Lithomimetics: lithosphere as inspiration for the design of novel materials

Materials scientists are quite used to looking at nature to find inspiration for new designs and functionalities. The living world is full of complex and sophisticated materials (from bones and seashells to cartilage and skin) that provide the most varied and extraordinary properties and functions. We have paid attention to this strategy of ‘architecturing materials’ before, for example by learning about how fish scales provide both excellent flexibility and mechanical resistance¹, how the toe-pads of geckos ‘nanoadhere’ to almost any kind of surfaces² or reviewing the diverse self-healing mechanisms of plants and animals³. What is not so usual is to draw our attention to the inanimate part of nature in search of that kind of inspiration. Is it less intriguing or compelling than living organisms? Or have we just been ignoring what it has to say?

A group of materials researchers from different universities have recently suggested⁴ that, indeed, we are probably overlooking a hidden treasure in the form of an immense repository of structural patterns sitting in the Earth’s lithosphere. According to these researchers, the morphology of inert matter could be just as inspirational for the design of novel materials as living beings have been for so long. This new way of looking at nature, which studies the structural patterns found in the lithosphere and assesses their potential for replication through artificial processes, is just taking its first steps and already has an appealing name: lithomimetics.

It is important to note that biomimetics and lithomimetics, although similar in concept and principle, have some crucial differences in approach and purpose. To begin with, the excellent properties of living organisms are the result of a long and complex evolutionary process, whereas inanimate nature does not undergo any kind of selection process in its development. The immediate consequence of this fact is that a given structural pattern found in the Earth’s crust does not necessarily imply an advantage for a given function. We could say that while animate nature offers us ready-made solutions for engineering materials, inanimate nature gives us hints or tips that could lead to promising material architectures.

Although some may hesitate to recognise the potential of this recently born approach, the authors put forward a good number of reasons to prove this new strategy interesting. One of these reasons is that the emergence of structural patterns in inanimate nature is not constrained by the limitations in temperatures and pressures present in the living world. This fact alone, the authors claim, leads to a huge variety of patterns that define a whole ‘atlas’ of potentially useful structures. A second good reason to take this new approach into account is that the physicochemical principles governing morphogenesis in the lithosphere may be easily replicated at the lab compared to their biological counterparts.

Most structural patterns in the lithosphere are the result of self-organisation induced in rocks by plastic deformation under high pressure. This process often involves adjacent geological bodies penetrating fractured ones due to bulk compression, which leads to whimsical geometries and arrangements of the different lithosphere’s constituents involved. This mixing of geological bodies is beautifully revealed in the pictures below, where formations such as vortices, chevron-like folds, spirals or boudinage are shown.

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The thermodynamic principle governing these transformations is that of the maximum rate of dissipation of mechanical energy, which is precisely the principle driving the formation of any meso-structure under plastic deformation. This parallelism has led the researchers to carefully examine several material architectures obtained by a suite of metal working techniques collectively known as severe plastic deformation (SPD). And the similarity between some natural and artificial patterns are indeed quite remarkable, as we can observe comparing the images above and below this paragraph.

Severe plastic deformation is a relatively simple top-down technique applied to metals in which the material’s microstructure is transformed through large shear strains under high pressure. The result of this process is an outstanding enhancement of mechanical properties due to the ultrafine grain refinement achieved, sometimes reaching grain sizes at the nanoscale. The main advantage of SPD compared to bottom-up techniques is that it produces microstructured bulkmaterial in a simple, economic way whereas bottom-up techniques only produce nano-grain materials in small volumes through more complex and expensive processes.

The simplicity of SPD techniques, which rely on self-organisation of matter to achieve multiscale hierarchical structures (in contrast to other techniques such as 3D printing or assembling), remind the authors the processes undergone by lithosphere structures and their extraordinary properties, including ultrafine grain structure, good bonding between components, high fracture resistance and good tensile ductility due to the presence of interlocked hard and soft constituents (a strategy frequently seen in biological structural systems too). The key factor here is the great variety of conditions in strain, pressure and heat found in the lithosphere, which leads to a huge variety of formations not yet explored in artificial processes.

Is it worth going down this road? Even though the deformation mechanisms of rocks might not be the same than those undergone by metals subjected to SPD forming techniques, the researchers take the similarity of the patterns under study (figures 2 and 3) as a good indication of how new processing routes could be developed by mirroring pattern formation in the lithosphere. To this end, they propose a new field of study, lithomimetics, which would dig deeper into the possibility of replicating geological patterns to achieve novel architectured materials.

Lithomimetics would then result in the cross-fertilization of materials science, metal working and structural geology. The accumulated knowledge of the morphogenesis of lithosphere patterns in geoscience together with the understanding of plastic deformation mechanisms of materials scientists and metallurgists could be of great value in order to find new processing routes and guiding principles for materials design. Lithomimetics has recently been considered⁵ as one of the most promising approaches for materials innovative design along with other current trends such as topological interlocking, lattice metamaterials or cluster and nanoparticle assembly. At any rate, if there is a very good reason to explore lithomimetics is that it introduces a fresh approach not considered so far in the long-stablished human practice of looking at nature in search of inspiration.

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