The biocompatibility and biodegradability of natural silk fibres and the benign conditions under which they (with impressive mechanical properties) are produced represent a biomimetic ideal. This ideal has inspired people in both academia and industry to prepare silk-mimetic polymers and proteins by chemical and/or biotechnological means. In the present paper, we aim to give an overview of the design principles of such silk-inspired polymers/proteins, their processing into various materials morphologies, their mechanical and biological properties, and, finally, their technical and biomedical applications.
- biomimetic material
- silk-inspired polymer
- silkworm silk
- spider silk
Arthropods have evolved to produce a variety of task-specific silk-protein-based fibres. Silkworms produce cocoons from silk-protein-based fibres as a means of protection during their metamorphosis into moths, and web-weaving spiders produce a number of different silk-protein-based fibres to capture prey (in webs), to protect/preserve their offspring/prey (in cocoons) and as lifelines to escape from predators; and certain silk fibres have mechanical properties superior to Nylon, Kevlar and high-tensile steel. Naturally occurring silkworm and spider silk fibres have been used by humans for millennia for applications as diverse as currency, hunting (bow strings, fishing lines or nets), paper, textiles and wound dressings owing to their mechanical properties and biocompatibility [1–3] (Table 1).
Silk fibres are typically composite materials primarily comprising proteins (to which they owe their mechanical properties) and other associated macromolecules (such as polysaccharides and lipids). Silkworm fibres are composed of two microfilaments embedded in a glycoprotein (sericin) coating. The microfilaments comprise a 6:6:1 complex of three different proteins: heavy-chain fibroin (which is hydrophobic with a molecular mass of approx. 350 kDa) that is linked to light-chain fibroin (which is relatively hydrophilic with a molecular mass of 25 kDa) via disulfide bonds, and P25 protein (molecular mass 30 kDa) that is linked via hydrophobic interactions. A spider's lifeline, in contrast, has a core-shell-type structure slightly different from the structure of silkworm silk; the core filament is an inhomogeneously distributed polymer blend of mainly two proteins (spidroins) that is coated with glycoproteins and lipids [1,4].
The natural process of production (known as spinning) for silkworm and spider silk is highly complex. Silk proteins are stored within the animal as remarkably highly concentrated protein solutions (up to 50 wt%) without the occurrence of undesirable aggregation. When necessary, the protein solution can be transported through a duct in which the protein solution is subjected to chemical and mechanical stimuli (such as ion exchange, extraction of water, acidification and elongational flow) that promote protein assembly into fibres.
The impressive mechanical properties of silkworm and spider silk fibres are due to the fact that their proteins form relatively elastic matrices (31-helices and/or β-turn spirals) toughened by anisotropic nanoparticulate inclusions (formed from stacks of β-sheets) (Figure 1). In the case of Bombyx mori silkworm fibroin, a Gly-Ala-Gly-Ala-Gly-Ser hexapeptide repeat is known to form β-sheets, whereas the fibroins of Antheraea pernyi and Samia cynthia ricini silkworms and spidroins of spiders are β-sheet-rich owing to repetitive stretches of poly(alanine). The toughness of spider silks is greater than that of silkworm silks because the anisotropic β-sheet inclusions in spider silk are smaller and better aligned with the long axis of the fibre due to the post-spin drawing process carried out by spiders.
Humankind has been able to cultivate B. mori silkworms in captivity and harvest their silk for thousands of years, but attempts to do the same with spiders have proven unsuccessful because of their cannibalistic nature. The mechanical properties as well as the biocompatibility and biodegradability of these high-performance fibres have inspired the preparation of silk-mimetic polymers and proteins.
Natural silk fibres can be crudely described as matrices of relatively elastic protein toughened by β-sheet nanoparticles, formed by the controlled self-assembly of silk proteins under specific chemical/mechanical conditions. Therefore silk fibres reflect nanoparticle-reinforced composite materials. Studies of the self-assembly of a number of silk-protein-inspired copolymers (containing blocks of β-sheet-forming peptides [Gly-Ala]n or Alan) have been reported in the last decade. β-Sheet-rich fibres or films have been prepared from: triblock copolymers composed of PEG [poly(ethylene glycol)] and either [(Ala-Gly)3-Glu-Gly]10 or [(Ala-Gly)3-Glu-Gly]20 [5–7]; multiblock copolymers composed of poly(isoprene) and Ala5-spacer-Ala5 ; and brush copolymers prepared from acrylonitrile and silk peptides (derived from the reaction of acryloylchloride with chymotrypsin-degraded B. mori fibroin) [9–11]. However, the effect of the non-peptidic blocks on the mechanical properties or biodegradability of these materials has not yet been reported.
The Sogah group published the most comprehensive studies of silk-inspired polymers [12–15]. Multiblock copolymers (with molecular masses between 27 and 40 kDa) were composed of a rigid aromatic spacer (derivatives of phenoxathiin or xanthene) that templated parallel or antiparallel β-sheet formation of two pendant Gly-Ala-Gly-Ala tetrapeptides, linked to flexible blocks (of short alkane or ethylene glycol chains). Films cast from methanoic (formic) acid solution were β-sheet-rich, but very brittle [12,13]. Replacement of the rigid aromatic spacer with a flexible PEG spacer improved the solubility of the polymers to up to 50 wt% in trifluoroethanol. Films cast from trifluoroethanol solution had markedly improved mechanical properties (elastic modulus ∼225 MPa, tensile strength ∼14 MPa, elongation at break ∼21%) owing to the increased flexibility of the PEG matrix .
The same group prepared multiblock copolymers (molecular mass of approx. 20 kDa) composed of PEG and either Ala4 or Ala6 peptides. Films cast from 40 wt% HFIP (1,1,1,3,3,3-hexafluoropropan-2-ol) solution of the polymer containing the Ala4 tetrapeptide had better mechanical properties (elastic modulus ∼310 MPa, tensile strength ∼17 MPa, elongation at break ∼26%) than those of the equivalent polymer containing the Gly-Ala-Gly-Ala tetrapeptide. Increasing the length of the poly(alanine) stretch to six amino acids improved the elastic modulus and tensile strength to approx. 490 and 19 MPa respectively, yet decreased the elongation at break to 12%. β-Sheet-rich fibres with micrometre scale diameters were prepared by extrusion of 5–15 wt% HFIP solutions into a methanol/acetone coagulant bath (a process known as wet-spinning); the elongation at break of such fibres was similar to natural silks, yet their tensile strengths (approx. 14 MPa) were much poorer than natural silk .
The high strength and directionality of hydrogen-bonding interactions in organic solvents can be used to direct the self-assembly of molecules into supramolecular polymers. For example, conjugation of alkyl chains to the termini of the Gly-Ala-Gly-Ala tetrapeptide yielded molecules that form supramolecular polymers in organic solvents (such as acetonitrile or toluene). The supramolecular polymer nanofibrils (of less than 20 nm) hierarchically assembled into bundles (with widths of approx. 100 nm) and subsequently into entangled networks of fibres with lengths of several micrometers, ultimately resulting in gelation . Alternatively, conjugation of the Gly-Ala-Gly-Ala-Gly pentapeptide to either one or both termini of an oligothiophene (tetramer) yielded molecules that form supramolecular polymers in organic solvents (such as dichloromethane) that may have interesting nanoelectronic applications [17,18].
Although solid-phase synthesis would theoretically allow the preparation of proteins with accurate primary sequences, the high molecular masses of naturally occurring silk proteins (>100 kDa) make this approach impractical. In contrast, fermentation of recombinantly ‘engineered’ silks is a more practical approach owing to its potential for large-scale production of proteins with precisely designed primary sequences, potentially allowing the production of proteins incorporating entirely new functionality (bioactivity, catalytic properties, etc.).
Poly(Ala-Gly)64, poly(Ala-Gly)240 and poly([Ala-Gly]3–6-Glu-Gly) were produced by high-cell-density fermentation of recombinant Escherichia coli in yields of up to 1 g/l. Solutions of the proteins in methanoic acid formed gels upon dilution with water (to a 70% methanoic acid solution) and washing the gels with methanol yielded a β-sheet-rich powder [19–21]. Hydrophobic derivatives of the analogous poly([Gly-Ala]3-Gly-Xaa) (where Xaa is an amino acid such as phenylalanine) were insoluble in water even in strongly denaturing conditions, whereas those with polar or charged residues (where Xaa is an amino acid such as tyrosine or lysine) could be dissolved in aqueous solutions of denaturants and were found to remain soluble after dialysis against pH-buffered water, allowing their subsequent chemical modification. The assembly properties of the glutamic acid derivatives were demonstrated to be pH-responsive, aggregating swiftly because of hydrophobic interactions upon protonation of the glutamic acids. Moreover, adsorption of amphiphilic derivatives ([Gly-Ala]3-Gly-Glu-[Gly-Ala]3-Gly-Lys)28 to surfaces was shown to render hydrophobic surfaces hydrophilic, and hydrophilic surfaces hydrophobic [19,22,23].
Although a number of research groups have prepared spider-silk-inspired proteins, we are particularly interested in the production of proteins based on the consensus sequences of the major ampullate (lifeline) silks of Araneus diadematus spiders (ADF-3 and ADF-4) that contain β-sheet-forming blocks of Ala6 or Ala8 and are known as eADF-3 and eADF-4 respectively . Our proteins (molecular masses between 46 and 106 kDa) can be produced in high yield by high-density fermentation in E. coli (or other suitable hosts such as yeast) and purified without the need for chromatographic separation which can be expensive and time-consuming [25–27]. Our proteins are soluble in a variety of aqueous and non-aqueous solvents, which facilitates their processing into several different material morphologies , including fibres , films [29–32], foams , gels , capsules [35,36] and spheres [37–39] (Figure 2). We have shown that the non-repetitive peptide sequence at the C-terminus of the proteins encourages their self-assembly , and that materials composed of blends of our proteins have improved materials properties . We have shown our materials to be biodegradable  and furthermore demonstrated the ability to chemically modify our proteins, allowing us to either reinforce our materials by cross-linking , or impart novel functionality via immobilization of bioactive moieties such as enzymes .
Triggered assembly of silk-inspired polymers/proteins
Natural silk proteins can be stored at high concentrations without the onset of undesirable aggregation inside the silkworm or spider. When necessary, the silkworm or spider exposes the proteins to chemical and mechanical stimuli, triggering protein assembly into fibres in a precisely controlled manner . In vitro, self-assembly of silk-inspired polymers/proteins owing to the formation of β-sheets is typically induced by exposure to a solvent such as an aqueous solution of potassium phosphate known to salt out the protein (as occurs during the natural spinning process) [28,37,39] or methanol which dehydrates the β-sheet-forming peptides within the polymer/protein . Other stimuli such as stretching (mimicking post-spin draw applied by spiders) , storage , heat  or UV radiation  have also been shown to induce β-sheet assembly.
An alternative to this is self-assembly triggered by exposure to chemical stimuli that subsequently lead to a change of the chemical structure of the polymer/protein. For example, as synthesized, silk-inspired triblock copolymers (composed of methyl methacrylate and t-butoxycarbonyl-Ala-Gly-Ala-Gly-ethyl methacrylate) had no well-defined secondary structure, as the t-butoxycarbonyl group acted as a steric barrier to β-sheet formation. Exposure of the polymers to trifluoroacetic acid led to acid-catalysed deprotection of the t-butoxycarbonyl group, allowing the formation of β-sheets [5,43]. Proteins incorporating methionine residues near the β-sheet-forming Ala5 peptides were demonstrated to assemble in solution into nanofibrils because of β-sheet formation. Oxidation of the methionine residues to sulfoxides disrupted the β-sheets, yielding fully soluble protein, and reduction of the oxidized residues triggered β-sheet formation and protein assembly [44–46].
Another alternative is self-assembly triggered by exposure to biological stimuli such as enzymes. This principle was first proved with proteins incorporating phosphorylation sites near the β-sheet, forming Ala5 peptides. Phosphorylation of the serine residues (with cAMP-dependent kinase) yielded highly soluble protein, and subsequent dephosphorylation (with calf intestinal alkaline phosphatase) triggered β-sheet formation and protein assembly ([46,47], but see ). Subsequently, short proteins incorporating the consensus repeats of the major ampullate silks of Euprosthenops australis spiders [containing long stretches of polyalanine (Ala12–15) that are highly prone to aggregation], the non-repetitive C-terminus from Araneus diadematus spiders (which we have shown previously to play a proactive role in silk protein assembly processes)  and thioredoxin (a solubility enhancing fusion protein which prevents assembly of the proteins in solution) were prepared. Proteolytic cleavage of thioredoxin with thrombin triggered the assembly of β-sheet-rich fibres that were hundreds of centimetres in length and had micrometre-scale diameters .
Silk-based chimaeric/hybrid proteins
A variety of silk-based chimaeric/hybrid proteins have been produced in the hope of generating proteins incorporating the attractive properties of both compounds, leading to proteins with enhanced solubility, with improved cell adhesion or capable of inducing biomineralization.
Natural S. cynthia ricini silk (although biocompatible and biodegradable) is virtually insoluble in both aqueous and nonaqueous solvents (due to repetitive stretches of Ala12–13) and therefore difficult to process into materials. Hybrid proteins (molecular masses between 17 and 36 kDa) based on S. cynthia ricini and B. mori fibroins were sufficiently soluble to be able to process the resulting protein into fibres [50,51]. Natural elastins (present in connective tissues) tend to be highly insoluble, rendering processing into materials a challenge. Hybrid proteins (molecular masses between 45 and 85 kDa) incorporating silk-like (Gly-Ala-Gly-Ala-Gly-Ser) and elastin-like (Gly-Val-Gly-Val-Pro) blocks are soluble in a variety of solvents facilitating their processing into fibres, films and gels that are potentially suitable for various biomedical applications .
Hybrid proteins combining β-sheet-forming peptide sequences (of either B. mori or S. cynthia ricini fibroins) and cell-adhesive peptide sequences [such as the RGD (Arg-Gly-Asp) peptide sequence of fibronectin] were processable into fibres (with either micrometre- or nanometre-scale diameters) or films. In vitro cell-adhesion studies demonstrated markedly improved cell adhesion over natural silk proteins with mouse fibroblast (Balb/3T3) cells, NHDF (normal human dermal fibroblast) cells or African green monkey kidney (Vero) cells [53–56]. Incorporation of the RGD sequence into engineered proteins based on N. clavipes spider dragline silk was subsequently shown to improve cell adhesion of mouse osteoblast MC3T3-E1 cells .
Hybrid proteins based on the combination of N. clavipes spider dragline silk and the R5 peptide (which is derived from the repetitive motif found in silaffin proteins that are involved in silica formation at neutral pH in in vitro studies) were processed into fibres or films. Incubation of these materials with a water-soluble silicic acid species led to biomineralization of their surface . Likewise, chimaeric proteins based on N. clavipes spider dragline silk and dentin matrix protein 1 (which is involved in the nucleation and oriented crystallization of hydroxyapatite) could be processed into films. Incubation of the films in simulated body fluid led to the growth of hydroxyapatite crystals on their surface, whereas silk films without the dentin matrix protein 1 domain did not induce biomineralization .
We are convinced that the production of silk-inspired polymers/proteins aids fundamental research of natural self-assembly processes of silk proteins, and has significant potential for technological and biomedical applications. We envisage that in the future de novo designed proteins inspired by silks will have a significant positive impact on our daily lives.
J.G.H. gratefully acknowledges financial support from the Alexander von Humboldt Foundation, and T.S. acknowledges financial support from Army Research Office [grant number W911NF-0810284].
Bionanotechnology II: from Biomolecular Assembly to Applications: Biochemical Society Focused Meeting held at Robinson College, Cambridge, U.K., 7–9 January 2009. Organized and Edited by Tony Cass (Imperial College London, U.K.) and Dek Woolfson (Bristol, U.K.).
Abbreviations: HFIP, 1,1,1,3,3,3-hexafluoropropan-2-ol; PEG, poly(ethylene glycol)
- © The Authors Journal compilation © 2009 Biochemical Society