Instead of needles or nasal sprays, annual flu shots could one day come in the form of a skin patch embedded with tiny pain-free needles.

That's the hope of researchers at the Georgia Institute of Technology. They say they've developed a patch that contains microneedles that are so small, they can't be felt.

When the patch is attached to the skin like a Band-Aid, the little needles penetrate the skin and release their vaccine. The tiny needles then simply dissolve away, leaving only the backing.

In tests of the microneedle patches that contained no vaccine, most users said using it was painless. Those that said they did feel something rated the pain at about one-tenth of a traditional vaccine needle.

The developers of the patch say their invention could simplify immunization programs by eliminating the need to dispose of hypodermic needles. User could apply the patches themselves, without the aid of nurses. And, since the vaccine in the patch is present as a dry formulation, it can remain stable during storage.

"The dissolving microneedle patch could open up many new doors for immunization programs by eliminating the need for trained personnel to carry out the vaccination," Mark Prausnitz, a professor in the Georgia Tech School of Chemical and Biomolecular Engineering, said in a news release.

"This approach could make a significant impact because it could enable self-administration as well as simplify vaccination programs in schools and assisted living facilities."

Gerogia Tech researchers have already tested the vaccine-carrying patch on mice and say the system works well, they report online in the journal Nature Medicine.

In the study, one group of mice received the influenza vaccine using traditional hypodermic needles injecting into muscle; another group received the vaccine through dissolving microneedles applied to the skin, while a control group had microneedle patches containing no vaccine applied to their skin.

When infected with influenza virus 30 days later, both groups that had received the vaccine remained healthy while mice in the control group contracted the disease and died.

"In this study, we have shown that a dissolving microneedle patch can vaccinate against influenza at least as well, and probably better than, a traditional hypodermic needle," Prausnitz said.

While the patch doesn't place the vaccine deep into the skin the way that most needles do, the outer layers of skin contain calls that also help to deliver vaccine.

"The skin is a particularly attractive site for immunization because it contains an abundance of the types of cells that are important in generating immune responses to vaccines," explained Richard Compans, professor of microbiology and immunology at Emory University School of Medicine.

So far, they've tested the microneedle patches only with flu vaccine, but they say the technique should be useful for other immunizations.