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Injecting a stream of water into a drop of liquid might sound like fun, but if done accurately and properly understood, the splash exercise could help scientists identify ways to inject fluids such as vaccines through. the skin without using needles.
This is the motivation behind a new study by engineers at MIT and the University of Twente in the Netherlands. The study involves shooting small jets of water through many types of droplets, hundreds of times, using high-speed cameras to capture each aqueous impact. The team videos are reminiscent of the famous strobe light photographs of a bullet piercing an apple, launched by Harold “Doc” Edgerton of MIT.
Edgerton’s footage captured sequential footage of a bullet fired through an apple, in explosive detail. The MIT team’s new videos of a jet of water fired through a droplet reveal a surprisingly similar dynamic of impact. Because the droplets in their experiments are transparent, the researchers were also able to track what happens inside a droplet when a jet is shot through.
Based on their experiments, the researchers developed a model that predicts the impact of a fluid jet on a droplet of a certain viscosity and elasticity. Since human skin is also a viscoelastic material, they say the model can be adjusted to predict how fluids might be delivered through the skin without the use of needles.
“We want to explore how needle-free injection can be done in a way that minimizes damage to the skin,” says David Fernandez Rivas, MIT affiliate researcher and professor at the University of Twente. “With these experiments, we get all this knowledge, to explain how we can create jets with the right speed and the right shape to inject into the skin.”
Rivas and colleagues, including Ian Hunter, George N. Hatsopoulos Professor of Thermodynamics at MIT, published their findings in the journal Soft matter.
Penetrating pores
Current needleless injection systems use a variety of means to propel drug at high speed through the natural pores of the skin. For example, the spin-out of MIT Portal Instruments, from Hunter’s group, focuses on a design that uses an electromagnetic actuator to eject thin jets of medication through a nozzle at speeds high enough to penetrate through the skin. and in the underlying muscle.
Hunter is working with Rivas on a separate needleless injection system to deliver smaller volumes in shallower layers of skin, similar to the depths to which tattoos are inked.
“This regimen poses different challenges but also offers opportunities for personalized medicine,” says Rivas, who says drugs such as insulin and some vaccines can be effective when given in smaller doses to the surface layers of the body. skin.
Rivas’ design uses a low-power laser to heat a fluid-filled microfluidic chip. Similar to boiling a kettle of water, the laser creates a bubble in the fluid that pushes the liquid through the chip and through a nozzle, at high speed.
Rivas has previously used clear gelatin as a substitute for the skin, to identify the velocities and volumes of fluid that the system could effectively deliver. But he soon realized that the rubbery material is difficult to reproduce accurately.
“Even in the same lab and following the same recipes, you can have variations in your recipe, so if you are trying to find the critical stress or the speed that your spray needs to have to go through the skin, sometimes you have problems. values one or two magnitudes apart, says Rivas.
Beyond the ball
The team decided to study in detail a simpler injection scenario: a jet of water, shot into a drop of water in suspension. The properties of water are better known and can be better calibrated than those of gelatin.
In the new study, the team set up a laser-based microfluidic system and fired thin jets of water at a single drop of water, or “pendant,” suspended from a vertical syringe. They varied the viscosity of each pendant by adding certain additives to make it as thin as water, or thick as honey. They then recorded each experiment with high speed cameras.
By playing the videos at 50,000 frames per second, the researchers were able to measure the speed and size of the jet of liquid that pierced and sometimes passed directly through the pendant. The experiments revealed some interesting phenomena, such as cases where a jet was drawn back into a pendant, due to the pendant’s viscoelasticity. Sometimes the jet also generated air bubbles when it pierced the pendant.
“Understanding these phenomena is important because if we inject into the skin in this way, we want to avoid, for example, introducing air bubbles into the body,” says Rivas.
The researchers sought to develop a model to predict the phenomena they saw in the laboratory. They were inspired by Edgerton’s bullet-pierced apples, which looked similar, at least outwardly, to the squad’s jet-pierced droplets.
They started with a simple equation to describe the energetics of a bullet fired through an apple, adapting the equation to a fluid scenario, for example by incorporating the effect of surface tension, which has no effect. in a solid like an apple but is the main force that can prevent a fluid from breaking. They worked assuming that, like a bullet, the jet fired would retain a cylindrical shape. They found this simple pattern to approximate the dynamics they observed in their experiments.
But the videos clearly showed that the shape of the jet, as it entered a pendant, was more complex than a simple cylinder. So the researchers developed a second model, based on an equation known to physicist Lord Rayleigh, which describes how the shape of a cavity changes as it moves through a liquid. They modified the equation to apply it to a jet of liquid moving through a droplet of liquid and found that this second model produced a more accurate representation of what they had observed.
The team plans to carry out other experiments, using pendants with properties even closer to those of the skin. The results of these experiments could help refine models to refine optimal conditions for injecting drugs, or even inking tattoos, without the use of needles.
This research was funded in part by the European Research Council as part of the European Union’s Horizon 2020 research and innovation program.
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