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This review examines size effects observed in the mechanical strength of biopolymers that are organized in microstructures such as fibrils, layered composites, or particle nanocomposites. We review the most important aspects that connect nanoconfinement of basic material constituents at critical length scales to the mechanical performance of the entire material system: elastic modulus, strength, extensibility, and robustness. We outline theoretical and computational analysis as well as experimentation by emphasizing two strategies found in abundant natural materials: confined fibrils as part of fibers and confined mineral platelets that transfer load through a biopolymer interface in nanocomposites. We also discuss the application of confinement as a mechanism to tailor specific material properties in biological systems.
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Download the Supplemental Material (PDF). Includes Supplemental Table 1 and Supplemental Figures 1-2 (also reproduced below). Supplemental Figure 1. Elastic modulus and crystallinity of nylon-6,6 electrospun fibers as a function of the fiber diameter. (a) The relative elastic modulus compared to the bulk modulus E* increases dramatically at a critical diameter of approximately 500 nm. This behavior is explained by the role of spatial confinement on entropy and the dominance of intermolecular interactions in thin nanofibers. (b) The alignment of crystallites and the degree of crystallinity in the fiber also improve with a smaller diameter, leading to higher strength and toughness. Figure adapted from Reference 8. Supplemental Figure 2. Effect of confinement on the toughness modulus in spider dragline silk MaSp1. Toughness modulus for tensile loading conditions (1) and (2), see Figure 8. By geometric confinement, the theoretical toughness modulus (190-200 MJ/m3) is approached which is in good agreement with experimental toughness moduli represented in the light-gray shaded area (110–350 MJ/m3). Figure adapted from Reference 47.