The Shape of an IV Tube Matters More Than You Might Think

The shape of an IV tube matters more than you might think
January 12, 2017

When delivering IV medicine or precisely timing a chemical reaction, controlling the speed at which a chemical enters the system is of paramount importance. Now new research from UNC Chapel Hill shows that controlling those chemicals is more about the shape of the injecting tube than the chemicals themselves.

Roberto Camassa and Richard McLaughlin, both professors in UNC’s mathematics department found that the aspect ratio—the ratio of the width to the height—of a tube controls how quickly a dissolved chemical will reach its target. They published their findings in the journal Science.

DropletTubes with a circular cross section deliver dissolved chemicals slowly at first and then pile up more and more over time. Tubes with a more elliptical or egg-shaped cross section start off with a strong punch of the dissolved chemical before tapering off over time. The more elliptical the tube, the stronger the initial punch and the more quickly the chemical tapers off. The researchers used mathematical calculations, computer models and lab experiments to confirm these findings. 

You may be wondering why such a seemingly simple finding made its way to a major journal like Science, but it is precisely because of its simplicity that this research is important.

The story of its significance begins with the fact that there's more to chemistry than stirring two liquids together to make the compound you want. Think about making a pot of chicken noodle soup—which actually involves a ton of chemistry. You need to heat water, adding chicken bones, to make your stock; you remove the chicken bones once the stock is ready. Then you add exactly the right amount of vegetables and spices to make the soup taste just right. The noodles have to cook for a specific amount of time to make sure they are neither too hard nor too mushy, and the chicken needs to cook outside the pot first and go into the soup right at the end, lest it get too chewy.

All of that temperature control, precise timing and purification is difficult enough to do in a kitchen or on a lab bench. But in the human body, or in a factory doing hyper-controlled chemistry on a large scale, chemists rely on the help of engineers to keep the chemistry running smoothly.

A major component of chemical engineer's job involves fluid dynamics, the science of how liquids and gases flow and mix with each other. Fluid dynamics are notoriously tricky, and can make even simple actions tricky to predict.

Laminar Flow and Taylor DispersionTake, for instance, the problem that Camassa and McLaughlin observed within this study. Imagine you have a tube filled with water and you gently inject red dye into one point in the tube so that if you looked at it from the side, you would see a red line. If the water started to flow, you might expect that line to move in a relatively uniform manner. It might spread out a little, as dye does in water, but you might think a reasonably well-defined block of red would move through the tube.

This assumption would be incorrect. Fluids without too much pressure on them exhibit something called laminar flow, which means that the water flows most quickly at the center of the tube and gradually slows as you move outward and does not move at all at the tube wall. So instead of a block of red dye, you would see more of a bullet, gradually spreading out over time.

But even that is not the whole story, because while the water flows through the tube, the dye also diffuses outward, turning the bullet into more of a knuckle in a process called Taylor dispersion. Shapes change depending on how quickly the water is moving and any change in shape affects how quickly the dye, or any other chemical reaches its target. Challenges like these are just the tip of the iceberg in fluid dynamics and chemical engineers have to account for all of it when designing systems to do the chemistry they want.

That is why this study is such a landmark. By changing something as simple as the shape of the tube, chemical and biomedical engineers can exercise precise control over when their chemicals reach a target. Instead of worrying about many variables, they can simplify to just the shape of the tube.

This technology could allow for specifically timed delivery of chemotherapy or other types of drugs and simple quality control measures in chemical manufacturing. Further, the shape effect works just as well for square and rectangular tubes as it does for circular and elliptical ones, respectively. This is important because rectangular tubes are much cheaper to make than elliptical ones.

Carrying out precise chemistry on very large or very small scales is extremely difficult, but given this research, it could be easier to control.

—Daniel Lane

Daniel Lane covers science, medicine, engineering and the environment in North Carolina. 

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