A hypodermic (hypo – under, dermic – the skin) needle is a hollow needle commonly used with a syringe to inject substances into the body or extract fluids from it. They may also be used to take liquid samples from the body, for example taking blood from a vein in venipuncture. Large-bore hypodermic intervention is especially useful in treating catastrophic blood loss or shock.
A hypodermic needle also provides for rapid delivery of liquids. It is also used when the injected substance cannot be ingested orally, either because it would not be absorbed, as with insulin, or because it would harm the liver.
Hypodermic needles also serve important roles in research requiring sterile conditions. The hypodermic needle significantly reduces contamination during inoculation of a sterile substrate in two ways. First, its surface is extremely smooth, preventing airborne pathogens from becoming trapped between irregularities on the needle’s surface, which could subsequently be transferred into the media as contaminants. Second, the needle’s point is extremely sharp, significantly reducing the diameter of the hole remaining after puncturing the membrane, which consequently prevents microbes larger than the hole from contaminating the substrate.