Many research groups are working on ways to overcome the challenge of generating suitable blood vessel networks for engineered tissue, as this is one of the major blocking issues in generating large tissue masses from scratch: "Researchers are hopeful that new advances in tissue engineering and regenerative medicine could one day make a replacement liver from a patient's own cells, or animal muscle tissue that could be cut into steaks without ever being inside a cow. Bioengineers can already make 2D structures out of many kinds of tissue, but one of the major roadblocks to making the jump to 3D is keeping the cells within large structures from suffocating; organs have complicated 3D blood vessel networks that are still impossible to recreate in the laboratory. Now [researchers] have developed an innovative solution to this perfusion problem: [rather] than trying to print a large volume of tissue and leave hollow channels for vasculature in a layer-by-layer approach, [they] focused on the vasculature first and designed free-standing 3D filament networks in the shape of a vascular system that sat inside a mold. As in lost-wax casting, a technique that has been used to make sculptures for thousands of years, the team's approach allowed for the mold and vascular template to be removed once the cells were added and formed a solid tissue enveloping the filaments. ... The researchers showed that human blood vessel cells injected throughout the vascular networks spontaneously generated new capillary sprouts to increase the network's reach, much in the way blood vessels in the body naturally grow. The team then created gels containing primary liver cells to test whether their technique could improve their function. ... Though these engineered tissues were not equivalent to a fully functioning liver, the researchers used cell densities that approached clinical relevance, suggesting that their printed vascular system could eventually be used to further research in lab-grown organs and organoids."