Two blood-vessel cross-sections run side by side. The top vessel is healthy tissue: endothelial cells are packed edge-to-edge, so nanoparticles simply flow past and never leave the bloodstream. The bottom vessel feeds a tumor: rapid, disorganized angiogenesis leaves gaps between endothelial cells, and the tumor's poor lymphatic drainage means anything that gets out struggles to leave again. This is the Enhanced Permeability and Retention (EPR) effect — nanoparticles of the right size passively accumulate in tumor tissue purely because the barrier is leaky and the drainage is weak.
In Passive only mode, particles that extravasate through a gap just keep diffusing randomly in the tumor interstitium — accumulation without discrimination between cells. In Active + passive mode, particles also carry surface ligands (small gold spikes) that recognize receptors over-expressed on cancer cells (yellow markers) — once close enough, a particle docks and locks onto that specific cell, the way antibody- or folate-conjugated nanoparticles do in real targeted-delivery systems. Active targeting always relies on passive EPR accumulation first — a ligand can't dock on a receptor it never reaches.
- Injection rate — how many nanoparticles enter the bloodstream per second.
- Extravasated — particles that crossed the leaky tumor endothelium via EPR.
- Bound to cancer cell — only rises in Active + passive mode, once a ligand-coated particle finds and docks on a receptor.
- Cleared (healthy) — particles that flowed through the healthy vessel without ever crossing its intact wall.