Checking whether a drug you are buying is real may become a lot easier as researchers have developed labels that reveal a hidden image when you breathe on them.

"You can verify that you have the real product with just a breath of air," said Nicholas Kotov, a professor of chemical engineering at University of Michigan in the US.

The researchers found a way to print tiny features 500 times smaller than human hair onto labels through nanotechnology, the image becomes visible when someone blows on it.

Tiny pillars on the surface hide the images printed which reappear when the pillars trap moisture.

The material they have developed is a special blend of polyurethane and adhesive and can stick to plastics, fabric, paper and metal.

"We use a moulding process," Terry Shyu, a doctoral student in chemical engineering at the University of Michigan said, noting that this inexpensive manufacturing technique is also used to make plastic cups.

The technique currently costs a dollar per square inch.

The study appeared in the journal Advanced Materials.