Scientists have developed a way to make bar codes so small they can tag individual molecules. Researchers at Pennsylvania State University and SurroMed Inc. fabricate the bar codes, which they call nano-bar code particles, out of cylinders of striped metallic nanoparticles. The stripes on the bar codes are made of metals with different reflective properties, enabling them to be read the same way as conventional bar codes. The process requires an optical microscope to scan the nano-bar codes, which are typically just 300 nanometers in diameter and 5 microns long.

The technology can be used to tag individual molecules to allow thousands to be tested at once in a sample of biological materials such as blood, making medical tests much more accurate and efficient. It also can be used to tag the silicon molecules in a computer chip to verify that the chip is authentic, says inventor Michael Natan, chief technology officer at SurroMed, in Mountain View, Calif.

Because the nano-bar codes are made of stable metals such as gold or platinum, they can be used in instances where environmental factors would destroy other types of markings, Natan says. The nano-bar codes can survive the final, high-temperature stage of chipmaking, while an organic tag, such as one made of paper, can't.

The bar codes are manufactured with an electrochemical process that uses templates with cylindrical holes to make each stripe in the final product. SurroMed can manufacture a couple of hundred stripe patterns, which can be combined with different combinations of four patterns each to form millions of unique codes.

By year's end, SurroMed expects to be able to manufacture at least 500 stripe patterns, capable of forming several billion unique codes. With plans to produce at least 1,000 stripe patterns, Natan says, by the end of next year, the ability to form unique codes will be practically limitless.