Molecular engineering of protein structures offers a uniquely versatile route for novel functionalities in materials. Here, we describe a method to form highly hydrophobic thin films using genetically engineered spider silk proteins. We used structurally engineered protein variants containing ADF3 and AQ12 spider silk sequences. Wetting properties were studied using static and dynamic contact angle measurements. Solution conditions and the surrounding humidity during film preparation were key parameters to obtain high hydrophobicity, as shown by contact angles in excess of 120°. Although the surface layer was highly hydrophobic, its structure was disrupted by the added water droplets. Crystal-like structures were found at the spots where water droplets had been placed. To understand the mechanism of film formation, different variants of the proteins, the topography of the films, and secondary structures of the protein components were studied. The high contact angle in the films demonstrates that the conformations that silk proteins take in the protein layer very efficiently expose their hydrophobic segments. This work reveals a highly amphiphilic nature of silk proteins and contributes to an understanding of their assembly mechanisms. It will also help in designing diverse technical uses for recombinant silk.

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In this paper, we propose a robotic fiber fabrication method based on solidification force control to achieve highly repeatable mechanical properties of fibers. Dextran material is used as the specimen in the experiments. It has been chosen because of its similar rheological behavior to silk protein at high mass concentrations. However, the viscosity of dextran material is very low at its liquid phase, so force control during fabrication is challenging. Here, we propose a novel approach that controls the mechanical properties of fiber by controlling the solidification force. We employ impedance control with force tracking to control the solidification force to carry out the threading experiments and examine the benefits of the proposed approach. The repeatability of the mechanical properties of the fabricated fibers has been studied and compared using three scenarios a) fiber fabrication without solidification force control, abbreviated as FFNC, b) fiber fabrication with solidification force control after 60 seconds of solidification from the beginning of the solidification force detection abbreviated as FFWC, and c) fiber fabrication with solidification force control immediately after the detection of the solidification force, abbreviated as FFSC. The experimental results show that fibers fabricated using FFSC scenario have the highest repeatability based on the coefficient of variation of properties of the fabricated fibers, where the obtained coefficient of variation of the toughness, stiffness, elongation, and strength are 12.8%, 13.6%, 14.8%, 12.7% respectively. The experimental results also showed that fibers' mechanical properties toughness, stiffness, elongation, and strength have a negative correlation with the fabrication pulling velocity.

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