Thin Is In

Long, skinny micelles last longer in the blood than their spherical counterparts. Dennis E. Discher and coworkers at the University of Pennsylvania compare thin cylindrical micelles called filomicelles (shown) with spherical nanoparticles with similar surface chemistry (Nat. Nanotechnol., DOI: 10.1038/nnano.2007.70). The filomicelles are made of polyethylene glycol (PEG) mixed with either polyethylethylene or biodegradable polycapralactone. When injected in rats, the filomicelles circulate in the blood 10 times longer than similar spherical particles with PEG on their surface. Up to about 8 µm (approximately the same size as blood cells), the longer filomicelles persist for longer times. Filomicelles that are 18 µm long quickly fragment into smaller pieces, with an 8-µm piece continuing to circulate and the larger piece being cleared from the system. Filomicelles also can enter cells, although flow conditions that mimic those in blood stretch the longer ones so that they have a tougher time interacting with cells. Filomicelles loaded with the cancer drug paclitaxel are able to enter cells and shrink tumors, with longer cylinders (up to 8µm) being more effective.