Silk Fibroin Bioink for 3D Printing in Tissue Regeneration: Controlled Release of MSC-extracellular Vesicles

Sodium alginate (SA)-based hydrogels are often employed as bioink for three-dimensional
(3D) scaffold bioprinting. They offer a suitable environment for cell proliferation and differentiation
during tissue regeneration and also control the release of growth factors and mesenchymal stem cell
secretome, which is useful for scaffold biointegration. However, such hydrogels show poor mechanical
properties, fast-release kinetics, and low biological performance, hampering their successful
clinical application. In this work, silk fibroin (SF), a protein with excellent biomechanical properties
frequently used for controlled drug release, was blended with SA to obtain improved bioink and
scaffold properties. Firstly, we produced a printable SA solution containing SF capable of the conformational
change from Silk I (random coil) to Silk II (β-sheet): this transition is a fundamental
condition to improve the scaffold’s mechanical properties. Then, the SA-SF blends’ printability and
shape fidelity were demonstrated, and mechanical characterization of the printed hydrogels was
performed: SF significantly increased compressive elastic modulus, while no influence on tensile
response was detected. Finally, the release profile of Lyosecretome—a freeze-dried formulation of
MSC-secretome containing extracellular vesicles (EV)—from scaffolds was determined: SF not only
dramatically slowed the EV release rate, but also modified the kinetics and mechanism release with
respect to the baseline of SA hydrogel. Overall, these results lay the foundation for the development
of SA-SF bioinks with modulable mechanical and EV-release properties, and their application in 3D
scaffold printing.