Silk features exceptional mechanical properties such as high tensile strength and great extensibility, making it one of the toughest materials known. The exceptional strength of silkworm and spider silks, exceeding that of steel, arises from beta-sheet nanocrystals that universally consist of highly conserved poly-(Gly-Ala) and poly-Ala domains. This is counterintuitive because the key molecular interactions in beta-sheet nanocrystals are hydrogen bonds, one of the weakest chemical bonds known. Here we report a series of large-scale molecular dynamics simulations, revealing that beta-sheet nanocrystals confined to a few nanometres achieve higher stiffness, strength and mechanical toughness than larger nanocrystals. We illustrate that through nanoconfinement, a combination of uniform shear deformation that makes most efficient use of hydrogen bonds and the emergence of dissipative molecular stick-slip deformation leads to significantly enhanced mechanical properties. Our findings explain how size effects can be exploited to create bioinspired materials with superior mechanical properties in spite of relying on mechanically inferior, weak hydrogen bonds.

Nanoconfinement controls stiffness, strength and mechanical toughness of β-sheet crystals in silk

Silk fibroin biomaterials for tissue regenerations.

We report a study of self-assembled beta-pleated sheets in B. mori silk fibroin films using thermal analysis and infrared spectroscopy. B. mori silk fibroin may stand as an exemplar of fibrous proteins containing crystalline beta-sheets. Materials were prepared from concentrated solutions (2−5 wt % fibroin in water) and then dried to achieve a less ordered state without beta-sheets. Crystallization of beta-pleated sheets was effected either by heating the films above the glass transition temperature (Tg) and holding isothermally or by exposure to methanol. The fractions of secondary structural components including random coils, alpha-helices, beta-pleated sheets, turns, and side chains were evaluated using Fourier self-deconvolution (FSD) of the infrared absorbance spectra. The silk fibroin films were studied thermally using temperature-modulated differential scanning calorimetry (TMDSC) to obtain the reversing heat capacity. The increment of the reversing heat capacity ΔCp0(Tg) at the glass transition fo...

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Determining Beta Sheet Crystallinity in Fibrous Proteins by Thermal Analysis and Infrared Spectroscopy

Structures, mechanical properties and applications of silk fibroin materials

A Perspective is presented on the history and current understanding of molecular gels and the factors that must be considered to characterize them. The abilities of the most important structural, dynamic, and rheological tools available currently to provide the information necessary to follow the formation of a molecular gel from its initial sol phase and then to define it at different distance and time scales are discussed. Approaches to determining a priori when a molecule will gelate a selected liquid, as well as possible methodologies for overcoming current limitations in understanding molecular gels, are presented. Finally, some of the many potential and realized applications for these materials are enumerated.

The past, present, and future of molecular gels. What is the status of the field, and where is it going?

http://people.physics.illinois.edu/selvin/prs/498ibr/spidersilk.bj.2006.pdf

Design of Superior Spider Silk: From Nanostructure to Mechanical Properties

Spider dragline silk, as a type of high-performance natural fiber, displays a unique combination of tensile strength and extensibility that gives rise to a greater toughness than any other natural or synthetic fiber. In contrast to silkworm silk, spider dragline silk displays a remarkable strain-hardening character for which the origin remains unknown. In this paper, the performance of silkworm silk and spider dragline fibers under stretching is compared based on a combined structural and mechanical analysis. The molecular origin of the strain-hardening of spider silk filaments is addressed in comparison to rubber and Kevlar. Unlike rubber, the occurrence of strain-hardening can be attributed to the unfolding of the intramolecular β-sheets in spider silk fibrils, which serve as “molecular spindles” to store lengthy molecular chains in space compactly. With the progressive unfolding and alignment of protein during fiber extension, protein backbones and nodes of the molecular network are stretched to support the load. Consequently the dragline filaments become gradually hardened, enabling efficient energy buffering when an abseiling spider escapes from a predator. As distinct from synthetic materials such as rubber (elastomers), this particular structural feature of spider draglines not only enables quick energy absorption, but also efficiently suppresses the drastic oscillation which occurs upon an impact. The mimicking of this strain-hardening character of spider silk will give rise to the design and fabrication of new advanced functional materials with applications in kinetic energy buffering and absorption.

Structural Origin of the Strain‐Hardening of Spider Silk

Ice nucleation inhibition: mechanism of antifreeze by antifreeze protein.

Zero-sized Effect of Nano-particles and Inverse Homogeneous Nucleation: PRINCIPLES OF FREEZING AND ANTIFREEZE *

We theoretically and experimentally study the mechanical response of silkworm and spider silks against stretching and the relationship with the underlying structural factors. It is found that the typical stress-strain profiles are predicted in good agreement with experimental measurements by implementing the “β-sheet splitting” mechanism we discovered and verified, primarily varying the secondary structure of protein macromolecules. The functions of experimentally observed structural factors responding to the external stress have been clearly addressed, and optimization of the microscopic structures to enhance the mechanical strength will be pointed out, beneficial to their biomedical and textile applications.

Ning Du

Papers

Design of Superior Spider Silk: From Nanostructure to Mechanical Properties

Structural Origin of the Strain‐Hardening of Spider Silk

Ice nucleation inhibition: mechanism of antifreeze by antifreeze protein.

Zero-sized Effect of Nano-particles and Inverse Homogeneous Nucleation: PRINCIPLES OF FREEZING AND ANTIFREEZE *

Unraveled mechanism in silk engineering: Fast reeling induced silk toughening