Modeling the printability of photocuring and strength adjustable hydrogel bioink during projection-based 3D bioprinting

As a projection based three-dimensional printing method, digital light processing bioprinting (DLPBP) has higher printing resolution and is suitable for constructing finer structures to mimic tissues when compared to extrusion based bioprinting. However, there is a lack of understanding about printing behavior during DLPBP. Herein, a photo crosslinking theory for ink was established and a specified amount of light absorber was added to control crosslinking depth. Then, a standardized methodology was established to quantitatively evaluate printing resolution using different parameters. Complex biostructures, such as the ear, hand, and heart, were precisely printed after understanding the mechanism. Additionally, the mechanical properties of printed samples were accurately adjusted by changing the hydrogel concentration, as well as the degree of substitution and photocrosslinking time. The tissue types printed were from ultra-soft tissues, such as liver (6-8 kPa) to soft tissue, such as the skin (0.3-0.4 MPa). A branching vessel with cells in a real tensile modulus was printed as a demonstration. After 1 week of culture, proliferation and function of human umbilical vein endothelial cells were characterized. Overall, we made it possible to print a mimic complex tissue with high precision, required physical properties and functionalized living cells.