Keywords

10.1 Introduction

Female orb weaving spiders such as Nephila clavipes and Araneus diadematus are able to produce up to seven different silk types for different applications, such as catching and wrapping prey, for protecting their offspring etc. Spider silks are mainly made of silk proteins (spidroins) and they are named after the glands the proteins are produced in. The best characterized silk is the Major Ampullate (MA) silk (also known as dragline silk). In order to analyze the properties of spider silk as well as to use them in distinct applications, sufficient quantities of the material are necessary.

Unfortunately, due to their cannibalistic behavior it is not possible to farm spiders. Further, collecting silk from individual spiders is time consuming and not very effective. Therefore, silk genes have been transferred from spiders to other host organisms to produce recombinant spider silk proteins. Besides employing cDNA, engineered silk genes have been utilized encoding proteins which are comparable to the natural ones concerning their amino acid sequence.

Here, we summarize efforts to recombinantly produce spider silk proteins in bacteria and highlight the possibilities of their application-driven modification, as well as the techniques of processing them into fibers.

10.2 Recombinant Production of Dragline Silk Proteins

Dragline silk comprises spidroins produced in the major ampullate gland and is used e.g. as the frame and the radii of an orb web, and also as the spider’s lifeline. In comparison to most man-made fibers like Kevlar or carbon fibers, dragline silk has superior mechanical properties especially concerning its toughness (Heim et al. 2009; Gosline et al. 1999). Two major ampullate spidroin classes can be defined named MaSp 1 and MaSp 2, which mainly differ in proline content (MaSp1 shows no or little proline residues, while MaSp2 is enriched in proline residues). Furthermore, MaSp1 tends to be more hydrophobic than MaSp2. The molecular setup of MaSp proteins is given in Fig. 10.1 showing exemplary the MaSp2 protein ADF4 of Araneus diadematus.

Fig. 10.1
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Molecular setup of the recombinant protein eADF4 (C16) (engineered ADF4). A consensus repeat motif (Module C) is derived from the repetitive core of the dragline protein ADF4. By reverse translation, synthetic oligonucleotides can be produced and seamlessly cloned

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Major ampullate spidroin monomers have a molecular weight of up to 350 kDa (Ayoub et al. 2007; Sponner et al. 2004). Despite of their different functions, spidroins comprise a highly repetitive core sequence, flanked by non-repetitive termini (Scheibel 2004; Eisoldt et al. 2011). The core domain accounts for approximately 90 % of the protein’s sequence and consists of repeating motifs, each formed by 30–150 amino acids which can be repeated up to 100 times in a single spidroin (Hayashi et al. 2004; Guerette et al. 1996).

While the repetitive core domain of a spider silk protein is important for its macromolecular properties, the non-repetitive termini play a key-role in storage and assembly. Compared to the core domain, the carboxy (NRC)- and aminoterminal (NRN) domains are highly conserved for each silk type throughout different species or sometimes even between different silk types (Motriuk-Smith et al. 2005; Rising et al. 2006; Garb et al. 2010), indicating their highly important function. Both terminal domains of MaSp form five-helix bundles (Hagn et al. 2010, 2011; Askarieh et al. 2010). The recently studied NRN domains of Latrodectus hesperus and Euprosthenops australis are monomeric at pH above 6.8 and dimerize in an antiparallel fashion upon slight acidification. In comparison to the aminoterminal domains, the NRC domain of Araneus diadematus is a disulphide-linked, parallel dimer with one of the five helices being domain swapped. Importantly, all solution structures were solved using recombinantly produced proteins.

First attempts to produce recombinant spider silk proteins by using cDNA from spiders revealed several problems. Bacterial hosts such as E. coli have a different codon usage than spiders lowering the protein yield. Further repetitive sequences are often removed in bacteria by homologous recombination creating a polydisperse set of proteins with different molecular weight (Xu and Lewis 1990; Arcidiacono et al. 1998). Likewise, the efforts to express silk genes in eukaryotic cells were not successful or yielded low protein amounts (Menassa et al. 2004; Lazaris et al. 2002). A short overview on different host organisms used for recombinant spider silk protein production is shown in Table 10.1.

Table 10.1 Host organisms used for expressing spider cDNA and cDNA fragments
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A more promising approach is the recombinant production of engineered spider silk spidroins with adapted DNA sequences. Based on the repetitive core sequence of spider silk spidroins, it is easy to design optimized genes by simple cloning techniques (Lawrence et al. 2004; Colgin and Lewis 1998; Huemmerich et al. 2004a; Lewis et al. 1996; Prince et al. 1995; Vendrely et al. 2008). An overview on recent attempts is shown in Table 10.2.

Table 10.2 Engineered spider silk proteins produced in different bacterial hosts
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10.3 Processing of Recombinant Spider Silk Proteins

Spider silk proteins can be processed into fibers, capsules, particles, hydrogels, foams, films or non-woven meshes (Schacht and Scheibel 2011; Spiess et al. 2010; Leal-Egana et al. 2012; Hardy et al. 2008). Assembly of the recombinant spidroins can be triggered by protein concentration, pH, temperature, ionic strength or mechanical stress, among others.

Here we will focus on processing of recombinant spider silk proteins into fibers and non-woven meshes.

10.3.1 Fibers

In principle, several techniques can be applied to produce fibers from solutions of recombinant spider silk proteins. Here a short overview on recent attempts of two prominent techniques, namely wet spinning and biomimetic spinning will be given.

10.3.1.1 Wet Spinning

In wet spinning processes, polymer or protein solutions are extruded into a coagulation bath. The proteins, dissolved in aqueous solution or organic solvents are extruded into water, methanol, isopropanol or acetone (Seidel et al. 1998; Hardy et al. 2008). An overview of approaches to wet-spin recombinant spider silk proteins as well as mechanical properties of the achieved fibers are given in Table 10.3.

Table 10.3 Wet spun fibers from recombinant spider silk protein
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The majority of the recombinant fibers have been made of MaSp1 and MaSp2 derivatives from Nephila clavipes. The molecular weight of the underlying proteins varied from 31 to 284 kDa with some tendency, but no strict relation between the molecular weight and tensile strength. Evidently, both the tensile strength and elasticity differed between fibers made of different recombinant spider silk proteins for reasons of molecular weight, but also protein sequence, concentration of the spinning solution, fiber diameter and the coagulation bath. There are also large deviations in the mechanical properties of the different fibers investigated in individual spinning approaches, as seen in Table 10.3. Such variability makes it hardly possible to exactly predict the fiber’s properties before spinning based on the amino acid sequence and/or the molecular weight. In principle the tensile strength of a fiber rises with decreasing diameter (Teulé et al. 2011) and increasing molecular weight of the protein (Xia et al. 2010), reaching a plateau above a specific protein size. Importantly, if compared to the mechanical properties of natural spider silk fibers, all man-made fibers show less mechanical stability, independent of the fiber diameter (Seidel et al. 2000). The reason for this finding could be related to molecular self-assembly of the spidroins. Therefore, a detailed analysis of this assembly is necessary, which is one basis of biomimetic spinning.

10.3.1.2 Biomimetic Spinning

Biomimetic spinning implements all factors that are important in the natural spinning process including molecular self-assembly of the spidroins. The main factors needed for a transition of a soluble state of the spidroins in the spinning dope into a solid fiber during natural spinning are a change in pH, ion exchange and internal water removal. In addition to chemical processes, extensional and shear forces in the duct are necessary to solidify the fiber (Fig. 10.2). Under storage conditions at pH 7, spidroins form micellar-like structures with liquid-crystalline properties to prevent aggregation (Knight and Vollrath 1999; Hu et al. 2007), whereby the terminal domains play a key role (Askarieh et al. 2010; Exler et al. 2007). Along the spinning duct the pH drops from around 7.4 to 6 and shear forces increase. Studies showed, that recombinant spidroins with NRC domains assemble into fibers while recombinant proteins without NRC domains only unspecifically aggregate (Hagn et al. 2010; Eisoldt et al. 2010; Rammensee et al. 2008). Further, the NRC domain has been shown to be important for pre-orientation of the core domains structure (Askarieh et al. 2010; Exler et al. 2007; Eisoldt et al. 2010).

Fig. 10.2
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Schematic overview of the parameters critical for the natural spinning process of spider dragline silk. During this process, the pH is lowered and phosphate and potassium ions are pumped into the silk dope, while water and chloride are extracted from the dope. Mechanical stress induces the formation of a silk fiber with high β-sheet content

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Such knowledge has been used to develop microfluidic devices mimicking the geometry of silk glands, and which allowed controlling the chemical and mechanical parameters necessary to produce silk fibers.

Rammensee et al. presented a microfluidic device which allowed to assemble recombinant spidroins in aqueous solution using solely the natural triggers (Rammensee et al. 2008). Next generation microfluidic devices allowed to produce functional spider silk fibers with predictable diameter (Kinahan et al. 2011).

The fiber diameter can be controlled by the flow rate of the protein solution within the microfluidic channel. The velocity along the silk gland of spiders rises due to a steady decrease in radius (Breslauer et al. 2009). Different spinning speeds have also a significant effect on the properties of the resulting fibers (Vollrath et al. 2001), likely because of higher molecular alignment caused by stretching of the fiber. In vitro, higher flow rates than in natural processes are needed based on less concentrated protein solutions in comparison to nature where increased concentration and thereby viscosity leads to fiber formation at lower elongational flow rates (Rammensee et al. 2008). Although microfluidic devices provide a simple method to produce uniformly sized fibers, there are still problems to produce endless and consistent fibers with properties similar to that of the natural blueprint.

10.3.2 Non-woven Meshes Made of Recombinant Spider Silk Proteins

Electrospinning of biopolymers is a cost effective and easy way to produce non-woven meshes with high inter-fiber spacing and fiber diameters ranging from micrometers down to a few nanometers (Frenot and Chronakis 2003). The advantage of electrospinning is the low amount of polymer solution needed to generate continuous fibers (Baumgarten 1971). If an electric field is applied to a pendant droplet of polymer solution a taylor conus is formed, and a polymer jet is created directing towards the counter electrode. The solvent evaporates before reaching the collector plate, and dried fibers can be collected e.g. as a non-woven mat (Huang et al. 2003). The fiber diameter can be controlled by the viscosity, the concentration of the polymer solution, the salt content, the surface tension of the solvent, the distance to the counter electrode, as well as the polymer itself (molecular weight, molecular weight distribution, solubility, and glass transition temperature) (Heikkila and Harlin 2008; Greiner and Wendorff 2007). Furthermore, the surrounding temperature and the relative humidity can significantly influence fiber formation (Fig. 10.3).

Fig. 10.3
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Scheme of an electrospinning setup. An electric field is applied to a pendant droplet of polymer solution leading to a taylor conus, from which a polymer jet emerges. The solvent evaporates before reaching the collector plate, and dried fibers can be collected e.g. as a non-woven mat (Huang et al. 2003)

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To guarantee the continuity of the fibers, the concentration of the silk solution must be high enough to permit the entanglement between the silk molecules. Interaction between the solvent and the silk molecules, as well as their molecular weight, will influence the minimal concentration needed (Chengjie et al. 2009). Depending on the collector set up, single fibers or non-woven meshes can be achieved, the later showing a high surface area to volume ratio and a high potential for applications in e.g. tissue engineering and wound healing.

Several groups have electrospun different silk types, including regenerated B. mori silk, regenerated N. clavipes dragline silk and silk-like polymers from organic solvents like formic acid (FA), Hexafluoroisopropanol (HFIP) or Hexafluoracetone (HFA) (Buchko et al. 1999; Jin et al. 2002; Zarkoob et al. 2004). Only a few electrospinning setups employed recombinant spider silk proteins as depicted below.

The recombinant spider silk protein eADF4(C16) (based on the dragline silk protein ADF4 of A. diadematus) was dissolved in HFIP at concentrations from 4 % (w/v) to 24 % (w/v). Below 8 % (w/v) nanoparticles were obtained (akin to electro-spraying), while homogeneous fibers could only be produced at concentrations above 16 % (w/v). With increasing protein concentration the fibers diameter increased from 150 to 700 nm (Leal-Egana et al. 2012). All as-spun eADF4(C16) fibers from HFIP were water soluble and predominantly consisted of random coil and α-helical structures. Since in most applications water stable fibers are necessary, the non-woven meshes can be post-treated with methanol vapor to induce the formation of β-sheet structures (Leal-Egana et al. 2012; Lang et al. 2013). Another way to promote β-sheet formation in non-woven meshes is temperature or humidity annealing (Wang et al. 2006; Zarkoob et al. 2004).

Cell culture experiments with BALB/3T3 mouse fibroblasts on post-treated eADF4(C16) non-woven meshes showed that cell adhesion and proliferation were strictly dependent on the diameter of the individual fibers. With increasing fiber diameter cells adhere much better to the meshes, and their doubling time decreases while their proliferation rate increases. One explanation for this phenomena is that the larger the fiber diameter, the larger the spacing between the fibers, making it easier for lamellipodia and filopodia to protrude (Leal-Egana et al. 2012).

Bini et al. created two engineered proteins based on the consensus sequence derived from MaSp 1 of Nephila calvipes. One of the proteins was further fused with an RGD motif to enhance cell interactions. Non-woven meshes comprising these proteins were spun from HFIP solutions, and the obtained fiber diameters ranged from 50 to 250 nm with an average diameter of 100 nm. To increase the β-sheet content non-woven meshes were post-treated with methanol (Bini et al. 2006) (cf. Sect. 10.4.1).

To generate organic–inorganic composite non-woven scaffolds, an R5 peptide (derived from the repetitive motif of silaffin proteins) was fused to N. clavipes spider dragline silk protein, either in presence or absence of an additional RGD motif (Foo et al. 2006). Fibers of the chimeric spider silk-silaffin proteins were spun from HFIP. Incubation of these non-woven meshes with silicic acid solution induced silica sphere formation on the non-woven mats with diameters ranging from 200 to 400 nm. Since the non-woven meshes were not post-treated, the fibers fused upon incubation in silicic acid. Contrarily, when non-woven meshes were treated with methanol before silification, silica nanospheres were sparsely observed. Silification reactions during electrospinning (concurrent processing) resulted in a non-uniform coating of the fibers but no particle formation (Foo et al. 2006).

10.4 Modification of Recombinant Silk Proteins

One advantage of recombinant spider silk proteins is the ease of genetic modification which allows the direct incorporation of functional groups into the silk proteins (as already depicted in some examples in Sect. 10.3.) Alternatively, chemical modification of distinct naturally occurring or artificially introduced specific amino acid side chains is feasible.

10.4.1 Genetic Engineering

Genetic engineering of silk genes allows to incorporate either individual amino acids, or even peptide sequences that enable enhanced cell adhesion or improved solubility (Table 10.4). In the following, several attempts to modify silk proteins for specific applications are summarized.

Table 10.4 Chimeric spider silk proteins for diverse biomedical applications
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10.4.1.1 Biomineralisation

Biosilica architectures in diatoms are produced under ambient conditions (aqueous solution, neutral pH and low temperatures). In vitro, the R5 peptide (derived from the repetitive motif of the silaffin protein of Cylindrotheca fusiformis) regulates and induces silica formation under similar conditions. To generate scaffolds for bone regeneration, the R5 sequence was genetically fused to an RGD containing N. clavipes spider dragline silk protein (as mentioned in Sect. 10.3.) (Foo et al. 2006). Besides fibers, the resulting chimeric silk-silica proteins were processed into films. By treating such films with silicic acid solution, the R5 peptide induces biomineralization on the surface (Foo et al. 2006). Osteogenic differentiation was analyzed culturing human mesenchymal stem cells (hMSCs) on such silk-silica protein films. The bound silica influenced osteogenic gene expression with upregulation of alkaline phosphatase (ALP), bone sialoprotein (BSP), and collagen type 1 (Col 1). Calcium deposits on silk-silica films further indicated enhanced osteogensis (Mieszawska et al. 2010).

Belton et al. determined the silica condensation using a range of silicifying peptides (R5: SSKKSGSYSGSKGSKRRIL; A1: SGSKGSKRRIL; Si4-1: MSPHPHPRHHHT, and repeats thereof) fused to the N-terminus of a recombinant N. clavipes spider dragline silk protein. The authors determined a strict relationship between silk solution properties (e.g. pH of the solution) and silica deposition, leading to silica silk chimera material formation (Belton et al. 2012).

Another approach for using silk scaffolds in bone formation was investigated by Huang et al., producing a chimeric protein based on the MaSp1 of N. clavipes and dentin matrix protein 1 which is involved in the nucleation and crystallization of hydroxyapatite. The recombinantly produced protein was processed into films, which showed no structural differences in comparison to films of non-modified silk. Incubation of processed films in simulated body fluids induced the growth of hydroxyapatite crystals on silk films with the fused dentin matrix protein 1, indicating their potential for applications in bone tissue engineering (Huang et al. 2007).

10.4.1.2 Cell Adhesion

For biomedical applications the interaction of a material’s surface with cells or tissue is highly important. One strategy to improve cell adhesion is to change the surface topography of a material (Leal-Egana et al. 2012; Leal-Egana and Scheibel 2012; Bauer et al. 2013). Another one is modification of the silk proteins with cell adhesive peptides. The recombinant spider silk protein eADF4(C16) was genetically modified with the linear cell adhesion sequence GRGDSPG. The RGD-modified protein was successfully processed into films, and cell adhesion and proliferation of mouse fibroblasts (BALB/3T3) was investigated thereon. In comparison to unmodified spider silk films, cells on RGD-modified films showed improved adhesion and proliferation (Wohlrab et al. 2012).

Bini et al. combined the consensus sequence derived from MaSp1 of N. clavipes with the cell binding motif RGD. The modified silk was processed into films and fibers, which were successfully used for culturing human bone marrow stromal cells (hMSCs). In comparison to the tissue culture plastic, silk surfaces showed increased calcium deposition, but surprisingly no impact on cell differentiation was observed (Bini et al. 2006). Since cell binding is dependent on the surface density of RGD, Morgan et al. blended RGD modified recombinant spidroin with B. mori silk in different ratios (10:90, 30:70, 50:50, 70:30, 90:10 RGD-spidroin: silk fibroin) to adjust the RGD concentration exposed on the silk film surface. Strikingly, the proliferation and differentiation of pre-osteoblasts (MC3T3-E1) was indistinguishable between the various blends (Morgan et al. 2008).

10.4.1.3 Gene Delivery

Engineered block copolymers of spider silk with poly(L-lysine) domains were investigated as gene delivery systems. The silk-poly(L-lysine) copolymer self-assembles into complexes with plasmid DNA (pDNA) through electrostatic interactions. The resulting particles delivered genes into human embryonic kidney cells (HEK) (Numata et al. 2009). To increase the delivery efficiency, incorportation of cell penetrating peptids (CPPs), like ppTG1, into silk hybrids was investigated. CPPs which are known to be amphipathic and positively charged (Zorko and Langel 2005) are able to deliver large-cargo molecules into cells (Madani et al. 2011). Complexes of the silk-poly(L-lysine)-ppTG1 protein with pDNA showed a transfection efficiency comparable to the transfection reagent Lipofectamine 2000 (Numata and Kaplan 2010). In another study the tumor-homing peptide (THP) was fused to a silk-poly(L-lysine) block copolymer. The pDNA complex of silk-poly(L-lysine)-THP block copolymers showed a significantly enhanced targeting of specific tumor cells (Numata et al. 2011, 2012).

10.4.1.4 Antimicrobial Silk

Silver containing silk materials could be used in applications in which antimicrobial activity is needed. Therefore, Currie et al. fused a silver binding peptide to a recombinant spider dragline silk protein derived from MaSp1 of N. clavipes. The resulting protein nucleated Ag ions from a silver nitrate solution (Currie et al. 2011). Regarding antimicrobial activity, Gomes et al. fused the consensus sequence derived from the MaSp1 of N. clavipes with three different antimicrobial peptides (the human antimicrobial peptides human neutrophil defensin 2 (HNP-2), human neutrophil defensin 4 (HNP-4) and hepcidin). The recombinantly produced chimeric proteins were processed into films, and it was demonstrated that the silk domain retained its self-assembly properties. The antimicrobial activity against E. coli and S. aureus was analyzed, and the microbial activity was demonstrated. Furthermore, cell studies with a human osteosarcoma cell line (SaOs-2) demonstrated the compatibility of these films with mammalian cells (Gomes et al. 2011).

10.4.1.5 Others

To control the solubility of spider silk proteins Winkler et al. incorporated methionine residues next to polyalanine sequences, found in the dragline silk of N. clavipes, to trigger β-sheet formation. This methionine residues can work as redox triggers (Winkler et al. 1999). The oxidation to the sulfoxide state with phenacyl bromide yields a water-soluble protein and prevents the formation of β-sheets by disrupting the hydrophobic interactions between the overlaying sheets. Reduction with β-mercaptoethanol, in contrast, triggers self-assembly into β-sheets, without disturbing the general macromolecular assembly behavior (Valluzzi et al. 1999; Szela et al. 2000).

To control the β-sheet content, enzymatic phosphorylation and dephosphorylation reactions were performed with genetically engineered spider dragline silk proteins. By introducing charged phosphate groups using cyclic AMP-dependent protein kinase, hydrophobic interactions between the β-sheets were prevented, and thus solubility of the proteins increased. Concomitantly, dephosphorylation with calf intestinal alkaline phosphatase induced self-assembly and β-sheet formation (Winkler et al. 2000).

In addition to functional peptide sequences, individual amino acid residues with chemically specific side chains (such as thiols of cysteine residues) can be incorporated in engineered spider silk proteins. Due to the fact that thiol side chains of cysteines are ideally suited for modifications at neutral pH, a single cysteine was introduced in eADF4(C16) (Spiess et al. 2010). In comparison to the cysteine free variant, no differences concerning protein structure or assembly were detectable in solution and in the films. It could be shown that target molecules like maleimide-conjugated fluorescein were successfully coupled to the cysteine-modified silk proteins in processed films as well as in solution.

10.4.2 Chemical Functionalization

Chemical modification of silk proteins is an alternative route towards the functionalization of silk materials. Due to the fact that most chemical reactions occur in the presence of other functional groups, there must be a high selectivity thereof. Further, the coupling should in the best case take place at physiological conditions (Sletten and Bertozzi 2009). The functionalization of silk proteins is often much easier than that of globular proteins, since silk proteins are mostly intrinsically unfolded in aqueous solution. Typically, modifications of amino acid side chains are achieved by modifications using electrophilic or nucleophilic groups (Carrico 2008). Most recombinantly produced spidroins comprise non-reactive glycine and alanine residues and lack amino acids residues with functional groups like lysines or cysteines. Nevertheless glutamic acid, aspartic acid as well as tyrosine residues allow specific chemical modifications of spidroins (Vendrely and Scheibel 2007).

Modification of recombinant spider silk proteins is possible in solution and after processing into different morphologies (Fig. 10.4a). For example small organic molecules as well as biological macromolecules like enzymes can be immobilized on eADF4(C16) films by modification of glutamic acid residues. After activation of the carboxyl groups (one per repetitive unit in addition to the C-terminus) with 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide/N-hydroxysulfosuccinimide (EDC/NHS), the enzyme β-galactosidase was efficiently coupled (Huemmerich et al. 2006). The activity of the enzyme after coupling was demonstrated using the specific substrate 5-bromo-4-chloro-3-indolyl-β-D-galactopyranoside (X-gal). Employing an ethylenediamine linker after EDC/NHS activation allowed coupling of fluorescein-5-isothiocyanat (FITC) to the eADF4(C16) film’s surface.

Fig. 10.4
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Chemical modification of engineered spider silk proteins; (a) Two routes of functionalization of silk fibers can be achieved: processing of the fibers followed by chemical modification or chemical functionalization in aqueous solution followed by fiber production; (b) Modification of recombinant silk proteins with N-hydroxysuccinimde (NHS)-activated carboxylic acid targets at primary amines (1) or functionalization of terminal carboxyl groups and glutamic acid residues with EDC/NHS activation followed by functionalization with different primary amines (2) (Huemmerich et al. 2006) or at thiol groups in cysteine modified variants through maleimide chemistry (3) (Spiess et al. 2010; Humenik et al. 2011; Schacht and Scheibel 2011)

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To create a single chemical bond between the recombinant spider silk protein eADF4(C16) and a target, one way is the modification of the N-terminus (since eADF4(C16) does not contain any other primary amines) (Schacht and Scheibel 2011). The one-step reaction of N-hydroxysuccinimde (NHS)-activated carboxylic acid targets with primary amines is, however, limited by a low specificity (Thordarson et al. 2006). Due to the fact that thiol side chains of cysteine residues are ideally suited for modifications at neutral pH, a single cysteine residue was introduced in eADF4(C16) (Spiess et al. 2010) (Fig. 10.4b, see also Sect. 10.4.1.5). Thiol modification was performed using maleimide, which reacts selectively and quantitatively in a single step procedure with thiol groups forming a stable thioether bond (Partis et al. 1983; Heitz et al. 1968). It has been shown that after eADF4(C16) film formation, target molecules like maleimide-conjugated fluorescein, monomaleimido-nanogold particles and β-galactosidase (in combination with NHS-PEO12-Maleimide crosslinker) could be successfully coupled (Spiess et al. 2010). Since there are no side-reactive complications, like in the case of carbodiimide activation (glutamic acid residues), the reaction was further used to couple a maleimide modified cyclic RGD peptide (Wohlrab et al. 2012).

10.4.3 Silk-Polymer Hybrids

Silk peptides can be combined with synthetic polymers like poly(ethylene glycol) (PEG) or poly(isoprene) to achieve new functionalities (Zhou et al. 2006; Rathore and Sogah 2001). Rathore and Sogah designed a N. clavipes- inspired block-copolymer by selective replacement of the amorphous peptide domain with flexible non-peptidic poly(ethylene glycol) (PEG) blocks, while retaining the poly(alanine) sequences (Rathore and Sogah 2001). The silk-polymer hybrids were synthesized with varying chain length of the poly(alanines) blocks via prefabricated blocks which were linked block by block (Lego method) (Winningham and Sogah 1997; Rathore and Sogah 2001). Despite the replacement, the silk peptide blocks achieved β-sheet conformation in the resulting polymer. Next, the mechanical properties of silk-polymer films and fibers were determined. Compared with films made of a B. mori-inspired block copolymer, the spider silk-inspired analogue films were tougher and stronger. Furthermore, increasing the length of the poly(alanine) block resulted in increased elastic modulus and tensile strength. As a result of the higher stiffness, the elongation at break decreased with increasing length of the poly(alanine) block (Rathore and Sogah 2001).

Since oligo- and polythiophenes are an interesting class of (semi)conducting materials, Klok et al. established the synthesis of diblock oligomers with oligo(3-alkylthiophene) (HT-O3AT) and the silk inspired pentapeptide (Gly-Ala-Gly-Ala-Gly) via solid phase acylation of the resin-bound pentapeptide for electronic applications. In addition to β-alkylated oligothiophenes which usually form organized lamellar assemblies, silk peptide conjugate assembly is driven by directed hydrogen bonding interactions (Klok et al. 2004).

In one approach a low-molecular-weight gelator (LMWGs) based on the silk-like tetrapeptide Gly-Ala-Gly-Ala was designed. To introduce additional van der Waals interactions and to regulate the solubility, apolar alkyl tails (C12) were added as terminal groups (Escuder and Miravet 2006). Gels with a β-sheet fibril network were obtained in several organic solvents like tetrahydrofuran, chloroform, cyclohexane or toluene. The fibrils (less than 20 nm) assembled into a network of bundles with a length of several micrometers and a width of ∼100 nm (Escuder and Miravet 2006). By varying the alkyl chain length (from C3 to C12) or by adding aromatic groups (phenyl or 4-nitrophenyl), the gelation behavior in different polar and apolar solvents could be controlled. For example, only compounds with similar alkyl chains on both ends formed gels in acetone, whereas compounds with dissimilar alkyl chains were not able to form gels (Iqbal et al. 2008).

10.5 Outlook

Spider silks are known for their outstanding mechanical properties and their biocompatibility since ages. Over the last decades, scientists developed several approaches to produce spider silk proteins recombinantly. Although major improvements have been made, certain questions still remain, concerning assembly, solubility and storage of spider silk proteins. Nevertheless, the possibility to process spider silk proteins into various morphologies together with the ability to produce different spider silk chimera and spider silk inspired polymers will allow various applications in medical and technical fields.