of a helix it will tendto try and unwind and straighten out when loaded along its long axis.

The geometry of helical structures also results in a high axial strength and alow bending stiffness. As a helical fiber oscillates from tension at the top tocompression at the bottom.

The nature of the fiber bundle gives rise to another advantage in terms ofdamage tolerance: crack isolation. Given a sufficiently weak interfacebetween fibers, a crack will not pass from one fiber to the next.

Additionally, because of lateral forces between fibers caused by the helixstructure pulling itself together in tension, a break in the fiber will graduallytake up the load until at some distance from the break it regains its full shareof the load. In contrast to a simple parallel fiber composite, because there isactually a force between the fibers this mechanism will occur even if theinterface between the fibers is a viscous fluid or nothing (i.e., relying oninter-fiber frictional forces).

Fibrous bundles are also capable of absorbing tremendous amounts ofenergy. Using the inter-fiber shear mechanism, a tensile structure can bedesigned with significant levels of hysteresis within a load cycle, which maybe time-dependent viscoelastic behavior (in the case of a viscous fluidinterface) or time-independent Coulomb-type damping (for a frictionalinterface).

The Role of Mechanics in Biological and Biologically Inspired Materials

Examples in Nature - Wood

Wood is another ubiquitous biological material that represents adaptation ofa plant species to exploit an ecological niche through a multi-scalestructure.

Wood makes use of a variety of helical structures to create a structure thathas both stiffness and strength adapted to ecological needs. In particular,represents a genetic adaptation to the success of a particular competitivestrategy for growth. For example, early in the life of the tree the woodemphasizes the need to grow quickly and compete for light.

Later in life, wind and other environmental loads pose the greatest risk to thetree. The mature wood that is produced is therefore stronger, but less heightresults from the energy investment. Both the molecular structure and thephysical orientation of the structural elements adapt, as a result of thechanging needs during growth.

The engineering equivalent of this approach would require that not only theshape of a component be designedfor an application, but the structure of the material at the microscopic andthe molecular level would adapt for each application as well.

The engineering equivalent to adaptation during the life would be in-servicechanges in the material. So, for example, if the structure of an engineeredcomponent were made of a truly tree-like material, the material would beadapted across length scales for the application as well as for the locationwithin the structure.

In one location of a part, the material could emphasize fracture toughness.Fracture toughness might gradually become less important in another areawhere greater strength would be required.

Springs or mounting points could be integrated into a single part by simplyadapting the modulus. In the case of a composite material, this type ofdesign could be implemented by orienting the polymer chains orcrystallinity in the matrix structures, as well as by varying the percentage ofreinforcement.

However, to fully implement the biomimicry of wood, the material wouldhave to adapt to the application. So, if a system encountered unexpectedloading in use, the material would adapt to the new loading by emphasizingthe need for increased strength or modulus.

The environment would trigger active control and dynamic repair. Thebiological world can provide inspiration and direction for developing thistype of integrated system and material design. Additionally, the designprocess can mimic the genetic adaptation to environmental demands for thedevelopment of shape and materials for a particular application.

Furthermore, it is notable that wood serves not only as a structural materialfor the tree. Wood is multi-functional, serving also as a transport andstorage medium for nutrients, connecting the roots to the leaves.The multiple roles that wood plays may also be applicable to engineeringstructures that require fluid transport as well as structural integrity.

However, first an increased engineering understanding of the performanceand structure of wood is needed. This understanding can help to guide notonly the optimization of materials but also the process of design for the newmaterials.

If mirrored in engineered material, this type of adaptation across lengthscales has the potential to cross performance barriers that exist in currentmaterial synthesis and engineering design.

Wood, because of its relative abundance and low cost, is often notrecognized as a high performance material. However, if used in aconfiguration that approximates that of the natural loading, it is only in recentyears that human-made materials have exceeded the performance of woodfor many structures.

Based on the combined effects of weight and strength in a cantilever, onlyengineered composites have better performance indices than wood. Forexample, even a glass epoxy material is comparable to wood, withperformance factors in stiffness-based design of 2.3-7.6 versus 1.9-4.5 forwood (in units of ~ m3Mg - 1).37

Furthermore, if environmental effects and fracture toughness areconsidered, wood becomes an even more attractive material. As anexample, end grain balsa is commonly used as a core material for sandwichcomposites. This is not only because of the low cost of the material, but alsobecause of impact resistance.

The Performance of Wood in Engineering Applications


However, wood design is by nature typically very conservative. This isbecause the material axes in wood are often misoriented relative to thegeometric axes. Wood grows in a manner such that the material axes areoriented in the principal stress direction. Because of the asymmetry of windloading this results in spiral grain growth as the standard condition insoftwoods.

In addition to grain angle effects, defects such as knots may be in evidencein wood. These defects serve non-structural functions in the tree. However,when used as a construction material they represent defects.

Finally, as a part of the competitive strategy of the tree, wood from early in atree's life (juvenile wood) has inferior material properties to wood producedlater in the life of the tree (mature wood). This difference is of the order of aone-third reduction in the modulus. In general, the variation in materialproperties between individuals is also quite large

This variability suggests why wood is not the solution to advanced design,but instead provides a model of how engineering materials can bedeveloped to break existing design barriers.

The possibility exists then to create a synthetic wood in which the variabilityis directed at meeting engineeringobjectives instead of the ecological need of the tree.

Such a material would depend on optimization in the same hierarchicalfashion as wood. The material is both inhomogeneous and anisotropic fromthe millimeter scale of the cell structure to the angstrom scale of theglucose.

The variation of the material shows functional adaptation as well at all scalesfrom the polymeric up to the microscopic orientation of the material axes ofthe tree. The adaptation at the scale of the polymer chains that build up thecellular composite wall is the smallest structural level.

The polymer chains themselves have limited adaptation during the life of thetree and within a single growth year to provide the properties required. As inother structures, adaptation of the chain length and crystallinity is observed.

Multi-scale Structure of Wood

Experimental Mechanics

The structural materials of the cell wall include carbohydrates andphenolics, linear polymers and a three-dimensional molecule. The celluloseis a simple linear glucose polymer that makes up nearly half of the cell wallmaterial. Hemicellulose makes up approximately 30% of the cell wall and isgenerally in an amorphous state. The remaining 25% or so of the cell wall isthe three-dimensional polymer, lignin, which is also amorphous.

The cell wall structure of wood at the first level is assembled from sub-nanometer fiber bundles of long cellulose polymer chains with some degreeof crystallinity in an amorphous matrix material. Additional componentscalled extractives also exist in wood but only have an indirect impact on themechanical properties of the wood.

The cellulose molecules are seldom found as individual entities in the cellwall, but rather are located in discrete bundles of parallel polymer chains.These discrete bundles in their matrix are further aggregated into a largerunit to create the microfibrils.

The microfibril may be the most important scale for optimization of thematerial. The nanometer scalemicrofibrils are assembled into the material that makes up the cell wall.While this small structure has in turn a number of layers, typically only thelayers that contain the oriented cellulose molecules contribute significantlyto the structural properties of the material.

The layers differ in the angles that the polymer chains make with the centralaxis of the cylinder as well as with the amount of lignin matrix relative to theoriented polymer chains. The thickest of the structural layerscontains nearly 70% of the cellulose and thus forms the main support for thecell. However, the outer layer of the microfibril has the ability to support thefibers that are oriented in the cell wall direction to avoid buckling.

The fibers that would be located in the layers outside of the structural layerwould tend to be oriented at a shallower angle to avoid the buckling, servingas a column wrap. At this level an additional key adaptation takes place.

This is the angle that the microfibrils make to the central axis of the cell. Thismicrofibril angle varies not only within the tree, but also within a single yearof growth. By orienting the microfibrils in the direction of the cell wall, a tallertree is obtained with less energetic cost.


Another key element to the performance of this material is the open porestructure that is formed with these materials. Rather than assembling theseparts into a fully dense structure, the materials form into a series of hollowtubes. This open pore structure is the transport system for the tree nutrientsand creates the low-density high-strength material.

At the same time as the growth at the polymeric level is responding to pre-programmed genetic needs, the cellular structure responds to theenvironment of the individual tree.

The resulting material anisotropy has considerable variation in properties atthis cellular level just as it does at the molecular Ievel.

Wood develops with the growth of layers of cells on a cylindrical innerstructure. As intuition would suggest, the wood has significantly differentproperties in the direction of the tubular structures compared with otherdirections.

The orientation of these axes is able to adapt to the loading on the tree toalign the principal material axes in the directions of the principal stresses.The evolutionary adaptation of spiral grain is thought to result from shearstresses in the tree. Shear stress results from the torsional loading appliedto the tree as a result of asymmetric wind and other loading.

Because mature wood is highly adapted to resist bending from the windloading of the crown, products produced from trees generally assume thatthe principal material axes are in the verticaldirection or the axis of maximum bending stress.

However, in the case of asymmetric loading of the tree, the torsional loadingcan significantly alter the orientation of the principal axis. The adaptation is adynamic response to current conditions on the tree, however only the newgrowth can change in response to the applied stress.

As a result, significant variation in the grain angle can also occur through theradius of the tree, which represents adaptation to changes in applied load onthe tree. Furthermore, the wood will also respond tothe particular load in a region if it is nearly pure compression, as well asreacting to the existence of stress concentrations in the form of damage orknots.

The use of multi-scale material optimization in wood is thus clearlyextensive. The optimization occurs at scales from the molecular to themicroscopic. The optimization is both genetic, representing the exploitationof a niche by the species, as well as individual.

However, what makes it potentially useful as a model for an engineeringsystem is the relative simplicity of the building blocks and the use of a basicstrategy for design that assumes certain ecological objectives.

Biological materials have served as inspiration for a new class of materialsthat has become of significant interest to the mechanics and materialscommunities: functionally graded materials (FGMs). FGMs are defined asmaterials featuring engineered gradual transitions in microstructure and/orcomposition, the presence of which is motivated by functionalrequirements that vary with location within the component.

Recent advances in the selection of materials over the past few decadeshave provided engineers with new opportunities to engineer materials usingFGM concepts. Currently, most structures are engineered by using a largenumber of uniform materials that are selected based on functionalrequirements that vary with location.

Abrupt transitions in material properties within a structure that result fromthese functional requirements leadto undesirable concentrations of stress capable of compromising structuralperformance by promoting crack growth along the interface.

In nature, these stresses are controlled by gradually varying the materialbehavior through a structure, resulting in a FGM. The gradual materialvariation results in a functionally graded architecture described with acontinuously graded or discretely layered interlayer that has several relevantlength scales, as seen in the adjacent figure, and a variation in functionalityin the interlayer.

In a variety of biological structures, from insect wings to bamboo, evidencecan be found that FGMs have been selected through natural evolution tooptimize structural performance through the unique coupling of materialand stress distribution.

However, the advent of new materials and manufacturing processes nowpermits approaches to developing "engineered" materials with tailoredfunctionally graded architectures, such as the "inverse design procedure" .

Biological Inspiration for Functionally Graded Materials


Thus, component design and fabrication have been synergisticallycombined, not just for the manufacture of FGMs, but also for theestablishment of an entirely new approach to engineering structures. Theend resultis the capability of engineering parts that correspond to designer-prescribedproperties.

Most of the work on synthetic materials has focused on controlling stressand crack growth in metal/ceramic composites, while the natural materialshave been limited to understanding the relationship between stress andmicrostructural distributions in bone, bamboo and shells.

In many of these research investigations, characterization has focused onrelating the distribution of material properties, such as hardness, to themicrostructure, independent of the stress distribution in the structure. Foradvanced structural systems, known as smart structures, the materialproperties can actually depend on this stress distribution.

In particular, smart structures can be fabricated from materials, such asshape memory alloys (SMAs), that experience stress-induced phasetransformations that can change the amount of deformation they recoverwhen they are heated.

Functionally grading the distribution of SMA wire reinforcement in smartcomposite panels has already been demonstrated to theoretically increasebuckling strength by controlling the distribution of recovery stresses thatare generated when the wires are heated.

To further advance the development of FGMs, it will be absolutely essentialto use experimental mechanics to characterize the coupling betweenmaterial and stress distributions. In particular, full-field deformationmeasurement techniques, such as Moire interferometry and digital imagecorrelation, combined with advanced microscopy techniques, such aselectron microscopy and atomic force microscopy, and localized propertycharacterization techniques, such as microtensile testing andnanoindentation, will play an important role inelucidating the unconventional structure/property/stress relationshipproduced by this coupling.

Characterization of Functionally Graded Materials

Future Mechanics Research in Functionally Graded Materials

Non-traditional testing methods, such as hybrid numerical-experimentaltechniques, will also be required because of the inherent inhomogeneousbehavior of these materials.

In addition to novel experimental characterization techniques, newtheoretical and computational mechanics concepts are also necessary tounderstand the performance of FGMs

ell as individual.


As explained in the previous chapters, the engineering principles ofbiological systems are characterized by a high level of redundancy andcomplex differentiation in the material hierarchy. It is exactly the redundancyand differentiation, rather than the optimization and standardization, thatcreates robust systems that successfully respond and adapt to differentstresses anf dynamic loads.

The patterns or the structures of biological systems such as waterlilies,sponges and dragonflies inform how the organism behave and performunder a specific range of environmental conditions.

Another type of system that is commonly used in biology and architecture ispolymorphism. Polymorphism describes systems that are made osdifferent elements, forms or individuals. In architecture, polymorphicsystems implies complex interrelations of form, materials and structure inthe design, which is in turn materialized by deploying the logic ofmanufacturing processes.

The leaves of the Amazon water lily gain structural support via girder-likesupport ribs.In still or slowly-moving waters there is one easy way to collect light: a plantcan float its leaves upon the surface. No plant does this on a morespectacular scale or more aggressively than the giant Amazon water-lily.

A leaf first appears on the surface as a huge fat bud, studded with spines.Within a few hours, it bursts open and starts to spread. Its margin has an up-turned rim, six inches high, so that as it expands it is able to shoulder asideany other floating leaf that gets in its way.

Beneath, it is strengthened with girder-like ribs which make the wholestructure rigid. They also contain air-spaces within them that keep it afloat.

Expanding at the rate of half a square yard in a single day, the leaf grows untilit is six feet across.

The underside of the leaf is a rich purple colour and armoured with abundantsharp spikes, perhaps as a defence against leaf-eating fish. One plant canproduce forty or fifty of such leaves in a single growing season andmonopolise the surface so effectively that few plants of other kinds cangrow alongside or below it…

Victoria Amazona (Waterlily)

In 1847, viable seeds arrived at Kew and there the gardeners managed to getthem to germinate. One of the seedlings was sent to Joseph Paxton whowas in charge of the Duke of Devonshire's splendid gardens atChatsworth…Paxton was not only a gardener of great skill but an architectof near-genius. He built one of the first big glasshouses.

When he came to design the cast-iron supports for his hithertounprecedented expanse of glass, he remembered the ribs and struts of hisgiant water-lily that supported the gigantic leaves and used them as thebasis of his designs not only for the glass-houses at Chatsworth but also, afew years later, for his architectural masterpiece, the Crystal Palace inLondon." (Attenborough 1995:290)

In the process of searching for sub divisible modular structure in theWaterlily, the method here described and employed was form finding withforce diagrams.

Architectural devices

Architectural devices1 2

3

1. Branching structure of Victoria Amazonica

2. diagrid structure of the Venus Flower Basket

3. Extraction of the rules of Victoria Amazonica structure

The membrane is the primary aerodynamic structure of the wings. It is avery thin structure, with a thickness of only 2 to 3 m2. Because it is sucha thin structure, the membrane is thought to carry only tensile loading inthe wings, while buckling under the slightest compressive stress.

The primary overall structural property of wings is span wise stiffnessand chord wise flexibility. The leading edge of the wing is comprised of avery stiff structure with three dimensional relief in order to provide highrigidity to the span of the wing. This causes the flexural stiffness alongthe span to be 1- 2 orders of magnitude greater than alongthe chord5.

It is considered that wing corrugation increases not only the warpingrigidity but also the flexibility. The wing of a dragonfly has somecharacteristic structures, such as "Nodus", "Stigma". Nodus is located inthe center of the leading edge, and stigma like a mark is located near theend of the wing. It is considered that these structures not only increasethe flexibility of the wing, but also prevent fatigue fracture of wings.One interesting characteristic to note about a dragonfly wing is thatthere are several different kinds of patterns present in the wing veinframework. The leading edge consists primarily of rectangular frameswhereas the trailing surface is largely formed of hexagons and someother polygons with more than 4 sides.

The patterns seen in the wing would tend to supporting the overallstructural model of a wing with a stiff leading edge and a moreflexible trailing edge, especially considering how the vein sizealsodecreases from the leading edge of the wing to the trailing edgeof the wing as can be seen in the adjacent figure.

Another notable characteristic of wing structure is the three-dimensional structure present in the wing. Although from mostphotographs of wings, they may appear to lay on a flat plane, inactuality the wings are full of three dimensional relief. One example ofthis, as mentioned before is in the leading edge.

The three leading edge veins form a sort of angle bracket structure asshown in the adjacent figure which contributes greatly to span wisewing stiffness.

In addition to this three dimension structure, the wing possesses anoverall camber.


Venus Flower Basket

Dragonfly - Wing Structure

Skeleton of sponge provides strength with lightweight material via itssiliceous composition. Despite its inherent mechanical fragility, silica iswidely used as a skeletal material in a great diversity of organismsranging from diatoms and radiolaria to sponges and higher plants. Inaddition to their micro- and nanoscale structural regularity, many ofthese hard tissues form complex hierarchically ordered composites.

One such example is found in the siliceous skeletal system of theWestern Pacific hexactinellid sponge, Euplectella aspergillum. In thisspecies, the skeleton comprises an elaborate cylindrical lattice-likestructure with at least six hierarchical levels spanning the length scalefrom nanometers to centimeters.

The basic building blocks are laminated skeletal elements (spicules)that consist of a central proteinaceous axial filament surrounded byalternating concentric domains of consolidated silica nanoparticles andorganic interlayers.

Two intersecting grids of non-planar cruciform spicules define a locallyquadrate, globally cylindrical skeletal lattice that provides theframework onto which other skeletal constituents are deposited. Thegrids are supported by bundles of spicules that form vertical, horizontaland diagonally ordered struts.

The overall cylindrical lattice is capped at its upper end by a terminalsieve plate and rooted into the sea floor at its base by a flexible cluster ofbarbed fibrillar anchor spicules. External diagonally oriented spiralridges that extend perpendicular to the surface further strengthen thelattice. A secondarily deposited laminated silica matrix that cements thestructure together additionally reinforces the resulting skeletal mass.

The wing of a dragonfly can be broken into a variety of basic structures.The overall two types of structure present are the veins and themembrane. Both consist of cuticle which is composed of the materialchitin.

The veins provide the primary structural support for the wings. As theirname suggests, the veins are hollow and carry hemolymph which servesto prevent the cuticle of the wing from becoming brittle.


Dragonfly wing with structures of interest

Basal Wing Section

costasubcosta

radius

triangle/supertriangle(levers trailing edge

downward)

.

Distal Wing Section

nodus(provides

stress relief)

pterostigma(sort of counterweight

to control wingflapping)

1. Structure of the Dragonfly wings



These examples, fiberglass structure of the sponge knownas Venus Flower Basket and the structure os the wing of theDragonfly, are interesting not only because of the diamondand polygonal grid, but also because of how these grids areusing a minimal amount of materials to achieve maximumperformance in very versatile and aggressive environments.

Looking at all these patterns and how their structural logicoverlaps give rise to interesting possibilities in assemblinglayers of data and pattern. Balmond cals this template ofideas informal, because there is no hierarchy, onlyinterdependence in the natural patterns.

In overlaying the patterns new 2d frameworks emerges. It isnot a process of simple replication, but of morphogenesis.Although these 2d patterns on a level are fixed, they give anopportunity to be viewed as a projection of more complex 3dnetworks.

These 3d networks begin to represent not just overlayingpatterns, but also compositional field, connectivity,geometry, system, proportion, form and structure.

The patters turns into structure, which becomes architecturaldevices.

Strategy: Jelly substance provides structural support: jellyfish

The composite mesoglea of jellyfish and sea anemonesprovides structural support using collagen fibers in a complex gel matrix.

Scyphozoa

Industrial Sector(s) interested in this strategy: Building, construction,shipping, marine facilities, safety

Quick-set materials, underwater bumpers for ships, protective materials forunderwater structures, personal flotation devices.

Inspiring organism

Bioinspired products and application ideas

Application Ideas

Nature strategies for mantaining physical integrity - Prevention of structural failure

Strategy: Structure protects against compression loading: staghorncoral

The structure of staghorn coral is just one example of a natural branchedsystem that protects against compression loading using scaled strutsjoined in a common lattice.

In nature we notice trees, branching corals, and other fairly stiff items. Inthese systems, all of the struts join in a common lattice, and no motion ispermissible at joints--they're simple, statically determined systems.

If our systems branch, they're usually equipped with lots of triangularelements, although some crude cases (frame houses) use an array ofmechanisms braced against any possible deformations by a structural skinof plywood or something similar.

In nature, more often than not, the branches of a system diverge withoutrejoining, although struts are sometimes joined into trusses--the arms ofsome sand dollar larvae and some bones in the wings of large birds havealready been mentioned

Acropora cervicornisHabitat(s): Marine Neritic

Industrial Sector(s) interested in this strategy: Architecture, structuralengineering, transportation

New building designs that minimize material use without sacrificingstability, new designs for cell and communication towers

Inspiring organism


Application Ideas

Strategy: Leaves resist tearing: brown algae

Leaves of brown algae survive extreme mechanical battering because oftheir sandwiched structure.

The leaves of the brown algae can endure extreme physical stress withoutsuffering any damage, due to its construction according to the sandwichmethod.

Its coarse outer skins were braced against each other by a meticulouslystructured layer of polygonal honeycombs.

Sandwich structures are construction elements in which two thinmembranes are linked to each other by a very loosely and lightly builtsupporting layer. The flexible membranes thus become a mechanical unitwhich is not only much more stable, but also to a great extent protectedagainst local breaks, cracks, and deformations.

The sandwich structure owes all these advantages to the loosely assembledfiller, which transmits the mechanical forces and distributes them quiteevenly over large areas of the membrane surface. In this way, no stressgreat enough to destroy the membrane can appear anywhere.

Durvillaea antarctica

Industrial Sector(s) interested in this strategy: Construction, manufacturing

Creating structures and materials that resist tearing.

Inspiring organism


Application Ideas

Strategy: Flexural, torsional stiffness with minimal material use:organisms

Nature achieves high flexural and torsional stiffness in support structures,with minimum material use, by using hollow cylinders as struts and beams.

Hollow cylindrical tubes. The way these give high flexural and torsionalstiffness with minimal material hasn't been lost on either nature orengineers. We use them as subsystems when building bicycles and racingcars and as entire systems in so-called monocoque aircraft fuselages,cylindrical storage tanks, glass jars, and metal cans.

Nature also uses them in diverse places--bamboo stems; vertebrate longbones; insect, spider, and crustacean appendages; the wing veins ofinsects; and the feather shafts of birds.

Sometimes they contain the entire organism, as in lots of threadlike algae,although it's unclear how much of the stiffness of these last comes fromfluid pressure rather than from their tubular solids--hydrostatic systems andhollow cylindrical beams aren't mutually exclusive.

Microtubules, stiffening elements in cells, are also hollow cylindricalbeams, although they may derive additional stiffness from ordered watermolecules at their surfaces.

Industrial Sector(s) interested in this strategy: Furniture, transportation,construction

Tubular furniture (parts); tubular train cars, buses, etc.; hollowcylindricalbuilding materials, such as beams; all with the potential to reducematerial use.


Application Ideas


All vertebrates and invertebrates with closed circulatory systems havearteries with this non-linear behaviour, but specific tissue properties vary togive correct function for the physiological pressure range of each species.

In all cases, the non-linear elasticity is a product of the parallel arrangementof rubbery and stiff connective tissue elements in the artery wall, anddifferences in composition and tissue architecture can account for theobserved variations in mechanical properties.

This phenomenon is most pronounced in large whales, in which very highcompliance in the aortic arch and exceptionally low compliance in thedescending aorta occur, and is correlated with specific modifications in thearterial structure.

Cephalopoda

Industrial Sector(s) interested in this strategy: Medical, mechanicalengineering, building, textiles

First aid supplies, deployable lightweight pipes or hoses, tents and otherlightweight building materials, fabrics.

Inspiring organism


Application Ideas

Strategy: Arterial walls resist stretch disproportionately: cephalopods

The arterial walls of cephalopods and arthropods resist stretchdisproportionately as stretch increases due to tissue architecture.

In effect, Laplace's law rules out the use of ordinary elastic materials forarterial walls, requiring that an appropriate material fight back againststretch, not in direct proportion to how much it's stretched, butdisproportionately as stretch increases. Which, again in obedience to thedictates of the real world, our arterial walls do--aneurysms, fortunately,remain rare and pathological.

We accomplish the trick first, by incorporating fibers of a non-stretchymaterial, collagen, in those walls, and second, by arranging those fibers in aparticular way. Thus, as the wall expands outward, more and more of theseinextensiblefibers are stretched out to their full lengths and add their resistance to stretchto that of the wall as a whole.

Arterial walls that resist stretch disproportionately as they extendcharacterize circulatory systems that have evolved within lineages quitedistinct from ourown--in cephalopods and arthropods, for instance.

Recruitable collagen fibers don't represent the only possible solution to thebasic problem, and they're not nature's inevitable choice.

The most important mechanical property of the artery wall is its non-linearelasticity. Over the last century, this has been well-documented in vessels inmany animals, from humans to lobsters.

Arteries must be distensible to provide capacitance and pulse-smoothing inthecirculation, but they must also be stable to inflation over a range of pressure.

These mechanical requirements are met by strain-dependent increases inthe elastic modulus of the vascular wall, manifest by a J-shapedstress–strain curve, as typically exhibited by other soft biological tissues.


Strategy: Systems allow changes in mechanical properties: organisms

Systems in nature allow organisms to change shape or their mechanicalproperties without changing the properties of given materials thanks toarticulated struts.

These share the common lattice of compression-resisting elements, buttheir joints (articulations) permit motion. We use them infrequently, but wedo deliberately build joints into many bridges, for example, so the resultingmechanisms can distort safely under changing wind loads, varied 'live' orfunctional loads, or thermal size changes.

Nature often uses the arrangement--major portions of vertebrate skeletonscan be best viewed as mechanisms of articulated struts. The hard elements(ossicles) and their connections in echinoderms such as starfish provideanother example.

Systems build around articulated struts combine nicely with muscles;sometimes, as in insect skeletons, the muscles are on the inside, but theprinciple is the same.

Among the best features of these systems is their ability to alter shape oroverall mechanical properties rapidly without having to change theproperties of specific materials. But even tensile tissues other than musclemay sometimes change properties fairly quickly in response to somechemical signal. These alterations have been studied most extensively in theso-called catch connective tissue of echinoderms.

A starfish undergoes an impressive mechanical transformation as it shiftsfrom being limp enough to crawl with its tube feet on an irregular substratumto being stiff enough so the same tube feet have adequate anchorage whenpulling open the shell of a clam.

Asteroidea

Industrial Sector(s) interested in this strategy: deployable structures

: Cell and utility towers that are more resistant to wind,deployable structures that can alter shape with changing seasonal winds andtemperature.

Inspiring organism:


Application Ideas

Strategy: Vines repair themselves: pipevine

Stems of pipevines repair fissures and ruptures in theirstrengthening tissues by parenchyma cells from surrounding tissuesswelling into the fissure to seal it.

The ability to heal fissures and injuries is characteristic, and an essentialfeature, of living organisms. A lot of research is currently being aim of ourresearch is to analyse fast healing processes in plantsquantitatively and to transfer the insights into biomimetic self-repairingtechnical materials.

A cursory pilot study revealed that some lianas, e.g. various Aristolochiaspecies, are especially suitable models as they exhibit very efficient rapidrepair mechanisms in their stems

[Aristolochia] macrophylla reacts to fissures and ruptures in its peripheralstrengthening tissues by a rapid repair mechanism which seals the lesionsvery effectively and secures the functional integrity of the plant structure. Assoon as a tiny fissure occurs, parenchyma cells from the surroundingcortex tissue swell into this fissure and immediately start to seal it.

At least four discernable phases might be involved in the fast repairmechanism of the sclerenchyma ring in stems of A. macrophylla: (1)elastic/vasoelastic deformation of the walls of the fissure-sealingparenchyma cells, (2) plastic deformation of the cell walls, (3) cell divisionin radial and tangential direction, and (4) lignification of the (mostperipheral) fissure-sealing cells.

The initial phases of fast repair of fissures take place without cell divisionand (significant) cell wall biosynthesis but mainly by physico-chemicalprocesses altering the mechanical properties of the cell walls of the fissure-sealing parenchyma cells.

This finding is encouraging for attempting to tranfer this repair mechanisminto technical applications.

Industrial Sector(s) interested in this strategy: Manufacturing, construction,medical, textiles

Lightweight architecture that self-seals. Self-sealing pneumatic structures.Self-repairing clothing, medical technology, pipelines, etc.


Application Ideas


Strategy: Microscopic holes deter fractures: starfish

Ossicles of starfish resist fractures via microscopic holes in the structure.

Use 'foamy' materials in which any threatening crack will be in short order runinto a hole. Not only does this reduce the chance of cracking, but it savesmaterial--less can be more.

The little hard bits of echinoderms, the ossicles, develop as single crystals,but they avoid the excessive brittleness typical of crystals by being especiallyholey.

Wood gains some material benefit from similar voids. Such materials comeunder the heading of 'cellular solids,' the term having no connection with'cellular' in the strictly biological sense but in the sense thatHooke…originally used the word for the microscopic holes in cork.

Asteroidea

Industrial Sector(s) interested in this strategy: Construction, ceramics,materials science, building science, pipes

Concrete and other building materials that better resist fractures, ceramicsthat resist fracture, cans and other packaging that fracture more easily(maybe to save energy during recycling). Plastics (computer cases, etc.)that prevent cracks from spreading; building materials, such as concrete,that stop cracks from spreading; pipes that "self-arrest" any cracks.

Inspiring organism


Application Ideas

Strategy: Elastic ligament provides support, shock absorption: largegrazing mammals

The nuchal ligament of large grazing mammals provides support for thehead and seems to act as a shock absorber, due to the presence of theprotein elastin

Our own rubber, elastin, occurs mainly as a component of two composites,skin and arterial wall. The nearest thing to pure elastin is the nuchal ligamentof large grazing mammals.

It runs from a ridge on the rear of the skull back along the top of the neck tothe thoracic vertebrae; it seems to act as a shock absorber as well as asupport for the head.

Bovidae

Industrial Sector(s) interested in this strategy: Structural engineering,materials science, telecommunications, utilities, transportation

Flexible yet resilient cables and hoses, composite materials, improvedmaterials for shock absorption (e.g. seatbelts).

About the inspiring organism


Application Ideas


Strategy: Continuous fibers prevent structural weakness: trees

Knotholes in wood do not crack because the fibers around them arecontinuous.

There has been relatively little attempt to produce an artificial analogue towood because wood is cheap, lightweight, tough, moldable, and easilyshaped.

However, when a hole is drilled in timber, it weakens the structure. The tree,however, drills no holes, even though it must disrupt the trunk's wood wherea new branch pushes through. The fibers deform around a knothole,remaining continuous.

Research is been carried out into how this can be used in fibrous compositematerials.

Plantae

Industrial Sector(s) interested in this strategy: Construction

Creating construction materials with the properties of wood.



Application Ideas


Nature strategies - Bucklingfor mantaining physical integrity

Inspiring organism


Application Ideas

Equisetum hyemale

Industrial Sector(s) interested in this strategy: Construction, transportation

Technical smart materials with stiffness varying as needed using pneumaticstructures. Uses could include airplane wings, spoilers on cars, and buildingshells.

Strategy: Stems vary stiffness: scouring horsetail

Stems of scouring horsetail vary their stiffness by having rings of supportivetissues that react to changes in turgor.

Plants with hollow axes, e.g. various horsetails and grasses, serve asgenerators of biological concepts for technical structures with variablestiffness. Their structure is characterised by a thin outer ring of strengtheningtissue stabilised by a lining of parenchyma cells.

The hollow stems are divided into shorter segments (internodes) bytransverse walls and stem thickenings at the so called nodes. The nodessignificantly reduce the danger of local buckling in these light-weightstructures.

The stability of these stems depends significantly on the internal pressure(turgor) of the parenchymatous cells. If the turgor pressure is reduced, e.g.by water deficiency, stiffness and stability of the stems decrease.

In some species--such as the Brazilian Giant Horsetail (Equisetumgiganteum) the resistance to ovalisation is extremely turgor-dependent. Inother horsetail species--such as the Dutch Rush (E. hyemale)--the outer ringof strengthening tissue is connected via wedge-shaped elements with aninner ring of strengthening tissue forming a mechanically resistant sandwichstructure.

These stems are also stabilised by the pressurised lining of parenchymatouscells but depend much less on the turgor pressure of the parenchyma cells.The mechanical stability resisting stem ovalisation is diminished by onlyabout 20% due to reduction of the turgor pressure.

Potential technical implementations are manifold, inspired by plants withmechanical properties of the stem varying with the internal pressure of thepressurised cellular lining.

These include light-weight structures with chambered pressure-stabilisedpneumatic structures that feature a segmental variation of stiffness and theability to adapt their stiffness or form to changing outer conditions, facilitatedeither adaptively or via integrated active control.

Envisaged technical applications for these types of biomimetic technicalsmart materials include: (1) shells of airplane wings and other aircraft(adaptation to changing aerodynamics); (2) shells of buildings of innovativeconstruction; (3) car parts, e.g. aerodynamically adjustable spoilers.

Strategy: Shape of feather shafts protect from wind: birds

The shafts of feathers and petioles of leaves protect from wind by havingnon-circular cross sections.

In cross section, feathers look like grooved petioles upside down. Again, thatmakes functional sense. If an elongated structure must have a groove toraise EI/GJ ('twistiness-to-bendiness ratio'), the groove should be on the sidethat's loaded in tension. That location won't increase the structure's tendencyto buckle, since tensile loading is nearly shape-indifferent.

A leaf blade bends its petiole downward; its aerodynamic loading bends afeather upward--leaf blades hang from the ends of their petioles; flying birdshang from bases of their wing feathers.

Aves

Industrial Sector(s) interested in this strategy: Architecture, construction,structural engineering

Incorporating materials with noncircular (and nonrectangular) crosssections into building design, construction, and engineering applications,such as fenceposts or bridge supports.

Inspiring organism


Application Ideas

Strategy: Skeleton provides support: sponges

The spicular skeleton of sponges provides structural support in the form ofdispersed struts.

In nature, the [dispersal strut] scheme is commoner but still far fromwidespread—the clearest example, is the spicular skeleton of sponges, inwhich tiny rigid elements are laced together by collagen . And there areoccasional forays in this direction among sea anemones (coelenterates) andsea cucumbers (echinoderms).

It ought to be reemphasized that the arrangement is not intrinsically flawed insome way; the limitation is more likely to lie in problems of compatibility withattachment surfaces for muscles.

Porifera

Industrial Sector(s) interested in this strategy: Refugee camps, military,recreation

More wind-resistant tents that require fewer poles.

Inspiring organism


Application Ideas


Strategy: Quills resist buckling: porcupine

Quills of porcupines resist buckling because they are made of a dense outershell surrounding an elastic, honeycomb-like core.

Thin walled cylindrical shell structures are widespread in nature: examplesinclude porcupine quills, hedgehog spines and plant stems. All have an outershell of almost fully dense material supported by a low density, cellular core.

In nature, all are loaded in some combination of axial compression andbending: failure is typically by buckling. Natural structures are oftenoptimized.

Mechanical models recently developed to analyze the elastic buckling of athin cylindrical shell supported by a soft elastic core (G.N. Karam and L.J.Gibson, Elastic buckling of cylindrical shells with elastic cores, I: Analysis,submitted to Int. J. Solids Structures, 1994, G.N. Karam and L.J. Gibson,Elastic buckling of cylindrical shells with elastic cores, II: Experiments,submitted to Int. J. Solids Structures, 1994) were used to study themechanical efficiency of these natural structures.

It was found that natural structures are often more mechanically efficientthan equivalent weight hollow cylinders. Biomimicking of natural cylindricalshell structures may offer the potential to increase the mechanical efficiencyof engineering structures.

common porcupineErethizon dorsatum (Linnaeus, 1758)Habitat(s): Forest, Grassland, Shrubland

Industrial Sector(s) interested in this strategy: Engineering, structuralengineering

Design of architectural structures with high buckling resistance, withpotential application in earthquake-proofing.



Application Ideas


Inspiring organism


Application Ideas

Equisetum hyemale

Industrial Sector(s) interested in this strategy: Construction, transportation

Technical smart materials with stiffness varying as needed using pneumaticstructures. Uses could include airplane wings, spoilers on cars, and buildingshells.

Strategy: Stems vary stiffness: scouring horsetail

Stems of scouring horsetail vary their stiffness by having rings of supportivetissues that react to changes in turgor.

Plants with hollow axes, e.g. various horsetails and grasses, serve asgenerators of biological concepts for technical structures with variablestiffness. Their structure is characterised by a thin outer ring of strengtheningtissue stabilised by a lining of parenchyma cells.

The hollow stems are divided into shorter segments (internodes) bytransverse walls and stem thickenings at the so called nodes. The nodessignificantly reduce the danger of local buckling in these light-weightstructures.

The stability of these stems depends significantly on the internal pressure(turgor) of the parenchymatous cells. If the turgor pressure is reduced, e.g.by water deficiency, stiffness and stability of the stems decrease.

In some species--such as the Brazilian Giant Horsetail (Equisetumgiganteum) the resistance to ovalisation is extremely turgor-dependent. Inother horsetail species--such as the Dutch Rush (E. hyemale)--the outer ringof strengthening tissue is connected via wedge-shaped elements with aninner ring of strengthening tissue forming a mechanically resistant sandwichstructure.

These stems are also stabilised by the pressurised lining of parenchymatouscells but depend much less on the turgor pressure of the parenchyma cells.The mechanical stability resisting stem ovalisation is diminished by onlyabout 20% due to reduction of the turgor pressure.

Potential technical implementations are manifold, inspired by plants withmechanical properties of the stem varying with the internal pressure of thepressurised cellular lining.

These include light-weight structures with chambered pressure-stabilisedpneumatic structures that feature a segmental variation of stiffness and theability to adapt their stiffness or form to changing outer conditions, facilitatedeither adaptively or via integrated active control.

Envisaged technical applications for these types of biomimetic technicalsmart materials include: (1) shells of airplane wings and other aircraft(adaptation to changing aerodynamics); (2) shells of buildings of innovativeconstruction; (3) car parts, e.g. aerodynamically adjustable spoilers.


Strategy: Sclereid cells prevent soft tissue collapse: plants

Sclereid cells in vascular plants help prevent the collapse of soft tissuesduring water stress via thick, lignified walls.

Sclereids are also cells with thick, lignified walls. They are grouped withfibres under the general term sclerenchyma. They differ from fibres ingenerally being shorter in relation to their length, but there is some overlap inthe range of cells.

They may be branched, sinuous or short -- often more or less isodiametric.The longer ones commonly feature in the sheaths to veins, particularly nearthe ends of the finer branches.

They can be pit-prop-like when they extend between the upper and lowersurfaces of leaves, and appear to help prevent collapse of softer tissues attimes of water stress, as in olive leaves and the leaves of many mangroveplants.

These plants, and many of the hard-leaved plants found in arid habitats, oftenhave abundant elongated or branched sclereids.

Plantae

Industrial Sector(s) interested in this strategy: Musical instruments, building

Material design applications to prevent cracking in musical instrumentsas drying occurs, structural or material designs that prevent cracking in wallsorfoundations at moisture levels fluctuate.

Inspiring organism


Application Ideas

Strategy: Bones self-heal: vertebrates

Osteoclasts/osteoblasts of bones maintain skeletalhomeostasis by resorbing bone/forming newly synthesized matrix.

"Bone remodeling is the result of the coordinated activity of osteoblasts,which form new matrix, and osteoclasts, which resorb bone…Boneremodeling is a temporally and spatially regulated process that results in thecoordinated resorption and formation of skeletal tissue.

Bone remodeling is carried out in basic multicellular units in whichosteoclasts resorb bone and osteoblasts form newly synthesized matrix in acoordinated process that takes about 4 months.

The number and function of osteoclasts and osteoblasts are regulated byextracellular and intracellular signals acting in a coordinated fashion tomaintain skeletal homeostasis.

Vertebrata

CAO and SKO design software, Lightweighting software reduces resourceuse, saves energy, Industrial Sector(s) interested in this strategy:Construction, manufacturing

Self-healing material such as concrete and ceramics. Building materialsthat adjust to internal and external stresses by adding or removing material asneeded.

Creating lightweight, yet strong materials by taking away unneeded materialand adding material where stresses are greatest.

Inspiring organism


Application Ideas

Nature strategies - Compressionfor mantaining physical integrity

Strategy: Support cells resist compression: nasturtium

The leaf stalks of Nasturtiums resist compression viaorthotetrakaidecahedron-shaped support cells.

The compressive core [of herbaceous stems] holds especial interest. Theinner, thin-walled, cells (parenchyma) of such a stem had a particular shape--a so-called orthotetrakaidecahedron, a fourteen-sided solid with eight six-edged faces and six four-edged faces .

If a set of distortable spheres of equal size are squeezed together so theycompletely fill some large volume, each will (ideally at least) take on thisparticular shape.

It's the shape that permits each to expose the minimum surface area. So thepeculiar shape of these cells supports the idea that they're being squeezedand thus that they function as a compression-resisting core.

Only thin cell walls are needed—pressure shouldn't differ among theindividual cells, and most of the motile animal systems lack any partitions atall. Still, these flimsy-looking cells have a crucial supportive function.

The fourteen-sided shape, incidentally, idealizes somewhat--the real cellsactually vary a bit.

Tropaeolum[Nasturtium]

Industrial Sector(s) interested in this strategy: Engineering, building, textilesand absorbent materials

Compression-resistant pipes, compression-resistant concrete thatincorporates orthotetrakaidecahedron shape, compression-resistant textilesand absorbentmaterials.

Iinspiring organism


Application Ideas

Strategy: Matrix stiffens connective tissue: sponges

The connective tissue of sponges is a matrix stiffened by embeddedspicules.

Putting small pieces of brittle material into a pliant matrix gives a compositecalled a 'filled polymer'--it amounts to a kind of random array of mechanisms.Koehl (1982) looked into the extent to which the connective tissues ofanimals that had embedded spicules behaved like proper filled polymers--embedded spicules are fairly widespread, not just in sponges, but in somecoelenterates, echinoderms, mollusks (the chitons), arthropods (stalkedbarnacles), and ascidians.

She took isolated animal spicules of various kinds and concentrations,embedded them in gelatin (raspberry flavored), and performed variousmechanical manipulations on the products.

Since the normal function of spicules is to stiffen tissue (although we're stillconsidering relatively unstiff structures), she used that as a criterion ofeffectiveness.

Even a relatively small proportion of spicules dramatically increasesstiffness; more spicules or more elongate or irregularly shaped spicules givemore stiffness, and small spicules are more effective than are large ones for agiven added mass.

One factor that matters a lot is the area of contact between spicules andmatrix, not unlike other composites. So whether the spicules are in a specificframework or in a random array, roughly the same rules seem to apply.

Porifera

Industrial Sector(s) interested in this strategy: Building, recycling, research

Composite concrete, recycling brittle materials to create composites,enhance functionality of existing composites.

Iinspiring organism


Application Ideas


Strategy: Structural composition provides strength in changingconditions: plants

The cell walls of vascular plants provide mechanical strength during differentstages of growth by adjusting their structural composition.

Plant cells need to be fully hydrated to work properly (except in periods ofdormancy, as for example in many seeds). Individual vegetative cells inplants, unlike those in animals, are encased in a cellulose cell wall.

The cellulose cell wall may be very thin, in cells that are actively dividing, asfor example, in growing shoot or root tips.

However, once developed into their mature form, the cell walls may becomethicker, and additional substances, mainly lignins, incorporated into theirstructure.

The cells themselves, then, contribute to the mechanical strength of theplant. Thin-walled cells when fully hydrated, are like small, pressurisedcontainers. Mature cells, especially those with thick walls, have mechanicalstrength of their own, even without watery contents. Indeed, many fibres lackliving contents when mature.

Plantae

Industrial Sector(s) interested in this strategy: Construction, structuralengineering

Example of how elements that serve one purpose can be adapted orincorporated to serve a later functional purpose within the same context: e.g.,scaffolding that becomes part of a building's frame or materials whosethickness can fluctuate based on changing temperatures or load-bearingrequirements.

Inspiring organism


Application Ideas

Strategy: Hole structure strengthens bone: horse

A metacarpal bone of a horse avoids structural weakness caused by a holevia stress-dispersing microstructure. Zebras, horses and other equinespecies put substantial stress upon their central forefoot bones, particularlythe third metacarpal, bones with remarkable strength despite having holes inthem for blood vessels to pass through.

The presence of a hole (or foramen) in a structural element offers thepotential for it to act as a site of stress concentration and initiation of cracks,yet these foramina do not weaken the bone nor act as fracture initiation sites.Hence the foramen in the third metacarpal of equine species has been ofinterest to engineers to learn how to design openings in structures in a waythat avoids cracking.

The key features investigators have found that minimize cracking at thesesites are: their location in regions predominantly experiencing compression,their elliptical rather than round shape (oriented parallel to the long axis of thebone and the lines of force), the 'softening' of the material discontinuity byincreased compliance of the tissue surrounding the opening that shifts peakstresses away from the foramen edge, and a ring of increased stiffnessreinforcing the foramen at some distance from it to absorb those stressesshifted inward from the compliant foramen edge.

Many human-made structures, such as airplane wings, need to have holes inthem to accommodate wires, fuel lines or hydraulic system elements andhence inspiration from the design of foramina in bones could have wideapplication.The third metacarpus bone in a horse's leg supports much of theforce conveyed as the animal moves. On one side of the cucumber-size boneis a pea-size hole where blood vessels enter.

As a rule, drilled holes weaken structures, causing them to break more easilythan solid structures when pressure is applied, but nature has found a way ofcircumventing this rule, the (horse) bone was configured in such a way that itpushed the highest stresses away from the foramen into a region of higherstrength…the bone's hole is also tougher than a typical drilled hole—moreresistant to initial cracks growing to catastrophic lengths.

Inspiring organism: Horse Equus caballa

Bioinspired products and application ideas: industrial Sector(s) interested inthis strategy: Construction, manufacturing

Application Ideas: Use for increasing strength of airplanes, boats,automobiles, other structures that have holes for wiring or fuel and hydrauliclines. (A quick rule of efficiency in the aerospace industry is that one pound ofweight saved in a plane can save 10 pounds of fuel.)


Strategy: Fibers reinforce hydrostatic skeletons:sunflowers

Hydrostatic structures found in sunflowers and other many other organismsserve various functions but almost always use helical fibers asreinforcement.

With few exceptions, nature uses the second arrangement of fibers for herinternally pressurized, water-filled cylinders. These structures (often termed'hydrostatic skeletons' or 'hydroskeletons' as well as 'hydrostats') havehelical reinforcing fibers.

And this particular arrangement is no rare or once-evolved thing. It occurs inthe stems of young herbaceous (nonwoody) plants such as sunflowers; itprovides a wrapping for flatworms (platyhelminths and nemerteans),roundworms (nematodes), and segmented worms (annelids); it stiffens thebody wall of sea anemones; it determines the response to musclecontraction of the outer mantle of squids; and it's a major functionalcomponent of shark skin.

The material of the fibers varies widely, the functions of these hydroskeletonsare even more diverse, but the wrapping is almost always helical.

Asteraceae

Industrial Sector(s) interested in this strategy: Water storage, materialsscience, building materials, textiles.

Water storage containers that resist fouling and leaks; building materials,tents, and other structural materials that include reinforcing helical fibers; no-rip fabrics, such as mosquito nets.

Inspiring organism


Application Ideas

Strategy: Collenchyma cells provide strength, flexibility: plants

Collenchyma cells in vascular plants support growing parts due to flexiblecellulosic walls, which lignify once growth has ceased.

In addition to the 'mechanical' cells - fibres and lignified parenchyma - a thirdcell type has mechanical functions. This is collenchyma. Collenchyma cellshave walls which during their development and extension are mainlycellulosic.

They grow with the surrounding tissue as it expands or lengthens. They aremore flexible than fibres, and if they remain unlignified, as they might inassociation with leaf veins or midribs, or in leaf stalks (petioles), they allowfor a high degree of flexibility in the organ itself.

Often, after growth in length of stems has occurred, and more mechanicalrigidity is an advantage, we find that the collenchyma cells become lignified,and function more as fibres.

Plantae

Industrial Sector(s) interested in this strategy: Architecture, building,nanotechnology, materials science

Design ideas for adding strength to structures or materials.

Inspiring organism


Application Ideas


In trees, junctions between main trunks and branches, for instance, areplaces of concentrated stresses. Trees compensate for this extra stress byadding more material to the shoulder.


Pinus sylvestris[Scots pine, Scotch pine]IUCN Red List Status: Least ConcernHabitat(s): Forest


CAO and SKO design software, Lightweighting software reduces resourceuse, saves energy, Industrial Sector(s) interested in this strategy:Construction, manufacturing, medicine.

Application Ideas

Lightweighting for manufacture and construction of vehicles, buildings,bridges, prosthetics.

Strategy: Lightweighting: Scots pine

Trunks and branches of trees withstand external stresses through load-adaptive growth.

Trees and bones achieve an even distribution of mechanical tension throughthe efficient use of material and adaptive structural design, optimizingstrength, resilience, and material for a wide variety of load conditions.

For example, to distribute stress uniformly, trees add wood to points ofgreatest mechanical load, while bones go a step further, removing materialwhere it is not needed, lightweighting their structure for their dynamicworkloads.

At the scale of the cell, trees arrange fibers in the direction of the flow of force,or principal stress trajectories, to minimize shear stress.

Engineers have incorporated these and other lessons learned from trees andbones into software design programs that optimize the weight andperformance of fiber-composite materials.

For example, car parts and entire cars designed with these principles haveresulted in new vehicle designs that are as crash-safe as conventional cars,but up to 30% lighter.

The analogy that [Claus] Mattheck wants to pursue, though, is not thatbetween trees and other organisms, but between trees and engineeredartefacts. If trees achieve longevity and structural stability, aren't these thequalities of reliability and integrity that engineers want to design intoproducts?

The key to this is Mattheck's contention that the structural optimisation intrees and apparent in other natural structures such as animal bones is allabout making the external and internal stresses as uniform across the wholestructure as possible.

Mattheck calls this the 'axiom of uniform stress' and adds that, though he cancite plenty of examples of it, he cannot prove it exists…Mattheck'scontention is that trees are constantly readjusting this balance by addingmore material at points of high stress and adding no material at points of lowor no stress. (Bones, he contends, go one stage further by actually shrinkingat points of low stress.)


Strategy: Thickness stabilizes tall trees: baobob

The trunk of the tall baobab tree compensates for the weak, water-storingstem via thick bark.

The adaptive significance of building a stem out of weak, low density wood isnot immediately apparent, particularly as extensive use of stored water doesnot appear possible under this strategy.

Baobab trees, however, have very thick bark, a design feature thatcontributes significantly to overall structural stability of the stem and maycompensatefor the reductions in stem stiffness that would otherwise occur throughmoderate use of stem water.

More detailed biomechanical and energetic analyses may demonstrate thatbaobab trees can actually achieve greater strength for a given energyinvestment that other trees, and the possibility that the maintenance of a largequantity of living parenchyma cells is somehow advantageous, whether forcarbohydrate storage or recovery from traumatic injury, cannot bediscounted

Inspiring organism

Adansonia[baobob tree]


Industrial Sector(s) interested in this strategy: Building industry

Application Ideas: Structural engineering of large buildings.

Strategy: Rod-like reinforcements provide strength: plants

Vascular bundles in plants provide mechanical strength, serving as rod-likereinforcements.

Part of a stem of a robust grass, in cross section. Here mechanical strengthofthe stem is provided by the vascular bundles set in a matrix of thinner-walledcells, rather like rod reinforcements.

Each vascular bundle has an outer sheath of fibres, forming a strong tube inwhich the two wide vessels can conduct water, and the strand of thin-walled,narrow cells (phloem) can transport sugar solutions with little risk ofdamage.

Just to the inner side of the outer ring of smaller vessels the several layers ofnarrow cells eventually become thick-walled and provide additional strengthin the form of a cylinder to the whole stem.

Inspiring organism

Plantae


Industrial Sector(s) interested in this strategy: Materials science, structuralengineering

Application Ideas

Models for the arrangement of structural elements that provide variouslevels of mechanical support, models for composite materials. Alsomultifunctional – used for liquid transport as well as structural support.


Strategy: Fibers keep tall spikes upright: titan arum

The tall spadix of the titan arum plant remains upright because it is filled withcobweb-like support fibers.

The tall grey spadix [of the titan arum], which is filled with cobweb-likesupport fibres, becomes flaccid, topples forward and droops over themargin of the spathe.

The spathe itself contracts inwards and its upper margins start to twist roundthe lower part of the spadix, clasping it so tightly that a huge water-tight bag iscreated.

Inspiring organism

Amorphophallus titanumAmorphophallus titanum (Becc.) Becc.[Titan arum]IUCN Red List Status: Unknown


Industrial Sector(s) interested in this strategy: Mechanical engineering,automotive, aviation, building

Application Ideas: Lightweight building materials with high material strength.

Strategy: Pressure provides structural support: blackback land crab

The body of the blackback land crab functions duringexoskeletal molt using both gas and liquid pressure, or a pneumo-hydrostaticskeleton.

Here we show that whenever its exoskeleton is shed, the blackback land crabGecarcinus lateralis relies on an unconventional type of hydrostatic skeletonthat uses both gas and liquid (a 'pneumo-hydrostat').

To our knowledge, this is the first experimental evidence for a locomotorskeleton that depends on a gas. The aquatic blue crabC allinectes sapidusmaintains mobility by switching to a hydrostatic skeleton 10 — a fluid-basedskeleton that is common in soft-bodied invertebrates.

Hydrostatic skeletons are arranged so that the force of muscle contraction istransmitted by an essentially incompressible aqueous fluid . Musclecontraction increases the pressure in the fluid, causing the deformations orstiffening required for support, movement and locomotion.


blackback land crabGecarcinus lateralis (Freminville, 1835)[Blackback land crab]IUCN Red List Status: Unknown


Industrial Sector(s) interested in this strategy: Construction, packaging,transportation

Application Ideas

Hydrostatic skeletal support.


Strategy: Interwoven trees gain structural support: tropical trees

Trees gain support by growing together in an upward spiral.

In tropical jungles, with their great variety of species, we encounter amultitude of Mechanical ideas for construction.We also find thin trunksjoining into bundles, supporting each other and forming an upward windingspiral.

Obviously, the plants compete for the light at the top by sophisticatedtechnical means.


Industrial Sector(s) interested in this strategy: Construction

Application Ideas

More stable buildings in areas prone to high winds.

Strategy: Reinforced fibers provide strength: plants

Fibers in many woody plants provide mechanical strength via ligninreinforcements.

Plant fibres occur in the wood of many plants, and because of theirassociation with the xylem, are called xylary fibres. They are also often foundin the outer part of young stems, bark and leaves, where they are calledextraxylary fibres.

Their main functioning is in strengthening. The common feature of fibre cellsis that they are elongated and thick-walled, with lignins permeating thecellulose of the cell wall.

Fibre cells normally have pointed ends. They often extend in length duringdevelopment, growing between cells that may not be lengthening at the samerate.

Fibres may be only about 10 times longer than wide, but many are 20-30 andeven up to and exceeding 100 times longer than wide.

They may remain flexible, as in many extraxylary fibres, or have more limitedflexibility, as in xylary fibres.

Inspiring organism

Plantae


Industrial Sector(s) interested in this strategy: Architecture, building,nanotechnology, materials science

Application Ideas

Composite building material with properties of high lignin wood. Designideas for adding strength to structures or materials.


Strategy: Crystals and fibers provide strength, flexibility: bones

The composition of bones grants them strength, light weight, and someflexibility via small inorganic crystals and thin collagen fibers.

Nature has no reason for making a bone round or square. The outlines ofbones, therefore, follow the stress lines or are vertical to them so that theygive an indication of the pressures the bone has to withstand.

But this ideal distribution of bone material along the stress lines would havebeen to little avail were the material itself not so well adapted to extraordinarypressure.

Just like fiberglass made of synthetics threaded with glass fiber, bone tissueis made up of two constituents which greatly differ in their mechanicalproperties.

About half the bone volume is made up of inorganic crystalline material. Itconsists of phosphate, calcium, and hydroxyl ions and comes very close tohydroxylapatite in structure.

It appears in the bone in the form of tiny crystals, only about 200 atomicdiameters in size. They are inserted between thin fiber hairs of the elasticmaterial collagen and seem to be linked with them.

Many of these parallel inorganic and organic building blocks form fascicles,which may be interwoven in various ways.

The end product is a material that is considerably stiffer than collagen,though low in weight, but by far not asbrittle and inelastic as pure hydroxylapatite.

Besides, because of the continuous alternation between brittle and elasticmaterial, there is little chance for a fracture to spread unchecked

Mineralized collagen fibrils are highly conserved nanostructural buildingblocks of bone. By a combination of molecular dynamics simulation andtheoretical analysis it is shown that the characteristic nanostructure ofmineralized collagen fibrils is vital for its high strength and its ability to sustainlarge deformation, as is relevant to the physiological role of bone, creating astrong and tough material.

Strategy: Nest cells support heavy weights: bees and wasps

Hives of bees and wasps support heavy weights usinghexagonal cells in offset positions.

The hexagonal cells of bees and wasps create an extraordinarily strongspace-frame, in particular in the vertical bee comb with two cell layers backto back with half a cell's shift in the position to create a three-dimensionalpyramidal structure.

The extraordinary strength is exemplified by a comb 37 centimetres by 22.5centimetres in size, which is made of 40 grams of wax but can contain about1.8 kilograms of honey.

A bees' honeycomb is one of the wonders of the world. Layer upon layer ofhexagonal cells of identical size and shape are stacked together as preciselyas if the bees had worked to a grid drawn on graph paper.

But why should bees build hexagonal cells? Why should they not be square,like boxes, or circular?

Natural organization is economical, expending the least amount of energyand using the least material necessary for a task. Three-way junctions of120° angles occur quite widely in nature, being the most economical anglefor joining things together.

About the inspiring

organism


An analysis of the molecular mechanisms of protein and mineral phasesunder large deformation of mineralized collagen fibrils reveals a fibrillartoughening mechanism that leads to a manifold increase of energydissipation compared to fibrils without mineral phase.

This fibrillar toughening mechanism increases the resistance to fracture byforming large local yield regions around crack-like defects, a mechanism thatprotects the integrity of the entire structure by allowing for localized failure.

As a consequence, mineralized collagen fibrils are able to toleratemicrocracks of the order of several hundred micrometres in size withoutcausing any macroscopic failure of the tissue, which may be essential toenable bone remodelling.

The analysis proves that adding nanoscopic small platelets to collagen fibrilsincreases their Young's modulus and yield strength as well as their fracturestrength.

Inspiring organism

Chordata


Industrial Sector(s) interested in this strategy: Construction, manufacturing,nanotechnology, materials science, medical, building, automotive, CO2sequestration

Application Ideas

Building strong, lightweight materials that can take a lot of stress. Highstrength materials, composites to use as bone replacements or mechanicallimbs. Mineralized fibers as construction element for buildings handlingshear and torsional stresses (earthquake, hurricane, etc). Using fractureresistant, yet impact absorbing fiber structure material for structure ofautomobile. Utilizing CO2 calcification of natural or synthetic fibers to createnovel material while sequestering CO2.


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http://www.ocean-designresearch.net/index.php?option=com_content&view=article&id=15&Itemid=30

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http://www.biomimetics.org.uk/

http://www.extra.rdg.ac.uk/eng/BIONIS/

http://www.biokon.net/bionik/beispiele.html.en

http://biodigitalarchitecturaldesign.blogspot.com/ Bibliography

advanced sustainability research

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natural materials

biogenic inorganic crystals

combination of materials

inorganic components

way nature

single crystals

different properties

synthetic silica