A hypodermic needle is a hollow needle commonly used with a syringe to inject substances into the body. They may also be used to take liquid samples from the body, for example taking blood from a vein in venipuncture.
A hypodermic needle is used when the substance would not be reliably absorbed by the digestive system, as is the case with insulin and many other drugs or if the substance needs to be instantly delivered into the body. The substance can be injected just under the skin (subcutaneous injection), into a muscle (intramuscular injection), or directly into the bloodstream (intravenous injection). Less commonly, hypodermic needles are used for injection into joints (intra-articular injection), cerebrospinal fluid (spinal injection) or into the skin itself (intradermal injection).
Virtually all current hypodermic needles and their associated syringes are designed for single use because they are hard to decontaminate and require sharpening after repeat use. Re-using or sharing needles can transmit many blood-borne diseases including AIDS and hepatitis C. Needles are normally used only once and disposed of in a sharps container.
Normally made of stainless steel, the end of the needle is bevelled to create a sharp pointed tip. This allows the needle to easily penetrate the skin. When a hypodermic needle is inserted, the bevel should be facing upwards.
The size of a needle is commonly stated as a gauge relating to the outside diameter of the needle. Using the Stubs needle gauge, these range from about 7 gauge (the largest) to 33 (the smallest). Twenty-one gauge needles are most commonly used for taking blood. The length of the needle is stated on the packaging and the gauge is often indicated by the colour of the plastic connector (called a luer).
There are a number of systems for gauging needles, including the Stubs Needle Gauge, and the French Catheter Scale.
It is estimated that about 10% of the population may have a phobia of needles.
- How do they get the hole through a hypodermic needle? (http://www.straightdope.com/mailbag/msyringe.html) at The Straight Dope.