It’s the “arching” or “ bridging” problem when dealing with other “ink” types when 3d’ing. With PLA or ABS you can solve it with print speed, thickness and how fast the printed material drys. With tissue deposition, there might be a reason to have bio-dissolvable arches to support the cells while they adhere to each other and form vessicles that transport blood in and out. After the structure solidifies, the temporary scaffolding could be removed. Just not sure what works as the scaffolding. Yeah, I 3d, but not tissue.
In other words I think you mean some kind of temporary filler for the vessels.
With chemical engineering ingenuity I don’t know why that could not be done. I’m thinking of the process by which filled cordial cherries (in chocolate) are commonly made. While the innards could be frozen before covering (and that is a common homemade process) the commercial solution is an enzyme that turns the solid innards to liquid within a reasonable time.
In 20 years they will be creating terminators to go back in time and kill Sarah Palin.
The mother of the resistance.