Velvet worms eject a fluid capture slime that can be mechanically drawn into stiff biopolymeric fibres. Remarkably, these fibres can be dissolved by extended exposure to water, and new regenerated fibres can be drawn from the dissolved fibre solution—indicating a fully recyclable process. Here, we perform a multiscale structural and compositional investigation of this reversible fabrication process with the velvet worm Euperipatoides rowelli, revealing that biopolymeric fibre assembly is facilitated via mono-disperse lipid-protein nanoglobules. Shear forces cause nanoglobules to self-assemble into nano- and microfibrils, which can be drawn into macroscopic fibres with a protein-enriched core and lipid-rich coating. Fibre dissolution in water leads to re-formation of nanoglobules, suggesting that this dynamic supramolecular assembly of mechanoresponsive protein-building blocks is mediated by reversible non-covalent interactions. These findings offer important mechanistic insights into the role of mechanochemical processes in bio-fibre formation, providing potential avenues for sustainable material fabrication processes.

Velvet worms secrete a fluid hunting slime comprised of a dispersion of nanoglobules that form microfibers under small mechanical shear forces, facilitating the rapid formation of stiff biopolymeric fibers. Here, we demonstrate that the nanoglobules are held together and stabilized as a dispersion by electrostatic interactions reminiscent of coacervate-based natural adhesives. Variation of ionic strength and pH affects the stability of nanoglobules and their ability to form fibers. Fibers mainly consist of large (∼300 kDa), highly charged proteins, and current biochemical analysis reveals a high degree of protein phosphorylation and presence of divalent cations. Taken together, we surmise that polyampholytic protein sequences, phosphorylated sites, and ions give rise to transient ionic cross-linking, enabling reversible curing of ejected slime into high-stiffness fibers following dehydration. These results provide a deeper understanding of velvet worm adhesive fibers, which may stimulate new routes toward mechanoresponsive and sustainable materials.

Natural materials provide an increasingly important role model for the development and processing of next-generation polymers. The velvet worm Euperipatoides rowelli hunts using a projectile, mechanoresponsive adhesive slime that rapidly and reversibly transitions into stiff glassy polymer fibers following shearing and drying. However, the molecular mechanism underlying this mechanoresponsive behavior is still unclear. Previous work showed the slime to be an emulsion of nanoscale charge-stabilized condensed droplets comprised primarily of large phosphorylated proteins, which under mechanical shear coalesce and self-organize into nano- and microfibrils that can be drawn into macroscopic fibers. Here, we utilize wide-angle X-ray diffraction and vibrational spectroscopy coupled with in situ shear deformation to explore the contribution of protein conformation and mechanical forces to the fiber formation process. Although previously believed to be unstructured, our findings indicate that the main phosphorylated protein component possesses a significant β-crystalline structure in the storage phase and that shear-induced partial unfolding of the protein is a key first step in the rapid self-organization of nanodroplets into fibers. The insights gained here have relevance for sustainable production of advanced polymeric materials.

Onychophorans use a unique hunting and defense strategy, which involves the ejection of an adhesive slime secretion produced by a pair of specialized glands. So far, a comparative study on the anatomy of these glands has not been carried out among different species. In this article, we compare anatomical features of slime glands in representatives of two major onychophoran subgroups, the Peripatopsidae and the Peripatidae, from different parts of the world. Our data show that the musculature of the reservoir is conserved whereas the composition of the secretory duct displays taxon-specific variation. Major differences concern the arrangement of glandular endpieces, which are distributed along the duct in Peripatopsidae but condensed in numerous repeated rosettes in Peripatidae. In addition, there are differences in the attachment pattern of slime glands to the inner surface of the body wall and to the outer surface of the gut between the two major onychophoran subgroups. A tube-like structure with a putative valve-like function is found at the transition of the secretory duct and the reservoir in the five Peripatopsidae species studied whereas it is absent in the two representatives of Peripatidae. Our findings suggest that the arrangement of musculature in the reservoir of the slime gland has remained unchanged since the divergence of Peripatidae and Peripatopsidae, while the composition of the secretory duct has been altered in one of these groups. However, the direction of evolutionary changes in duct composition cannot be determined unambiguously due to current uncertainty regarding the phylogenetic relationships of Onychophora.

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