A new laser-based injection system that blasts microscopic jets of drugs into the skin could soon make getting a shot as painless as being hit with a puff of air.
The system uses an erbium-doped yttrium aluminium garnet, or Er:YAG, laser to propel a tiny, precise stream of medicine with just the right amount of force. The laser is combined with a small adaptor that contains the drug to be delivered, in liquid form, plus a chamber containing water that acts as a ‘driving’ fluid. A flexible membrane separates these two liquids. Each laser pulse, which lasts just 250 millionths of a second, generates a vapour bubble inside the driving fluid. The pressure of that bubble puts elastic strain on the membrane, causing the drug to be forcefully ejected from a miniature nozzle in a narrow jet a mere 150μm in diameter, just a little larger than the width of a human hair.
‘The impacting jet pressure is higher than the skin tensile strength and thus causes the jet to penetrate smoothly into the targeted depth underneath the skin, without any splashback of the drug,’ says Jack Yoh, professor of mechanical and aerospace engineering at Seoul National University in South Korea, who developed the device along with his graduate students.
Though various techniques have been developed in hopes of taking the pain out of injections, hypodermic needles are still the first choice for ease of use, precision, and control. Although other research groups have developed similar injectors they are mechanically driven using piston-like devices to force drugs into the skin, which gives less control over the jet strength and the drug dosage. Yoh says: ‘The laser-driven microjet injector can precisely control the dose and the depth of drug penetration underneath the skin. Control via laser power is the major advancement over other devices, I believe.’
Tests on guinea pig skin show that the drug-laden jet can penetrate up to several millimetres beneath the skin surface, with no damage to the tissue. Because of the narrowness and speed of the jet, it should cause little or no pain. Yoh says: ‘However, our aim is the epidermal layer,’ which is located closer to the skin surface, at a depth of only about 500μm. This region of the skin has no nerve endings, so the method ‘will be completely pain-free,’ he says.
Yoh’s findings were published in the Optical Society’s peer-reviewed journal Optics Letters, volume 37, issue 18. In Yoh’s previous studies, his researchers have used a laser wavelength that was not well absorbed by the water of the driving liquid, causing the formation of tiny shock waves that dissipated energy and hampered the formation of the vapour bubble. In the new work, Yoh and colleagues use a laser with a wavelength of 2,940nm, which is readily absorbed by water. This allows the formation of a larger and more stable vapour bubble.
That then induces higher pressure on the membrane; that is ideal for creating the jet and significantly improves skin penetration.
Yoh is now working with a company to produce low-cost replaceable injectors for clinical use. ‘In the immediate future, this technology could be most easily adopted to situations where small doses of drugs are injected at multiple sites,’ he says. ‘Further work would be necessary to adopt it for scenarios like mass vaccine injections for children.’
