Silk proteins are a promising material for drug delivery due to their aqueous processability, biocompatibility, and biodegradability. A simple aqueous preparation method for silk fibroin particles with controllable size, secondary structure and zeta potential is reported. The particles were produced by salting out a silk fibroin solution with potassium phosphate. The effect of ionic strength and pH of potassium phosphate solution on the yield and morphology of the particles was determined. Secondary structure and zeta potential of the silk particles could be controlled by pH. Particles produced by salting out with 1.25 M potassium phosphate pH 6 showed a dominating silk II (crystalline) structure whereas particles produced at pH 9 were mainly composed of silk I (less crystalline). The results show that silk I rich particles possess chemical and physical stability and secondary structure which remained unchanged during post treatments even upon exposure to 100% ethanol or methanol. A model is presented to explain the process of particle formation based on intra- and intermolecular interactions of the silk domains, influenced by pH and kosmotrope salts. The reported silk fibroin particles can be loaded with small molecule model drugs, such as alcian blue, rhodamine B, and crystal violet, by simple absorption based on electrostatic interactions. In vitro release of these compounds from the silk particles depends on charge – charge interactions between the compounds and the silk. With crystal violet we demonstrated that the release kinetics are dependent on the secondary structure of the particles.

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.

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Spider silk is a material consisting of very large (>200 kDa) proteins and has a high potential for biomedical applications as a result of its biocompatibility and biodegradability. We report on the influence of physicochemical factors on structure formation of the engineered spider silk protein eADF4(C16), which mimics the known sequence of the dragline protein ADF4 from the spider Araneus diadematus. Under certain experimental conditions, eADF4(C16) forms stable microspheres that have been analyzed with respect to sphere size, size distribution, and surface inertness upon different preparation methods (dialysis, pipette and micromixing). As a result of their material strength, biocompatibility, and the possibility of functionalization, spider silk microspheres have a high potential for the development of targeted drug-delivery systems.

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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