A syringe is composed of a hollow barrel, a tight-fitting plunger with a rubber stopper, and a needle or nozzle. When the mechanism fails, the issue typically stems from a physical fault, an error in technique, or a characteristic of the fluid being handled.
Physical Damage and Internal Obstruction
A common problem is a plunger that sticks or jams, often due to dried medication or residue remaining inside the barrel from a previous use. This residue increases friction, causing the smooth, controlled motion to become jerky or completely seized. A manufacturing defect, such as a slightly bent plunger shaft or a lack of proper lubrication, can also cause resistance, leading to a pulsed delivery or making the syringe feel stuck.
Another hardware failure involves the needle becoming clogged, which prevents the fluid from being drawn or expelled. The narrow bore of the needle can be blocked by particulate matter suspended in the fluid, or by medication that has crystallized or solidified inside the tip.
Cracks or leaks in the barrel, particularly near the hub where the needle attaches, can compromise the necessary airtight vacuum. Worn seals around the plunger can also cause this issue. This results in inaccurate dosing or leakage because the syringe cannot pull the fluid effectively.
Errors in Drawing Technique
One frequent error is pulling the plunger too quickly, which can create a vacuum lock or airlock within the barrel. This rapid action causes the internal pressure to drop too suddenly, making the plunger resist further movement and effectively stalling the drawing process.
Improper positioning of the needle is another source of technical failure, especially when drawing from a fluid container like a vial. If the needle tip is not kept fully submerged in the liquid, air bubbles can be drawn into the syringe instead of the intended fluid. These air bubbles can then disrupt the flow and lead to inconsistent dispensing.
If the plunger is not pulled straight, it can bind against the barrel wall, causing significant resistance that the user mistakes for a stuck syringe. Priming the syringe is also a foundational technique that prevents issues. This process involves drawing air equal to the desired fluid volume into the syringe and injecting it into the vial to equalize the pressure.
After drawing, holding the syringe upright and gently tapping the barrel helps to dislodge any trapped air bubbles. These bubbles must then be expelled before use to ensure accurate volume measurement.
Problems Related to Fluid Dynamics
The internal resistance of a liquid to flow is known as viscosity. High-viscosity fluids, or “thicker” liquids, require significantly more force to draw into the barrel. Attempting to draw a viscous fluid too quickly can put excessive strain on the plunger and lead to inconsistent flow rates.
A practical solution for handling viscous fluids is to draw the fluid much more slowly, allowing the internal friction to be overcome gradually. If permitted, switching to a larger gauge needle, which has a wider diameter, can also reduce the resistance and ease the drawing process.
The temperature of the fluid dramatically impacts its flow characteristics, as cold temperatures cause viscosity to increase, making the fluid thicker and harder to move. Certain components in the fluid may also precipitate or crystallize at cold temperatures, physically blocking the needle bore.
If the fluid source contains large, poorly mixed air pockets, these bubbles may be drawn in, preventing proper liquid uptake and leading to an interrupted flow. For temperature-sensitive fluids, ensuring they are at room temperature or slightly warmer can reduce viscosity and facilitate a smoother draw.