Although spider silks have been studied for decades, the assembly properties of the underlying silk proteins have still not been unravelled. Previously, the detection of amyloid-like nanofibrils in the spider's silk gland suggested their involvement in the assembly process.Recombinantly produced spider silk also self-assembles into nanofibrils. In order to investigate the structural properties of such silk nanofibrils in more detail, they have been compared to amyloid-like fibrils to highlight structural similarities.

Due to their extraordinary mechanical and biochemical properties, silks have long been in focus of research. In vivo, fibers are formed from silk proteins, in vitro, however, a variety of materials can be produced in addition to fibers including capsules, particles, films, foams, and gels. The versatility of silk proteins, along with their biocompatibility, biodegradability, and potential for processing in aqueous solution under ambient conditions make silk‐based materials good candidates for biomedical applications such as drug delivery systems and scaffolds for tissue engineering. Here, we summarize recent progress in research employing recombinantly produced engineered spider silk proteins with a focus on the fundamentals of silk protein processing. We highlight recombinant spider silk films and particles as morphologies that represent model systems with adjustable material properties controlled by process parameters.

The engineered spider silk protein eADF4(C16) reveals similarities to amphiphilic block copolymers. Drop cast of protein solutions on different hydrophobic as well as hydrophilic templates out of different starting solvents (hexafluoroisopropanol, formic acid and aqueous buffers) generated silk films varying in structure and surface properties. Here, the underlying secondary structure of the proteins, the mechanical integrity at increased temperatures, homogeneity and surface topography of silk films, as well as the wettability were investigated in detail. Interestingly, the used templates had impact on microphase separation of the silk molecules as seen by the content of b-sheet structures; as well as on silk film surface hydrophobicities.

Spider silk fibers are one of the most remarkable biopolymers displaying a unique combination of mechanical properties, biocompatibility and biodegradability. The recombinant production of spider silk proteins now allows the processing of silk proteins into novel materials with the aim of future applications. Here, we analyzed films made of a recombinantly produced, engineered spider silk protein with a sequence derived from the dragline silk protein ADF4 of the European garden spider Araneus diadematus. An influence of different initial solvents (aqueous buffer, hexafluoro-2-propanol and formic acid) on certain film properties was identified in as cast as well as methanol post-treated films: while no significant effects on the films' thermal stability were observed, a significant influence on their mechanical properties could be shown. Interestingly, solvent-induced effects were sustained after methanol post-treatment and could be correlated with the presence and arrangement of secondary structure elements. Insights into molecular orientation of individual structural elements within the films upon applied load were revealed by combined polarized IR spectroscopy and mechanical measurements.

Due to their biocompatibility, their extraordinary mechanical properties and the ability to be processed into various shapes, natural polymers like spider silk proteins are promising candidates for materials' applications. However, for many applications, additional specific functionalization is necessary. Here, we present recombinantly produced engineered spider silk proteins based on one dragline silk component of the European garden spider Araneus diadematus. The proteins have been engineered in order to incorporate cysteine which allows site-specific functionalization. These cysteine containing variant silk proteins are characterized in terms of structure, assembly and chemical reactivity in solution. Further, films composed of these proteins were structurally investigated by CD-and FTIRspectroscopy. Comparison of the variants with the original cysteine-free silk protein revealed no apparent differences in solution and in the films. Functionalization of the thiol groups of these silk protein-based films with molecules such as nanogold, dyes, biotin and b-galactosidase demonstrates the potential of such films for a broad range of applications which opens up new possibilities in materials research based on silk polymers.

Back Cover: (Bio)polymers, such as spider silk, allow applications in biomedicine due to their biocompatibility, biodegradability, and mechanical properties. Upon polymer processing, spider silk proteins can adopt various morphologies, for example, particles, non‐wovens, and membranes among others. These properties make silk‐based materials good candidates for biomedical applications, such as drug delivery systems and scaffolds for tissue engineering. Further details can be found in the feature article by K. Spiess, A. Lammel, and T. Scheibel* .

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