Empowered by evolutionary selective pressures that yield biopolymers with properties optimized for survival, Nature often proves to be the most creative synthetic organic chemist. Scientists across disciplines have been confronting the challenge of trying to improve on materials that Nature produces by focusing on the rational design of natural variants that exhibit predictably altered properties. These efforts to improve on nature are empowered by a growing arsenal of technical tools for manipulating the genomes of organisms, new chemical reagents for selectively modifying molecular structures, and controlled processing conditions. Silk represents one of the most attractive targets of such efforts. This special issue of Biopolymers explores the range of approaches being used byleading laboratories for characterizing and rationally manipulating natural and designer variants of this fascinating biopolymer.
Silk has been used in textiles for millennia, but in the past several decades has been subject to a surge of investigation into its synthesis (both natural and artificial), structure, function, and natural diversity. This research has been driven by both scientific curiosity with the aid of improved analytical tools as well as the ability to process silk into new materials that are well-suited for biomedical applications. Of particular interest are the impressive mechanical properties of spider silk as well as the ability of silkworms and spiders to produce fibers in an aqueous and “green” process with relatively low energy requirements.1
The origins of spider silk's high toughness (energy absorbed to breakage, a combination of strength and elasticity) are still unclear. The variable, metastable, and hierarchical nature of the fiber makes it difficult to study. Furthermore, inter-lab (and likely intra-lab) variability in silk sampling processes can confound results. To address this problem, Reed et al. discuss the variability in Bombyx mori silk mechanical properties, and determine the variables necessary to control when handling these fibers before study. Furthermore, through the development of new analytical tools with high spatial and temporal resolution new insight has been gainedinto the molecular structure of silk fibers. The paper by Holland et al. reports on the combination of birefringence measurements and mechanical testing to study molecular orientation during the tensile testing of Nephila edulis dragline fibers. Lefèvre et al. describe the recent advances in linearly polarized Raman microspectroscopy,2, 3 and the insight gained from its use into the structure of single spider silk fibers and silk solution. Advances in computational modeling are now also allowing insight into the origins of strength in silk at many different structural hierarchies, from crystallites to fibrils to fibers to webs.4–6 Herein, Bratzel and Buehler present molecular dynamics simulations of the β-sheet crystals in Nephila clavipes dragline silk, and their response to various mechanical loading conditions.
In their review, Lefèvre et al. describe the use of linearly polarized Raman microspectroscopy to study the structure of silk dope in its native, solution state. Within the gland, silk proteins are secreted from the epithelial lining and are stored at concentrations over 30% w/w, in an aqueous environment.1 The stability of silk proteins at such high concentrations for extended periods of time is unique and intriguing. As evidenced by several recent publications, the N- and C- terminal domains are largely responsible for this stability.7–10Eisoldt et al. outline and summarize the recent progress in understanding how these highly evolutionarily conserved terminal domains assist in aqueous processability, storage, and assembly of silk proteins.
These terminal domains are pH responsive and, within the silk gland, a gradual acidification along the gland length contributes to the assembly process. The work by Leclerc et al. uses NMR spectroscopy and dynamic light scattering to study the contents of the Nephila clavipes major ampullate gland, solubilized major ampullate fibers, and recombinant versions of the MaSp1 and MaSp2 proteins and investigate the role of pH in secondary structure formation. Yazawa et al. approach the problem by synthesizing a model dragline peptide and use solid-state NMR to investigate the role of acidification in its conformational change.
In addition to changes in pH, as the spinning dope traverses through the gland and undergoes a phase transition from liquid to fiber, it experiences stresses from the force of the fiber being drawn, and stresses exerted by the gland geometry.11, 12 Under these conditions, silk dope exhibits liquid crystalline textures, thought to assist in the low-energy spinning process. Rey et al. provide a comprehensive review of liquid crystal models, how they apply to biological materials, and how they relate to silk processing and help us understand the spinning process. Such models are often complemented by rheological measurements, which have recently become a powerful tool for gaining insight into the spinning processes in both spiders and silkworms.13, 14Holland et al. have provided a unique perspective for utilizing rheology as a tool to compare the phylogenetic relationships of four species of silkworm (domesticated and wild silkworms) in regards to their energetic input toward silk production. Further exploring less conventional silkworm silks, Kundu et al. reviews non-Mulberry silks and their utility for biomedical applications.
In addition to silkworms and spiders, there are many more silks that exist in nature, presumably the vast majority of which are still undescribed. These likely have unique and interesting material properties, each having evolved for a specific ecological function while minimizing production energy. As evidenced by the recent discovery of the Darwin Bark Spider through “bioprospecting,” which has dragline fibers with two times the toughness of other spiders, it is likely that we are only just beginning to characterize the diversity of natural silks.15 To this end, this issue contains two papers on “alternative” silks. The paper by Sutherland et al. reviews the coiled coil silk produced by bees, ants, and hornets that are used in fiber or sheet form for mechanical structures, thermal regulation, and humidification. Ashton et al. report on an underwater silk adhesive made by Caddisflies for protective shelters and food harvesting nets.
Just as one finds many different uses of silk in nature, reprocessing and reengineering of silk into new materials and structures has broadly expanded the uses of silk materials. Both reconstituted silkworm silk and genetically engineered spider silk can be formed into a multitude of materials ranging from fibers to films to sponges to microspheres to fibrous mats.16 These materials have been used extensively for biomedical applications because of silk's exceptional biocompatibility and biodegradability. Pritchard et al. discuss the particular benefits of aqueous processability, and the emerging unique features of silk in stabilizing a range of different labile molecules in reconstituted silk materials. Due to its widespread availability, the majority of the work into silk materials has been performed with Bombyx mori silkworm silk. However, other silks could have unique medical roles, and Widhe and Herrera-Valencia discuss the more recent use of natural and recombinant spider silk proteins for biomedical applications.
As Widhe and Herrera-Valencia conclude, the scale of recombinant silk production systems is still too limited for all but the most low-volume applications. However more advanced recombinant hosts are being engineered to tackle these problems (metabolically engineered E. coli to produce full-length silk-like proteins17 and engineered salmonella to export silk-like monomers18). In addition, in 2007, we saw the publication of the first full length silk sequences,19 which provided tremendous insight into the structure and evolution of dragline proteins and will hopefully help lead to the faithful synthetic replication of natural silk. Despite the difficulty in sequencing silks due to their long lengths and repetitiveness, with improvements in sequencing technology, we should expect to see the sequences of many more silks shortly. This will provide additional insight into the composition and structure of silk, as well as the translational machinery necessary to produce such large amounts of highly glycine and alanine-biased protein.
As genetic engineering and gene sequencing become more sophisticated, enabling the rapid creation of custom structural and functional silks at high yield, and the processing mechanisms of native silks are further elucidated, one can only speculate what the future applications of silk will be and how silk itself will be redefined. For example, Teulé et al. have created a chimeric “silk” composed of Nephila clavipes flagelliform and MaSp2 domains, expressed in E. coli. In addition, we have already seen several recombinant, functionalized silk block copolymers.20–22 Perhaps soon the vision of designer protein polymers, from plug-and-play sequence modules, will be realized.16, 23 Combined with the discovery of new silks through bioprospecting, the future application space of the field seems almost impossibly broad.
As the first published compilation since the 1994 ACS Symposium Series “Silk Polymers: Materials Science and Biotechnology,” we hope this special issue of Biopolymers will invigorate a more general scientific interest in silk polymers and serve as a guide to those looking to learn about the latest knowledge in the field. We also hope the enduring interest in and continued insight into this unique family of protein polymers will attract further students and investigators to explore the many unanswered questions in the field.