The work is a major step toward creating human tissue constructs realistic enough to test drug safety and effectiveness, researchers said.
The method will also help bring closer the building of fully functional replacements for injured or diseased tissue that can be designed from CAT scan data using computer-aided design (CAD), printed in 3D at the push of a button.
"This is the foundational step toward creating 3D living tissue," said Jennifer Lewis, senior author of the study, from the Wyss Institute for Biologically Inspired Engineering at Harvard University.
Tissue engineers have printed human tissue before, but they have been limited to thin slices of tissue about a third as thick as a dime.
When scientists try to print thicker layers of tissue, cells on the interior starve for oxygen and nutrients, and have no good way of removing carbon dioxide and other waste. Nature solves the problem by permeating tissue with a network of tiny thin-walled blood vessels that nourish tissue and remove waste, so researchers mimicked this function.
To print 3D tissue constructs with a predefined pattern, researchers needed functional inks with useful biological properties, so they developed several tissue-friendly inks containing key ingredients of living tissues.
To create blood vessels, they developed an ink which melted as it is cooled, rather than as it warmed. This allowed the scientists to first print an interconnected network of filaments, then melt them by chilling the material and suction the liquid out to create a network of hollow tubes, or vessels.
The Harvard team then road-tested the method to assess its power and versatility.
They printed 3D tissue constructs with a variety of architectures, resulting in an intricately patterned construct containing blood vessels and three different types of cells - a structure approaching the complexity of solid tissues.
When they injected human endothelial cells into the vascular network, those cells regrew the blood-vessel lining.
Keeping cells alive and growing in the tissue construct represents an important step toward printing human tissues, researchers said.
Lewis and her team are now focused on creating functional 3D tissues that are realistic enough to screen drugs for safety and effectiveness.
The study is published in the journal Advanced Materials.