According to the authors of a research paper in Science, "Although strong and stiff human-made composites have long been developed, the microstructure of today's most advanced composites has yet to achieve the order and sophisticated hierarchy of hybrid materials built up by living organisms in nature." Steel and metal alloys have high strength and flaw tolerance, but are heavy; ceramics have strength but poor flaw tolerance; polymers are flaw-tolerant but they deform under applied loads.
"Nature has found its way around this dilemma by combining plateletlike ceramic building blocks with polymeric matrices to render hybrid materials that are both strong and flaw-tolerant. Examples include mineralized tissues of vertebrates, such as bone, teeth, and calcified tendons, as well as the outer skeleton of invertebrates, such as the nacreous layer of mollusk shells.
The exquisite structure of these biological materials and the underlying concepts leading to their mechanical behavior have been extensively studied. Although substantial progress has been made on understanding the mechanical response of such structures, the manufacture of artificial composites that copy nature's designs remains a challenging goal."
Mother-of-pearl is 97% lime, but its microstructure gives it a breaking strength a thousand times higher. (Source: go here)
The authors go on to report on their interesting work with layered hybrid films. This research has stimulated a commentary article by Ortiz and Boyce, which is really the focus of this blog. My purpose here is to draw attention to three of the many excellent points made in their article.
1. The natural world does not just provide examples of materials that are interesting to scientists and engineers - they demonstrate "mechanical design principles". There is an underpinning rationale which, when we have grasped it, is of both theoretical and practical importance. It is not just a case of finding a feature in the natural world that works, but of recognising holistic, information-rich designs in living things.
"Using materials available in the environment that typically exhibit poor macro-scale mechanical properties (brittle biological ceramics and compliant macromolecules), they [living things] can achieve orders-of-magnitude increases in strength and toughness; in many cases, this "mechanical property amplification" occurs in a nonadditive manner that goes beyond the simple composite rule of mixture formulations. Synthetic structural materials that take advantage of the mechanical design principles found in nature could transform many fields; e.g., materials science, mechanical and civil engineering, and aeronautics and astronautics."
2. The design principles are applied to systems. There are many different components and structural variants in biological materials.
"Biological composites make use of local chemistry, compositional gradients, macromolecular supramolecular structure, length scale effects, geometry, and other factors to design robust interfaces and inter-phases that bond together different material phases, even in the presence of water."
3. Biological materials have inherent specificity. We like generic materials, but the natural world works on the basis that although design principles are generic, the applications are specific. Materials in nature are customised to be fit for purpose:
"We have yet to fully understand and take advantage of the inherent specificity of natural mechanical design principles. For example, multilayered armored fish scales serve as protection from predatory penetrating impacts, mussel byssal threads are hysteretic yet resilient to large strain deformation in order to maintain adhesion to rocks in the face of the pounding surf, and graded layer junctions in teeth resist catastrophic fracture during mastication. Each of these systems experiences, and has been designed to endure, very different loading conditions in their environment and during their function."
Biomimetic research is based on the premise that the natural world is information-rich and reverse engineering methodologies are likely to be fruitful. The significance of the two articles discussed here is that both recognise that the information concerns design principles embedded in specific applications. There is a conceptual gulf between this and the "evolutionary tinkering" mechanism of Darwinism, which has no depth and knows only pragmatism as a rationale. Design-based methodologies in biomimetics are yielding tangible results - and evolutionary tinkering is revealed as irrelevant.
Bioinspired Design and Assembly of Platelet Reinforced Polymer Films
Lorenz J. Bonderer, Andre R. Studart, and Ludwig J. Gauckler
Science, 319, 22 February 2008: 1069-1073.
Although strong and stiff human-made composites have long been developed, the microstructure of today's most advanced composites has yet to achieve the order and sophisticated hierarchy of hybrid materials built up by living organisms in nature. Clay-based nanocomposites with layered structure can reach notable stiffness and strength, but these properties are usually not accompanied by the ductility and flaw tolerance found in the structures generated by natural hybrid materials. By using principles found in natural composites, we showed that layered hybrid films combining high tensile strength and ductile behavior can be obtained through the bottom-up colloidal assembly of strong submicrometer-thick ceramic platelets within a ductile polymer matrix.
Bioinspired Structural Materials
Christine Ortiz and Mary C. Boyce
Science, 319, 22 February 2008: 1053-1054.
Materials scientists are seeking to create synthetic materials based on the mechanical design principles found in biological materials such as seashell nacre.
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