How It Works: Set up and load your pen

1. 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.

Your order arrives as hardware only. We don't include printed instructions with orders — this Help Center is the manual, so it's always current and we can fix or expand it the day someone asks a good question.

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.

Know the parts

Six parts, called the same thing on every page of this Help Center.

Cap	The outer sleeve over the cartridge holder. It pulls straight off. It doesn't unscrew.
Cartridge holder	The clear tube under the cap. It unscrews from the pen body, and you can see the cartridge through it.
Pen body	The aluminum half with the dial knob.
Dial knob	At the back of the pen body. Turn it to set the amount: 60 clicks, 0.01 mL each, clockwise to add, counterclockwise to back off. Press its end to dispense.
Plunger	The black threaded rod inside the body. It pushes the cartridge's stopper, and it turns as it moves, so never force it straight in.
Tip	The screw-on pen needle. Sold separately. A fresh one every session.

Put together, the pen is 6.3 × 0.79 × 0.79 in and weighs about 1.76 oz. It fits in a fridge door with the cartridge loaded.

“Where's my cartridge holder?”

It's in there. Promise. The order of operations just isn't obvious the first time:

1. Pull the pen apart first. Don't unscrew anything yet — pull straight, and the cap comes off to expose the cartridge holder.
2. Then unscrew the cartridge holder from the pen body.

If you unscrewed it first by mistake, the cartridge holder is now hidden up inside the cap. No harm done — screw the pen back together, then start again from step 1.

Practice with saline first

How much a cartridge holds

Full cartridge information Compare pen models

2. 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.

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. 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.
2. 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. Set the pen parts down on a clean surface.
3. 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.
4. Expose and wipe the cartridge top. Peel the pouch open at the metal-cap end — the end with the silver ring — and slide the cartridge forward until only that end pokes out. Wipe it with an alcohol wipe and let it dry. The rest of the cartridge stays in the pouch, protected, until it goes into the pen.
5. 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.
6. Set the vent. Push the vent needle through the silver-ring end of the cartridge. That open barrel is now a pressure relief valve.
7. 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.
8. Remove the vent first, then the syringe. The self-sealing top closes behind both.
9. Seat the cartridge and close up. Take the filled cartridge out of its pouch and drop it into the holder metal cap first, so the gray stopper faces the pen body. Thread the holder back onto the body. A new pen 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 4, and you should not skip it.

Never fill past 3 mL

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3. 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 exactly what the nine steps above describe: a 1 mL syringe with its plunger removed acts as an air outlet while the solution goes in alongside it. The result is 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.

Which one to use

4. Priming the pen

Priming clears the air out of the cartridge and the tip. Do it every time you load a fresh cartridge and every time you put on a new tip — a new tip starts full of air, even on a cartridge you primed last session.

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 dial knob. 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

Choosing the right pen tips Troubleshooting guide

5. Using it day to day

Every session after setup

Once the pen is loaded, the routine is the same every time:

1. Take the pen out of the refrigerator. Cold solution is thicker, so if it presses stiffly, give it time to warm up.
2. Screw on a fresh, unused pen tip.
3. Prime the new tip. Needle up, dial a few clicks and press until solution appears at the tip. Every new tip starts full of air. See priming.
4. Dial your amount. Turn the dial clockwise. Each click is 0.01 mL, the same as one unit on an insulin syringe. Went one too far? Turn it back counterclockwise.
5. Press the end of the dial knob to dispense. When the dial reaches zero, hold still for about ten seconds before you move the pen. The mechanism has stopped; the fluid hasn't.
6. Take the tip off and put it straight into a sharps container. The pen goes back in the fridge with no tip on.

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 one.

6. Swapping in a new cartridge

When the stopper stops moving and nothing more comes out at the tip, that cartridge is done. The swap is the loading routine again, starting from a used pen.

1. Take the tip off and put it in the sharps container.
2. Pull the pen straight apart, then unscrew the cartridge holder.
3. Tip the empty cartridge out into the sharps container. It's glass, so it goes in sharps, not the bin.
4. Reset the plunger. Guide it back into the body and let it turn under your finger.
5. Fill a fresh cartridge with a vent (steps 3 to 8), drop it in metal cap first, and thread the holder back on.
6. Fresh tip, then prime. A new cartridge always needs priming.

7. 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

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.

Shop MODTECH Classic cartridges Cartridge information

8. When something goes wrong

Full troubleshooting guide Safety and warnings

9. Why a pen instead of a vial and syringe

1. 01

   Prep time

   Vial + syringeGather supplies, draw, measure, tap out bubbles, double-check the amount

   MODTECH penFresh tip, quick prime, dial. The same short routine every session

2. 02

   Measuring

   Vial + syringeTiny graduation marks that are easy to misread

   MODTECH penClick-stop dial with 60 exact increments

3. 03

   Consistency

   Vial + syringeDepends on technique, angle, and experience

   MODTECH penSame click, same amount, every time

4. 04

   On the shelf

   Vial + syringeVial, loose syringes, wipes, and a sharps bin to store

   MODTECH penOne pen in the fridge door, a box of tips, and the sharps container you'd have anyway

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10. Where to go next

• Cartridge information
• Pen tips
• Pen models
• Troubleshooting
• Safety and warnings
• Shipping & guarantee
• Full FAQ

Still stuck?

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.