Tuning the Physical Properties of Carboxylated Cellulose Nanocrystal (cCNC) Microspheres by Hybridizing with Silk Fibroin (SF)

A new class of hydrophobic/lipophilic cellulose microspheres are made from carboxylated cellulose nanocrystals (cCNC) by adding silk fibroin (SF) protein in the course of spray-drying from aqueous suspension. We found mere 2% SF addition could leverage the surface energy with an increase of contact angle from 27.5° to 60.4°. Besides the complete altered surface energy from cellulose beads, the hybrid SF-cCNC microspheres also show improved mechanical properties and prolonged diffusion kinetics for transporting water-soluble ions / molecules (e.g., methylene blue). Depth profiling of the SF-cCNC microspheres reveals that SF is more concentrated at the surface in comparison with the core, and this surface localization is the reason for the tuned properties. Moreover, post methanol treatment of the SF-cCNC hybrid microspheres induces a β-sheet phase transition to the Silk II structure, which can further enhance the mechanical properties and slow down the small molecule transport of the microspheres. Therefore, a new method has been established that could tune the physical properties of functional cellulose microspheres through the control of SF structural transformation, which could significantly benefit for controlled drug release and microplastic beads replacement applications.