Assessing the Interfacial Dynamic Modulus of Biological Composites

Biological composites (biocomposites) possess ultra-thin, irregular-shaped, energy dissipating interfacial regions that grant them crucial mechanical capabilities. Identifying the dynamic (viscoelastic) modulus of these interfacial regions is considered to be the key toward understanding the underlying structure–function relationships in various load-bearing biological materials including mollusk shells, arthropod cuticles, and plant parts. However, due to the submicron dimensions and the confined locations of these interfacial regions within the biocomposite, assessing their mechanical characteristics directly with experiments is nearly impossible. Here, we employ composite-mechanics modeling, analytical formulations, and numerical simulations to establish a theoretical framework that links the interfacial dynamic modulus of a biocomposite to the extrinsic characteristics of a larger-scale biocomposite segment. Accordingly, we introduce a methodology that enables back-calculating (via simple linear scaling) of the interfacial dynamic modulus of biocomposites from their far-field dynamic mechanical analysis. We demonstrate its usage on zigzag-shaped interfaces that are abundant in biocomposites. Our theoretical framework and methodological approach are applicable to the vast range of biocomposites in natural materials; its essence can be directly employed or generally adapted into analogous composite systems, such as architected nanocomposites, biomedical composites, and bioinspired materials.

Cortical Bone Model With a Microcrack Under Tensile Loading

BackgroundBone is a biological material whose mechanical properties are outstanding. The fracture mechanics research of cortical bone is a major challenge to fully understand the complex behavior of biological composites and for the design of future bioinspired materials. MethodsIn order to characterize the fracture mechanics behavior of cortical bone, the plane problem for the cortical bone with a microcrack located in the interstitial tissue under tensile loading was considered. Using the solution for the continuously distributed edge dislocations as Green's functions, the problem was formulated. ResultsThe singular integral equations with Cauchy kernels were obtained. And the numerical results indicate that the stress intensity factor of the microcrack is dominated by the material constants and the geometric parameters of the cortical bone.ConclusionThe numerical results suggest that a soft osteon promotes the microcrack propagation while stiff one repels it, but this interaction effect is limited near the osteon. Some of the numerical results are in accordance with the results obtained and additional numerical results predicted need to be confirmed.

Interrogating helical nanorod self-assembly with fractionated cellulose nanocrystal suspensions

The helical self-assembly of cholesteric liquid crystals is a powerful motif in nature, enabling exceptional performance in many biological composites. Attempts to mimic these remarkable materials by drying cholesteric colloidal nanorod suspensions often yield films with a non-uniform mosaic-like character, severely degrading optical and mechanical properties. Here we show—using the example of cellulose nanocrystals—that these problems are due to rod length dispersity: uncontrolled phase separation results from a divergence in viscosity for short rods, and variations in pitch can be traced back to a twisting power that scales with rod length. We present a generic, robust and scalable method for fractionating nanorod suspensions, allowing us to interrogate key aspects of cholesteric self-assembly that were previously hidden by colloid dispersity. By controlled drying of fractionated suspensions, we can obtain mosaic-free films that are uniform in colour. Our findings unify conflicting observations and open routes to biomimetic artificial materials with performance that can compete with that of nature’s originals.

Fracture Toughness of Biological Composites With Multilevel Structural Hierarchy

It is well known that the biological composites have superior mechanical properties due to their exquisite multilevel structural hierarchy. However, the underlying mechanisms of the roles of this hierarchical design in the toughness of the biocomposites remain elusive. In this paper, the deformation and fracture mechanism of multilevel hierarchical structures are explored by molecular dynamics simulations. The effects of the multilevel design on fracture toughness, nonlinear deformation of soft matrix, and the crack path pattern were quantitatively analyzed. We showed that the toughness of composites is closely associated with the pattern of the crack path and the nonlinear deformation of the matrix. Additionally, the structure with a higher level of hierarchy exhibit higher toughness, which is less sensitive to the geometrical change of inclusions, such as the aspect ratio and the staggered ratio. This work provides more theoretical evidence of the toughening mechanism of the multilevel hierarchy in fracture toughness of biological materials via new methods of analyzing fracture of multilevel structures and provides guidelines for the design of high-performance engineering materials.

Biological composites—complex structures for functional diversity

The bulk of Earth’s biological materials consist of few base substances—essentially proteins, polysaccharides, and minerals—that assemble into large varieties of structures. Multifunctionality arises naturally from this structural complexity: An example is the combination of rigidity and flexibility in protein-based teeth of the squid sucker ring. Other examples are time-delayed actuation in plant seed pods triggered by environmental signals, such as fire and water, and surface nanostructures that combine light manipulation with mechanical protection or water repellency. Bioinspired engineering transfers some of these structural principles into technically more relevant base materials to obtain new, often unexpected combinations of material properties. Less appreciated is the huge potential of using bioinspired structural complexity to avoid unnecessary chemical diversity, enabling easier recycling and, thus, a more sustainable materials economy.