The secret, it seems, is to be average. Mid-sized nanomaterials ?about 27 to 30 nanometers in diameter, or about 1,000 times thinner than a human hair ?are optimal for cellular entry. In a research article to be posted this week in the online edition of the Proceedings of the National Academy of Sciences, the researchers note that this information is significant for developing gene and drug delivery tools as well as assessing the safety of nanotubes ?infinitesimal bits of carbon and other materials used in everything from cameras to clothing.
"If you know how viruses get into cells, you know how to better design drugs to keep them out," said L.B. Freund, professor of engineering at Brown. "Or if you do want molecules to get in ?those in medication, say ?knowing an optimal particle size for entry is also helpful.
"With nanotubes, we may be able to manufacture ones of a certain size to minimize chances that they'll enter and perhaps harm cells."
The type of cell entry the team studied is called receptor-mediated endocytosis.
Here's how it happens: A virus or other particle arrives at the cell membrane. Protein receptors on the membrane act like hooks, grabbing onto hooks, or ligands, on the particle, much like two pieces of Velcro. As more and more chemical hooks are recruited for the task, the membrane wraps around the particle until it is completely engulfed. This is how herpes and influenza viruses get inside cells.
The process is believed usually to involve clathrin, a protein that coats the invader to aid in the Velcro-like fastening. Yet scientists have shown that flu viruses can invade cells even without a clathrin coat. Along with Huajian Gao and