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Recombinant spider silk proteins, such as eADF4(C16), can be used for various applications. Colloidal particles of eADF4(C16) show potential as drug delivery systems. Tuning the colloidal properties of suspensions of eADF4(C16) particles represents a major prerequisite for their use in pharmaceutical formulations. In this study we determined the surface properties concerning inter-particle interactions by means of electrophoretic mobility and direct force measurements. The surface charge of eADF4(C16) spider silk particles was determined as a function of ionic strength and pH, respectively. The resulting electrophoretic mobility can be described using the O'Brien and White theory and is directly related to the amino acid sequence of the protein. We determined the extension of a fuzzy protein layer protruding into the solution by direct force measurements using a colloidal probe technique. This soft layer leads to deviations in the electrophoretic mobility and is responsible for additional repulsive forces at small separation distances. These steric forces lead to a stabilization of the particle suspension at high ionic strength. † Electronic supplementary information (ESI) available. See

Spider silk fi bres are well known for their high tensile strength in combination with high elasticity. Based on the possibility of recombinant production of spider silk proteins, technical applications of spider silk materials are nowadays feasible. The engineered recombinant spider silk protein eADF4(C16) is based on the sequence of ADF4 ( Araneus diadematus fi broin), one out of at least three proteins of the dragline silk of the European garden spider A. diadematus . The protein eADF4(C16) can be processed into different morphologies. Here, capsules of eADF4(C16) are assembled at an oil/water interface. These microcapsules are mechanically stable and can be used as a transport system for higher molecular weight compounds such as enzymes or chemical catalysts. Further, they can be regarded as a small enclosed reaction chamber with a semi-permeable membrane. Reactions can be initiated by diffusion of the reactants through the silk membrane. The eADF4(C16) capsules protect the enzyme β -galactosidase, used as model, against proteolysis. Functional α -complementation of β -galactosidase visualizes the controllable activation of an enzyme within such spider silk capsule, highlighting the broad applicability thereof as reaction containers, e.g., for enzymes.

a Spider silk fibers are well known for their mechanical properties, and they are therefore in the focus of materials scientists. Additionally, silks display biocompatibility making them interesting materials for applications in medicine or cosmetics. Due to the low abundance of natural spider silk proteins because of the spider's cannibalism, the recombinant spider silk protein eADF4 has been established for material science applications. Once processed into micron-sized particles by controlled salting-out, these particles can be used as drug delivery vehicles. For any application of the silk particles it is important to know their mechanical characteristics for processing and storage reasons. Here, we examine the swelling behavior and mechanics of these particles. Upon hydration, a drastic drop in elastic modulus occurs by orders of magnitude, from 0.8 GPa in the dry state to 2.99 MPa in the wet state. Importantly, the elastic modulus of recombinant silk particles can be tuned by varying the molecular weight of the used proteins, as well as chemical crosslinking thereof.

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