September/October 2008

Striped Camouflage

Researchers find stealthy way to enter cells

By Kristina Grifantini

An artist’s rendering of nanoparticles conveying imaging agents to cells. Credit: Francesco Stellacci, Darrell Irvine, and colleagues, MIT

Nanotechnology promises ways to deliver cancer-killing drugs or imaging agents directly to diseased cells. But cell-targeting nanoparticles will need a way to get through the cell's main line of defense against foreign entities: its membrane. While some nanoparticles can enter by poking holes in the membrane, that can destroy the cell.

Now a group at MIT has created the first synthetic nanoparticles that are large enough to hold drugs or imaging agents but can still slip through cells' membranes without breaking them. The researchers' breakthrough was to coat gold nanoparticles with alternating bands of hydrophobic and hydrophilic molecules.

The idea sprang from a conversation between Darrell Irvine, associate professor of biological engineering, and Francesco Stellacci, associate professor of materials science and engineering. Stellacci mentioned that nanoscale mixtures of hydrophilic and hydrophobic materials naturally separate into stripes. Irvine suggested that they test how particles coated with such stripes interact with cell membranes, which are made up of hydrophobic and hydrophilic layers.

In research reported in Nature Materials in May, Irvine, Stellacci, and their colleagues described how their striped nanoparticles, which were 10 to 12 nanometers in diameter and carried fluorescent dye, successfully penetrated mouse cells without rupturing them. Nanoparticles coated randomly with the same hydrophobic and hydrophilic material could not penetrate the cells.

"That this was dependent on the structure of the chemical groups on the surface of the particle rather than some other property was a major, unexpected finding," says Irvine.

The team plans to explore what cargo the particles can successfully carry and whether striped nanoparticles made of materials other than gold have the same properties.