Most people never think about the cartridge until one leaks or cracks. But that small glass tube is the part that holds everything, and it’s built to a tighter spec than the pen around it. Here’s what you’re actually holding.
Four parts, one job
A standard 3 mL pen cartridge has four pieces:
- The glass barrel. The clear tube that holds the solution.
- The plunger (stopper). The rubber piece at the back end. Your pen’s plunger rod pushes on it.
- The septum. A thin rubber disc at the front that the pen needle pierces.
- The crimp cap. The aluminum ring that locks the septum to the glass.
Cartridge patents describe it the same way: the back end is sealed by a piston of elastomeric material, and the front has a pierceable septum held in place by a cap. When you screw on a pen needle, the back end of the needle goes through that septum. Nothing else in the pen touches the solution.
The size is standardized
Cartridges are made to an international standard, ISO 13926. Part 1 covers the glass cylinders, Part 2 the plunger stoppers and Part 3 the seals. The first part was updated in August 2026.
That standard is why a 3 mL cartridge from one major glass maker fits a pen designed around another’s. SCHOTT, one of the largest cartridge makers, lists its 3 mL cartridge at 11.6 mm outside diameter, 9.7 mm inside diameter and 62.3 mm long. Gerresheimer, another major supplier, says its pen cartridges are designed according to ISO 13926-1 and come in a 3.0 mL size.
It’s also why every PepPen takes the same cartridge. We don’t support 1.5 mL cartridges on any model.
Why borosilicate glass
Pen cartridges are made from Type I borosilicate glass, the same class used for most lab and pharmaceutical glassware. The U.S. Pharmacopeia describes borosilicate glass as having “a high hydrolytic resistance and a high thermal shock resistance.” In plain terms, it doesn’t react much with what’s inside it, and it handles temperature changes without cracking easily.
Why the plunger glides
The inside of the barrel is coated with a very thin layer of silicone. Without it, the rubber plunger would grab the glass and move in jerks instead of sliding smoothly. Many makers bake the silicone on with heat so it bonds to the glass instead of floating free.
That smooth slide is what lets each click of your pen’s dial move the plunger the same distance every time.
A little history
Cartridge pens aren’t new. The first reusable pen injector launched in 1985 and used a replaceable 1.5 mL cartridge. Today most cartridge pens hold up to 3 mL, which is the size the whole PepPens line is built around.
How to handle them
Glass is glass. A few habits go a long way:
- Inspect before filling. Look for chips at the crimp end and cracks along the barrel. One cartridge patent notes that micro cracks are “hardly visible,” so check under good light.
- Don’t drop them. Repeated small knocks can weaken glass even when nothing looks broken. If a cartridge hits a hard floor, replace it.
- Check the plunger. It should sit straight in the barrel. A tilted plunger is the most common cause of a leak.
- Fill with a vent needle. Pressure from filling without one is the number one reason plungers get pushed out the back. Our fill guide walks through it.
- Don’t overfill. 3 mL is the maximum.
Our cartridges arrive in factory-sealed sterile packaging from the manufacturer. We don’t independently test or guarantee sterility once a package is opened or handled.
Need more? Shop 3 mL glass cartridges or see the FAQ.
All PepPens products are sold for research use only.
Sources
- ISO 13926-1: Glass cylinders for pen-injectors and 2026 edition
- ISO 13926-2: Plunger stoppers
- ISO 13926-3: Seals
- SCHOTT cartriQ 3 mL cartridge datasheet
- SCHOTT Pharma: pharmaceutical cartridges
- Gerresheimer: glass cartridges for pen systems
- USP <660> Containers: Glass
- Nest Scientific: baked-on siliconization
- Pen injector history review, PMC
- Cartridge patent US 2018/0017531
