The notion of 3D printed biological tissue holds all kinds of possibilities for drug testing and the reparation of damaged cells, though replicating the complexities of human tissue in a lab presents some very big challenges. A new bioprinting method developed by researchers from the Wyss Institute for Biologically Inspired Engineering at Harvard University has enabled the creation of tissue constructs with small blood vessels and multiple cell types, marking important progress toward the printing of living tissue.

While 3D printed human tissue has been printed before, researchers have been limited to producing relatively thin layers. Efforts to create layers thicker than around a third of a dime have encountered problems due to the cells on the interior starving of oxygen and nutrients, while also having no way to dispose of waste, ultimately causing them to suffocate and die.

To combat this problem, the Wyss Institute researchers used three specially developed "bio-inks," that is, inks borrowing certain biological properties from real living tissue. The first used extracellular matrix, which joins the cells together to form tissue, while the second ink used a combination of the extracellular matrix and living cells.

The third ink was designed to melt, not as it warms, but as it cools. This meant that once the team had used it to create a network of cells, it could be chilled, melted and ultimately sucked out of the tissue, leaving a network of hollow tubes in its place and a pathway for blood vessels to travel.

This mimics a key characteristic of living tissue, where the interior cells are sustained by a network of tiny, thin-walled blood vessels providing oxygen and nutrients while also removing waste. The team tested the replica and using the model were able to construct printed tissues of various architectures and ultimately an intricate construct containing blood vessels and three types cell types, a structure that it says is approaching the complexity of human tissue.

"Tissue engineers have been waiting for a method like this," said Don Ingber, M.D., Ph.D., Wyss Institute Founding Director. "The ability to form functional vascular networks in 3D tissues before they are implanted not only enables thicker tissues to be formed, it also raises the possibility of surgically connecting these networks to the natural vasculature to promote immediate perfusion of the implanted tissue, which should greatly increase their engraftment and survival".

The team's short term ambitions for the technology are centered on creating 3D tissues that mimic living tissue closely enough to be useful in screening drugs for safety and effectiveness. "That's where the immediate potential for impact is," said Jennifer Lewis, Care Faculty Member of the Wyss Institute and a senior author of the sudy.

The researched was published this month in the journal Advanced Materials.

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