Modular Assembly of a Conserved Repetitive Sequence in the Spider Eggcase Silk: From Gene to Fiber.

Spider silk features extraordinary toughness in combination with great biocompatibility and biodegradability, fascinating researchers to prepare artificial silk fibers inspired from the natural art of spinning. In addition to C- and N-terminal domain, a repeat unit from Lactrodectus mactans spider eggcase silks displays substantial sequence conservation across species. Herein, we attempt to spin the engineered tubuliform spidroin 1 (eTuSp1) by microfluidics in a mode of modular assembly comprising the genetic construction, micellar formation, phase separation, and further solidification. Based on the conserved gene sequence, a unique amphiphilic behavior was predicted and then verified by combined techniques of dynamic light scattering, transmission electron microscopy, and synchrotron radiation X-ray diffraction to reveal the formation of micelle-like structure. Through the employ of biomimetic microfluidic devices, desolvation of eTuSp1 was simplified by the nonsolvent induced phase separation in place of the conventional ions exchange and acidification. Both controlled by protein concentrations and flow rate ratios, silk fibers were assembled similar to these reported in other studies of spheres/spherical aggregates observed as intermediates. Because of the applied shear and elongational flow in microfluidic systems, these intermediates were forced to form fibrillar assemblies accompanied by the conformational transformation from α-helix to β-sheet. The resultant mechanical properties were investigated in response to the change of secondary structures and morphologies during spinning process. This work studies the sequence-structure-property relationship, providing comprehensive and systematic insight into the design rational on the preparation of artificial silk fibers from microscale to macroscale.