The mechanical properties of natural substances such as bone and shell are envied by those involved in the fabrication of materials. A 'bricks-and-mortar' structure, assembled layer by layer, is the key to making sea shells.
For a materials scientist, cross-sectional images of the complex microstructures of naturally occurring hard materials such as bones and sea shells are awe-inspiring. Over many millions of years, nature has devised schemes to combine seemingly incompatible building-blocks — 'soft' organic proteins and 'hard' inorganic particles of calcium carbonate — in a manner that produces composite materials with the unusual combination of high strength, hardness and toughness. Imagine, however, that you could build such a structure as a mason would, one layer at a time, from the bottom up. Writing in Nature Materials, Tang and colleagues1 explain how it can be done, using a molecular-level processing scheme known as layer-by-layer assembly2,3.
Flexible soft materials that can undergo energy-absorbing molecular rearrangements during deformation are tough, but also very compliant. In contrast, rigid hard materials are stiff but often also very brittle, and they have little ability to absorb energy, so their toughness is low. To be strong, hard and tough, a material must be able to absorb a large amount of energy during mechanical deformation and also maintain high stiffness. In bone or shell, this desirable combination of properties is made possible by one key attribute — a bricks-and-mortar-like structure, made up of strongly interacting, nanometre-size building-blocks. The 'hard' bricks and 'soft' mortar are complementary in their response to stress and strain.
So far, attempts to mimic these structures with synthetic building-blocks have failed to produce a material with similarly impressive mechanical properties, because most conventional processing techniques simply do not offer the nanoscale level of control needed to create a highly regular bricks- and-mortar-type arrangement. Nature has no such difficulty with nanoengineering: it can assemble, in a regular manner, building-blocks of the right dimensions that interact strongly enough at their interfaces to allow the transfer of deformation energy between the rigid bricks and the softer mortar. Reproducing these elements synthetically is a challenge.
But this is exactly what Tang et al.1 have achieved, through the alternating sequential deposition of negatively charged, nanometre-thick clay platelets (the bricks) and a positively charged polymer (the mortar). The primary driving force for this adsorption-based assembly, which is carried out entirely from dilute aqueous solutions of the materials, is electrostatic: the positively charged polymer chains are attracted to the negatively charged clay platelets, and vice versa. By assembling the clay platelets (in this case, a material called montmorillonite) and polymer chains one deposition step at a time, the authors are able to create a bricks-and-mortar-type arrangement that mimics the natural structure of nacre, the material known as mother-of-pearl (Fig. 1).
The polymer chains are arranged in coils and folds, physically pinned by relatively weak electrostatic interactions. As the material is deformed and the clay platelets begin to slide over each other, the polymer chains can undergo molecular rearrangements, through the breaking of 'sacrificial' ionic bonds and the concomitant unfolding of the coiled polymer chains. And this process, in turn, allows the material to absorb a lot of deformation energy. The authors report that thin films assembled in this way have tensile strength and stiffness that approach those of seashell nacre. Their stiffness is significantly higher than the more disordered composites previously fabricated from similar materials using conventional methods.
As a step towards creating synthetic materials that truly mimic the mechanical behaviour of naturally occurring materials, this is an important advance. The true potential of this approach — to construct more complicated layered structures containing many types of building-blocks — has yet to be pursued. But, for example, this group has also shown that layer-by-layer assembled films containing carbon nanotubes have exceptional mechanical properties4. So this seems a promising way of fabricating multilayered, multi-component thin films, with molecular architectures designed to take full advantage of the complex interactions possible between many different types of materials. And as this assembly process simply involves alternately dipping a substrate into dilute aqueous solutions of oppositely charged materials, it is easy to control the number, type and sequence of layers added to the film5.
There is a price to pay, however, for assembling the building-blocks one layer at a time. Such structures, particularly if they need to reach thicknesses in the micrometre range, require far longer fabrication times than more conventional processes such as spin-coating. Indeed, it is impressive that Tang et al.1 have succeeded in creating free-standing, micrometre-thick films in this way (Fig. 1c).
Another challenge in working with charged polymers is that they are capable of adsorbing a lot of water, which in turn can degrade the mechanical properties by screening the ionic interactions that lend strength to the material. Tang et al. show that there is a significant decrease in mechanical performance when films are tested in high-humidity environments. But this problem can be addressed, as many different types of materials can be assembled in these multilayer films, including those that are hydrophobic (water resistant) or that can be rendered hydrophobic by subsequent chemical or thermal treatments.
In any event, these model structures are sure to provide new insight into the behaviour of naturally occurring materials. Their applications could be widespread, from synthetic engineering of biological hard tissue to thin-film protective coatings.
Tang, Z., Kotov, N. A., Magonov, S. & Ozturk, B. Nature Mater. 2, 413–418 (2003).
Decher, G., Hong, J. D. & Schmitt, J. Thin Solid Films 210, 831–835 (1992).
Decher, G. Science 277, 1232–1237 (1997).
Mamedov, A. A. et al. Nature Mater. 1, 190–194 (2002).
Joly, S. et al. Langmuir 16, 1354–1359 (2000).
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