advanced sustainability research

Advanced Sustainability - Ss09 Porfessor Matias del Campo Deborah Kaiser

Structural Integrity in nature

Reverence for the beauty and usefulness of the naturally occurring materials around us has been felt and expressed ever since man learned to use them for improving the quality of life and standard of living. Various stages, in the growth of our civilization are therefore aptly named after stone, iron and bronze. It is customary to refer to the current millennium as the age of materials.

This is no less a measure due to the rapid strides that the field of materials science and technology has made over the last fifty years or so. In our persistent attempts to improve the performance and versatility of a given material or combination of materials as in composites, we have learnt to observe and analyse the way nature has so successfully developed materials in living organisms and systems.

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

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

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

For example, silica in the sponge f.SiO2/3_H2Og monorhaphis is found in the form of a spicule of up to 3_0m length. In the cross section,it is composed of several concentric layers varying in thickness from 10 _m in the centre to 3_m at the periphery. The layers are deposited on an axial organic fibre. Under three-point bend testing, it exhibits strength of 593MPa as against a value of 155MPa for synthetic silica. The work to fracture is also 30 times higher in the spicule (Levi et al 1989). Intercalation of acidic macromolecules into the crystal lattice appears to be common.

Similarly, spider dragline silk is far superior to steel of comparable dimensions and the energy to fracture on equivalent weight basis is 100 times 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 single crystal of calcite (Jackson et al 1988; Currey 1977).

Such examples together with increasing pressure towards the conservation of the environment have led materials scientists and engineers to carefully study natural systems, their design and their methods for the synthesis of constituent materials.

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

Natural materials are self-generating, hierarchical, multifunctional, nonlinear, composite, adaptive, self-repairing and biodegradable. Bones in animals illustrate some of these charac- reduces in density and weight. This phenomenon is indicative of the ability of bone to adapt itself to the changing demands of the sustained stress levels to which it is subjected (Lakes 1993). In this respect bone is a smart material even if the response time is longer.

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

Biomimicry - Intro

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

Perceptions regarding the scope of biomimetics appear to vary very widely depending upon the specialized discipline of the investigator. Japanese electronic companies are supporting biomimetic research with a view to learning the way biological systems process information.

On the whole, the field of biomimetics addresses more than one issue.

Those engaged in this field of research activity try to mimic natural methods of manufacture of chemicals in order to create new ones (imitate barnacles for producing a natural glue), learn new principles from phenomena observed in nature (flight of birds and insects, swimming of fish and aquatic animals), reproduce mechanisms found in nature and copy the principles of synthesising materials under ambient conditions and with easily available raw materials.

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

Biomedical engineers consider biomimetics as a means of conducting tissue engineering and trace the origins of biomimetics to ancient times when Mayan, Roman and Chinese civilizations had learnt to use dental implants made of natural materials.

Material scientists view biomimetics as a tool for learning to synthesise materials under ambient conditions and with least pollution to the environment.

Chemists have always wondered at the ease with which ammonia is produced in biological nitrogen fixation, methanol is produced in biological oxidation of methane and oxygen is generated in photosynthesis (Shilov 1996). They hope to learn the synthesis of polymers that can perform the roles of enzymes in such processes. Biologists study biomimetics not only for an understanding of the biological processes but also to trace the evolution of various classes of organisms.

Figure 1. (a) The crossed lamellar structure of an abalone shell. (b) Co-aligned aragonite rods in mollusc shell.

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

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

Engineers and architects attempt to explore the relationship between structure and function in natural systems with a view to achieve analogous synthetic design and manufacture.

Definition and scope

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

Similarly, the origins of the design of the Eiffel Tower are traced to the inspiration derived from the structure of the head of the femur in the thigh.

Hermann Von Meyer, professor of anatomy at Zurich observed that the head of the femur has many fingers of bone arranged in curving lines.

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

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

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

In sharks, skin scales possess tiny ridges that run parallel to the longitudinal body axis and reduce the drag (Dickinson 1999). In porpoises, the skin is designed and made to absorb pressure fluctuations thus preventing turbulence.

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

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

The Palace design was based on the observation of the structure, unusual size and great strength of the leaves of a water lily called Victoria amazonica.

Joseph Paxton, a gardener by profession, was fascinated by the intricate ribs and cross ribs at the back of the leaf of this lily and built a greenhouse the roof of which incorporated a similar scaffold.

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

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

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

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

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

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

Designs from nature

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

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

Members of phylum Echinodermata have calcite that contains magnesium and 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 biomineralization process are aimed at synthesising nanoparticles, polymer mineral composites and templated crystals.

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

Microstructure resulting from mineralization The properties of natural materials arise in a large measure due to the ordered spatial structures that exist over many length scales. We find a bewildering variety of microstructures arising from biomineralization. The grain 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 simple and spherulitic; (ii) nacreous both columnar and sheet; (iii) gross-lamellar, both simple and complex; (iv) homogeneous.

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

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

In type I composites (Chiton teeth, algae) the matrix is inert and does not possess specific nucleation sites. Growth of the crystals is spatially restricted. Consequently, there is no control of the matrix on crystal size, orientation and morphology and it acts as a support encouraging heterogeneous nucleation of the inorganic phase.

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

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

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

The simple prismatic microstructure consists of mutually adjacent prisms while the spherulitic prismatic structure consists of coarse first-order prisms with second-order prisms originating from spherulitic sectors at the surface. In the columnar nacreous structure columns of uniform-sized tablets are formed.

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

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

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

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

Biomineralization

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

The natural proteins in the shell provide a modular fibre in which different domains are held together with intermediate strength bonds as compared to the bonding within the molecule. On increasing the stress and before molecule's backbone can break, the modules unfold and begin to yield.

Such events occur repeatedly in long molecules that are compacted into number of domains. The work required for fracture is thus enhanced considerably (Smith et al 1999).

Structure– junction relationships in bio-composites Tissues in living organisms have specific roles to perform. These composite 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 macroscopic scale the nodes, which occur periodically along its length, impart stability and rigidity to the plant.

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

Observations and studies made on trees, teeth and bone reveal the following aspects of their design. (a) Biological structural members optimize themselves to maintain uniform stress across the cross section (b) Both trees and bones add material in the overloaded areas to compensate 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 seaurchin.

The cleavage and tensile strengths across the fiber direction are also raised considerably in the presence of the nodes. An analysis of the stresses experienced by the bamboo during bending indicates that the maximum stresses are generated at the outer part of the culm.

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

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

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

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

Biomineralization

Plant surfaces and their multifunctional properties Approximately 460 million years ago, the first plants moved from their aqueous environment to the drier atmosphere on land, and they needed a protective outer coverage. The key innovation was the plant cuticle, a continuous extracellular membrane, which covers the primary aboveground 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, such as wood and bark, can forgo this protective layer.

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

Plants settled into nearly all conceivable habitats. In their specific environments, the cuticle serves as the crucial multifunctional layer representing one of the largest interfaces between biosphere and atmosphere, 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. Cuticles reduce the loss of water to the same, or even higher degree, as a synthetic polymer of comparable thickness would.

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

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

Cell forms, sizes and their fine structures have a great influence on several functional 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 water loss and leaching of ions from the inside of the cells to the environment, the reduction or increase of surface wetting and reduction of particle adhesion, as for example, pathogen adhesion.

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

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

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

Stratification of the outer part of epidermic cell

Some of these structures, such as special morphological types of cells like hairs or epicuticular waxes (introduced later), are characteristic for a special group of plants, thus they are useful features for grouping of plants in systematic orders (taxons).

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

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

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

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

Function and diversity of plant surface structures

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

A) Transport barrier: limitation of uncontrolled water loss/leaching from interior and foliar uptake; B) surface wettability; C) anti-adhesive, self-cleaning properties: reduction of contamination, pathogen attack and reduction of attachment/locomotion of insects; D) signaling: cues for host–pathogen/ insect recognition and epidermal cell development; E) optical properties: protection against harmful radiation; F) mechanical properties: resistance against mechanical stress and maintenance of physiological integrity; G) reduction of surface temperature by increasing turbulent air flow over the boundary air layer.

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

Diversity of structure, morphology and wetting of plant surfaces

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 outermost layer a thin extracellular membrane, called cuticle. The plant cuticle is a compositematerial mainly built up of a cutin network and hydrophobic waxes.

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

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

In terms of functional aspects, which were summarized in Fig. 1, the most important part of the epidermis cell is the cuticle. Hence the cutin and its integrated (intracuticular) and overlying (epicuticular) waxes will be introduced in more detail in the following paragraphs.

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

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

Surfaces and wetting Wetting is the fundamental process of liquid interaction at solid–gaseous interfaces. It describes how a liquid comes into contact with a solid surface.

Wetting is important in many everyday situations, for example, liquid paintings 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 several cleaning procedures.

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

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

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

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

Plant surfaces and their wetting behavior The plant cuticle with its integrated and exposed waxes is in general a hydrophobic material, but structural and chemical modifications induce variations in surface wetting, ranging from superhydrophilic to superhydrophobic.

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

Why are the leaves of most flowers hydrophilic? There is a simple explanation 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 three dimensional waxes forms a slippery surface for most insects, and would lead to less pollination.

Additionally, it is important to notice that hydrophobic leaves might become hydrophilic by the accumulation of environmental, hydrophilic contaminations like spores, bacteria, dirt particles and chemical aerosols on their surfaces.

Superhydrophilic plant surfaces.

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

Diversity of structure, morphology and wetting of plant surfaces

However, wetting is also influenced by the chemistry of the surface, e.g., secretion of hydrophilic compounds by epidermal glands. Optimized organs for the uptake of water are the roots of the plants. Most roots are characterized by papillae and hairy cells, but also porous surface structures have 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. In those species, superhydrophilicity of the surface is the basis for the uptake of water and nutrients from the environment.

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

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

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

Some genera in this group form funnels by arranging their leaves in form of a rosette. In these funnels, water can be stored after a rain shower and organic litter can accumulate and decay, so hairs at the funnel surface also absorb nutrients dissolved in the water Another reason why plants profit from dry surfaces is that the growth of most pathogen microorganisms, including bacteria and fungi, is limited by water shortage.

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

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

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

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

The leaves of the Lotus plant afford an impressive demonstration of selfcleaning14. This self-cleaning effect was found to be a result of the intrinsic hierarchical surface structure built by randomly oriented small hydrophobic wax tubules on the top of convex cell papillae.

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

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

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

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 not only the removal of particles is of interest, but also the maintenance of flight capability, which may be lost due to a load of weight on the wings.

Superhydrophobic plant surfaces and the Lotus leaf

Superhydrophobic hairy surfaces.

Many terrestrial plants and animals are water repellent due to their hydrophobic surface components in connection with microscopic roughness.

Besides the superhydrophobic leaf structures described above, a second method of water repellence has been developed in plants. Hairy leaf surfaces, such as those on the leaves of the lady's mantle can very efficiently repel water.

Diversity of structure, morphology and wetting of plant surfaces

On such surfaces, a deposited drop bends the fibers (hairs), but the stiffness of the hairs prevents contact with the substrate, and promotes a fakir 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 may also be extremely useful for underwater systems because they minimize the wetted area of immersed surfaces and therefore may greatly reduce drag, as well as the rate of biofouling.

Technical applications of biomimetic surfaces Bionics contains a wide spectrum of research fields such as, for example, lightweight constructions, fluid dynamics, robotics, micro- and nanoelectromechanical systems (MEMS, NEMS), and sensors. The dimensions of interest reach from the molecular level up to the function of complex organisms.

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

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

Today, well described examples are the feet of the gecko, which are perfectly adapted for reversible attachment on surfaces, and the selfadhesive 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 to animals. Recently the structure of shark skin has been used as a model for the development of swimming dresses with reduced surface drag when diving into water.

Self-repairing processes in plants sealing fissures serve as concept generators for the development of biomimetic coatings for membranes of pneumatic structures.

Diversity of structure, morphology and wetting of plant surfaces

This chapter intends to clarify the role mechanics has assumed in understanding the correlation of structure to properties and functionality in biological materials, and the biologically inspired materials that can be developed from this knowledge.

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

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

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

Fibrous Tissues

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

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

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

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

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

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

The matrix materials are globular proteins or glycoproteins and water in uncalcified soft tissues. In hard tissues, hydroxyapitite and lignin are the most common matrix/filler materials.

The small variation in the basic components of biological materials implies that the large variations in observed properties are the direct result of structural variations.

Mechanics of Helical Structures

Inter-fiber forces arise because the asymmetrical nature of a helix it will tend to 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 a low bending stiffness. As a helical fiber oscillates from tension at the top to compression at the bottom.

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

Additionally, because of lateral forces between fibers caused by the helix structure pulling itself together in tension, a break in the fiber will gradually take up the load until at some distance from the break it regains its full share of the load. In contrast to a simple parallel fiber composite, because there is actually a force between the fibers this mechanism will occur even if the interface between the fibers is a viscous fluid or nothing (i.e., relying on inter-fiber frictional forces).

Fibrous bundles are also capable of absorbing tremendous amounts of energy. Using the inter-fiber shear mechanism, a tensile structure can be designed with significant levels of hysteresis within a load cycle, which may be time-dependent viscoelastic behavior (in the case of a viscous fluid interface) or time-independent Coulomb-type damping (for a frictional interface).

The Role of Mechanics in Biological and Biologically Inspired Materials

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

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

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

Furthermore, it is notable that wood serves not only as a structural material for the tree. Wood is multi-functional, serving also as a transport and storage medium for nutrients, connecting the roots to the leaves.

The multiple roles that wood plays may also be applicable to engineering structures that require fluid transport as well as structural integrity.

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

However, first an increased engineering understanding of the performance and structure of wood is needed. This understanding can help to guide not only the optimization of materials but also the process of design for the new materials.

Examples in Nature - Wood

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

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

The Performance of Wood in Engineering Applications

The engineering equivalent to adaptation during the life would be in-service changes in the material. So, for example, if the structure of an engineered component were made of a truly tree-like material, the material would be adapted across length scales for the application as well as for the location within the structure.

Wood, because of its relative abundance and low cost, is often not recognized as a high performance material. However, if used in a configuration that approximates that of the natural loading, it is only in recent years that human-made materials have exceeded the performance of wood for many structures.

In one location of a part, the material could emphasize fracture toughness.

Fracture toughness might gradually become less important in another area where greater strength would be required.

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

Springs or mounting points could be integrated into a single part by simply adapting the modulus. In the case of a composite material, this type of design could be implemented by orienting the polymer chains or crystallinity in the matrix structures, as well as by varying the percentage of reinforcement.

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

Furthermore, if environmental effects and fracture toughness are considered, wood becomes an even more attractive material. As an example, end grain balsa is commonly used as a core material for sandwich composites. This is not only because of the low cost of the material, but also because of impact resistance.

The Role of Mechanics in Biological and Biologically Inspired Materials

However, wood design is by nature typically very conservative. This is because the material axes in wood are often misoriented relative to the geometric axes. Wood grows in a manner such that the material axes are oriented in the principal stress direction. Because of the asymmetry of wind loading this results in spiral grain growth as the standard condition in softwoods.

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

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

The cell wall structure of wood at the first level is assembled from subnanometer fiber bundles of long cellulose polymer chains with some degree of crystallinity in an amorphous matrix material. Additional components called extractives also exist in wood but only have an indirect impact on the mechanical properties of the wood.

Finally, as a part of the competitive strategy of the tree, wood from early in a tree's life (juvenile wood) has inferior material properties to wood produced later in the life of the tree (mature wood). This difference is of the order of a one-third reduction in the modulus. In general, the variation in material properties 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 be developed to break existing design barriers.

Multi-scale Structure of Wood The possibility exists then to create a synthetic wood in which the variability is directed at meeting engineering objectives instead of the ecological need of the tree.

Experimental Mechanics Such a material would depend on optimization in the same hierarchical fashion as wood. The material is both inhomogeneous and anisotropic from the millimeter scale of the cell structure to the angstrom scale of the glucose.

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

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

The cellulose molecules are seldom found as individual entities in the cell wall, but rather are located in discrete bundles of parallel polymer chains.

These discrete bundles in their matrix are further aggregated into a larger unit to create the microfibrils.

The microfibril may be the most important scale for optimization of the material. The nanometer scale microfibrils are assembled into the material that makes up the cell wall.

While this small structure has in turn a number of layers, typically only the layers that contain the oriented cellulose molecules contribute significantly to the structural properties of the material.

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

The fibers that would be located in the layers outside of the structural layer would tend to be oriented at a shallower angle to avoid the buckling, serving as 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. This microfibril angle varies not only within the tree, but also within a single year of growth. By orienting the microfibrils in the direction of the cell wall, a taller tree is obtained with less energetic cost.

The Role of Mechanics in Biological and Biologically Inspired Materials

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

At the same time as the growth at the polymeric level is responding to preprogrammed genetic needs, the cellular structure responds to the environment of the individual tree.

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

Wood develops with the growth of layers of cells on a cylindrical inner structure. As intuition would suggest, the wood has significantly different properties in the direction of the tubular structures compared with other directions.

The orientation of these axes is able to adapt to the loading on the tree to align the principal material axes in the directions of the principal stresses.

The evolutionary adaptation of spiral grain is thought to result from shear stresses in the tree. Shear stress results from the torsional loading applied to the tree as a result of asymmetric wind and other loading.

Because mature wood is highly adapted to resist bending from the wind loading of the crown, products produced from trees generally assume that the principal material axes are in the vertical direction or the axis of maximum bending stress.

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

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

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

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

Biological Inspiration for Functionally Graded Materials Biological materials have served as inspiration for a new class of materials that has become of significant interest to the mechanics and materials communities: functionally graded materials (FGMs). FGMs are defined as materials featuring engineered gradual transitions in microstructure and/or composition, the presence of which is motivated by functional requirements that vary with location within the component.

Recent advances in the selection of materials over the past few decades have provided engineers with new opportunities to engineer materials using FGM concepts. Currently, most structures are engineered by using a large number of uniform materials that are selected based on functional requirements that vary with location.

Abrupt transitions in material properties within a structure that result from these functional requirements lead to undesirable concentrations of stress capable of compromising structural performance by promoting crack growth along the interface.

In nature, these stresses are controlled by gradually varying the material behavior through a structure, resulting in a FGM. The gradual material variation results in a functionally graded architecture described with a continuously graded or discretely layered interlayer that has several relevant length scales, as seen in the adjacent figure, and a variation in functionality in the interlayer.

In a variety of biological structures, from insect wings to bamboo, evidence can be found that FGMs have been selected through natural evolution to optimize structural performance through the unique coupling of material and stress distribution.

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

The Role of Mechanics in Biological and Biologically Inspired Materials

Thus, component design and fabrication have been synergistically combined, not just for the manufacture of FGMs, but also for the establishment of an entirely new approach to engineering structures. The end result is the capability of engineering parts that correspond to designer-prescribed properties.

Non-traditional testing methods, such as hybrid numerical-experimental techniques, will also be required because of the inherent inhomogeneous behavior of these materials.

In addition to novel experimental characterization techniques, new theoretical and computational mechanics concepts are also necessary to understand the performance of FGMs

Characterization of Functionally Graded Materials ell as individual.

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

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

In particular, smart structures can be fabricated from materials, such as shape memory alloys (SMAs), that experience stress-induced phase transformations that can change the amount of deformation they recover when they are heated.

Functionally grading the distribution of SMA wire reinforcement in smart composite panels has already been demonstrated to theoretically increase buckling strength by controlling the distribution of recovery stresses that are generated when the wires are heated.

Future Mechanics Research in Functionally Graded Materials To further advance the development of FGMs, it will be absolutely essential to use experimental mechanics to characterize the coupling between material and stress distributions. In particular, full-field deformation measurement techniques, such as Moire interferometry and digital image correlation, combined with advanced microscopy techniques, such as electron microscopy and atomic force microscopy, and localized property characterization techniques, such as microtensile testing and nanoindentation, will play an important role in elucidating the unconventional structure/property/stress relationship produced by this coupling.

The Role of Mechanics in Biological and Biologically Inspired Materials

As explained in the previous chapters, the engineering principles of biological systems are characterized by a high level of redundancy and complex differentiation in the material hierarchy. It is exactly the redundancy and differentiation, rather than the optimization and standardization, that creates robust systems that successfully respond and adapt to different stresses anf dynamic loads.

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

Another type of system that is commonly used in biology and architecture is polymorphism. Polymorphism describes systems that are made os different elements, forms or individuals. In architecture, polymorphic systems implies complex interrelations of form, materials and structure in the design, which is in turn materialized by deploying the logic of manufacturing processes.

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

When he came to design the cast-iron supports for his hitherto unprecedented expanse of glass, he remembered the ribs and struts of his giant water-lily that supported the gigantic leaves and used them as the basis of his designs not only for the glass-houses at Chatsworth but also, a few years later, for his architectural masterpiece, the Crystal Palace in London." (Attenborough 1995:290) In the process of searching for sub divisible modular structure in the Waterlily, the method here described and employed was form finding with force diagrams.

Victoria Amazona (Waterlily) The leaves of the Amazon water lily gain structural support via girder-like support ribs.

In still or slowly-moving waters there is one easy way to collect light: a plant can float its leaves upon the surface. No plant does this on a more spectacular 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 upturned rim, six inches high, so that as it expands it is able to shoulder aside any other floating leaf that gets in its way.

Beneath, it is strengthened with girder-like ribs which make the whole structure 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 until it is six feet across.

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

Architectural devices

3

1. Branching structure of Victoria Amazonica 2. diagrid structure of the Venus Flower Basket 3. Extraction of the rules of Victoria Amazonica structure

1

2

Architectural devices

Venus Flower Basket Skeleton of sponge provides strength with lightweight material via its siliceous composition. Despite its inherent mechanical fragility, silica is widely used as a skeletal material in a great diversity of organisms ranging from diatoms and radiolaria to sponges and higher plants. In addition to their micro- and nanoscale structural regularity, many of these hard tissues form complex hierarchically ordered composites.

One such example is found in the siliceous skeletal system of the Western Pacific hexactinellid sponge, Euplectella aspergillum. In this species, the skeleton comprises an elaborate cylindrical lattice-like structure with at least six hierarchical levels spanning the length scale from nanometers to centimeters.

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

Two intersecting grids of non-planar cruciform spicules define a locally quadrate, globally cylindrical skeletal lattice that provides the framework onto which other skeletal constituents are deposited. The grids are supported by bundles of spicules that form vertical, horizontal and diagonally ordered struts.

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

Dragonfly - Wing Structure 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 the membrane. Both consist of cuticle which is composed of the material chitin.

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

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

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

It is considered that wing corrugation increases not only the warping rigidity but also the flexibility. The wing of a dragonfly has some characteristic structures, such as "Nodus", "Stigma". Nodus is located in the center of the leading edge, and stigma like a mark is located near the end of the wing. It is considered that these structures not only increase the flexibility of the wing, but also prevent fatigue fracture of wings.

One interesting characteristic to note about a dragonfly wing is that there are several different kinds of patterns present in the wing vein framework. The leading edge consists primarily of rectangular frames whereas the trailing surface is largely formed of hexagons and some other polygons with more than 4 sides.

The patterns seen in the wing would tend to supporting the overall structural model of a wing with a stiff leading edge and a more flexible trailing edge, especially considering how the vein size alsodecreases from the leading edge of the wing to the trailing edge of the wing as can be seen in the adjacent figure.

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

The three leading edge veins form a sort of angle bracket structure as shown in the adjacent figure which contributes greatly to span wise wing stiffness.

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

Architectural devices

.

Basal Wing Section

costa triangle/ subcosta supertriangle (levers trailing edge downward)

Distal Wing Section

pterostigma radius

nodus (provides stress relief)

(sort of counterweight to control wing flapping)

Dragonfly wing with structures of interest

1. Structure of the Dragonfly wings

Architectural devices

Architectural devices These examples, fiberglass structure of the sponge known as Venus Flower Basket and the structure os the wing of the Dragonfly, are interesting not only because of the diamond and polygonal grid, but also because of how these grids are using a minimal amount of materials to achieve maximum performance in very versatile and aggressive environments.

Looking at all these patterns and how their structural logic overlaps give rise to interesting possibilities in assembling layers of data and pattern. Balmond cals this template of ideas informal, because there is no hierarchy, only interdependence in the natural patterns.

In overlaying the patterns new 2d frameworks emerges. It is not a process of simple replication, but of morphogenesis.

Although these 2d patterns on a level are fixed, they give an opportunity to be viewed as a projection of more complex 3d networks.

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

The patters turns into structure, which becomes architectural devices.

Architectural devices

Strategy: Structure protects against compression loading: staghorn coral The structure of staghorn coral is just one example of a natural branched system that protects against compression loading using scaled struts joined in a common lattice.

Strategy: Jelly substance provides structural support: jellyfish The composite mesoglea of jellyfish and sea anemones provides structural support using collagen fibers in a complex gel matrix.

Inspiring organism

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

Scyphozoa

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

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

In nature, more often than not, the branches of a system diverge without rejoining, although struts are sometimes joined into trusses--the arms of some sand dollar larvae and some bones in the wings of large birds have already been mentioned

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

Bioinspired products and application ideas

Application Ideas

Inspiring organism Acropora cervicornis Habitat(s): Marine Neritic Bioinspired products and application ideas Industrial Sector(s) interested in this strategy: Architecture, structural engineering, transportation Application Ideas New building designs that minimize material use without sacrificing stability, new designs for cell and communication towers

Nature strategies for mantaining physical integrity - Prevention of structural failure

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 torsional stiffness with minimal material hasn't been lost on either nature or engineers. We use them as subsystems when building bicycles and racing cars 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 long bones; insect, spider, and crustacean appendages; the wing veins of insects; 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 from fluid pressure rather than from their tubular solids--hydrostatic systems and hollow cylindrical beams aren't mutually exclusive.

Microtubules, stiffening elements in cells, are also hollow cylindrical beams, although they may derive additional stiffness from ordered water molecules at their surfaces.

Strategy: Leaves resist tearing: brown algae Leaves of brown algae survive extreme mechanical battering because of their sandwiched structure.

The leaves of the brown algae can endure extreme physical stress without suffering any damage, due to its construction according to the sandwich method.

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

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

The sandwich structure owes all these advantages to the loosely assembled filler, which transmits the mechanical forces and distributes them quite evenly over large areas of the membrane surface. In this way, no stress great enough to destroy the membrane can appear anywhere.

Inspiring organism

Bioinspired products and application ideas

Durvillaea antarctica

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

Bioinspired products and application ideas Industrial Sector(s) interested in this strategy: Construction, manufacturing

Application Ideas Application Ideas Tubular furniture (parts); tubular train cars, buses, etc.; hollow cylindricalbuilding materials, such as beams; all with the potential to reduce material use.

Creating structures and materials that resist tearing.

Nature strategies for mantaining physical integrity - Prevention of structural failure

Strategy: Arterial walls resist stretch disproportionately: cephalopods The arterial walls of cephalopods and arthropods resist stretch disproportionately as stretch increases due to tissue architecture.

In effect, Laplace's law rules out the use of ordinary elastic materials for arterial walls, requiring that an appropriate material fight back against stretch, not in direct proportion to how much it's stretched, but disproportionately as stretch increases. Which, again in obedience to the dictates 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-stretchy material, collagen, in those walls, and second, by arranging those fibers in a particular way. Thus, as the wall expands outward, more and more of these inextensible fibers are stretched out to their full lengths and add their resistance to stretch to that of the wall as a whole.

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

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

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

Inspiring organism Cephalopoda Bioinspired products and application ideas

Arterial walls that resist stretch disproportionately as they extend characterize circulatory systems that have evolved within lineages quite distinct from our own--in cephalopods and arthropods, for instance.

Industrial Sector(s) interested in this strategy: Medical, mechanical engineering, building, textiles Application Ideas

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

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

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

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

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

Nature strategies for mantaining physical integrity - Prevention of structural failure

Strategy: Vines repair themselves: pipevine

Strategy: Systems allow changes in mechanical properties: organisms

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

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

The ability to heal fissures and injuries is characteristic, and an essential feature, of living organisms. A lot of research is currently being aim of our research is to analyse fast healing processes in plants quantitatively and to transfer the insights into biomimetic self-repairing technical materials.

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

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

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

[Aristolochia] macrophylla reacts to fissures and ruptures in its peripheral strengthening tissues by a rapid repair mechanism which seals the lesions very effectively and secures the functional integrity of the plant structure. As soon as a tiny fissure occurs, parenchyma cells from the surrounding cortex tissue swell into this fissure and immediately start to seal it.

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

The initial phases of fast repair of fissures take place without cell division and (significant) cell wall biosynthesis but mainly by physico-chemical processes altering the mechanical properties of the cell walls of the fissuresealing parenchyma cells.

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

Among the best features of these systems is their ability to alter shape or overall mechanical properties rapidly without having to change the properties of specific materials. But even tensile tissues other than muscle may sometimes change properties fairly quickly in response to some chemical signal. These alterations have been studied most extensively in the so-called catch connective tissue of echinoderms.

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

Inspiring organism: Asteroidea

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

Bioinspired products and application ideas

Bioinspired products and application ideas

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

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

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

Application Ideas Lightweight architecture that self-seals. Self-sealing pneumatic structures.

Nature Self-repairing clothing, medical technology, pipelines, etc.

strategies for mantaining physical integrity - Prevention of structural failure

Strategy: Elastic ligament provides support, shock absorption: large grazing mammals

Strategy: Microscopic holes deter fractures: starfish Ossicles of starfish resist fractures via microscopic holes in the structure.

The nuchal ligament of large grazing mammals provides support for the head and seems to act as a shock absorber, due to the presence of the protein 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 ligament of large grazing mammals.

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

Use 'foamy' materials in which any threatening crack will be in short order run into a hole. Not only does this reduce the chance of cracking, but it saves material--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 especially holey.

About the inspiring organism

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

Bovidae

Inspiring organism

Bioinspired products and application ideas

Asteroidea

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

Bioinspired products and application ideas Industrial Sector(s) interested in this strategy: Construction, ceramics, materials science, building science, pipes

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

Application Ideas Concrete and other building materials that better resist fractures, ceramics that 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.

Nature strategies for mantaining physical integrity - Prevention of structural failure

Strategy: Continuous fibers prevent structural weakness: trees Knotholes in wood do not crack because the fibers around them are continuous.

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

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 where a 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 composite materials.

About the inspiring organism Plantae Bioinspired products and application ideas Industrial Sector(s) interested in this strategy: Construction Application Ideas Creating construction materials with the properties of wood.

Nature strategies for mantaining physical integrity - Prevention of structural failure

Strategy: Stems vary stiffness: scouring horsetail

Inspiring organism

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

Equisetum hyemale Bioinspired products and application ideas

Plants with hollow axes, e.g. various horsetails and grasses, serve as generators of biological concepts for technical structures with variable stiffness. Their structure is characterised by a thin outer ring of strengthening tissue stabilised by a lining of parenchyma cells.

The hollow stems are divided into shorter segments (internodes) by transverse walls and stem thickenings at the so called nodes. The nodes significantly reduce the danger of local buckling in these light-weight structures.

Industrial Sector(s) interested in this strategy: Construction, transportation Application Ideas Technical smart materials with stiffness varying as needed using pneumatic structures. Uses could include airplane wings, spoilers on cars, and building shells.

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 (Equisetum giganteum) the resistance to ovalisation is extremely turgor-dependent. In other horsetail species--such as the Dutch Rush (E. hyemale)--the outer ring of strengthening tissue is connected via wedge-shaped elements with an inner ring of strengthening tissue forming a mechanically resistant sandwich structure.

These stems are also stabilised by the pressurised lining of parenchymatous cells but depend much less on the turgor pressure of the parenchyma cells.

The mechanical stability resisting stem ovalisation is diminished by only about 20% due to reduction of the turgor pressure.

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

These include light-weight structures with chambered pressure-stabilised pneumatic structures that feature a segmental variation of stiffness and the ability to adapt their stiffness or form to changing outer conditions, facilitated either adaptively or via integrated active control.

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

Nature strategies for mantaining physical integrity - Buckling

Strategy: Skeleton provides support: sponges

Strategy: Shape of feather shafts protect from wind: birds

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

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

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

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

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

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

Inspiring organism

Inspiring organism

Porifera

Aves

Bioinspired products and application ideas

Bioinspired products and application ideas

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

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

Application Ideas

Application Ideas

More wind-resistant tents that require fewer poles.

Incorporating materials with noncircular (and nonrectangular) cross sections into building design, construction, and engineering applications, such as fence posts or bridge supports.

Nature strategies for mantaining physical integrity - Buckling

Strategy: Quills resist buckling: porcupine Quills of porcupines resist buckling because they are made of a dense outer shell surrounding an elastic, honeycomb-like core.

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

In nature, all are loaded in some combination of axial compression and bending: failure is typically by buckling. Natural structures are often optimized.

Mechanical models recently developed to analyze the elastic buckling of a thin 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 the mechanical efficiency of these natural structures.

It was found that natural structures are often more mechanically efficient than equivalent weight hollow cylinders. Biomimicking of natural cylindrical shell structures may offer the potential to increase the mechanical efficiency of engineering structures.

About the inspiring organism common porcupine Erethizon dorsatum (Linnaeus, 1758) Habitat(s): Forest, Grassland, Shrubland Bioinspired products and application ideas Industrial Sector(s) interested in this strategy: Engineering, structural engineering Application Ideas Design of architectural structures with high buckling resistance, with potential application in earthquake-proofing.

Nature strategies for mantaining physical integrity - Buckling

Strategy: Stems vary stiffness: scouring horsetail

Inspiring organism

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

Equisetum hyemale Bioinspired products and application ideas

Plants with hollow axes, e.g. various horsetails and grasses, serve as generators of biological concepts for technical structures with variable stiffness. Their structure is characterised by a thin outer ring of strengthening tissue stabilised by a lining of parenchyma cells.

The hollow stems are divided into shorter segments (internodes) by transverse walls and stem thickenings at the so called nodes. The nodes significantly reduce the danger of local buckling in these light-weight structures.

Industrial Sector(s) interested in this strategy: Construction, transportation Application Ideas Technical smart materials with stiffness varying as needed using pneumatic structures. Uses could include airplane wings, spoilers on cars, and building shells.

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 (Equisetum giganteum) the resistance to ovalisation is extremely turgor-dependent. In other horsetail species--such as the Dutch Rush (E. hyemale)--the outer ring of strengthening tissue is connected via wedge-shaped elements with an inner ring of strengthening tissue forming a mechanically resistant sandwich structure.

These stems are also stabilised by the pressurised lining of parenchymatous cells but depend much less on the turgor pressure of the parenchyma cells.

The mechanical stability resisting stem ovalisation is diminished by only about 20% due to reduction of the turgor pressure.

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

These include light-weight structures with chambered pressure-stabilised pneumatic structures that feature a segmental variation of stiffness and the ability to adapt their stiffness or form to changing outer conditions, facilitated either adaptively or via integrated active control.

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

Nature strategies for mantaining physical integrity - Buckling

Strategy: Bones self-heal: vertebrates

Strategy: Sclereid cells prevent soft tissue collapse: plants

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

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

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

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

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

They may be branched, sinuous or short -- often more or less isodiametric.

The longer ones commonly feature in the sheaths to veins, particularly near the ends of the finer branches.

The number and function of osteoclasts and osteoblasts are regulated by extracellular and intracellular signals acting in a coordinated fashion to maintain skeletal homeostasis.

They can be pit-prop-like when they extend between the upper and lower surfaces of leaves, and appear to help prevent collapse of softer tissues at times of water stress, as in olive leaves and the leaves of many mangrove plants.

Inspiring organism Vertebrata

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

Bioinspired products and application ideas

Inspiring organism

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

Plantae

Application Ideas

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

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

Application Ideas

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

Bioinspired products and application ideas

Material design applications to prevent cracking in musical instruments as drying occurs, structural or material designs that prevent cracking in walls or foundations at moisture levels fluctuate.

Nature strategies for mantaining physical integrity - Compression

Strategy: Matrix stiffens connective tissue: sponges

Strategy: Support cells resist compression: nasturtium

The connective tissue of sponges is a matrix stiffened by embedded spicules.

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

Putting small pieces of brittle material into a pliant matrix gives a composite called a 'filled polymer'--it amounts to a kind of random array of mechanisms.

Koehl (1982) looked into the extent to which the connective tissues of animals that had embedded spicules behaved like proper filled polymers-embedded spicules are fairly widespread, not just in sponges, but in some coelenterates, echinoderms, mollusks (the chitons), arthropods (stalked barnacles), and ascidians.

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

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

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

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

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

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

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

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

Iinspiring organism

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

Tropaeolum [Nasturtium]

Iinspiring organism

Bioinspired products and application ideas

Porifera

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

Bioinspired products and application ideas Application Ideas Industrial Sector(s) interested in this strategy: Building, recycling, research Application Ideas Composite concrete, recycling brittle materials to create composites, enhance functionality of existing composites.

Compression-resistant pipes, compression-resistant concrete that incorporates orthotetrakaidecahedron shape, compression-resistant textiles and absorbent materials.

Nature strategies for mantaining physical integrity - Compression

Strategy: Hole structure strengthens bone: horse A metacarpal bone of a horse avoids structural weakness caused by a hole via stress-dispersing microstructure. Zebras, horses and other equine species put substantial stress upon their central forefoot bones, particularly the third metacarpal, bones with remarkable strength despite having holes in them for blood vessels to pass through.

The presence of a hole (or foramen) in a structural element offers the potential 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 of interest to engineers to learn how to design openings in structures in a way that avoids cracking.

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

Strategy: Structural composition provides strength in changing conditions: plants The cell walls of vascular plants provide mechanical strength during different stages of growth by adjusting their structural composition.

Plant cells need to be fully hydrated to work properly (except in periods of dormancy, as for example in many seeds). Individual vegetative cells in plants, 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, as for example, in growing shoot or root tips.

However, once developed into their mature form, the cell walls may become thicker, and additional substances, mainly lignins, incorporated into their structure.

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

Inspiring organism

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

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

Inspiring organism: Horse Equus caballa

Plantae Bioinspired products and application ideas Industrial Sector(s) interested in this strategy: Construction, structural engineering Application Ideas Example of how elements that serve one purpose can be adapted or incorporated to serve a later functional purpose within the same context: e.g., scaffolding that becomes part of a building's frame or materials whose thickness can fluctuate based on changing temperatures or load-bearing requirements.

Bioinspired products and application ideas: industrial Sector(s) interested in this strategy: Construction, manufacturing Application Ideas: Use for increasing strength of airplanes, boats, automobiles, other structures that have holes for wiring or fuel and hydraulic lines. (A quick rule of efficiency in the aerospace industry is that one pound of weight saved in a plane can save 10 pounds of fuel.)

Nature strategies for mantaining physical integrity - Compression

Strategy: Collenchyma cells provide strength, flexibility: plants Collenchyma cells in vascular plants support growing parts due to flexible cellulosic walls, which lignify once growth has ceased.

In addition to the 'mechanical' cells - fibres and lignified parenchyma - a third cell type has mechanical functions. This is collenchyma. Collenchyma cells have walls which during their development and extension are mainly cellulosic.

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

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

Strategy: Fibers reinforce hydrostatic skeletons: sunflowers Hydrostatic structures found in sunflowers and other many other organisms serve various functions but almost always use helical fibers as reinforcement.

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

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

Inspiring organism Plantae

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

Bioinspired products and application ideas

Inspiring organism

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

Asteraceae Bioinspired products and application ideas

Application Ideas Design ideas for adding strength to structures or materials.

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

Application Ideas Water storage containers that resist fouling and leaks; building materials, tents, and other structural materials that include reinforcing helical fibers; norip fabrics, such as mosquito nets.

Nature strategies for mantaining physical integrity - Compression

Strategy: Lightweighting: Scots pine Trunks and branches of trees withstand external stresses through loadadaptive growth.

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

About the inspiring organism

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

For example, to distribute stress uniformly, trees add wood to points of greatest mechanical load, while bones go a step further, removing material where it is not needed, lightweighting their structure for their dynamic workloads.

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.

Pinus sylvestris [Scots pine, Scotch pine] IUCN Red List Status: Least Concern Habitat(s): Forest Bioinspired products and application ideas CAO and SKO design software, Lightweighting software reduces resource use, saves energy, Industrial Sector(s) interested in this strategy: Construction, manufacturing, medicine.

Application Ideas

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

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

For example, car parts and entire cars designed with these principles have resulted 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 that between trees and other organisms, but between trees and engineered artefacts. If trees achieve longevity and structural stability, aren't these the qualities of reliability and integrity that engineers want to design into products?

The key to this is Mattheck's contention that the structural optimisation in trees and apparent in other natural structures such as animal bones is all about making the external and internal stresses as uniform across the whole structure as possible.

Mattheck calls this the 'axiom of uniform stress' and adds that, though he can cite plenty of examples of it, he cannot prove it exists‌Mattheck's contention is that trees are constantly readjusting this balance by adding more material at points of high stress and adding no material at points of low or no stress. (Bones, he contends, go one stage further by actually shrinking at points of low stress.)

Nature strategies for mantaining physical integrity - Compression

Strategy: Rod-like reinforcements provide strength: plants

Strategy: Thickness stabilizes tall trees: baobob

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

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

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

The adaptive significance of building a stem out of weak, low density wood is not immediately apparent, particularly as extensive use of stored water does not appear possible under this strategy.

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

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

Inspiring organism

Baobab trees, however, have very thick bark, a design feature that contributes significantly to overall structural stability of the stem and may compensate for the reductions in stem stiffness that would otherwise occur through moderate use of stem water.

More detailed biomechanical and energetic analyses may demonstrate that baobab trees can actually achieve greater strength for a given energy investment that other trees, and the possibility that the maintenance of a large quantity of living parenchyma cells is somehow advantageous, whether for carbohydrate storage or recovery from traumatic injury, cannot be discounted

Plantae Inspiring organism Bioinspired products and application ideas Industrial Sector(s) interested in this strategy: Materials science, structural engineering

Adansonia [baobob tree] Bioinspired products and application ideas

Application Ideas Industrial Sector(s) interested in this strategy: Building industry Models for the arrangement of structural elements that provide various levels of mechanical support, models for composite materials. Also multifunctional – used for liquid transport as well as structural support.

Application Ideas: Structural engineering of large buildings.

Nature strategies for mantaining physical integrity - Compression

Strategy: Pressure provides structural support: blackback land crab

Strategy: Fibers keep tall spikes upright: titan arum

The body of the blackback land crab functions during exoskeletal molt using both gas and liquid pressure, or a pneumo-hydrostatic skeleton.

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

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

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

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

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

Inspiring organism

Hydrostatic skeletons are arranged so that the force of muscle contraction is transmitted by an essentially incompressible aqueous fluid . Muscle contraction increases the pressure in the fluid, causing the deformations or stiffening required for support, movement and locomotion.

Amorphophallus titanum Amorphophallus titanum (Becc.) Becc.

[Titan arum] IUCN Red List Status: Unknown

About the inspiring organism Bioinspired products and application ideas blackback land crab Gecarcinus lateralis (Freminville, 1835) [Blackback land crab] 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.

Bioinspired products and application ideas Industrial Sector(s) interested in this strategy: Construction, packaging, transportation Application Ideas Hydrostatic skeletal support.

Nature strategies for mantaining physical integrity - Compression

Strategy: Reinforced fibers provide strength: plants

Strategy: Interwoven trees gain structural support: tropical trees

Fibers in many woody plants provide mechanical strength via lignin reinforcements.

Trees gain support by growing together in an upward spiral.

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

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

In tropical jungles, with their great variety of species, we encounter a multitude of Mechanical ideas for construction.We also find thin trunks joining into bundles, supporting each other and forming an upward winding spiral.

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

Bioinspired products and application ideas

Fibre cells normally have pointed ends. They often extend in length during development, growing between cells that may not be lengthening at the same rate.

Industrial Sector(s) interested in this strategy: Construction

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

More stable buildings in areas prone to high winds.

Application Ideas

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

Inspiring organism Plantae Bioinspired products and application ideas Industrial Sector(s) interested in this strategy: Architecture, building, nanotechnology, materials science Application Ideas Composite building material with properties of high lignin wood. Design ideas for adding strength to structures or materials.

Nature strategies for mantaining physical integrity - Compression

Strategy: Nest cells support heavy weights: bees and wasps

Strategy: Crystals and fibers provide strength, flexibility: bones

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

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

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

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

The extraordinary strength is exemplified by a comb 37 centimetres by 22.5 centimetres in size, which is made of 40 grams of wax but can contain about 1.8 kilograms of honey.

A bees' honeycomb is one of the wonders of the world. Layer upon layer of hexagonal cells of identical size and shape are stacked together as precisely as 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 energy and using the least material necessary for a task. Three-way junctions of 120째 angles occur quite widely in nature, being the most economical angle for joining things together.

About the inspiring organism

But this ideal distribution of bone material along the stress lines would have been to little avail were the material itself not so well adapted to extraordinary pressure.

Just like fiberglass made of synthetics threaded with glass fiber, bone tissue is made up of two constituents which greatly differ in their mechanical properties.

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

It appears in the bone in the form of tiny crystals, only about 200 atomic diameters in size. They are inserted between thin fiber hairs of the elastic material 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 as brittle and inelastic as pure hydroxylapatite.

Besides, because of the continuous alternation between brittle and elastic material, there is little chance for a fracture to spread unchecked Mineralized collagen fibrils are highly conserved nanostructural building blocks of bone. By a combination of molecular dynamics simulation and theoretical analysis it is shown that the characteristic nanostructure of mineralized collagen fibrils is vital for its high strength and its ability to sustain large deformation, as is relevant to the physiological role of bone, creating a strong and tough material.

Nature strategies for mantaining physical integrity - Compression

An analysis of the molecular mechanisms of protein and mineral phases under large deformation of mineralized collagen fibrils reveals a fibrillar toughening mechanism that leads to a manifold increase of energy dissipation compared to fibrils without mineral phase.

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

As a consequence, mineralized collagen fibrils are able to tolerate microcracks of the order of several hundred micrometres in size without causing any macroscopic failure of the tissue, which may be essential to enable bone remodelling.

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

Inspiring organism Chordata Bioinspired products and application ideas Industrial Sector(s) interested in this strategy: Construction, manufacturing, nanotechnology, materials science, medical, building, automotive, CO2 sequestration Application Ideas Building strong, lightweight materials that can take a lot of stress. High strength materials, composites to use as bone replacements or mechanical limbs. Mineralized fibers as construction element for buildings handling shear and torsional stresses (earthquake, hurricane, etc). Using fracture resistant, yet impact absorbing fiber structure material for structure of automobile. Utilizing CO2 calcification of natural or synthetic fibers to create novel material while sequestering CO2.

Nature strategies for mantaining physical integrity - Compression

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Bibliography