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A universal design paradigm in biology is the use of hierarchies, which is evident in the structure of proteins, cells, tissues, and organisms, as well as outside the material realm in the design of signaling networks in complex organs such as the brain. A fascinating example of a biological structure is that of diatoms, a microscopic mineralized algae that is mainly composed of amorphous silica, which features a hierarchical structure that ranges from the nano-to the macroscale. Here, we use the porous structure found at submicron length scales in diatom algae as a basis to study a bioinspired nanoporous material implemented in crystalline silica. We consider the mechanical performance of two nanoscale levels of hierarchy, studying an array of thin-walled foil silica structures and a hierarchical arrangement of foil elements into a porous silica mesh structure. By comparing their elastic, plastic, and failure mechanisms under tensile deformation, we elucidate the impact of hierarchies and the wall width of constituting silica foils on the mechanical properties, by carrying out a series of large-scale molecular dynamics (MD) simulations with the first principles based reactive force field ReaxFF. We find that by controlling the wall width and by increasing the level of hierarchy of the nanostructure from a foil to a mesh, it is possible to significantly enhance the mechanical response of the material, creating a highly deformable, strong, and extremely tough material that can be stretched in excess of 100 pct strain, in stark contrast to the characteristic brittle performance of bulk silica. We find that concurrent mechanisms of shearing and crack arrest lead to an enhanced toughness and are enabled through the hierarchical assembly of foil elements into a mesh structure, which could not be achieved in foil structures alone. Our results demonstrate that including higher levels of hierarchy are beneficial in improving the mechanical properties and deformability of intrinsically brittle materials. The findings reported here provide insight into general material design approaches that may enable us to transform a brittle material such as silicon or silica into a ductile, yet strong and tough material, solely through alterations of its structural arrangement at the nanoscale.

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The interest in nanoscale biomimetics and biologically inspired designs for structural and mechanical applications has significantly increased over the past few years. One reason for this lies in the apparent benefits that come from nanomaterials, including enhanced mechanical properties and multifunctionality. Yet, many of these materials are produced from generally expensive methods with possible toxic byproducts. But what if we could harness the benefits of nanomaterials through efficient and environmentally friendly mass production? Here, we use diatoms as a design paradigm to create novel nanoscale materials that encompass surprising mechanical properties, by utilizing a merger of structure and Biology implements intriguing structural design principles that allow for attractive mechanical properties-such as high strength, toughness, and extensibility despite being made of weak and brittle constituents, as observed in biomineralized structures. For example, diatom algae contain nanoporous hierarchical silicified shells, called frustules, which provide mechanical protection from predators and virus penetration. These frustules generally have a morphology resembling honeycombs within honeycombs, meshes, or wavy shapes, and are surprisingly tough when compared to bulk silica, which is one of the most brittle materials known. However, the reason for its extreme extensibility has not been explained from a molecular level upwards. By carrying out a series of molecular dynamics simulations with the first principles-based reactive force field ReaxFF, the mechanical response of the structures is elucidated and correlated with underlying deformation mechanisms. Specifically, it is shown that for wavy silica, unfolding mechanisms are achieved for increasing amplitude and allow for greater ductility of up to 270% strain. This mechanism is reminiscent to the uncoiling of hidden length from proteins that allows for enhanced energy dissipation capacity and, as a result, toughness. We report the development of an analytical continuum model that captures the results from atomistic simulations and can be used in multiscale models to bridge to larger scales. Our results demonstrate that tuning the geometric parameters of amplitude and width in wavy silica nanostructures are beneficial in improving the mechanical properties, including enhanced deformability, effectively overcoming the intrinsic shortcomings of the base material that features extreme brittleness.

Diatoms are unicellular algae that exhibit highly intricate, silicified cell walls called frustules. Frustules consist of hierarchical nanostructures composed of amorphous silica and organic protein. Earlier work has suggested diatoms as inspiration for novel molecular sieves, resins, and optical coatings because of their unique mechanical, structural, and optical properties. Here the present authors report studies of the mechanics of helical silica structures inspired by the geometry found in diatom frustules consisting of both crystalline and amorphous silica. Molecular dynamics simulations are reported with the reactive force field ReaxFF, which is a powerful model to describe interatomic interactions derived directly from first principles quantum mechanics. It is found that introducing a helical nanostructural geometry gives the typically brittle silica a highly ductile, yet softer, mechanical behaviour: extensions of 100 per cent to roughly 300 per cent are observed for varying helical geometries. The present authors show that hydroxide termination markedly affects the mechanical properties of the silica, lowering both the Young's modulus and the ultimate strength of the structures.The results reported here demonstrate the potential to drastically alter, and possibly improve, the properties of an inherently brittle material by confining structural features to the nanoscale and by altering its hierarchical geometry without introducing any additional material components, solely relying on geometrical changes.