Ethanol-mediated electrospinning of casein-only bead-free nanofibers

Fabrication of electrospun nanofibers by blending casein with electrospinnable polymers and/or additives is well reported. However, the electrospinnablility of pure casein has not been described due to the tendency of casein proteins to self-aggregate and form large colloidal structures. Here we analyze the influence of pH, ethanol content, ionic environment, and casein concentration on the solubility, solution viscosity, surface tension, and conductivity of casein-based spinning dopes. Further, the influence of casein dope characteristics on our ability to electrospin bead-free nanofibers was evaluated. A homogenous dispersion of casein was observed for mixtures with 20 wt % casein prepared using 60 % ethanol/water mixture at pH(eth) 10. A minimum number of bead defects (BN, 6 x 10(-3)/mu m(2)) and bead area (BA, 8.7 x 10(-2)) with minimum average fiber diameter (FD, 424 nm) and porosity (52 %) were observed for casein fibers fabricated using dopes containing 2 wt % of tetrasodium pyrophosphate (TSPP) on a casein basis. Further, the viscosity dependence on casein concentration at constant relative TSPP content was like that expected for a polyelectrolyte, and bead-free nanofibers were obtained at a concentration similar to 2.5 times the entanglement concentration (C-e). Our results indicate that micellar dissociation is a necessary condition for the electrospinning of casein-only fibers free of beads and other polymers to fabricate novel biodegradable, and biocompatible, casein-based nanostructured mats, with the potential for food, cosmetic, packaging, and biomedical applications.

Automated summary

Fabrication of bead-free nanofibers from pure casein is challenging due to its self-aggregation tendency. In this study, the influence of pH, ethanol content, ionic environment, and casein concentration on the solubility and solution characteristics of casein-based spinning dopes was analyzed. The addition of tetrasodium pyrophosphate (TSPP) reduced bead defects and improved the properties of the nanofibers. The viscosity of the solution depended on the casein concentration, and bead-free nanofibers were obtained at a concentration higher than the entanglement concentration (C-e).