Characterization of Ultralow‐Density Cellular Solids: Lessons from 30 years of Bone Biomechanics Research

Abstract

Recent advances in manufacturing of microarchitectured materials have begun to overcome longstanding challenges in materials engineering to create porous materials that are simultaneously lightweight, strong, stiff, and flaw-tolerant. Novel manufacturing techniques now enable materials with lattice structures to be constructed at lower densities with higher resolutions than previously possible. In parallel, extensive materials characterization of lightweight porous biological materials-such as bone, wood, bamboo, and sponges-has enabled identification of common strategies to achieve high specific stiffness, strength, and impact energy absorption. [1,2] Although the details of the structures of these materials vary considerably, these key mechanical properties arise from two common microstructural features: 1) a multiscale hierarchical structure that spans the nano-to the microscale (Figure 2) and 2) tolerance of flaws below a critical characteristic material length scale. [3] Materials engineers are now poised to combine novel manufacturing techniques with biomimetic materials design strategies to develop synthetic engineering materials with properties that were previously unachievable.Understanding the load-bearing properties of these novel low-density microarchitectured materials is necessary to translate them to engineering design applications. In general, the microscale properties of the solid constituent material are known, but the millimeter-scale mechanical properties of the porous cellular solid are unknown and require assessment. Although standard experimental and computational analyses are adequate for high-and medium-density microarchitectured materials, a substantial literature from the bone biomechanics community demonstrates that 1) standard approaches can result in large errors in characterization of low-and ultralow-density porous materials, and 2) adjustments to experimental and computational methods for low-and ultralow-density porous materials improve accuracy and precision of these approaches.Solutions from the bone biomechanics community are applicable to low-density synthetic materials. We aim to offer information on the relationship between microarchitecture and mechanical properties in low-density bone that is primarily reported in the medical literature, often in the context of bone biology or clinical research and not summarized in a manner that is immediately useful to materials scientists. The goal of this article is to provide a review of the past 30 years of cancellous bone biomechanics with the intent of presenting an example of methodologies for characterizing the structure and mechanical performance of a low-density porous microarchitectured material. In particular, we answer three key questions: 1