Not really. At least if you consider a fully artificial creation alive (which bio-ships implies). No known physical limitations on building artificial shells that have fully artificial organism grown inside of them. Whether or not its practical is another thing entirely. Plus, while natural organisms probably wouldn't grow in space--or we'd see them around one of our gas giants--artificially created ones are another matter entirely. We've just barely figured out it might be possible. claiming to know the limitations is pre-mature.