Fig. 3

Delineation showing the size dependent physiological barriers against magnetic nanoparticles blood circulation. Even the smallest magnetic nanoparticles do not cross the blood-brain barrier and the blood vessel epithelium in the muscles, as long as the tissues are not cancerous (A). Nanoparticles with a diameter of about 5 nm or less are able to penetrate through small pores such as in the epithelium of the lungs and skin (B). Sinusoidal capillaries in the liver are fenestrated (100–180 nm) and lined with the Kupffer cells which quickly uptake large nanoparticles (> 100 nm) or agglomerates tagged with opsonins, whereas smaller nanoparticles (< 100 nm) are captured and hidden in the Disse space from where they can be collected by hepatocytes (C). Nanoparticles larger than about 200 nm get trapped in the marginal zones and the red pulp of the spleen, where they are absorbed by splenic macrophages (D). In the kidneys, nanoparticles with d_H < 10–15 nm in diameter are filtered out, whereas nanoparticles with d_H < 50–60 nm can penetrate through the pores in the intestines and glands (E)