wileyonlinelibrary.combiomedicines. [ 1 ] The mesoscale phenomena and architectures of soft materials are essential in generating the next generation of technology opportunities, societal benefi ts, and scientifi c advances. [ 2 ] The accumulating facts indicate that the functionality of soft materials that is critical to macroscopic performance begins to manifest itself not at the atomic or nanoscale but at the mesoscale. [ 3 ] Animal silks, including both spider and silkworm silks, show the hierarchical structures of amorphous chains and stacked β-sheets. [ 4 ] Due to the unique mechanical, optical, and biological behavior, [ 5 ] silk materials are found to have a broad range of applications in tissue engineering, bioelectronics, optics, and other areas. [ 6 ] In particular, the toughness of spider dragline silk fi bers overrides Kevlar, steel, and most man-made fi bers available today. [ 6c , 7 ] The most recent studies reveal that both spider dragline and silkworm silk fi bers consist of bundles of twisted nanofi brils, [ 2,8 ] which in turn consist of amorphous molecular chains and β-sheet nanocrystallites. [ 4a , 9 ] The great mechanical strength and substantial elasticity of silk fi bers are believed to be in connection with a special arrangement of amorphous molecular chains and β-sheet nanocrystallites in nanofi brils. [ 4a , 7c , 10 ] Unfortunately, although there are a number of speculations concerning such an arrangement, [ 4a,g , 7c , 10a , 11 ] none of them have been directly verifi ed experimentally.As a type of soft materials, the performance of silk fi bers should also be determined by four factors of hierarchical network structures ( Figure 1 ): [ 2,12 ] (1) Topology: The topology of nodes describes how the joints/ points are associated with each other. (2) Correlation length: The average of the distance between two adjacent building blocks in the same structural level. (3) Ordering/symmetry of building blocks: The symmetry or ordering of the nodes (or the representing blocks) of the network structure determine the performance of the materials. (4) Strength of linkage or interactions: It refers to the strength of linkage or interactions between the adjacent structural units at the same level. The linkage can be physical, chemical bonding, or virtual connection/association. [ 13 ] Based on the combined technologies of atomic force microscopy, X-ray diffraction/scattering, Fourier transform infrared spectra analysis, etc., it is demonstrated that the nano-fi shnet-like networks, one of the most fl exible but toughest structures, turn out to be the basic structure of silk fi laments. The force patterns of pulling individual fi brils allow the identifi cation of the pathways of unfolding protein segments in stacking β-crystallites, which reveal the fi shnetlike topology. The calculation shows that the β-crystallites in silk nanofi brils are the cross-linking points of the nano-fi shnets, which may enhance the toughness of silk fi laments up to 1000 times, compared with amyloid-like and unl...

Spiders employ different combinations of silk proteins to produce up to seven types of silks with distinct mechanical properties for various purposes, ranging from prey capture to offspring protection in egg cases. [ 1 ] Generally, spider silks are semicrystalline protein polymers that contain both crystalline and amorphous regions. [ 2 ] Silk fi bers have attracted the attention of the popular media because of their extraordinary mechanical properties which are unusual in comparison with synthetic non-protein-based fi bers. [ 3 ] Silk biomaterials have been developed for various biomedical and industrial applications, such as sutures for micro surgery, scaffolds for tissue engineering, silk particles for drug delivery, and other materials which require strength, elasticity, biodegradability and biocompatibility. [ 4 ] However, farming of spiders is limited by their highly cannibalistic and territorial behavior. [ 5 ] Consequently, spinning artifi cial silk from spider silk proteins that are produced by recombinant biotechnology is one of the most promising alternatives. [ 6 ] Studies on spider silk genes have demonstrated that silk proteins consist of multiple repetitive domains and non-repetitive terminal domains and have very high molecular weights (MWs), ranging from ∼ 250 to ∼ 366 kDa, [ 7 ] implying that the size of silk proteins is a key determinant of the silk's outstanding mechanical properties. In order to biomimic native spider silks, the recombinant proteins used to spin artifi cial silks should be similar to their native counterparts in size and composition. Due to the difficulties in producing high MW silk proteins in large quantity, attempts to spin artifi cial silks have mainly focused on smallto medium-sized recombinant proteins with simplifi ed amino acid (AA) sequences and MWs of < 120 kDa, which always result in fi bers with much lower tenacity compared to their corresponding natural silks. [ 6a , 8 ] Recently a large silk-like protein ( ∼ 285 kDa), which is similar to the repetitive region of a dragline silk protein in size but different in sequence and composition, was produced in high yield in metabolically engineered Escherichia coli . [ 9 ] The artifi cial fi ber spun from this protein had a tenacity of 508 ± 108 MPa, which is still much lower than its native form's strength of 1215 ± 233 MPa measured from the same spider species. [ 10 ] To overcome the problems in cloning and expression of native-sized spider silk proteins, an inteinbased protein fusion method has been proposed very recently for producing multiple repetitive domains of silk proteins in plant systems. [ 11 ] With this method, a mixture of multimers ranging from two to ten domains was generated, but homogeneous large protein could not be obtained. Similarly, such non-uniform multimers can be synthesized by introducing two cysteine residues into one silk protein fragment and then by chemically linking one cysteine in one domain with one cysteine in another domain. [ 12 ] It was found that the fi bers containing the multim...