Such a system could one day be used to provide more control when battling diseases commonly treated with more than one drug, according to the researchers.

"With a lot of diseases, especially cancer and AIDS, you get a synergistic effect with more than one drug," said Kimberly Hamad-Schifferli, senior author of a paper on the work that appeared in the current issue of journal ACS Nano.

Delivery devices that can release two drugs already exist, but the timing of the release must be built into the device -- it cannot be controlled from outside the body. The new system is controlled externally and theoretically could deliver up to three or four drugs.

The new technique takes advantage of the fact that when gold nanoparticles are exposed to infrared light, they melt and release drug payloads attached to their surfaces.

Nanoparticles of different shapes respond to different infrared wavelengths, so "just by controlling the infrared wavelength, we can choose the release time" for each drug, said the researchers.