Materials in nature. Composites

"The walls of the palace are composed of adobe -- sticky clay mixed with stalks of corn, animal stools, all fermented together -- and conical stones pressed into it, so as to form an elastic base -- the stones can move independently in their matrix of adobe, absorbing, dispersing, the force of an earthquake.  I am fascinated by this, and draw a diagram in my notebook: the discovery of composites for added strength, for resisting shock, millennia ago. Since nothing so singular can be passed over by the group, a vigorous discussion at once breaks out about composites in nature -- the interweaving, at a microscopic level, of two different materials, one crystalline or amorphous, perhaps, and one fibrous, in order to get something harder, tougher, yet more elastic than either component alone. Nature has employed composites in all sorts of biological structures: horses' hooves, abalone shells, bone, the cell wall of plants. We use the same principle for reinforced concrete, and new synthetic ceramics or reinforced plastics; the Zapotec used it for adobe."

- Oaxaca Journal. Oliver Sacks

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It is fascinating indeed. Nature has been at work for billions and billions of years now. Playing with different systems of assembly, organisation. It has kept an ongoing note-trail in DNA of all the current living species, and keeps continuously working at things, improving, learning, exploring, changing, adapting, changing the environment, and then again changing the being. It seems to keep adopting and reinforcing best practices and lessons it has learnt which keeps her in good stead across species. One of them seems to be composite materials. (Most of human thinking is new, early, young, and logic-linear. Nature's is relatively more disparate. And so beautiful. See following, from one paper on Nature's materials)

"Another major difference between materials from Nature and the Engineer is in the way they are made. While the Engineer selects a material to fabricate a part according to an exact design, Nature goes the opposite direction and grows both the material and the whole organism (a plant or an animal) using the principles of (biologically controlled) self-assembly. Moreover, biological structures are even able to remodel and adapt to changing environmental conditions during their whole lifetime. This control over the structure at all levels of hierarchy is certainly the key to the successful use of polymers and composites as structural materials."

Talk about form/function. Or content/form issues.

Adapting the form (of a whole part or organ, such as a branch or a vertebra) is one aspect of functional adaptation. A second, which relates more directly to Materials Science, is the functional adaptation of the microstructure of the material itself (such as the wood in the branch or the bone in the vertebra). This dual optimization of the part’s form and of the material’s microstructure is well known for any engineering problem. However, in natural materials shape and microstructure are intimately related due to their common origin, which is the growth of the organ. Growth implies that “form” and “microstructure” are created in the same process. The shape of a branch is created by the assembly of molecules to cells, and of cells to wood with a specific shape. Hence, at every size level, the branch is both form and material – the structure becomes hierarchical.

It is not evident at all that the lessons learned from hierarchical biological materials will be applicable immediately to the design of new engineering materials. The reason arises from striking differences between the design strategies common in Engineering and those used by Nature. These differences are contributed by the different sets of elements used by Nature and the Engineer – with the Engineer having a greater choice of elements to choose from in the “toolbox”. Elements such as iron, chromium, nickel, etc. are very rare in biological tissues and are certainly not used in metallic form as, for example, in steels. Iron is found in red blood cells as an individual ion bound to the protein hemoglobin: its function is certainly not mechanical but rather chemical, to bind oxygen. Most of the structural materials used by Nature are polymers or composites of polymers and ceramic particles. Such materials would not be the first choice of an engineer who intends to build very stiff and long-lived mechanical structures. Nevertheless, Nature makes the best out of the limitations in the chemical environment, adverse temperatures and uses polymers and composites to build trees and skeletons.

Perhaps the biggest difference is selection and adaptation. There is equity when it is not about selection, but adaptation to the way things are. Also, adaptation allows to equate a self to multiple functions. While selection that way is narrow and relatively static, to the dynamic vitality of nature.

If not in material, perhaps the principles of the way nature works can guide our thinking.

Different strategies in designing a material result from the two paradigms of “growth” and “fabrication” are shown in Fig. 1. In the case of engineering materials, a machine part is designed and the material is selected according to the functional prerequisites taking into account possible changes in those requirements during service (e.g. typical or maximum loads, etc.) and considering fatigue and other lifetime issues of the material. Here the strategy is a static one, where a design is made in the beginning and must satisfy all needs during the lifetime of the part. The fact that natural materials are growing rather than being fabricated leads to the possibility of a dynamic strategy. Taking a leaf as an example, it is not the exact design that is stored in the genes, but rather a recipe to build it. This means that the final result is obtained by an algorithm instead of copying an exact design. This approach allows for flexibility at all levels. Firstly, it permits adaptation to changing function during growth. A branch growing into the wind may grow differently than against the wind without requiring any change in the genetic code. Secondly, it allows the growth of hierarchical materials, where the microstructure at each position of the part is adapted to the local needs. Functionally graded materials are examples of materials with hierarchical structure. Biological materials use this principle and the functional grading found in Nature may be extremely complex. Thirdly, the processes of growth and “remodeling” (this is a combination of growth and removal of old material) allow a constant renewal of the material, thus reducing problems of material fatigue. A change in environmental conditions can be (partially) compensated for by adapting the form and microstructure to new conditions. One may think about what happens to the growth direction of a tree after a small land-slide occurs. In addition to adaptation, growth and remodeling, processes occur which enable healing allowing for self-repair in biological materials.