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WIREs Nanomed Nanobiotechnol
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Silk nanofibril self‐assembly versus electrospinning

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Natural silk fibers represent one of the most advanced blueprints for (bio)polymer scientists, displaying highly optimized mechanical properties due to their hierarchical structures. Biotechnological production of silk proteins and implementation of advanced processing methods enabled harnessing the potential of these biopolymer not just based on the mechanical properties. In addition to fibers, diverse morphologies can be produced, such as nonwoven meshes, films, hydrogels, foams, capsules and particles. Among them, nanoscale fibrils and fibers are particularly interesting concerning medical and technical applications due to their biocompatibility, environmental and mechanical robustness as well as high surface‐to‐volume ratio. Therefore, we introduce here self‐assembly of silk proteins into hierarchically organized structures such as supramolecular nanofibrils and fabricated materials based thereon. As an alternative to self‐assembly, we also present electrospinning a technique to produce nanofibers and nanofibrous mats. Accordingly, we introduce a broad range of silk‐based dopes, used in self‐assembly and electrospinning: natural silk proteins originating from natural spinning glands, natural silk protein solutions reconstituted from fibers, engineered recombinant silk proteins designed from natural blueprints, genetic fusions of recombinant silk proteins with other structural or functional peptides and moieties, as well as hybrids of recombinant silk proteins chemically conjugated with nonproteinaceous biotic or abiotic molecules. We highlight the advantages but also point out drawbacks of each particular production route. The scope includes studies of the natural self‐assembly mechanism during natural silk spinning, production of silk fibrils as new nanostructured non‐native scaffolds allowing dynamic morphological switches, as well as studying potential applications. This article is categorized under:  Biology‐Inspired Nanomaterials > Peptide‐Based Structures  Nanotechnology Approaches to Biology > Nanoscale Systems in Biology  Biology‐Inspired Nanomaterials > Protein and Virus‐Based Structures
Morphological switches of hybrid silk proteins. (a) AFM tip stretches SELP adsorbed on a mica surface (i) resulting in nucleation and fibril growth perpendicularly to the scanning direction (white arrows) enabling regular fibril micropatterns (ii). (b) Genetically engineered collagen‐silk like triblock copolymers CSC are in a random coil conformation in solution, controlled by the protonation of histidines (indicated by +) in the silk block at pH 2. The polypeptides assemble upon charge neutralization at pH 8 into micelles in the presence of a short silk block (S8–16) (i) and β‐sheet structured fibrils in the presence of a long silk block (S24–48) (ii). (c) Schematic structure of DNA‐spider silk “click” conjugates, which were hybridized in branched topologies according to the designed DNA sequence (i), and they self‐assembled into fibrillar nanoribbons (ii, Cryo‐TEM) and micro‐rafts (iii, confocal florescence microscopy) in an annealing process depending on the employed temperature gradients. ((a) Reprinted with permission from Varongchayakul et al. (). Copyright 2013 American Chemical Society; (b) Reprinted with permission from Beun et al. (). Copyright 2014 American Chemical Society; (c) Reprinted with permission from Humenik, Drechsler, et al. (). Copyright 2014 American Chemical Society; (a) Reprinted with permission from Johnson et al. (). Copyright 2012 Royal Society of Chemistry)
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Morphologies made of the recombinant spider silk protein eADF4(C16). (a) Nanofibrils assembled at low phosphate concentrations. (b) Hydrogels formed due to interactions and entanglements of the fibrils at higher protein concentration. (c) Particles made upon salting out at high phosphate concentrations. (d) Fibril growth seeded at the particle surface. ((a) Reprinted with permission from Humenik, Drechsler, and Scheibel (). Copyright 2014 American Chemical Society; (b) Reprinted with permission from Schacht and Scheibel (). Copyright 2011 American Chemical Society; (c) Reprinted with permission from Slotta, Rammensee, Gorb, and Scheibel (). Copyright 2008 Wiley‐VCH; (d) Reprinted with permission from Humenik, Smith, Arndt, and Scheibel (). Copyright 2015a Elsevier)
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Self‐assembled fibrils from natural silk proteins. (a) 3‐Dimensional confocal rheology images (i) of silk dope before (top left, insert) and after shear (bottom right) demonstrating shear induced fibrillization and a representative horizontal slice (ii) of fibrillization in a x,y‐plain of the sample. Green arrows represent the direction of shear. (b) AFM (i) and transition electron microscopy (TEM) (ii) images of the nanofibers assembled from the peptide GAGSGAGAGSGA, a hydrolysis product from the crystalline domain of regenerated SF. The insert in (i) represents a height profile analysis of the nanofibers. (c) and (d) AFM Structural characterization of fibrils assembled from regenerated SF and on graphene, respectively. The insert in C represents 2D–WAXS patterns of the β‐sheets oriented in parallel with the fibril axis in (c). (e) Schematic disintegration of B. mori fibers using a combination of sonication, solubilization, and a heat treatment resulting in uniform nanofibrils (i), which were processed into ultrathin membranes (ii) with pore sizes of 5–20 nm as represented by cross‐sectional SEM image in the middle. The inserts show cross‐sectional high magnification (left) and top view (right). ((a) Reprinted with permission from Holland, Urbach, and Blair (). Copyright 2012 Royal Society of Chemistry; (b) Reprinted with permission from Hao et al. (). Copyright 2013 Royal Society of Chemistry; (d) Reprinted with permission from Ling, Li, Adamcik, Wang, et al. (). Copyright 2014 American Chemical Society; (c) Reprinted with permission from Ling,Li, Adamcik, Shao, et al. (). Copyright 2014 Wiley‐VCH; (e) Reprinted with permission from Ling, Li, Jin, Kaplan, and Buehler (). Copyright 2016 Wiley‐VCH)
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Hierarchical molecular models of different silks. (a) In the spinning dope, noncanonical silk proteins possess prevalently helical secondary structure elements, for example, forming coiled coils or collagen like superhelices (i) which are part of a rigid repetitive domain terminated with small unstructured termini (ii). Due to typically clustered distribution of charges and respective ionic interaction, the rod‐like molecules self‐assemble into filaments and fibrils (iii) with 2–4 nm in diameter, which align in micrometer scaled tactoids (iv) yielding an ordered liquid crystalline mesophase (v). (a) In the noncanonical fiber, aligned tactoids [i] occur due to the presence of shear stress during the spinning process. The fiber (ii) solidifies upon extrusion due to oxidative cross‐linking of Cys residues and/or dehydration. The protein's conformation, however, remains unchanged, forming helical crystallites embedded in an amorphous phase (iii) (Walker et al., ). (c) Canonical silk proteins typically reveal a highly repetitive unfolded amphiphilic core domain flanked by hydrophilic globularly folded termini (i and ii) in the dope. The proteins, stored at high concentration, self‐assemble into micellar nematic mesophases (iii and iv) being stabilized by a chaotropic environment. (d) During the spinning process, the proteins stretch and align in coalescing mesophases (i) resulting in a formation of fibrils (ii) with 10–100 nm in diameter in the fiber (iii). The influx of kosmotropic ions and pH drop in the spinning duct trigger a transformation of the core domain into β‐sheet rich structures (iv), which align and stack into nanocrystallites crosslinking an amorphous matrix (Eisoldt, Smith, & Scheibel, ; Jin & Kaplan, )
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Relationship of concentration and fiber diameter obtained by electrospinning of regenerated SF. The curves represent different published approaches as described in the legend (fibrillar: spinning dope contains fibrils, Dou & Zuo, ; Liu et al., ; Zhang, Zuo, et al., , spherical: spinning dope contains spheres, Zhang, Zuo, et al., , no structure defined: the authors did not determine any structural features in the spinning dope, Amiraliyan et al., ; Ayutsede et al., ; Sukigara et al., )
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Production of Bombyx mori silk fibroin solutions for subsequent electrospinning and the relevant parameters for preconditioning of the spinning dope. Sericin must be removed by degumming at conditions affecting the resulting microstructure morphologies in the regenerated silk fibroin solution. Furthermore, an intermediate film casting‐ and dissolution step can be applied to adjust the solvent conditions and to trigger the occurrence of spherical or fibrillar structures in solution, strongly determining the viscosity of the spinning dope and thus the electrospinning process
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Schematic overview of the electrospinning setup. Basic components (left), detailed view on Taylor cone, the processes leading to fiber formation (middle), and main process parameters (right) are presented
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Biology-Inspired Nanomaterials > Protein and Virus-Based Structures
Nanotechnology Approaches to Biology > Nanoscale Systems in Biology
Biology-Inspired Nanomaterials > Peptide-Based Structures

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