From Diatom Frustule to the Design of Novel Bioinspired Lightweight Materials

Diatoms are the most prevalent creatures in all aquatic environments and among the most important from an ecological viewpoint. The characteristic feature of these nano-sized unicellular organisms, which for years has stimulated the interest of the scientific community and beyond, is their protective biosiliceous exoskeleton known as the frustule. The diatom frustule, besides being a sophisticated three-dimensional physical barrier that separates the diatom’s internal organelles from their surroundings, is a classic example of multifunctionality achieved in Nature through the hierarchical assembly of a few simple constituents. This three-dimensional cell wall has been optimized by Nature to survive very challenging environments: It controls nutrient gaining and diatom sinking rate, it acts as a filter against viruses, it controls light absorption, it provides mechanical protection, and many other functions that promote the spread of diatoms in Nature. Previous studies of diatoms have focused primarily on the role of their shells for filtration, drug delivery, sensing, and scaffolding. Recently, there has been a growing interest in the mechanical performance of the frustule, with potential implications in nanotechnology and large-scale bioinspired structures. Here, we investigate the role of the morphological features of diatoms on frustule mechanical properties and functionality through a comprehensive analytical, numerical, and experimental approach, with experimental tests carried out on 3D-printed diatom-inspired samples. We focus on the <i>Coscinodiscus sp.</i> and on the out-of-plane behavior of its multilayer shell. To simplify the study, we consider only the middle layer, characterized by a honeycomb-like structure, and we build several 3D geometric models (random, Voronoi, etc) to probe the effect of shape- and size-gradients on the overall out-of-plane mechanical behavior. As mechanical parameters, we focus on the stiffness of the resulting structures, the peak stress, and the absorbed energy. We notice how the shape of the unit cell and the gradient affect the macroscopic behavior–although keeping a constant connectivity– thus allowing a transition from a bending-dominating behavior to a stretching dominating one. These bioinspired models suggest the possibility of manufacturing geometries with locally tunable properties to achieve a global behavior optimized for the mechanical function they have to perform. These results build off our previous studies on the effect of other morphological features (e.g., pore size and distribution, wall thickness) on both mechanical and fluid-dynamic properties. The outcome of our studies reveals how the natural geometry is optimized to simultaneously provide lightweight, bending stiffness, and structural integrity, limiting local buckling and providing different dissipation mechanisms to absorb energy, thus preventing catastrophic failure. The optimal combination of mechanical and fluid-dynamic properties confirms the multifunctionality of such structures and makes them models for the future design of multifunctional templates. The transfer of Nature's design principles to engineering, and specifically the functional gradients and heterogeneities present in biological materials, can open new venues towards increasing the efficiency and performance of man-made materials. Thus, this research may provide new insights for the design of novel biomimetic diatom-like materials and engineering device innovation based on multifunctional biological nanomaterials.