Researchers continue to take incremental steps towards the creation of engineered living tissues containing the vascular networks needed to support it. Absent blood vessels, numerous varieties of functional tissue can be generated from cell samples: lung, liver, kidney, and so forth. These organoids are limited in size to a few millimeters, however, the distance the nutrients and oxygen can perfuse. Generating blood vessel networks is a serious technical challenge, and the major obstacle to the production of entire organs for transplantation. Consider that natural capillary networks exhibit a density of hundreds of vessels passing through every square millimeter of tissue cross-section. Even the best of present efforts are distant from that scale, though in laboratory demonstrations they suffice to produce essentially functional larger tissue sections.
Researchers have developed a way to 3D print living skin, complete with blood vessels. The advancement is a significant step toward creating grafts that are more like the skin our bodies produce naturally. "Right now, whatever is available as a clinical product is more like a fancy Band-Aid. It provides some accelerated wound healing, but eventually it just falls off; it never really integrates with the host cells."
A significant barrier to that integration has been the absence of a functioning vascular system in the skin grafts. Researchers have been working on this challenge for several years, previously showing that they could take two types of living human cells, make them into "bio-inks," and print them into a skin-like structure. Researchers now show that if they add key elements - including human endothelial cells, which line the inside of blood vessels, and human pericyte cells, which wrap around the endothelial cells - with animal collagen and other structural cells typically found in a skin graft, the cells start communicating and forming a biologically relevant vascular structure within the span of a few weeks.
"As engineers working to recreate biology, we've always appreciated and been aware of the fact that biology is far more complex than the simple systems we make in the lab. We were pleasantly surprised to find that, once we start approaching that complexity, biology takes over and starts getting closer and closer to what exists in nature." Once the team grafted the engineered tissue onto a mouse, the vessels from the printed skin began to communicate and connect with the mouse's own vessels. In order to make this usable at a clinical level, researchers need to be able to edit the donor cells using something like the CRISPR technology, so that the vessels can integrate and be accepted by the patient's body. "We are still not at that step, but we are one step closer,."