Biomaterials Science PAPER Cite this: Biomater. Sci., 2018, 6, 146 Received 21st July 2017, Accepted 3rd November 2017 DOI: 10.1039/c7bm00649g rsc.li/biomaterials-science The role of hierarchical design and morphology in the mechanical response of diatom-inspired structures via simulation Alejandro Gutiérrez, a Metin G. Guney, b Gary K. Fedder b,c and Lilian P. Dávila * a Diatoms are microscopic algae with intricate shell morphologies and features ranging from the nano- meter to the micrometer scale, which have been proposed as templates for drug delivery carriers, optical devices, and metamaterials design. Several studies have found that diatom shells show unique mechanical properties such as high specific strength and resilience. One hypothesis is that these properties stem from the structural arrangement of the material at the nanometer and micrometer length scales, challenging the concept between what constitutes a “material” versus a “structure”. In this work, we have conducted a systematic simulation-prototyping study to shed light on the mechanics of diatom-inspired hierarchical microstructures. The Finite Element Method (FEM) was used to replicate three-dimensional diatom shells under compressive forces. The intricate hierarchical shell structure of the Coscinodiscus sp. diatom frus- tule observed in nature was reproduced in detail. Simulation parameters were selected to reproduce com- pression experiments, with a force distributed on the surface of the diatom shell. A frustule diameter of 50 μm was used with pore diameters ranging from 0.25 to 1.2 μm across different layers. A “unit cell” FEM model was also created to focus on the basic structural element of a diatom frustule. Both of these models were used to elucidate the relation between morphology and mechanical response. Additionally, select designs were prototyped using 3D Direct Laser Writing (DLW) lithography to evaluate the feasibility of manufacturing diatom-inspired devices at the micro-scale. Distinct correlations between pore size in each frustule layer, or pore shape in the basal layers, and the mechanical response of the diatom shell were established in this study. The 3D-DLW prototypes exhibit a similar level of intricate morphological traits observed in real diatoms, opening the possibility of a simulation-based process for the design and fabrication of diatom-inspired microdevices. This research helps explain how morphology features are central to the mechanical performance of hierarchically arranged structures and biomaterials in general, and it represents a step toward the manufacture of emerging metamaterials and microarchitectures. Introduction Manipulating materials at the nanometer scale has been con- sistently proposed as a grand challenge of science, 1 with impli- cations on the fabrication of materials and structures with unique properties. 2 One path towards nanoscale manipulation is focusing on biomaterials, taking advantage of the results of millions of years of evolution to develop effective solutions. Specifically, this work focuses on one type of biostructure: diatoms. These microscopic algae have intricate porous shell morphologies and features 3 ranging from the nanometer to the micrometer scale. Diatoms have been proposed 4–8 as tem- plates for drug delivery carriers, optical devices, and metama- terials design. Several studies 9–13 have found diatom shells show unique mechanical properties such as high specific strength and resilience. Indeed, silica biostructures produced by microbial diatoms or radiolarians have complex mor- phologies tailored by evolution that appear to optimize specific functions such as protection from predators or filtering. 14 A diatom shell (called frustule) consists of two overlapping valves joined by one or more girdle bands as shown in Fig. 1, giving the shell a resemblance to a Petri dish. Frustules can vary in size from a few microns to over a millimeter 3 and often exhibit features (e.g. pores, slits, ribs) as small as a few nano- meters. There are two general types of diatoms depending on symmetry: 3 centric diatoms are often circular and have radial symmetry while pennate diatoms are elongated and often have a Materials Science and Engineering, School of Engineering, University of California Merced, 5200 N. Lake Road, Merced, CA 95343, USA. E-mail: ldavila@ucmerced.edu b Electrical and Computer Engineering, College of Engineering, Carnegie Mellon University, USA c The Robotics Institute, Carnegie Mellon University 5000 Forbes Ave., Pittsburgh, PA 15213, USA 146 | Biomater. Sci. , 2018, 6, 146–153 This journal is © The Royal Society of Chemistry 2018