A Staff Report from the Straight Dope Science Advisory Board

How do they get the hole through a hypodermic needle?

March 29, 2005

Dear Straight Dope:

A friend of mine (really) wants to know how they can make those tiny needles. He has a point (so to speak); how can they be so small yet the stuff in the syringe still gets through. Any ideas?

Syringes are a special interest of mine. In addition to having an engineer's interest in such things, I've been diabetic for some time and have given myself perhaps 30,000 or so injections. So I venture to say I know whereof I speak. 

The key to syringe making is forming the hollow tube, or cannula. Processes to make small tubes and hollow needles are quite old, and almost always begin by forming a large tube. This large tube is formed by either rolling a sheet of metal into a tube and welding the seam, or by taking a solid billet of metal and boring a hole through the center while the metal is heated (creating "seamless tubing").

This large tube is softened by heating it (called annealing), then drawn through a tool called a die--which in this application of the word is essentially a hardened piece of metal with a small hole. As the tube is drawn through the die it both stretches, increasing the tube length, and shrinks, decreasing the tube diameter. The tube is passed through smaller and smaller dies, continuing to stretch in length and shrink in diameter, until the desired size is reached. The last drawing through the die is often done without heat, therefore cold-working the tube to increase its strength and hardness. Sometimes a stiff piece of wire or a mandrel is placed inside the tube to prevent the walls from collapsing while it is being drawn, but often the process relies on incredibly consistent steel quality and high-tolerance equipment to manufacture cannulae that meet the specifications.

Becton Dickinson, one of the largest manufacturers of syringes in the world, says it makes syringes using 3/4-inch-wide stainless steel rolled and welded into a 1/4-inch diameter tube, then heated, drawn, and finally cold-drawn to its final size. Some of those sizes are pretty small. For typical insulin syringes, popular sizes include:

Needle gaugeOutside diameter, inchesInside diameter, inches Nominal wall thickness, inches
270.0160 - 0.01650.0075 - 0.00950.004
280.0140 - 0.01450.0065 - 0.00800.0035
290.0130 - 0.01350.0065 - 0.00800.003
300.0120 - 0.01250.0055 - 0.00700.003
320.0090 - 0.00950.0035 - 0.00500.002

The tubes are straightened, cut to the proper length, cleaned, and then the tip or bevel is cut onto the end. The piercing end of the bevel, right on the very tip, is called the lancet or as I like to call it, the "&$#%@*"! Many different styles of lancet can be cut; which one is selected depends upon the application--to be blunt (no pun intended), what portion of your body it's designed to poke holes into. Contrary to popular belief, sharper is not always better--medical professionals rely on the slight resistance of the needle through the skin to ensure that it penetrates to the correct depth and doesn't go bouncing off a bone, into your brain, etc.

As to your last question, the simple answer is that the "stuff" is able to get through because it is small enough to get through. Needles certainly must be matched to the application--try pushing honey through a 32-gauge needle, and you're going to need a lot of force if you can do it at all. This is one of several reasons why most injectable drugs have a recommended needle and syringe size or size range.

References

Avitzur, B., Tubing and Tubing Products, Handbook of Metal-Forming Processes, 1983.

Siekman, Philip, "Becton Dickinson Takes a Plunge with Safer Needles," Fortune, Sept. 19, 2001

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