Mucus membranes, such as those in the intestine, nose, eyes, vagina and mouth, are good targets for therapeutic drug delivery thanks to their large surface area and rich blood supply. But these membranes, which can be several hundreds of microns thick, are important barriers to drug penetration. Previous delivery devices focused on microscale structures that were chemically modified with molecules like lectins so that they would better adhere to mucus cells. Yet the problem here is that such devices are eliminated in a matter of hours as the mucus naturally turns over.

Now, a team led by Tejal Desai and Kayte Fischer of the University of California at San Francisco, has shown that silicon nanowires can penetrate the mucus layer and adhere to the underlying epithelium. This means that the nanowires remain on the cells for up to three days (the cell turnover time). "Our result supports the notion that decreasing the size of surface structures increases adhesion – whether through van der Waals adhesion or other nanoscale interactions with cells," said Desai.

The team, which includes researchers from the Lawrence Berkeley National Laboratory, The Charles Stark Draper Laboratory in Cambridge, MA, and Nanosys Inc in Palo Alto, CA, obtained its result by testing the conditions under which the nanowires adhered to epithelial cells. The scientists also quantified the amount of shear the devices could withstand before being eliminated. This is important because mucosal tissue experiences lots of shear flow. For example, intestinal flow typically has shears between 0.1 and 10 dynes/cm2 (compared with 100 dynes/cm2 when a person has diarrhoea).

"Previous mucoadhesives aimed to do just that – adhere to mucus – but nanowire-coated beads, on the other hand, adhere to cells underneath the mucus layer," Desai told "While we have not published in vivo work yet, in vitro flow studies with mucin show that the nanowire-coated devices adhere more strongly than those modified with tomato lectin."

Desai added that being able to use nanostructures as adhesives without chemical modifications potentially reduces the processing steps involved. Moreover, silicon nanowires are robust and biocompatible and might be integrated into other silicon-based systems, such as microchips or circuitry. "The ultimate hope for this technology is to create devices that allow proteins and other macromolecules (for example insulin) to be delivered via pills instead of syringes," she explained.

The researchers are currently investigating potential mechanisms of adhesion, such as active cell restructuring. "We would eventually like to work with animal models to determine whether these devices could be used in humans," said Desai.

The work was published in Nano Letters.