Interplay of structure and mechanics in silk/carbon nanocomposites
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Introduction Animal silks, such as spider dragline silks, feature superior toughness, with toughness values that are several times that of steel and Kevlar fibers.1–3 Carbon nanomaterials (CNMs) such as graphene oxide (GO), graphene, and carbon nanotubes (CNTs), by contrast, are the strongest materials ever tested. For example, graphene has a tensile strength of 130.5 GPa and a Young’s modulus of 1 TPa,4 which is 200 times stronger than steel. Silk materials, especially silk fibroin (SF), derived from mulberry Bombyx mori silkworm cocoons, are often integrated with CNMs to construct mechanically enhanced materials. Applications for these SF/CNM nanocomposites have also expanded, from structural materials to high-tech fields, such as biomedicine, ultrafiltration membranes, environmentally adaptive materials, as well as electronic and optical devices.5 As with biological nanocomposites such as diatoms, seashells, and bone, the macroscopic mechanical properties of SF/CNM nanocomposites are governed by the size, shape, structure, and arrangement of the “building blocks” (the repetitive units that constitute the superior structure, such as basic crystal units and nanofibrils). However, there is growing evidence that the mechanical behavior of these composite materials is largely modulated by the interfacial interactions, even though they comprise a very small volume fraction in the materials.6 For example, weak interfaces typically
induce nonlinear deformations and deflect cracks in materials into configurations in which propagation is more difficult.7 Gradient interfaces usually channel programmed deformation of materials to adapt to environmental (e.g., humidity and temperature) changes.8 The regulation of interactions between SF and CNMs provides a rational route to tailor the mechanics of the materials to fulfill task-specific needs without involving additional components for mechanical enhancement. Different applications of the materials often require distinct mechanical properties. For instance, soft regenerated engineered tissues (e.g., artificial skin and ligaments9,10) and wearable electronics (e.g., electronic skins and sensors11) usually require SF/CNM nanocomposites that are mechanically compatible with biological tissues, with low modulus, good flexibility, and stretchability. In contrast, in some hard regenerated engineered tissues (e.g., bones and teeth6,12) and biosensor systems (e.g., diagnostic and biochemical sensors13), the SF/CNM nanocomposites are required to exhibit high strength and high structural stability. This article provides a summary of structure–mechanics relationships in SF/CNM nanocomposites, including SF/GO, SF/graphene, and SF/CNT systems. We mainly focus on the effects of interactions between SF and CNMs on mechanical properties of the composite materials. We demonstrate the challenges involved in developing these materials and provide
Jing Ren, School of Physical Science and Technology, ShanghaiTech University, China; [email protected] Yawen Liu, School of Physical Science and Tech
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