What Is a Hypodermic Needle?
A hypodermic needle is a hollow needle used to deliver substances into the body or withdraw fluids. The needle passes through the skin, while the syringe attached to it holds the fluid and moves it through the needle using a plunger.
Medical professionals use hypodermic needles to take blood samples, administer medications, give vaccines and more.
“You'll only feel a slight pinch,” says the nurse as she brings the needle towards your skin. You see her insert the needle and push the end of the device. Suddenly, it's all over.
If you've ever gone through an experience like this, you've likely come into contact with a hypodermic needle. But how does it actually work? Keep reading to explore the needle, the syringe and the parts that make them work together.
What Does Hypodermic Mean?
The term hypodermic comes from two Greek words: hypo, meaning “under”, and derma, meaning “skin”. Together, they mean “under the skin”.
Needle vs Syringe: What's the Difference?
The needle and syringe are connected, but they aren't the same part. The needle is the hollow component that enters the skin. The syringe consists of a barrel and plunger, with a separate plunger seal in three-part designs.
You may hear the complete assembly called a hypodermic syringe. Some syringes have a fixed needle, while others accept a separate, detachable needle.
Why Do Medical Professionals Use Hypodermic Needles?
Injections are one way to deliver medication. Other routes include oral delivery and delivery through the skin, known as transdermal delivery.
An injection bypasses the digestive system, so the medication does not pass through the stomach. How quickly it reaches the bloodstream and starts working depends on the medication and the injection route; delivery under the skin is not always faster than other methods.
What Are the Parts of a Syringe?
Medical syringes are commonly described as two-part or three-part designs. This count does not include the needle, whether it is supplied separately or already attached.
Two-Part Syringes: Barrel and Plunger
A two-part syringe has two main components:
- Barrel: the main tube that holds the liquid being collected or administered.
- Plunger: the moving part that fits inside the barrel and draws fluid in or pushes it out.
The plunger fits against the inside of the barrel to form a seal. Pulling it back draws fluid into the barrel; pushing it forwards expels fluid through the syringe tip.
Three-Part Syringes: An Additional Seal
A three-part syringe also has a barrel and a plunger, but includes a separate rubber or synthetic rubber seal at the end of the plunger inside the barrel.
Sometimes described as a cap or plunger head, this component forms the seal between the plunger and the barrel to help prevent liquid passing around it.
How Does the Needle Attach to the Syringe?
Now that you understand the syringe, it's time to look at the connection. Two common fittings for detachable needles are:
- Luer lock: a threaded fitting that secures the needle with a twisting action.
- Luer slip: a tapered fitting onto which the needle hub is pushed, with friction holding the connection together.
With fixed needle syringes, the needle is already attached, so neither connection method is needed.
Luer Lock Fittings
A Luer lock connection uses a thread to secure the needle hub. The connection must be assembled according to the manufacturer's instructions to reduce the risk of leakage or disconnection.
Luer Slip Fittings
A Luer slip connection relies on the matching tapers of the syringe tip and needle hub. Depending on the manufacturer's instructions, a slight twisting motion may be used when fitting the needle.
Luer tapers are covered by international standards, but that does not mean every needle fits every syringe. Check the specified connection and compatibility rather than assuming that any two products will work together.
Luer slip and Luer lock are different fittings and should not be described interchangeably.
Hypodermic Needle Sizes and Gauges
Needle gauge describes the needle's outside diameter, rather than its circumference. The higher the gauge number, the finer the needle.
Injection needles are available in gauges such as 32G through to 16G, with sizes outside that range also available. Gauge and length are separate measurements: gauge tells you how thick the needle is, while length tells you how long it is.
For example, short 4–12 mm needles are grouped by length, not by gauge alone.
Needle Gauge Colours
Colour coding helps identify needle gauge. Examples include:
- 33G = Black
- 32G = Dark green
- 31G = White
- 30G = Yellow
- 29G = Red
- 28G = Blue-green
- 27G = Grey
- 26G = Brown
- 25G = Orange
- 23G = Dark blue
- 22G = Black
- 21G = Dark green
- 20G = Yellow
- 19G = Cream
- 18G = Pink
- 16G = White
Needle dimensions and colour coding are covered by international standards. However, colours repeat across some gauges, so always read the gauge and length on the product label rather than relying on colour alone.
What Is the Needle Bore?
The bore is the hollow passage down the centre of the needle. Fluid travels through this passage into or out of the syringe.
In general, needles with a larger outside diameter can have a larger bore. However, the bore also depends on the thickness of the needle wall, so gauge alone does not tell you the exact internal diameter.
If the bore were too large for a particular needle design, its outer walls could be too thin, compromising the needle's strength.
What Are Hypodermic Needles and Syringes Made From?
Different designs use different materials. Hypodermic needle tubes are typically made from stainless steel. Syringe barrels may be made from plastic or glass, while many disposable syringes have plastic plungers and, in three-part designs, synthetic rubber seals.
Materials used in medical needles and syringes must be suitable for their intended contact with the body and the substances they deliver. Biocompatibility concerns how a material interacts with the body; it does not guarantee that a reaction can never occur.
Check the manufacturer's information for the exact materials, sterility and intended use of a particular product. A disposable, single-use needle or syringe should not be cleaned for reuse.
How Do Manufacturers Check Needle Quality?
Needle and syringe components are produced in large quantities, so quality checks are important for identifying defects that could affect their performance.
Checks during manufacture can include:
- Visual inspection for deformed or incorrect parts.
- Measurement of component length, width and thickness.
- Checks that the correct pieces have been assembled.
- Inspection of packaging for errors or defects.
Manufacturers use these checks as part of their quality processes to assess conformity with applicable standards, regulatory requirements and their own specifications. Visual and dimensional checks alone do not establish sterility or guarantee that a product is defect-free.
How Does a Hypodermic Needle Work?
The needle provides a hollow pathway, while the syringe plunger moves fluid along it.
Drawing Fluid into the Syringe
When a medical professional pulls back the plunger, fluid can travel through the hollow needle and into the barrel. Keeping the needle opening within the fluid being drawn helps avoid drawing air through that opening.
Graduation marks on the side of the barrel show the volume collected and help the professional judge when the required amount has been drawn.
Delivering Fluid from the Syringe
When the plunger is pushed forwards, it displaces the fluid in the barrel, moving it through the needle. For an injection, a trained professional selects the appropriate site and technique for the medication and route.
Buying Hypodermic Needles and Syringes
Hypodermic needles are essential equipment for medical professionals and for some people managing chronic conditions under clinical guidance.
Before choosing a product, check:
- The required needle gauge and length.
- The syringe capacity and graduation markings.
- Whether the needle is fixed or detachable.
- The connection type and stated compatibility.
- The manufacturer's intended use and instructions.
If you're looking for both components together, explore our needle and syringe kits and check each product's details against your requirements. If you're unsure which size or fitting you need for a prescribed treatment, ask your healthcare professional.