Biopolymer nanofibrils: structure, modeling, preparation, and applications

Abstract

Biopolymer nanofibrils exhibit exceptional mechanical properties with a unique combination of strength and toughness, while also presenting biological functions that interact with the surrounding environment. These features of biopolymer nanofibrils profit from their hierarchical structures that spun angstrom to hundreds of nanometer scales. To maintain these unique structural features and to directly utilize these natural supramolecular assemblies, a variety of new methods have been developed to produce biopolymer nanofibrils. In particular, cellulose nanofibrils (CNFs), chitin nanofibrils (ChNFs), silk nanofibrils (SNFs) and collagen nanofibrils (CoNFs), as the four most abundant biopolymer nanofibrils on earth, have been the focus of research in recent years due to their renewable features, wide availability, low-cost, biocompatibility, and biodegradability. A series of top-down and bottom-up strategies have been accessed to exfoliate and regenerate these nanofibrils for versatile advanced applications. In this review, we first summarize the structures of biopolymer nanofibrils in nature and outline their related computational models with the aim of disclosing fundamental structure-property relationships in biological materials. Then, we discuss the underlying methods used for the preparation of CNFs, ChNFs, SNF and CoNFs, and discuss emerging applications for these biopolymer nanofibrils.

Keywords: biopolymers; cellulose; chitin; collagen; nanofibrils; silk.

Figure 1.

Hierarchical structures of CNFs, ChNFs, SNFs and CoNFs in wood, arthropod exoskeleton, spider silk and bone.

Figure 2.

Timeline of the structural characterization and preparation of biopolymer nanofibrils.

Figure 3.

The structure of the Type I collagen. A, the chemical structure of glycine, proline and hydroxyproline. B, the structure of tropocollagen. C, AFM image of tropocollagen [58]. Copyright 2013. Reproduced with permission from Elsevier Ltd.

Figure 4.

Molecular deformation mechanisms of the wood cell wall. A, The H-bonding network, indicated by the dash lines, in the crystalline cellulose [73]. Copyright 2008. Reproduced with permission from American Chemical Society. B, The atomistic structure of wood cell wall material, in which the crystalline cellulose phases are sandwiched with an amorphous matrix composed of hemicellulose and lignin. Shear loading on the model reveals two deformation mechanisms of the model: (M1) the reorganization of the molecules within the matrix and (M2) the sliding of matrix molecules along the cellulose surface with breaking and reconstruction of H-bonds [98]. Copyright 2015. Reproduced with permission from Elsevier Ltd.

Figure 5.

Mechanical properties of chitin-protein interfaces. A, The crystal structure of chitin and the internal H-bonding network (red dotted lines). B, Atomistic models for studying the interaction between protein and chitin with a-helix and b-sheet being detached from the chitin surface [102]. Copyright 2013. Reproduced with permission from Springer.

Figure 6.

Computational modeling of natural SNFs. A, Fracture mechanism of small and large β-sheet nanocrystals during pull-out simulations. B, Schematic phase diagram to show the interplay of the parameters h and L (h is strand length and L is the nanocrystal size) in defining the properties of nanocrystals. The formation of confined β-sheet nanocrystals with critical strand length h* and critical nanocrystal size L* provides maximum strength, toughness, and stiffness. S=schematic plot of the strength of a β-strand as a function of strand length h, F=strength of nanocrystal as a function of crystal size L, T=toughness of nanocrystal as a function of crystal size L. C, The relationship between β-sheet nanocrystal dimension and mechanical properties. A-C reproduced with permission from [7]. Copyright 2010. Springer Nature. D, Force-displacement curves of the β-sheet nanocrystals with different dimensions under lateral loading. E, Stress-strain curves of spider silk based on different β-sheet nanocrystal sizes, ranging from 3 to 10 nm. D and E reproduced with permission from [13]. Copyright 2010. American Chemical Society. F, Finite element simulation of fibril interactions, showing deformation and the maximum principal stress. Shear is applied from left to right on the lower fibril [123]. Copyright 2011. Reproduced with permission from American Chemical Society. G, Loading conditions used in the simulations to understand the impact of the defects on fiber mechanical properties, implementing tensile [mode I, (1) and (2)] and shear [mode II, (3) and (4)] loading with varied aspect ratios. H, Dependence of the failure strain and failure stress on the fibril size H, as well as a direct comparison with experimental results and the mechanical behavior of a defect-free silk fiber. G and H reproduced with permission from [112]. Copyright 2011. American Chemical Society.

Figure 7.

The structure and atomistic model of collagen. A, The hierarchical structure of collagen fibers. Three polypeptide chains composed of amino acids win around each other to form tropocollagen at the nano-scale. Tropocollagens further assemble into fibrils in a staggered fashion and fibrils assemble collagen fibers [134]. Copyright 2006. Reprinted with permission from National Academy of Sciences. B, The full atomistic model of tropocollagen with various content of minerals [133]. Copyright 2011. Reprinted with permission from American Chemical Society.

Figure 8.

Preparation of CNFs through mechanical nanofibrillation methods. A and B, Digital photograph and working mechanism of a high-pressure homogenizer. B reproduced with permission from [143]. Copyright 2011. John Wiley & Sons, Inc. C and D, TEM images of CNFs isolated from (C) bamboo and (D) rice straw by nanofibrillation of the purified cellulose pulps using a high-pressure homogenizer. C and D reproduced with permission from [144]. Copyright 2014. Elsevier Ltd. E and F, Digital photograph and working mechanism of a grinder. E and F reproduced with permission from [151]. Copyright 2013. Elsevier. G and H, FE-SEM images of wood CNFs isolated by nanofibrillation of the cellulose pulps using a grinder. G and H reproduced with permission from [152]. Copyright 2007. American Chemical Society. I, Digital photograph a high-speed blender. J, FE-SEM image of wood CNFs isolated by nanofibrillation of the cellulose pulps using a high-speed blender. I and J reproduced with permission from [153]. Copyright 2011. American Chemical Society. K, Digital photograph a high-intensity ultrasonicator. L, FE-SEM image of wood CNFs isolated by nanofibrillation of the cellulose pulps using a high-intensity ultrasonicator. L reproduced with permission from [160]. Copyright 2014. John Wiley & Sons, Inc.

Figure 9.

Preparation of CNFs through chemical modification combined with mechanical nanofibrillation methods. A, Digital photograph of carboxymethylated CNF gel at 2 wt% in water. A reproduced with permission from [178]. Copyright 2013. Cambridge University Press. B, TEM image of wood carboxymethylated CNFs. The scale bar is 500 nm. B reproduced with permission from [176]. Copyright 2008. American Chemical Society. C-F, TEM images of CNFs disintegrated after TEMPO-mediated oxidation of never-dried samples: (C) bleached sulfite wood pulp, (B) cotton, (C) tunicin, and (D) BC. Inset in C is a transparent suspension of 0.1% CNFs of never-dried sulfite wood pulp TEMPO-oxidized with 2.5 mmol NaClO per gram of cellulose. C-F reproduced with permission from [181]. Copyright 2006. American Chemical Society.

Figure 10.

Preparation of CNCs through strong acid hydrolysis methods. A-C, TEM images of CNCs obtained by H2SO4 hydrolysis of (A) cotton, (B) Avicel, and (C) tunicate cellulose. Insets: enlarged views of some individual particles. A-C reproduced with permission from [192]. Copyright 2008. American Chemical Society. D and E, TEM images of CNCs obtained by hydrochloric acid hydrolysis of (D) tunicate and (E) cotton. D reproduced with permission from [194]. Copyright 2007. American Chemical Society. E reproduced with permission from [160]. Copyright 2014. John Wiley & Sons, Inc. F, AFM image of CNCs obtained by phosphoric acid hydrolysis of cotton. F reproduced with permission from [195]. Copyright 2013. American Chemical Society.

Figure 11.

Preparation of BC through bottom-up methods. A, Scheme for the formation of BC. A reproduced with permission from [208]. Copyright 1998. Elsevier. B, BC layers grown with different culture time (maximum 4 weeks). B reproduced with permission from [199]. Copyright 2000. Kluwer Academic Publishers. C-E, Synthesis of cellulose in Acetobacter xylinum. C-E reproduced with permission from [211]. Copyright 1976. National Academy of Sciences. F and G, FE-SEM images of BC pellicles: (F) shows the planar extensively cross-linked structure, whereas (G) shows the relatively weakly crosslinked layers in the thickness direction. F and G reproduced with permission from [212]. Copyright 2008. John Wiley & Sons, Inc.

Figure 12.

Overview of mechanical nanofibrillation methods for the preparation of ChNFs.

Figure 13.

Representative microscopy images of ChNFs prepared by mechanical nanofibrillation. A, SEM images of α-ChNFs from crab shell [228]. Copyright 2009. Reproduced with permission from American Chemical Society. B, SEM images of α-ChNFs from mushroom [229]. Copyright 2011, Reproduced with permission from MDPI. C, TEM image of β-ChNFs from squid pen.

Figure 14.

Elucidation of the mechanism and process of the preparation of ChNFs by TEMPO-mediated oxidation or partial deacetylation combined with mechanical treatment.

Figure 15.

Representative microscopy images of ChNFs prepared by chemical modification combined with mechanical treatment. A, crab shell α-chitin nanowhiskers prepared by TEMPO-mediated oxidation [236]. Copyright 2008. Reproduced with permission from American Chemical Society. B, crab shell α-ChNFs and ChNCs mixtures prepared by partially deacetylation [239]. Copyright 2010. Reproduced with permission from Elsevier Ltd. C, crab shell α-ChNFs prepared by esterification followed by mechanical treatment [238]. Copyright 2016. Reproduced with permission from Elsevier Ltd. D, β-ChNFs prepared by TEMPO-mediated oxidation [237]. Copyright 2009, Reproduced with permission from Elsevier Ltd.

Figure 16.

A, TEM image of chitin nanowhiskers prepared by acid hydrolysis from shrimp shell [221]. Copyright 2007. Reproduced with permission from American Chemical Society. B, AFM image of chitin nanowhiskers prepared by acid hydrolysis from crab shell [251]. Copyright 2012. Reproduced with permission from Elsevier Ltd.

Figure 17.

The methods for production of SNFs. A and B, Schematic of the top-down approaches to produce SNFs [273]. Copyright 2016. Reproduced with permission from John Wiley & Sons Inc. C, Schematic of the bottom-up approaches to produce SNFs [276] (Copyright 2014. Reproduced with permission from Royal Society of Chemistry) and [281] (Copyright 2017. Reproduced with permission from American Association for the Advancement of Science).

Figure 18.

Structural features of the SNFs obtained from top-down methods, bottom-up methods and assembled on inorganic surface. A, SEM image of SNF produced from ultrasound processing [159]. Copyright 2007. Reproduced with permission from AIP Publishing LLC. B, SEM image of SNF produced from FA/CaCl₂ dissolution [275]. Copyright 2014. Reproduced with permission from The Royal Society of Chemistry. C, SEM image of SNF produced from HFIP-based liquid exfoliation [273]. Copyright 2016. Reproduced with permission from John Wiley & Sons Inc. D, AFM image of SNFs obtained from tuning solvent systems [274]. Copyright 2016. Reproduced with permission from John Wiley & Sons Inc. E, AFM image of SNFs produced from 7 vol% EtOH induced self-assembly of silk fibroin [267]. Copyright 2014. Reproduced with permission from John Wiley & Sons Inc. F, AFM image of SNFs produced from heating induced self-assembly of silk fibroin [281]. Copyright 2017. Reproduced with permission from American Association for the Advancement of Science. G, silk fibroin self-assembly on the reduced GO surface [299]. Copyright 2014. Reproduced with permission from American Chemical Society. H, Top, the self-assembly of silk fibroin on the reduced graphene, GO and silicon dioxide surfaces. Bottom, the simulation of the interaction between silk fibroin and inorganic surfaces [298]. Copyright 2016. Reproduced with permission from John Wiley & Sons Inc.

Figure 19.

A typical route for extracting collagen from the calcified tissue. The flow chart was redrawn according to reference [308].

Figure 20.

Development of CNFs and CNCs as nanobuilding blocks for advanced composite applications. A-D, Composite preparation by a CNC template approach. A, Schematic of the template approach to well-dispersed CNC/polymer composites. i, A non-solvent is added to a CNC dispersion in the absence of any polymer. ii, Solvent exchange promotes the self-assembly of a CNC gel. iii, The gelled CNC scaffold is imbibed with a polymer by immersion in a polymer solution, before the nanocomposite is dried (iv) and compacted (v). B, Digital photograph of a CNC aerogel, prepared by supercritical extraction of a CNC acetone gel (A, ii) with a CNC density of 15 mg ml⁻¹. C, Same object as in B, imaged through crossed polarizers. D, SEM image of the same material (scale bar is 200 nm). A-D reproduced with permission from [340]. Copyright 2007. Springer Nature. E-G, Stimuli-responsive polymer composites inspired by the sea cucumber dermis. E and F, Digital photograph of sea cucumber in relaxed (E) and stiffened (F) state demonstrating the firming of dermal tissue in the vicinity of the contacted area. G, Schematic representation of the architecture and switching mechanism in the artificial composites with dynamic mechanical properties. In the “on” state, strong hydrogen bonds between rigid, percolating CNCs maximize stress transfer and in addition to that the overall modulus of the composite. The interactions are switched “off” by the introduction of a chemical regulator that allows for competitive hydrogen bonding. E-G reproduced with permission from [341]. Copyright 2008. American Association for the Advancement of Science. H-J, Fabrication of flexible magnetic aerogels using CNFs as templates. H, Schematic showing the synthesis of flexible magnetic aerogels. I, SEM image of a 98% porous magnetic aerogel containing cobalt ferrite nanoparticles after freeze-drying. Right inset: nanoparticles surrounding the CNFs. Left insets: photograph and schematic of the aerogel. J, HRTEM image of a single particle from magnetic aerogels showing the lattice fringes corresponding to the (111) reflections of the spinel structure, and the corresponding distance. H-J reproduced with permission from [348]. Copyright 2010. Springer Nature. K-O, Fabrication of thermally insulating and fire-retardant anisotropic foams based on CNFs and GO. K, Schematic illustration of the freeze-casting process, highlighting the growth of anisotropic ice crystals surrounded by walls of the dispersed nanoparticles. L, Digital photograph of the nanocomposite foam. M, SEM cross-section image of a freeze-cast nanocomposite foam. N, Three-dimensional reconstruction of the tubular pore structure of the nanocomposite foam derived from X-ray microtomography. O, X-ray microtomography image showing that the tubular pores are straight and several millimeters long in nanocomposite foam. K-O reproduced with permission from [349]. Copyright 2015. Springer Nature.

Figure 21.

Development of CNFs for optical applications. A, Digital photograph of a transparent and flexible BC reinforced polymer composites. A reproduced with permission from [206]. Copyright 2005. John Wiley & Sons, Inc. B, Digital photograph of a transparent and flexible wood CNF nanopaper. B reproduced with permission from [353]. Copyright 2009. John Wiley & Sons, Inc. C, Luminescence of an organic light-emitting diode deposited onto a transparent BC nanocomposite. C reproduced with permission from [212]. Copyright 2008. John Wiley & Sons, Inc. D, Luminescence of an organic light-emitting diode deposited onto a flexible and transparent wood CNF nanocomposite. D reproduced with permission from [351]. Copyright 2009. Elsevier Ltd. E, Schematic of the chiral nematic ordering present in H2SO4-hydrolyzed CNCs, along with an illustration of the half-helical pitch P/2 (~150–650 nm). F, POM image of the mesoporous silica film obtained from the calcination of the H₂SO₄-hydrolyzed CNCs/silica composite film. G, Photograph showing the different colours of mesoporous silica films obtained from the calcination of various composite films. H, Transmission spectra of the mesoporous silica films. I, Synthetic route to mesoporous photonic cellulose. E-H reproduced with permission from [360]. Copyright 2010. Springer Nature. J, Photographs and K, UV/Vis (solid lines) and CD spectra (dashed lines) of mesoporous photonic cellulose soaked in EtOH/H₂O at different ratios as indicated. I-K reproduced with permission from [368]. Copyright 2014. John Wiley & Sons, Inc.

Figure 22.

Development of CNF-derived nanopapers for electronic applications. A, Digital photographs of original nanopaper (left), CNT@nanopaper (middle) and silver nanowire@nanopaper. B-D, Flexible performance of the transparent conductive nanopapers. (B) Resistance values (184Ω) of silver nanowire@nanopaper after mountain folding. (C) The lighting of a green LED placed between mountain- and valley-folded silver nanowire@nanopapers. (D) Paper craft by using transparent conductive papers. A-D reproduced with permission from [372]. Copyright 2014. Springer Nature. E, Schematic illustrations of the fabrication processes for stretchable CNF/graphene nanopapers. F, Example images of the free-standing flexible nanopaper and stretchable nanopaper. G-I, Example images of the CNF/graphene nanopaper sensors stretched in the X-, Y- and Z directions. J, Corresponding response curves for stretching in three directions. E-J reproduced with permission from [373]. Copyright 2014. John Wiley & Sons, Inc.

Figure 23.

Development of CNF-derived aerogels for electronic applications. A, Fabrication of CNF/CNT conductive aerogels. A reproduced with permission from [376]. Copyright 2013. John Wiley & Sons, Inc. B, SEM image of BC aerogel. C, SEM image of BC-derived carbon aerogel. D and E, The pressure response of BC-derived carbon aerogels. D, Plot of the electric resistance variation with compressive strain. The inset in (D) shows an in-situ measurement of the electric resistance during the compression process. E, The variation of electric current with cyclic compression in a closed circuit. B-E reproduced with permission from [379]. Copyright 2013. John Wiley & Sons, Inc. F and G, Resistance change of the BC-derived aerogel/PDMS composite under mechanical deformations. F, Variation of the normalized resistance (ΔR/R₀) of the composite as a function of tensile strain up to 80% in the first two stretch-release cycles. The inset shows the stretching process. (G) ΔR/R₀ of the composite at a bend radius of up to 1.0 mm in the first bending cycle. The inset shows the bending process. F and G reproduced with permission from [378]. Copyright 2012. Springer Nature.

Figure 24.

Development of CNF-derived composite papers for energy storage applications. A, Schematic representation of the overall fabrication procedure for unitized SEA. B, Schematic illustration of an h-nanomat cell with the unitized SEA configuration. A and B reproduced with permission from [392]. Copyright 2014. American Chemical Society. C, Schematic illustration depicting CNF/CNT-intermingled heteronet architecture of CM electrodes. D, Digital photographs showing the electrochemical activity of paper crane hetero-nanonet cell (I) and paper crane HN cell (II) (a left-bottom inset image). The left-top image is a digital photograph showing the operation of a mini toy car installed with the single-unit hetero-nanonet paper cell. C and D reproduced with permission from [393]. Copyright 2015. John Wiley & Sons, Inc.

Figure 25.

Development of CNF-derived carbon aerogels for energy storage applications. A-F, BC-derived carbon aerogels for supercapacitors. A and B, STEM images of the carbon aerogels@MnO₂ with corresponding elemental mapping images of (A) C and (B) Mn. C and D, EFTEM images of the nitrogen-doped carbon aerogels with corresponding elemental mapping images of (C) C and (D) N. E, Scheme of the asymmetric supercapacitor device. F, Ragone plots of the supercapacitors. A-F reproduced with permission from [400]. Copyright 2013. John Wiley & Sons, Inc. G-I, BC-derived carbon aerogels for lithium-ion batteries. G, TEM images of the BC-derived carbon aerogels. H, TEM image of the carbon aerogel/SnO₂ composites. I, Comparison of cycling performances of carbon aerogel/SnO₂ composites and aggregated SnO₂ nanoparticles over 100 cycles at 100 mA g⁻¹. G-I reproduced with permission from [401]. Copyright 2013. John Wiley & Sons, Inc.

Figure 26.

Development of CNF/CNC-derived materials for energy conversion applications. A, An organic solar cell on conductive CNF nanopaper. A reproduced with permission from [409]. Copyright 2013. Royal Society of Chemistry. B, Solar cell based on foldable and transparent conductive CNF nanopaper. B reproduced with permission from [410]. Copyright 2015. Springer Nature. C, Device structure of solar cells on H₂SO₄ hydrolysis CNC substrates: CNC/Ag/PEIE/PBDTTT-C:PCBM/MoO₃/Ag. D, J–V characteristics of the solar cell on H₂SO₄ hydrolysis CNC substrate in the dark (thin black line) and under 95 mW/cm² of AM1.5 illumination (thick red line). C and D reproduced with permission from [411]. Copyright 2013. Springer Nature.

Figure 27.

Development of CNFs for environmental applications. A-E, CNF-derived nanopaper filters for filtration. A, Schematic of the size-exclusion CNF nanopaper filter for nanoparticle removal. A reproduced with permission from [412]. Copyright 2014. John Wiley & Sons, Inc. B, SEM image of removed 20 nm gold nanoparticles using the CNF nanopaper filter. B reproduced with permission from [415]. Copyright 2016. Royal Society of Chemistry. C, AFM and TEM of CNF/silk fibroin nanostructure. z-scale:10 nm. D, The model of periodically assembled silk backbones on CNF surface. E, The as-prepared 200 nm nanopaper supported on PC filter before and after filtrating R₆G. C-E reproduced with permission from [417]. Copyright 2017. American Chemical Society. F, G, CNF-derived carbon aerogels for oil/water separation. F, Digital photograph showing the absorption of gasoline by a piece of BC-derived carbon aerogel from the water. F reproduced with permission from [379]. Copyright 2013. John Wiley & Sons, Inc. G, Digital photograph showing the absorption of chloroform by a piece of wood CNF-derived carbon aerogel from the water. G reproduced with permission from [394]. Copyright 2016. John Wiley & Sons, Inc.

Figure 28.

Biomedical applications of ChNF-based materials. A, hydroxyapatite (HA)-coated nanofibrous chitin microspheres (NCM) induced bone regeneration. B, the original rabbit bone defects of and the defects treated with HA, NCM, NCMH2, and NCMH3 for 12 weeks.A and B reproduced with permission from [434]. Copyright 2017. American Chemical Society. C, Representative gross images of the wounds treated with the sham operation, bone marrow mesenchymal stem cells (BMSCs), empty hydrogel and BMSCs-encapsulated CNFs based hydrogel, the scale bars = 1 cm [440]. Copyright 2016. Reproduced with permission from John Wiley & Sons Inc. D-F, Histopathological changes in dextran sulfate sodium (DSS)-induced acute ulcerative colitis (UC) mice, and G-I, myeloperoxidase-positive cells in the colons of DSS-induced acute UC mice on day 6. The labels were indicated with the control (+) (D, G), ChNFs administration (+) (E, H), and chitin powder suspension (F, I). The scale bars =100 μm. D-I reproduced with permission from [444]. Copyright 2016. Elsevier Ltd. J and K, ChNFs reinforced chitosan beads for the investigation of β-glucosidase immobilization efficiency [247]. Copyright 2015. Reproduced with permission from The Royal Society of Chemistry.

Figure 29.

Optical and electronic applications of ChNF-based materials. A, Optically transparent nanocomposites using fibrillated ChNFs with three different types of acrylic resins [230]. Copyright 2012. Reproduced with permission from Hindawi. B, Photograph (with AFM image inset) of a chitin reflective optical grating replica on a silicon substrate, molded from a grating with 1200 grooves/mm[264]. Copyright 2011. Reproduced with permission from John Wiley & Sons Inc. C, Fabrication approach of the N-doped ChNF aerogel from prawn shells[455]. Copyright 2015. Reproduced with permission from Elsevier Ltd. D, All-chitin derived flexible electric circuit with the thickness of carbon layer approximately 1 μm [456]. Copyright 2017. Reproduced with permission from John Wiley & Sons Inc. E, Organic light-emitting diodes device on ChNF paper to fabricate substrates for flexible green electronics [452]. Copyright 2016. Reproduced with permission from John Wiley & Sons Inc.

Figure 30.

Environmental applications of ChNF-based materials. A, Observation of ChNCswith stabilizing properties in oil/water emulsions after 24 h of storage at room temperature [462]. Copyright 2011. Reproduced with permission from Elsevier Ltd. B-C, Hydrogels via gas phase coagulation (top) indicated B) adsorption of Reactive Blue 19 (bottom) owing to negatively charged ChNFs and C) adsorption of Basic Green 4 (bottom) owing to positively charged ChNFs [241]. Copyright 2016. Reproduced with permission from American Chemical Society.

Figure 31.

Regulation of silk fibril alignments in one to three dimensions. A, Schematic of dry-spinning of silk microfibril/HFIP dope. B and C, the polarized optical (B) and SEM image (C) of regenerated silk fibers produced from silk microfibril spinning dope. D, SEM image of a yarn-like spiral regenerated silk fiber produced by rotating the collector in a plane direction that perpendicular to the fiber axis. E, Three-dimensional cell patterns generated on yarn-like spiral and as-spun regenerated silk fibers. Fluorescent images show the preferential alignment of HDFs (green) along the axes of the yarn-like spiral and as-spun regenerated silk fibers (red). A-E reproduced with permission from [115]. Copyright 2017. Springer Nature. F, Schematic and visual images of hydrogel-forming process in the electric field. G, polarized optical images of SNF hydrogels before electric field treatment (left) and after treatment (right). H, High-resolution SEM image of the nanostructure on the layers. The layers are composed of aligned SNFs. I, Alignment of stem cells on the on the surface of the electronically oriented silk hydrogels for 3 days incubation. F-I reproduced with permission from[470]. Copyright 2016. John Wiley & Sons Inc. J, Birefringence and corresponding internal nanofibrillar morphology of a ring-anchored fiber. Increasing stress and birefringence correspond to increasing nanofibrillar alignment and orientation. K, buckling of high-aspect-ratio beams with increasing birefringence/alignment of fibers induced via contraction. L-M, Visual image of a web taken between two polarizer films. This six-anchor nanofibrillar web (~2.5 mg) supporting an 11 g point load. J-N reproduced with permission from [296]. Copyright 2017. Springer Nature.

Figure 32.

Injectable SNF hydrogels. A, Thixotropic SNF-based hydrogel obtained by centrifugation of SNF dispersion [276]. Copyright 2014. Reproduced with permission from The Royal Society of Chemistry. B, Injectable and pH-responsible SNF hydrogels generated from heating induced self-assembly and their applications in sustained anticancer drug delivery [279]. Copyright 2016. Reproduced with permission from American Chemical Society. C, SNF/laponite nanoplatelets injectable hydrogels produced from sonication processing with enhancing mechanical performance and bioactivity [477]. Copyright 2016. Reproduced with permission from American Chemical Society.

Figure 33.

SNF assisted synthesis of the functional inorganic nanomaterials. A, SEM image of gold nanoplatelets synthesized from liquid exfoliated SNF solution [273]. Copyright 2016. Reproduced with permission from Wiley & Sons Inc. B, The gold nanocrystals produced from EtOH-induced SNF solution [487]. Copyright 2014. Reproduced with permission from Springer. C, the oriented growth of gold nanoparticles following SNFs. D, One-step synthesis of Fe₃O₄⁻silk composite nanoparticles. E, SNF-modulated morphology control of CaCO₃ nanocrystals using silk nanofibrils with the length of about 50 nm. C-E, reproduced with permission from [272]. Copyright 2014. Reproduced with permission from American Chemical Society. F-H, SNF/HAP nanocomposites. G and H indicate that HAP nanocrystals can homogeneously disperse on the SNF network without aggregates. F reproduced with permission from [488]. Copyright 2016. The Royal Society of Chemistry. G and H reproduced with permission from [281]. Copyright 2017. American Association for the Advancement of Science.

Figure 34.

SNF based nanoporous membranes. A, Schematic of the vacuum filtration devices. B, Visual image of the SNF membranes produced from liquid exfoliated SNF dispersion. C, and D, A cross-sectional SEM image of the liquid exfoliated SNF membrane under low and high magnification. E and F, Visual appearance of the membrane with 5:5 SNFs: amyloid fibrils under visual (E) and cross-polarized light (F) observation. G, A cross-sectional SEM image of the membrane with 100% SNFs. H, Quantum dot patterned SNF membranes under UV light. I) Visual image of gold single crystal nanoplatelets (92 wt%) patterned SNF membranes. J, Visual images show the attached SNF based flexible electronic devices deformed with the deformation of pig ear. K, SNF/ Kevlar nanofibril based electronic devices [489]. Copyright 2017. Reproduced with permission from American Chemical Society. L, Magnetic functionalization and tensile properties of the membrane (SNFs:amyloid fibrils:magnetic nanoparticles weight ratio of 70:10:20), as prepared by vacuum filtration. M, Shape-memory properties of the magnetic composite membrane when exposed to the combined presence of an external magnetic field and water. B-D and H-J reproduced with permission from [273]. Copyright 2016. John Wiley & Sons Inc. E-G, L and M reproduced with permission from [267]. Copyright 2014. John Wiley & Sons Inc.

Figure 35.

SNF based filtration membrane for water purification. A, pristine SNF ultrathin filtration membrane. B, Schematic of the rejection process of SNF membrane during the filtration. A and B, reproduced with permission from [280]. Copyright 2016. Reproduced with permission from American Chemical Society. C, SNF/amyloid nanofibril nanoporous membrane with tunable pore size [267]. Copyright 2014. Reproduced with permission from John Wiley & Sons Inc. D, Coarse-grained computational MD simulations for SNF/HAP assembly and deposition. E, the route to fabricate the SNF/HAP membranes, and visualization of typical multilayer structures formed. D and E reproduced with permission from [281]. Copyright 2017. Reproduced with permission from American Association for the Advancement of Science.

Figure 36.

Structure of CoNF-based materials. A, CoNFs based multi-layered scaffold was fabricated to mimic the composition and microstructural properties of the superficial, intermediate and deep layers of the osteochondral region [526]. Copyright 2016. Reproduced with permission from Elsevier Ltd. B, Hierarchical structure of the tendon [16]. Copyright 2007. Reproduced with permission from Nature Springer. C, Anisotropic alignment of CoNF laminar in cornea [530]. Copyright 2003. Reproduced with permission from Association for Research in Vision and Ophthalmology. D, 2D (a,b,c,d) and 3D (e,f,g,h) reconstructions of CoNFs nano-sized bioactive glass and CoNFs hybridizing rolls with MicroCT images. More homogenous mineralization (i.e., greater carbonated hydroxylapatite formation) was achieved in red phase of these rolls with longer conditioning time in simulated body fluid [535]. Copyright 2011. Reproduced with permission from Elsevier Ltd. E, Fabrication of the hierarchical structure of hydroxyapatite (HA)-collagen composite. The self-assembling of composite materials was indicated with (I) CoNFs, (II) the HA crystals grow on the surface of organized CoNFs, and (III) paralleled organization of the mineralized CoNFs [536]. Copyright 2003. Reproduced with permission from American Chemical Society.

Figure 37.

Applications of CoNF-based materials. A, CoNFs glue applied in lung surgery to reduce air leaks from the lung. (i) Pneumothorax in a treated rabbit at day 4, the lung border is marked by arrows. (ii) The adhesion is showing a giant-cell granuloma containing CoNFs glue debris [544]. Copyright 2000. Reproduced with permission from Oxford University Press. B, Combined AlloDerm® and thin skin grafting for the treatment of (i) a scar contracture area on the anteromedial aspect of the left elbow, and (ii) postoperatively indicated the even releasing surface and adequate skin color of the elbow [545, 552]. Copyright 2014. Reproduced with permission from John Wiley & Sons, Inc. C, Strategy of dye and growth factors anchoring in CoNFs to enable wound assessment as well as to expedite wound healing [545]. Copyright 2014. Reproduced with permission from John Wiley & Sons, Inc. D, Schematic of representative examples of various classes of molecules that interact with CoNFs, along with the representation of the interaction network of the three major CoNFs (types I, II and III) [558]. Copyright 2015. Reproduced with permission from Elsevier Ltd. E, Biocompatible CoNFs paramagnetic scaffold induced drug release with magnetic field stimulus [566]. Copyright 2015. Reproduced with permission from American Chemical Society. F, Bandaging films (i) prepared with CoNFs recycled from the leather industry wastes, and (ii) application to skin [561]. Copyright 2016. Reproduced with permission from Royal Society of Chemistry. G, The SEM images of (i) CoNFs based hydrogels for metal ions adsorption. (ii) The smooth morphology of the metal-adsorbed surface is related to filling the surface pores by metal particles [588]. Copyright 2009. Reproduced with permission from Elsevier Ltd.