Right now, most researchers are working on either sensors or drug-delivery systems, but not both. "Everybody who is in this field is working toward closed-loop technology, but it's at least 10 years away," says Therafuse's Sage.

A very different kind of semiconductor drug-delivery system, the quantum dot approach, might be just as far away. Quantum dots are crystals—often of the II-VI semiconductor cadmium selenide—which, because they are mere nanometers across, retain the quantum properties of single atoms. Critical for biomedical applications, these properties include the ability to absorb and emit photons of a very specific wavelength.

In single atoms, the wavelength depends on the type of atom involved, but with quantum dots the stimulation wavelength is related to their size. Smaller dots absorb and give off a shorter wavelength—that is, the light they absorb and emit is closer to the violet end of the spectrum—whereas larger dots produce longer wavelengths closer to the red end.

Most of the current commercial investment in quantum dots is related to their use in computing applications and lasers. The market for quantum dot-based chemicals for biomedical research—now roughly US $720 million—has also been growing, says Stefanie Lattner, portfolio executive at the publicly funded venture capital firm Innovation Works, in Pittsburgh. But at the moment, no venture capital money is chasing drug-delivery applications, she says.

That may be because studies of the therapeutic uses of quantum dots are still mainly at the academic level. Shuming Nie, adjunct professor of biomedical engineering at Emory University in Atlanta, is one of the first researchers to investigate quantum dots as drug-delivery systems. Specifically, Nie has chemically bound the breast and prostate cancer drug Taxol to quantum dots in an effort to deliver it specifically to tumor cells, leaving the rest of the body unaffected [see illustration, "Dots Spell Doom for Tumors"].

The scheme could increase the drug's efficacy and reduce its side effects. The American Cancer Society, in Atlanta, estimates that in 2004, 15 percent of cancer deaths among U.S. women will be from breast cancer; similarly, 10 percent of cancer deaths among U.S. men will be from prostate cancer.

Nie's group started its research by studding the Taxol-bound quantum dots with a molecule that binds to folic acid receptors, which are present on tumor cells at concentrations roughly 1000 times those found on normal cells. The receptor-targeting molecule allows the nanoparticles to home in preferentially on cancer cells. In work published in August, the Emory scientists got even better results using antibodies against prostate cancer cells rather than folic acid binders.

Nie and his co-workers injected the Taxol-coated nanoparticles into mice that had been surgically implanted with human prostate tumors. After the injections, they illuminated the mice with infrared light, which penetrates into their tissues and excites the quantum dots [see photo, "Diagnosis by Dot"]. As the energy states of the dots fall back, they emit energy sufficient to cleave the bonds between the Taxol and the particles, releasing the drug to attack tumor cells. "We have evidence that our quantum dot conjugates can get into cancer cells and kill them," says Nie.

He acknowledges that the approach will be more difficult for human patients, whose bodies are thicker than that of the average mouse. Infrared light will penetrate only a few centimeters into living tissues. "I don't think there's any hope for this in treating cancers of the internal organs," says Nie, "but it might work for [the skin cancer] melanoma or for breast cancer."