According to this article, "The effect of nanoparticle size on the probability to cross the blood-brain barrier: an in-vitro endothelial cell model"[1]: "GNPs[

According to this article, "The effect of nanoparticle size on the probability to cross the blood-brain barrier: an in-vitro endothelial cell model"[1]: "GNPs[gold nanoparticles] of various sizes (20, 50, 70 and 110 nm) were synthesized and coated with barbiturate, which is a molecule that can easily penetrate the BBB [34]. Therefore, coating GNPs with barbiturate molecules will facilitate their penetration through the BBB, both for therapy and imaging applications. ... The results show that GNPs of size 70 nm are optimal for the maximum amount of gold within the brain cells, and that 20 nm GNPs are the optimal size for maximum free surface area." The nanowarming article reports using iron oxide nanoparticles with a final average diameter of 50 nm (after some chemical prep & coating operations). So getting a bunch of iron nanoparticles into a brain isn't out of the question. Bigger challenge is probably getting them back out! [1]: http://jnanobiotechnology.biomedcentral.com/articles/10.1186/s12951-015-0075-7 http://jnanobiotechnology.biomedcentral.com/articles/10.1186...