Physics: Principles and Problems

 

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An Injection You Can't Feel
March 2004

Everybody has to go to the doctor from time to time. Maybe you need a shot to keep you from getting the flu; maybe you need a booster for your vaccines. If you step on a rusty nail, a quick injection will prevent you from contracting tetanus. Nobody likes the “ouch” of hypodermic needles, but they are an invaluable tool in modern medicine.

Think for a second about that last example, though. Which has more “ouch”--the tetanus shot or the rusty nail? Although we all complain about having a needle poked through our skin, it sure beats having a nail driven in. That seems obvious . . . but why should a nail hurt more? The simple answer is nerves.

Building a Smaller Needle

You don’t actually feel your skin, you feel the nerves in your skin. The needle is smaller than the nail and thus activates fewer nerves. It follows that an even smaller needle would hurt still less. If modern crafting techniques could make a needle small enough, might it not hurt at all? Might you not even feel it?

As it turns out, the answer is yes--in principle. Human skin could be pierced without generating any sense of hurt because nerve endings don’t cover one hundred percent of our surface. There are enough of them to allow us to feel our environment, but from a microscopic perspective, there is still room left over. An extremely small needle might be inserted between the endings, something like the way you could thread a thin, straight wire through a screen.

No one has ever been able to precision-craft so small a device, however; but researchers think they are finally on the verge of it. They’re working on building an ouchless system of injection, called microneedles.

Microneedles?

At the Georgia Institute of Technology in Atlanta, Mark Prausnitz and his coworkers have built a device for micro-injection. It’s a patch only about the size of your thumbnail, but contained on its surface are tiny pointed tips. Each is no longer than the dot at the bottom of this exclamation point! Look carefully at that dot for a moment and imagine being afraid of it. You can’t do it.

But how could such a super-tiny needle inject anything useful into your body? By itself, it couldn’t; but Prausnitz and his team have developed silicon, metal and glass patches that contain upward of a thousand micro needles each. At a microscopic level, having such a patch on your skin is like rolling in a cactus bed; you are pierced over and over again. At the macroscopic level where we live, though, you don’t feel a thing.


Less Pain, More Control

The microneedles could be coated with a drug, so that placing the patch on your skin would in itself be a small injection. Another idea is to have an overlying patch that is filled with medication slowly seep through hollow microneedles, allowing a “drain” of medication into the body.

This is good news for anybody who hates that occasional needle stick at the doctor’s office. There are, however, more significant applications than that. A once-a-year flu shot doesn’t really hurt that much, but some people--say, folks with diabetes--need to give themselves injections on a daily basis. Microneedle technology could save these people a lot of discomfort, as well as wear and tear on their over-poked veins.

Moreover, researchers are looking into techniques for using microneedles to inject a small amounts of drugs at steady rates. Since you feel no pain, you could “wear” a microneedle patch for hours and receive small doses of medication in a controlled way, rather than the all-or-nothing approach of those big, ouchy hypodermics.


Activity:

Using your imagination, make a list of future tools that could be improved if they were much smaller. For example, would a tiny scalpel help a surgeon or not? Would a tiny pair of pliers help an electrician? How about a tiny fish hook? If it seems that making something smaller wouldn’t make it any better, can you imagine ways in which making it smaller might change the way it is used?

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Physics: Principles and Problems