Retired academic neurosurgical anesthesiologist [38 years; https://scholar.google.com/citations?user=5DdrMc8AAAAJ&hl=en] here. My best guess is that once inorganic materials such as nanoparticles enter the cerebral circulation and lodge in brain tissue, they are there permanently.
What kind of interaction would you guess the particles would have with the brain after passing the barrier? I know heavy metals have stong interactions, but I wonder how bad plastic is, really - not that it would be good of course.
Depends a lot on the size, polymer, and even geometry (thread, shard, or just blob). Unlike metals, aliphatic plastics have little electronic interaction. My intuition as a chemist is most will just hang out in the lipid membrane or in vacuoles not really interacting with biology much. But it could also be worse, such as disrupting protein folding, messing with DNA replication, or triggering chronic low-grade inflammatory response. We don't really know at this time which is why we need studies like TFA.
I wouldn't even guess as to the nature of interaction of cerebral cortex/hypothalamus/brain stem/etc. with plastic nanoparticles — but I doubt that it would be a benefit. As Deckard remarked in another context, more likely than not a hazard.
