Thursday, May 9, 2013

How Sharp is a Hypodermic Needle?


This week I had a group of students from a local high school visit the Microscopy Lab here at Eastfield College.  One of the students, who has a parent who is a physician, brought in some unopened hypodermic needles. Not being one to pass up an opportunity to use our scanning electron microscope I thought I would take a look.

I imaged two different sizes - a 19 gauge and 25 gauge needle.  It is going to seem weird to some of you (including me) but the lower the gauge the bigger the needle.  The 19 gauge needle has a surprisingly large bore needle.  It looks like the ones they use when I donate blood and is larger than one that would be used for a regular intramuscular injection, which is the 25 gauge needle. 

This side by side comparison will show you the apparent differences in the size of the two needles.

[20x] - 19 gauge 
Here is a look at the business end of the larger needle.  I strongly suspect that the specks on it are entirely my fault.  I photographed it and had it in the lab for a while before I got it in the SEM.  I am sure it was perfectly sterile when I first opened it.  Obviously, I do not run my lab under "clean room" conditions. 

[55x] - 19 gauge 
Two things show up in this image that I find interesting.  First of all, you can see that the point is made by two separate cuts.  The cut that makes the tip is more acute than the one above it. Second, you will see that the interior of the needle has a pretty rough texture.
Here is the tip of the 19 gauge needle.
The tip of the 19 gauge needle
At 1,600x the tip of this needle looks like a jagged piece of shrapnel.  But take a look at the scale at the bottom of the image.  The entire length of the scale is 30 microns which means the distance between marks is only 3 microns - that is 3 millionths of a meter.  Bottom line, this is really sharp.
Here is the inside of the barrel of the needle.
Same spot - higher magnification.

Same spot again.
Above is the smaller 25 g needle.  The base of the needle is cut at a steeper angle which would make it sharper.  You can see that the tip is also made with two cuts at different angles.

Here is the tip of the 25 g needle.  Looks kind of blunt but it really isn't on the macroscopic scale.

Same image but with a direct measurement.  Both of the lab's Hitachi SEMs are calibrated to make measurements on images.  Pretty cool.

I noticed that the top of the bevel was not a nice oval shape.  Lets look a little closer.

You can see that a thin layer of metal has been folded back during the manufacturing process. No worries - this is so small that you wouldn't feel the difference.

In this image you are looking straight into the sharp end of the 25g needle.  The tip is a little to the left of bottom center.  You can also see the two bevel cuts that make up the point.  I added the measurement to show you what a neat job of engineering this is.  That hole down the middle is 1/64th of an inch in diameter!!
Here is some not so pretty engineering.  To get these needles into my SEM I needed to cut them off.  I didn't have any wire nippers so I used a pair of pliers and simply bent the metal back and forth until it broke.  You are looking at metal fatigue.
The images made by Eastfield College Microscopy Lab may be used, saved, uploaded, modified, or linked just as as long as you give credit to Eastfield college.  The only restriction is that they may not be sold.

Murry Gans
Eastfield College
Mesquite, TX


  1. I can slide one through your skin and you won't even feel it!!! LOL!

  2. I would be interested to see single to lightly used 30 or 31 gauge needles compared with unused ones to see normal wear and tear to use as visual tools to why disposables should only be used once.

  3. This is super cool. Great work. Can you show the 25G needle in profile, the orthogonal view of the 57um tip dimension you post?