Painless microjet injections powered by laser could one day replace jabs from hypodermic needles in delivering annual flu shots, vaccines and other medications, according to researchers at Seoul National University in South Korea, who write about the design of their Er:YAG laser microjet transdermal device and how they tested it on guinea pigs in the 15 September issue of Optics Letters.

The device uses an erbium-doped yttrium aluminum garnet, or Er:YAG, laser, to drive a tiny, precise stream of liquid drug with just the right amount of force. It uses multiple pulses of laser beam at lower energy, thereby delivering a significantly higher dose than a previous version, the Nd:YAG system, report senior author Jack Yoh, professor of mechanical and aerospace engineering, and colleagues.

Although various techniques have been used to design new ways to make injections painless, for ease of use, control, and precision, the hypodermic needle is still the instrument of choice.

Other researchers have tried to develop similar microjet systems as that devised by Yoh and colleagues, but they are invariably mechanically powered, using piston-based methods to drive the drugs into the skin. However, those types don’t allow enough control over the jet strength and the dose, says Yoh in a press statement.

“The laser-driven microjet injector can precisely control dose and the depth of drug penetration underneath the skin. Control via laser power is the major advancement over other devices, I believe,” he explains.

The type of laser in their device is commonly used by dermatologists, particularly for facial cosmetic treatments.

Behind the miniature jet nozzle is a small chamber containing a liquid form of the drug to be injected, and behind that is another for the “driving” fluid, in this case, water. A flexible membrane keeps the two liquids separate.

A series of very short laser pulses, lasting no more than 250 millionths of a second each, generates a vapor bubble inside the driving fluid. The bubble creates a pressure or elastic strain on the membrane, which forces the drug to be ejected through the tiny nozzle as a narrow jet no more than 150 micrometers (millionths of a meter) wide, or slightly thicker than a human hair.

Yoh explains that the jet pressure is higher than the tensile strength of skin, so it penetrates smoothly into the targeted depth underneath, causing no splashback.

The team has tested the device on guinea pig skin. This showed the jet drives the drug up to several millimeters under the skin, without damaging surrounding tissue.

The speed and narrowness of the jet should be enough to make the procedure painless, says Yoh. But just the fact they are aiming for the epidermal layer just under the surface of the skin, about 500 micrometers down, where there are no nerve endings, should already ensure it is “completely pain-free”.

The team has been perfecting their new device for a while. For instance, in previous versions, the laser wavelength was not quite right in that it was not well absorbed in the driving liquid, and did not produce a good vapor bubble capable of delivering the right amount of elastic strain to the membrane.

In this latest version, described in the study, the team used a laser with a wavelength of 2,940 nanometers. This is well absorbed by the driving liquid, in this case water, forms a larger and more stable vapor bubble, which results in a higher pressure on the membrane.

“This is ideal for creating the jet and significantly improves skin penetration,” says Yoh.

A company is now working with the team to make low-cost replaceable micro-injectors for clinical use.

They hope soon to be able to use the technology in settings where small doses of drugs are injected at multiple sites.

Yoh says more work will be needed before the device can be used in settings like mass vaccination of children.