For as long as I have been watching progress in tissue engineering, the primary and most important barrier to building organs to order has been the inability to construct vascular networks. A network of capillaries must exist for blood, and thus nutrients and oxygen necessary to cell survival, to reach more than a few millimeters into a tissue. In live tissues, hundreds of minuscule capillaries pass through every square millimeter, considered in cross-section. Replicating this level of capillary density in engineered tissue has yet to be accomplished, with even the more advanced technology demonstrations falling well short of this goal.

Well funded initiatives such as the effort to produce genetically engineered pigs with organs that can be decellularized for transplantation into humans, or the application of decellularization to donor human organs, should be considered as attempts to work around the vascular challenge. That is why they exist. If a suitable vascular network cannot be produced from scratch, then the existing vascular network in an existing organ is the only viable alternative. It remains to be seen as to how long these approaches will be needed, how long it will take the research community to be able to grow larger tissues with sufficient vascular networks for practical use in medicine.

As the research community continues to wrestle with the production of vascular networks, scientists have become ever more proficient in the production of small sections of organ tissue from the starting point of a cell sample, known as organoids. Given the ability to reprogram patient cells into induced pluripotent stem cells, which can then be used to produce cells of any type, building functional organoids only requires a suitable protocol: the right signals and conditions to convince cells to form tissue as they do in the body. Discovering how to do this for the more important internal organs has proceeded apace over the past decade: livers, kidneys, lungs, the thymus, and more. As soon as a viable approach to vascularization of tissue emerges, scaled up and fully functional organs made to order will soon follow.

Sacrificial ink-writing technique allows 3D printing of large, vascularized human organ building blocks

Artificially grown human organs are seen by many as the "holy grail" for resolving the shortage of donor organs for transplant, and advances in 3D printing have led to a boom in using that technique to build living tissue constructs in the shape of human organs. However, all 3D-printed human tissues to date lack the cellular density and organ-level functions required for them to be used in organ repair and replacement. Now, a new technique called SWIFT (sacrificial writing into functional tissue) overcomes that major hurdle by 3D printing vascular channels into living matrices composed of stem-cell-derived organ building blocks (OBBs), yielding viable, organ-specific tissues with high cell density and function. "This is an entirely new paradigm for tissue fabrication. Rather than trying to 3D-print an entire organ's worth of cells, SWIFT focuses on only printing the vessels necessary to support a living tissue construct that contains large quantities of OBBs, which may ultimately be used therapeutically to repair and replace human organs with lab-grown versions containing patients' own cells." SWIFT involves a two-step process that begins with forming hundreds of thousands of stem-cell-derived aggregates into a dense, living matrix of OBBs that contains about 200 million cells per milliliter. Next, a vascular network through which oxygen and other nutrients can be delivered to the cells is embedded within the matrix by writing and removing a sacrificial ink. "Forming a dense matrix from these OBBs kills two birds with one stone: not only does it achieve a high cellular density akin to that of human organs, but the matrix's viscosity also enables printing of a pervasive network of perfusable channels within it to mimic the blood vessels that support human organs."

Biomanufacturing of organ-specific tissues with high cellular density and embedded vascular channels