Proton conductivity of the protein-based velvet worm slime

The properties of complex bodily fluids are linked to their biological functions through natural selection. Velvet worms capture their prey by ensnaring them with a proteinaceous fluid (slime). We examined the electrical conductivity of slime and found that dry slime is an insulator. However, its conductivity can increase by up to 10⁶ times in its hydrated state, which can be further increased by an order in magnitude under acidic hydration (pH ≈ 2.3). The transient current measured using ion-blocking electrodes showed a continuous decay for up to 7 h, revealing slime's nature as a proton conducting material. Slime undergoes a spontaneous fibrilization process producing high aspect ratio ≈ 10⁵ fibers that exhibit an average conductivity ≈2.4 ± 1.1 mS cm^-1. These findings enhance our understanding of slime as a natural biopolymer and provide molecular-level guidelines to rationally design biomaterials that may be employed as hygroscopic conductors.