Vascularization: the diffusion limit
Why engineered tissue thicker than a couple of hundred microns fails without a blood supply — the field's most stubborn bottleneck.
Oxygen diffuses only about 100–200 microns from a capillary. Any engineered construct thicker than that needs its own blood supply, or the cells at its centre die. This single constraint explains why thin tissues have progressed toward the clinic while thick, metabolically active organs have not.
Strategies fall into two families. One encourages host vessels to grow in, typically by delivering VEGF and related factors, sometimes in combination with PDGF to stabilise the new vessels. The other pre-forms a vascular network inside the construct before implantation, using printed channels or co-cultured endothelial cells. In bone specifically, the coupling of vessel growth and bone formation is so tight that the process has its own name — osteogenic–angiogenic coupling — and it is why large defects are so difficult.
Timing matters as much as dose. Vessels must arrive as the scaffold degrades and tissue forms, which makes this as much a release-kinetics problem as a biological one.
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