Due to the brain-blood barrier (BBB), a protective barrier separating the nervous tissue from the blood circulation, charged and hydrophilic drug molecules cannot reach the brain. Polymeric micelles represent an attractive solution to this problem as they can cross the BBB and thus deliver drugs to the central nervous system. Nevertheless, it is not known whether the interactions between these nanoparticles and the central nervous system are safe.

This issue prompted Jens Lykkesfeldt, University of Copenhagen, Denmark, and colleagues to examine the acute toxicity of well-characterized polyethylene glycol (PEG)ylated anionic and cationic polymeric micelles. When high doses of cationic micelles were administered to rats trough intracerebral injections, they caused brain lesions, ultimately provoking the death of neurons in the site of injection. These toxic effects seemed to be specific to cationic micelles since they did not occur when the researches administered anionic micelles.
The negative charge of polymeric micelles is, thus, essential to ensure their safe delivery to the brain.

• Differential toxicological response to positively and negatively charged nanoparticles in the rat brain,
  Kristina Bram Knudsen, Helle Northeved, Pramod Kumar Ek, Anders Permin, Thomas L Andresen, Steen Larsen, Karen M Wegener, Henrik Rye Lam, Jens Lykkesfeldt,
  Nanotoxicology 2013.
  DOI: 10.3109/17435390.2013.829589