Strands of cow cartilage substitute for ink in a 3D bioprinting process that may one day create cartilage patches for worn out joints, according to a team of Penn State University engineers. Cartilage is a good tissue to target for scale-up bioprinting because it is made up of only one cell type and has no blood vessels within the tissue. It is also a tissue that cannot repair itself. Once cartilage is damaged, it remains damaged. Previous attempts at growing cartilage began with cells embedded in a hydrogel - a substance composed of polymer chains and about 90 percent water - that is used as a scaffold to grow the tissue.