Recombinant pyriform-aciniform spidroin blends form enhanced silk fibers without discernible pre-assembly interactions

Spider silks are natural protein-based biomaterials that combine remarkable mechanical properties with biocompatibility and biodegradability, often outperforming synthetic materials. Orb-weaver spiders produce up to seven silk types, each composed of different proteins (spidroins) that are often composite materials. Materials based on recombinant spidroins have typically applied individual spidroins or chimeric fusion proteins, with limited investigation to date of composite fibers blending different spidroins. This study develops and characterizes composite fibers formed by blending recombinant proteins that are based on pyriform (Py) and aciniform (W) silk protein repeat units prior to spinning. Although pull-down, heteronuclear correlation solution-state NMR spectroscopy, and circular dichroism spectroscopy experiments showed no evidence of direct interactions between Py and W repeat units, relatively homogeneous nanoparticles self-assembled in the spin dope state and protein mixtures were readily wet-spun into fibers. A mixture of single Py and W repeat units produced fibers that could not withstand post-spin stretching. In contrast, mixtures of Py₂ + W₂ (i.e., tandem repeats of each silk type) yielded fibers with mechanical properties tunable through post-spin stretching, achieving an extensibility of ∼220% in air-stretched fibers. Under some conditions, fiber properties were comparable to chimeric Py₂W₂ fibers, but substantial mechanical variability was generally observed. This variability was echoed in secondary structure composition, implying that spinning conditions must be chosen with care and mechanical properties interpreted prudently. Blending distinct recombinant silk proteins thus represents a viable and flexible strategy for evaluating composite silk fiber performance, offering a potentially more straightforward alternative or precursor to development of chimeric spidroin-based biomaterials.