Nanoconfinement of spider silk fibrils begets superior strength, extensibility and toughness

Tristan Giesa,Nicola Pugno,Markus Buehler,Melis Arslan

doi:10.1038/npre.2011.5916.2

Tristan Giesa, Nicola Pugno + Show 2 more

Open Access

https://doi.org/10.1038/npre.2011.5916.2

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Abstract

AbstractSilk is an exceptionally strong, extensible and tough material made from simple protein building blocks. The molecular structure of dragline spider silk repeat units consists of semi-amorphous and nanocrystalline beta-sheet protein domains. Here we show by a series of computational experiments how the nanoscale properties of silk repeat units are scaled up to create macroscopic silk fibers with outstanding mechanical properties despite the presence of cavities, tears and cracks. We demonstrate that the geometric confinement of silk fibrils to diameters of 50±30 nm width is critical to facilitate a powerful mechanism by which hundreds of thousands of protein domains synergistically resist deformation and failure to provide enhanced strength, extensibility and toughness at the macroscale, closely matching experimentally measured mechanical properties. Through this mechanism silk fibers exploit the full potential of the nanoscale building blocks, regardless of the details of microscopic loading conditions and despite the presence of large defects. Experimental results confirm that silk fibers are composed of silk fibril bundles with diameters in the range of 20-150 nm, in agreement with our predicted length-scale. Our study reveals a general mechanism to map nanoscale properties to the macroscale and provides a potent design strategy towards novel fiber and bulk nanomaterials.

Highlights

The great appeal of exploring spider silk, an ancient hierarchical biological protein material (Fig. 1a), lies in its intriguing mechanical properties that emerge despite the material’s simple protein building blocks [1,2,3,4]
A critical insight derived from this finding is that molecular unfolding, beta-sheet crystal rupture and other failure mechanisms span the entire structural scale of silk fibrils and up to several micrometer in length given that the width of the fibrils is confined to 50-100 nm (Fig. 4c)
This explains why the predictions from molecular simulations [14,15,16] agreed well with experimental testing of entire silk fibers, because the molecular properties can be effectively upscaled under geometric confinement such that flaw-tolerance is reached

Summary

Introduction

The great appeal of exploring spider silk, an ancient hierarchical biological protein material (Fig. 1a), lies in its intriguing mechanical properties that emerge despite the material’s simple protein building blocks [1,2,3,4]. We focus on the silk of orb-weaving spiders which is known to be extremely strong, extensible and tough [5,6,7,8] These silk fibers typically feature an initial modulus up to an average of 10 GPa [5, 6, 9], a high extensibility exceeding 50% strain at failure [5, 6, 9,10,11], which results in toughness values of several times that of Kevlar’s [12]. Beyond an initial high-stiffness regime spider silk softens at the so-called “yield point” where the stress-strain response gives way to a plateau, eventually leading to an ultimate stiffening regime prior to failure [13] The combination of these mechanisms results in the characteristic softening-stiffening stress-strain response that is found for many different types of silk [5, 6, 9]. These beta-sheet nanocrystal cross-links facilitate the stretching and unfolding of semi-amorphous domains, to expose their hidden length, by providing a backbone structure [14,15,16]

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