Figure 4.

Main pathways for nanoparticles to cross the blood-brain barrier. Penetration of charged molecules (including cationic nanoparticles: liposomes and gold nanoparticles) depends on adsorptive-mediated endocytosis while molecules with high hydrophilicity (like transferrin conjugated nanoparticles, liposomes, poly(lactic-co-glycolic acid/poly (lactic acid)) needs receptor-mediated endocytosis. Carrier-mediated pathway is a substrate-specific process while small hydrophilic molecules penetrate through paracellular pathway. Passive diffusion of small lipophilic molecules, e.g., gold nanoparticles occur through transcellular pathway.