5. Biomimetics and Self-assembly

As some of these examples illustrate, nature's materials continue to be the envy of the materials scientist. But we are increasingly able to do far more than just stand back and admire. As more understanding emerges about how these materials are structured and put together, so the materials scientist acquires inspiration and guidance for making comparable fabrics by taking tips from nature. The buzzword here is "biomimetics" (sometimes called "bionics" in continental Europe): the mimicry of biological systems in engineering contexts. For the materials scientist, this is largely about making composite materials with superior properties-high toughness, for example-using cheap, readily available substances assembled into intricate microstructures through low-energy pathways.

Studies of biological materials have heightened an appreciation that, while optimizing a particular material parameter tends to involve the fine-tuning of a specific feature of the structure, the combination of several desirable properties is typically a matter of controlling structure and organization across several different length scales. In other words, natural materials display hierarchical structures. The strength of bone is more than a matter of marrying organic and inorganic materials (the protein collagen and the mineral hydroxyapatite); there are distinct types of organization at scales ranging from the primary structure of the collagen helices, through to the placement of the crystals along the fibrils at the 100-nm scale to the arrangement of osteon fibers at sub-millimeter scales and the macroporosity of the bulk substance.

Figure 5. The Hierarchical Structure of Bone Extends over at least
Four Orders of Magnitude in Size Scales

A comparable hierarchy of structure is evident in most of nature's structural materials, notably wood, tendon, cartilage and silk. In many of these instances the mechanical function of the components of the hierarchy can be understood according to familiar engineering principles.

Related to the idea of hierarchy is the use of modular structure, which in this case generally means building up materials through the assembly of identical smaller units. Wood, for example, with its compartmentalized cellular structure illustrates the mechanical advantages of a material composed of adjoining, closed cells. It has a high strength-to-weight ratio, and the modular architecture can help to localize damage: rupture of a few cells need not lead to a propagating crack, as it does in brittle materials. The stepwise unfolding of the titin protein, which holds the fibrous proteins together in muscle, is due to a kind of modular domain structure and gives it a stress-strain curve very different from a purely elastic filament. A similar sawtooth-like stress-strain curve has been measured for the polymeric filaments that are pulled out from the organic binder between the aragonite plates of nacre (Mother of Pearl) as the plates are separated. Because the area under the curve, equal to the work of fracture, is greater in this case than for either a stiff, strong filament or an elastic one, this modular extension behavior makes for a tough adhesive, and may prove to be a more general mechanism in biology.

One of the predominant leitmotifs of nature's structural materials is orientational control of fiber growth. In bone, the oriented packing of the collagen fibrils increases the elastic modulus, the work of fracture and the breaking strain of the hydroxyapatite. The high fracture toughness of wood- greater than that predicted for a simple fibrous composite of the fiber and matrix components- seems to be due largely to the helical winding of cellulose fibers in the multilayered cell walls. The orientation of the protein fibers in silk helps to give it a fracture strength greater than that of steel.

Many hard biomaterials are manufactured by controlled nucleation and growth of crystals. The archetype is nacre, a layered sandwich of mineral platelets and protein sheets. Bone is a much more intimate marriage of mineral and polymer, while tooth enamel has a woven texture that reveals control of crystal growth in all three dimensions. The layered structure of nacre lends itself most obviously to mimetic synthesis, since materials engineers already have considerable experience in preparing layered composites. The basic strengthening mechanism derives from the presence of weak interfaces between the mineral platelets. The energy of a crack is dissipated in pulling the platelets apart, and its forward progress is deflected laterally as this happens. The same principle has been exploited in a synthetic layered composite in which slabs of silicon carbide are separated by a thin film of graphite at the interfaces. The resulting material is tougher than a monolithic piece of the ceramic.

Figure 6. A Ceramic Composite made from Layers of Silicon Carbide
Coated with Graphite is much Tougher than a Hard,
but Brittle Block of the Pure Ceramic
This microstructure mimics that of Mother-of-Pearl (nacre)

Natural materials are, almost by definition, self-assembling-which generally also implies active control systems to guarantee self-repair, self-reinforcement and disassembly when needed. Biomimetic synthetic materials cannot yet achieve all of this, but the basic idea of self-assembly is one that has been enthusiastically imported from supramolecular chemistry. The essential feature of self-assembling chemical systems is that they are "pre-programmed" with the information needed for creation of the superstructure. That is to say, the component molecules are capable of interactions, often highly directional in space, that guide the assembly process into the required architecture.

Nowhere is this more elegantly illustrated than in the use of DNA as a fabric for materials synthesis. DNA is the "programmed" molecule par excellence-in chromosomes, it holds the basic information needed to create a replicating organism. The important point for the materials chemist is that this information is embodied in highly specific intermolecular interactions, which enable both complementary DNA and messenger RNA to be assembled piece by piece on single-stranded DNA. In short, the two strands recognize one another. This enables the use of DNA as a programmed molecular girder, in which the ends will link up in very well specified ways, determined by the terminal sequence of base pairs. Short strands of double-helical DNA can be given "sticky ends"-short single-stranded sections-that will recognize and bind to other ends with the complementary sequence. Moreover, the cell provides ready-made enzymes for making these sticky unions permanent through covalent linkage (ligation enzymes), or cleaving the resulting framework at locations precisely defined by sequence (restriction enzymes).

Using these principles and this molecular machinery, synthetic strands of DNA have been fashioned into topologically complex architectures such as a cube and other polyhedra, and into extended, ordered sheet-like arrays of DNA loops, rather like a kind of molecular chain mail.

Figure 7. A Molecule in the Shape of a Truncated Icosahedron,
made by the "Programmed" Self-assembly of Strands of DNA

One aim is to extend this approach to the formation of three-dimensional networks, like a kind of DNA zeolite. Suitably functionalized, these might serve as selective catalysts; or they might act as templates for mineralization or metallization, giving more robust porous frameworks. Perhaps the electrically conductive properties of DNA might even be brought to bear to good effect in these networks.

One of the most useful tricks deployed for natural self-assembly is the use of templates. Organic tissues imprint shape and pattern on biominerals such as bone and the exoskeletons of marine organisms such as radiolarians and diatoms. Some of the latter have the most exquisite designs at the microscopic scale, seemingly cast in the mineral phase around a mould of organic vesicles. This same idea of using (self-assembling) organic structures as a mould for making patterned inorganic materials is exemplified in the synthesis of the mesoporous silica MCM-41 and its relatives, where the organic structures are micelle-like aggregates of surfactants. The same principle has been scaled up still further by using bubble-like surfactant vesicles to imprint complex patterns on the surface of an aluminophosphate.

Figure 8. An Inorganic Material (an Aluminophosphate) Patterned by
Bubble-like Structures Self-assembled from Surfactant Molecules

And "colloidal crystals"-orderly stacks of microspheres made from silica or polymers-have been used as casts for making porous solids with voids an order of magnitude bigger than those of MCM-41.

Figure 9. An Ordered Porous Material made by Casting Silicon around
a "Colloidal Crystal," in which mm-sized Spheres are Packed in Regular Arrays
The spheres are subsequently broken down to create voids

Related to templating, is the idea of compartmentalization: of conducting materials synthesis in compartments that delimit the extent of growth, as well as providing a microenvironment in which parameters, such as supersaturation of a precipitating phase can be delicately controlled by active transport of ions. This is how much biomineralization takes place, perhaps most dramatically in the formation of exquisitely patterned coccolith plates in the soft tissues of the sea creatures called coccolithophores. The natural iron-storage protein ferritin has been used as an enclosed compartment for the formation of monodisperse iron oxide nanoparticles, while similar small particles of inorganic and polymeric materials have been cast inside the empty protein coats (virions) of viruses.

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