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printed Ladder of Life. True bioprinting, as we deine it, creates living tissue,
not inanimate replacement parts. Bioprinting involves placing living cells into
just the right location with a 3D printer, to fabricate functional, heterogeneous
living tissue.
Researchers deine “bioprinting” in different ways, and as the ield advances,
the term will likely take on even more meanings. One approach to bioprint-
ing involves the use of a “living ink” that's a printable gel with living cells
suspended inside. The special gel—called hydrogel—cushions and protects
the living cells as they are pushed through the printing nozzle. Once the liv-
ing ink has been printed out and laid down into the right place, the hydrogel
maintains the tissue's desired structure. The living cells will secrete a sub-
stance into the hydrogel that eventually forms a supportive matrix. As the
living cells continue to develop, the matrix develops into cartilage or some
other type of living tissue.
Depositing the right cell type into the right location is somewhat like the
process of planting the perfect vegetable garden, with each vegetable planted
precisely in the right location to receive the optimal amount of light. Not
all stem cells are the same. So far, nature is still much better than humans
or computers at creating the perfect garden of stem cells placed into precise
position.
One of the major advantages of 3D printing with living ink to make soft
tissue is a printer's ability to carefully squirt cells into precise patterns and
shapes. As printers get better at printing using multiple print heads, each print
head can be illed with a different cell type. The result will be that one nozzle
would print a different type of cell, and another nozzle would print hydrogel
with different material qualities. By borrowing the concept of multi-material
3D printing and applying it to the biological world, researchers are steps closer
to creating artiicial tissue that mimics nature's complicated shapes, internal
structures, and cellular diversity.
Cell placement is one challenge. Another challenge is making sure that
cells are placed in such a way that they will eventually form the right shape.
Cell location and the shape of the resulting tissue is critical for proper organ
functioning. Growth factors could also be printed and added into the mix.
For example, heart tissue requires high cell density to make sure that the
heart beats in a regular rhythm. If the cells seeded onto the scaffolding in artii-
cial cardiac tissue aren't tightly interconnected, the result is an irregular heart-
beat. Since a tissue design is based on a computer-guided design, bioprinted
tissue made using living ink will be precisely made and its design repeatable.
