Cartridge Information

Everything about the glass.

The cartridge is the part that actually holds your solution, and it's the part most likely to cause a problem if you buy the wrong one. This page covers the spec, the packaging, how much you really get out of a fill, and the handling rules worth following.

1. The spec that matters

Volume	3 mL / 3cc
Material	Borosilicate glass
Stopper	11mm long gray stopper — this is the dimension that decides whether it fits
Top	Self-sealing septum under a silver crimp ring
Fits	V1, V2 and V3 — all three generations, no adapters
Sold in	10 and 20 packs, each cartridge individually sealed

Buying from someone else

A MODTECH pen takes a 3 mL cartridge with an 11mm long stopper, and nothing else. Plenty of 3 mL cartridges on the market use a shorter stopper. They look almost identical in a photo and they will not seat correctly in the pen.

If you're sourcing elsewhere, check the stopper length before you order rather than after.

2. Sterile packaging

How they're sterilized

MODTECH cartridges are steam sterilized and then sealed individually into a CE / ISO 11607 certified self-sealing pouch — the same packaging standard used for surgical instruments. Each pouch carries a steam indicator that changes color once the cycle has run, so a sterilized pouch is visually distinguishable from an unprocessed one.

Reading the pouch

An intact seal means the cartridge inside is sterile. Open it immediately before you fill, not in advance.

If a pouch is torn, already open, wet, or the seal has lifted, treat that cartridge as non-sterile and set it aside. At ten to a pack there is no reason to gamble on a questionable one.

A useful habit when filling: slide the cartridge forward inside the pouch so only the sealed top end is exposed, fill it there, and only then take it fully out. The glass stays protected right up until it goes into the pen.

3. Capacity

The numbers

Maximum fill	3 mL. This is a ceiling, not a target.
Per click	0.01 mL — one unit on a standard 100-unit insulin syringe
Per full dial turn	0.6 mL (60 clicks)
Full cartridge	About five full dial turns' worth

Residual volume

No cartridge-and-needle system empties to zero. A small amount of solution always stays behind — in the neck of the cartridge above the stopper's final position, and in the pen tip itself. That leftover is called residual volume, or dead space.

Two practical consequences:

Don't plan the last fraction of a cartridge. Treat the final portion as unrecoverable rather than being surprised by it. If your total needs to come out exact, work it out from what you put in, not from squeezing the cartridge dry.
A finer tip holds less back. Dead space in the needle is small but real, and a shorter tip retains marginally less than a long one. It matters more the smaller your per-session amount is.

Chasing the last drops by dialing repeatedly against an empty cartridge won't recover them, and it puts needless load on the mechanism. When the dial stops producing anything at the tip, that cartridge is done.

4. Filling rules

Never fill past 3 mL

The stopper has to stay inside the glass barrel. Push it past the bottom edge and the cartridge will not thread onto the pen body properly — which usually shows up later as a pen that feels wrong or won't actuate at all.

If it happens: leave the syringe inserted and gently push the stopper back until it sits level with the bottom edge of the glass before you remove anything.

Why it needs a vent

A cartridge is a sealed glass tube. Push liquid into the top and the air already inside has nowhere to go — it compresses, pressure builds, and the stopper is what eventually gives, shooting out the bottom and taking your fill with it.

A vent is simply a second opening that lets that air out while liquid goes in. It costs one insulin syringe and it is the single most important step in loading a Classic cartridge.

The full loading and venting procedure

5. Cartridge types

Classic vs pre-purged

	Classic	Pre-purged / easy-load
Stopper position	Seated at the bottom	Positioned so it walks down as liquid enters
Vent needed	Yes	No
Main risk	Blowing the stopper if you skip the vent	Overshooting the stopper past the bottom edge
We sell it	Yes — this is all we stock	No

What we stock

MODTECH stocks Classic 3mL sterile glass cartridges only. Every instruction in this Help Center is written for the Classic cartridge, and our pens are specified around it.

If you've sourced pre-purged cartridges elsewhere they may well work, but we can't support hardware we don't sell — and the fit rule still applies regardless of type.

Shop 10 and 20 packs Loading walkthrough

6. Handling and storage

One cartridge, one fill

Cartridges are single-use. Once a pouch is opened and a cartridge is filled, that's its life. Don't wash and refill one, and don't try to re-sterilize it at home — there is no way to restore the sterile barrier the pouch provided, and repeated septum punctures degrade the seal.

At the price of a pack, a fresh cartridge is the cheapest part of the whole setup. Use one.

Chips and cracks

Inspect the glass before you fill and again before you seat it. A chipped rim, a hairline crack, or a stopper that looks nicked or deformed means that cartridge goes in the sharps bin, not in the pen.

Glass under pressure from the plunger is not something to take a chance on.

Storing them

Unopened pouches: somewhere clean, dry and out of direct sunlight. Room temperature is fine — the pouch is the sterile barrier, so keep it dry and don't crush the box.
A loaded cartridge in the pen: the pen is designed for refrigerated storage with the cartridge in place, and fits in the fridge door.
Handle the pen gently when cold. Glass and a sudden knock are a bad combination.

7. Where to go next

Still stuck?

Email tfortech@modtechsupply.net. A real person answers within one business day, and that person is the owner.

MODTECH products are precision hardware sold for laboratory and research use only. They are not medical devices. No substances are included with or sold alongside any product, and nothing on this page constitutes medical advice or instructions for administering any substance to humans or animals.