Troubleshooting

1. Solution clarity and complete dissolution

If a reconstituted solution contains even microscopic undissolved particles, they can partially restrict the needle lumen and increase back-pressure.

Increased resistance during delivery may:

• Make the injection feel firm
• Extend equalization time
• Contribute to minor post-withdrawal dripping

Best practice:

• Gently swirl — never shake.
• Confirm solution is fully clear before loading.
• Do not use solutions containing visible particulate.

2. Viscosity and temperature

Viscosity describes a fluid's resistance to flow.

• Higher viscosity = greater flow resistance.
• Lower temperature = increased viscosity.

Refrigerated solutions are naturally thicker and may require longer equalization time.

3. Needle gauge and solution compatibility

All standard pen-tip needle gauges are mechanically compatible with the Pen-Depot device. However, performance is influenced by the interaction between the needle and the solution being delivered.

Flow characteristics depend on:

• Needle internal diameter
• Solution viscosity
• Reconstitution volume
• Temperature

For example:

• 33G = very fine internal diameter, slower flow.
• 29–30G = slightly wider internal diameter, improved flow.

Thicker or colder solutions may perform more consistently with a slightly wider internal diameter. Selecting the appropriate gauge improves delivery smoothness, reduces resistance, and minimizes post-dose equalization drip.

See the Pen Tips Guide for full details on choosing needles.

4. Cartridge compatibility and alignment

Pen-Depot Research Injection Pens are engineered specifically for:

• 3 mL cartridges
• 11 mm long-stopper geometry
• Defined glass wall tolerances

Using off-spec, prefilled, or non-standard cartridges may introduce:

• Plunger drag
• Uneven pressure
• Mechanical misalignment
• Increased resistance

Cartridge geometry and stopper length directly influence drive alignment and pressure distribution. Always verify cartridge specifications prior to use.

5. Air and internal pressure stabilization (priming)

Air pockets within the cartridge can compress and expand during delivery, affecting pressure stability.

Before each use:

• Dial a small increment.
• Hold the pen upright with needle facing upward.
• Gently tap to move air to the top.
• Press until a small drop appears.

This removes trapped air and stabilizes internal pressure for smooth, accurate delivery.

6. Equalization timing

After the plunger stops advancing, internal pressure may still be normalizing.

Recommended hold time after dose completion:

• Low viscosity: 8–10 seconds
• Moderate viscosity: 10–12 seconds
• Higher viscosity or fine gauge: 12–15 seconds

Maintain light forward pressure while withdrawing. A small drop (one to two drops) after removal can be normal pressure equalization behavior, particularly when using finer-gauge needles or higher-viscosity solutions.

7. Needle construction and flow efficiency

Not all pen-tip needles perform identically. Variations in wall thickness, internal coating, bevel geometry, and hub precision can significantly influence flow rate and resistance.

Thin-wall, silicone-coated, multi-bevel pen needles reduce resistance and improve equalization efficiency. Always replace the needle after each use to maintain sharpness and prevent residue buildup.