Fibers on the Fly: Multiscale Mechanisms of Fiber Formation in the Capture Slime of Velvet Worms.

Many organisms have evolved a capacity to form biopolymeric fibers outside their bodies for functions such as defense, prey capture, attachment, and protection. In particular, the adhesive capture slime of onychophorans (velvet worms) is remarkable for its ability to rapidly form stiff fibers through mechanical drawing. Notably, fibers that are formed ex vivo from extracted slime can be dissolved in water and new fibers can be drawn from the solution, indicating that fiber formation is encoded in the biomolecules that comprise the slime. This review highlights recent findings on the biochemical and physicochemical principles guiding this circular process in the Australian onychophoran Euperipatoides rowelli. A multiscale cross-disciplinary approach utilizing techniques from biology, biochemistry, physical chemistry, and materials science has revealed that the slime is a concentrated emulsion of nanodroplets comprised primarily of proteins, stabilized via electrostatic interactions, possibly in a coacervate phase. Upon mechanical agitation, droplets coalesce, leading to spontaneous self-assembly and fibrillation of proteins-a completely reversible process. Recent investigations highlight the importance of subtle transitions in protein structure and charge balance. These findings have clear relevance for better understanding this adaptive prey capture behavior and providing inspiration toward sustainable polymer processing.