How It Works — Loading the Pen

Prep once. Dial. Done.

The old routine: a vial, a fresh syringe, and a careful draw against tiny graduation marks, every single session. The MODTECH routine: fill a sterile cartridge once, load the pen once, and every session after that takes about ten seconds. Grab it, dial it, dispense, put it back.

Already own a pen and just need the loading procedure? Skip straight to the step-by-step guide.

1. Why a pen instead of a vial and syringe

MODTECH pen vs vial and syringe compared on prep time, measuring, consistency, and storage

Vial + syringe MODTECH pen
Prep time Gather supplies, draw, measure, tap out bubbles, double-check the dose One click. About five seconds, every session
Measuring Tiny graduation marks that are easy to misread Click-stop dial with 60 exact increments
Consistency Depends on technique, angle, and experience Same click, same amount, every time
On the shelf Vial, loose syringes, wipes, and a sharps bin to store One pen. Minimal, and it fits in the fridge door

Every session after setup

Once the pen is loaded, the routine is the same every time and takes about ten seconds:

  1. Take the pen out of the refrigerator.
  2. Screw on a fresh, unused pen tip.
  3. Dial your exact amount. Every click of the 60-increment dial is a precise, repeatable step — 0.01 mL, the same as one unit on an insulin syringe.
  4. Press, dispense, done. Back in the fridge until next time.

Storage between sessions

The pen is built to live in the refrigerator with the cartridge loaded. The solid aluminum body handles cold storage without issue, and it fits in the fridge door. A 3mL cartridge covers many sessions — when it runs empty, swap in a fresh sealed one in seconds.

Compare pen models Shop sterile cartridges

2. Before you start

Everything below assumes your compound is already reconstituted. That part is on you — MODTECH sells hardware only, and nothing is included, prefilled, or sold alongside a pen.

What you'll need

  • Your reconstituted solution.
  • A 3–5 mL transfer syringe. This is what carries the solution into the cartridge.
  • A 1 mL (100U) insulin syringe. This becomes your vent, and it gets thrown away afterward. It is not optional — see why this needs a vent.
  • One sterile 3mL cartridge. Ours are sold in 10 and 20 packs, each individually sealed.
  • Your pen. V2 or V3 — the routine is identical.
  • A fresh pen tip, plus alcohol wipes and a clean flat surface to work on.

Practice with saline first

Run this whole routine once with sterile water or saline before you do it for real. Fill a cartridge, vent it, seat it, prime it, take it apart. Loading is muscle memory, and ten minutes of practice is the difference between a smooth first session and a wasted cartridge.

Please don't learn on your actual compound. MODTECH isn't responsible for material lost while you're getting the hang of the hardware.

How much a cartridge holds

Cartridge volume 3 mL / 3cc. That is a hard ceiling, not a target — see overfilling.
Per click 0.01 mL, the same as one unit on a standard insulin syringe.
Per full dial turn 0.6 mL — the dial has 60 click-stop increments.
A full cartridge About five full dial turns' worth.
Stopper 11mm long gray stopper. A short-stopper cartridge will not seat in a MODTECH pen, whoever sold it to you.

Full cartridge information Getting started guide

3. Loading a cartridge

These steps are written for the Classic sterile glass cartridge — the only kind MODTECH sells, and the one our pens are specified around.

The short version: fill a sterile cartridge with a vent in place, drop it into the cartridge holder, screw the pen back together, attach a fresh tip, and prime until liquid appears. About two minutes the first time.

Why this needs a vent

A cartridge is a sealed glass tube with a stopper at the bottom. When you push liquid in through the top, the air already inside has nowhere to go. It compresses, pressure builds, and eventually something gives — and what gives is the stopper, which shoots out the bottom and takes your solution with it.

A vent is just a second opening that lets that air escape while liquid goes in. It costs you one insulin syringe and it is the single most important step on this page.

The nine steps

  1. Draw up your solution. Pull the full volume into your 3–5 mL transfer syringe and set it down on a clean surface while you prep the cartridge.
  2. Open and wipe the cartridge. Peel the sterile pouch open and wipe the top of the cartridge — the end with the silver ring — with an alcohol wipe. Let it dry.
  3. Build your vent. Take the 1 mL insulin syringe, pull the plunger all the way out, and strip off every cap and needle guard. You want a bare hollow barrel with a needle on the end.
  4. Set the vent. Push the vent needle through the silver-ring end of the cartridge. That open barrel is now a pressure relief valve.
  5. Transfer slowly. With the vent still in place, insert your transfer syringe right alongside it and push the solution in slowly. You'll hear air hissing out of the vent as you go — that sound means it's working.
  6. Remove the vent first, then the syringe. The self-sealing top closes behind both.
  7. Open the pen. Pull it straight apart — do not unscrew it first, or the cartridge holder stays stranded inside the cap. Once the cap is off, unscrew the clear cartridge holder from the pen body.
  8. Reset the plunger. If the black plunger is sticking out of the body, guide it back in and let it rotate as it travels. It rides a corkscrew thread. Forcing it straight in will bend it, and that's avoidable damage rather than a warranty claim.
  9. Seat the cartridge and close up. Drop the filled cartridge into the holder and thread it back onto the pen body. A new V2 can need firm pressure to catch the first thread — start it straight and turn slowly so you don't cross-thread it.

Then prime it. That's section 5, and you should not skip it.

Never fill past 3 mL

3 mL is the maximum, not a goal. The stopper has to stay inside the glass barrel. Push it past the bottom edge and the cartridge won't attach to the pen body correctly, which shows up later as a pen that feels wrong or won't actuate.

If it does happen: 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.

Cartridge information Compare pen models

4. Venting methods

Two ways to solve the air problem. Both are closed systems, so the sterile interior of the cartridge stays protected. Pick one.

Method 1: Vent air displacement (recommended)

The most controlled of the two, and the method the nine steps above describe.

  1. Load your compound into a 3–5 mL syringe.
  2. Take a 1 mL standard insulin syringe. Unwrap it, remove every cap and guard, and pull the plunger completely out. You're left with an empty barrel.
  3. Insert that barrel into the self-sealing top of the cartridge. It is now your air outlet.
  4. Insert the compound syringe directly next to it and inject slowly. Displaced air escapes through the vent instead of building up behind the stopper.

Result: a steady fill with no back-pressure, and a stopper that stays exactly where you left it.

Method 2: Pump displacement

No second syringe required. Slower, and it needs a lighter touch.

  1. Load a 5 mL syringe with your compound and push every trace of air out of it first.
  2. Insert it into the self-sealing end of the cartridge.
  3. Gently draw the plunger up to pull air out of the cartridge, then release. The pressure difference starts moving compound in on its own.
  4. Repeat the cycle. Each pull-and-release moves a little more compound in and lets air flow back into the syringe.
  5. If flow slows or stalls, switch to short slow pumps and release fully between each one.

Don't rush this one. Fast or forceful plunger movement is the quickest way to dislodge the stopper and lose the fill.

Which one to use

Vent air displacement Pump displacement
Best for First-timers and full 3 mL fills Partial fills, or when you have no spare insulin syringe
Extra equipment One 1 mL insulin syringe, sacrificed as the vent None
Speed Faster Slower
Blowout risk Lowest Higher if you pump too hard
Our take Start here Fine once you've done a few

5. Priming the pen

Priming clears the air sitting above the liquid. Do it every time you load a fresh cartridge.

  1. Screw on a fresh pen tip and remove both the outer cover and the inner needle guard so the needle is exposed.
  2. Hold the pen needle up and thump the glass gently with a fingernail. Bubbles rise to the top.
  3. Dial about 5 clicks.
  4. Still pointing up, press the end of the pen. The plunger drives the stopper up and pushes air out through the needle.
  5. Repeat until you see solution at the needle tip and no more air comes out.

Why the first push is stiff

On a fresh cartridge the first push often takes noticeably more force. You're breaking the stopper loose from the bottom of the glass, and a little extra pressure there is normal.

If it stays stiff on every push after that, something else is going on — start with the troubleshooting guide.

Choosing the right pen tips Troubleshooting guide

6. About pre-purged easy-load cartridges

You'll come across “pre-purged” or “easy load” cartridges sold elsewhere, and it's worth understanding what they are even though they aren't what we stock.

The design positions the stopper so that no vent is required. You insert your syringe, apply steady pressure, and the stopper walks down the barrel on its own as liquid enters. It removes the vent step and reduces the chance of a blowout, at the cost of a different set of habits.

We don't sell them

MODTECH stocks Classic 3mL sterile glass cartridges only. We do not sell pre-purged easy-load cartridges. Every instruction on this page is written for the Classic cartridge, and that's the one our pens are specified around.

If you bought your cartridges from us, follow the nine steps and vent them.

If you use them anyway

We can't support hardware we don't sell, but the failure modes are worth knowing if you've sourced them elsewhere:

  • Start with an air-free syringe. Trapped air adds pressure and agitates the solution.
  • Insert straight into the self-sealing top. A nicked stopper won't reseal.
  • Expect resistance at the start. Steady, consistent pressure is what unseats the stopper. After a fill or two you'll know the feel.
  • Slow down the moment the stopper begins to move. Transferring fast agitates the compound.
  • Watch the stopper on full 3 mL fills and never let it travel past the bottom edge of the glass. If it does, leave the syringe in and gently push it back level with the bottom of the barrel.
  • Fill through the pouch. Slide the cartridge forward so only the self-sealing end is exposed through the paper side, fill it, then remove it. Keeps it as sterile as possible.

Fit still applies. Whatever style of cartridge you use, a MODTECH pen takes 3 mL cartridges with an 11mm long stopper and nothing else. A cartridge that doesn't match that spec won't seat, pre-purged or not.

Shop MODTECH Classic cartridges Cartridge information

7. When something goes wrong

What you're seeing Usually because What to do
Stopper pushed out the bottom during the fill No vent, or you injected too fast Start over with a fresh cartridge. Use Method 1 and inject slowly.
Cartridge won't thread onto the pen body Overfilled — the stopper travelled past the bottom edge Reinsert the syringe and push the stopper back level with the bottom of the glass.
You press and nothing happens No cartridge seated, or it hasn't been primed The mechanism needs resistance to actuate. Seat a cartridge, then prime it.
Plunger won't go back into the body You're pushing it straight in Let it rotate as it travels — it's a corkscrew thread. Guide, don't force.
The cartridge holder has disappeared You unscrewed the pen before pulling it apart Screw it back together, then pull straight apart first.
Still getting air at the needle tip Not enough priming cycles Keep going with the needle pointed up until solution appears.
First push feels very stiff The stopper is breaking loose from the bottom Normal on a fresh cartridge. See above.

Full troubleshooting guide Safety and warnings

8. 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.