Silk fibroin, gelatin, and human placenta extracellular matrix-based composite hydrogels for 3D bioprinting and soft tissue engineering.

Background: There is a great clinical need and it remains a challenge to develop artificial soft tissue constructs that can mimic the biomechanical properties and bioactivity of natural tissue. This is partly due to the lack of suitable biomaterials. Hydrogels made from human placenta offer high bioactivity and represent a potential solution to create animal-free 3D bioprinting systems that are both sustainable and acceptable, as placenta is widely considered medical waste. A combination with silk and gelatin polymers can bridge the biomechanical limitations of human placenta chorion extracellular matrix hydrogels (hpcECM) while maintaining their excellent bioactivity.

Method: In this study, silk fibroin (SF) and tyramine-substituted gelatin (G-TA) were enzymatically crosslinked with human placental extracellular matrix (hpcECM) to produce silk-gelatin-ECM composite hydrogels (SGE) with tunable mechanical properties, preserved elasticity, and bioactive functions. The SGE composite hydrogels were characterized in terms of gelation kinetics, protein folding, and bioactivity. The cyto- and biocompatibility of the SGE composite was determined by in vitro cell culture and subcutaneous implantation in a rat model, respectively. The most cell-supportive SGE formulation was then used for 3-dimensional (3D) bioprinting that induced chemical crosslinking during extrusion.

Conclusion: Addition of G-TA improved the mechanical properties of the SGE composite hydrogels and inhibited crystallization and subsequent stiffening of SF for up to one month. SGE hydrogels exhibit improved and tunable biomechanical properties and high bioactivity for encapsulated cells. In addition, its use as a bioink for 3D bioprinting with free reversible embedding of suspended hydrogels (FRESH) has been validated, opening the possibility to fabricate highly complex scaffolds for artificial soft tissue constructs with natural biomechanics in future.