Bioengineers have cleared a major hurdle on the path to 3D printing replacement organs with a breakthrough technique for bioprinting tissues.The new innovation allows scientists to create exquisitely entangled vascular networks that mimic the body's natural passageways for blood, air, lymph and other vital fluids.The research is featured on the cover of this week's issue of Science. It includes a visually stunning proof-of-principle -- a hydrogel model of a lung-mimicking air sac in which airways deliver oxygen to surrounding blood vessels. Also reported are experiments to implant bioprinted constructs containing liver cells into mice.The work was led by bioengineers Jordan Miller of Rice University and Kelly Stevens of the University of Washington (UW) and included 15 collaborators from Rice, UW, Duke University, Rowan University and Nervous System, a design firm in Somerville, Massachusetts."One of the biggest road blocks to generating functional tissue replacements has been our inability to print the complex vasculature that can supply nutrients to densely populated tissues," said Miller, assistant professor of bioengineering at Rice's Brown School of Engineering. "Further, our organs actually contain independent vascular networks -- like the airways and blood vessels of the lung or the bile ducts and blood vessels in the liver. These interpenetrating networks are physically and biochemically entangled, and the architecture itself is intimately related to tissue function. Ours is the first bioprinting technology that addresses the challenge of multivascularization in a direct and comprehensive way."