Nothing. That's air trapped in the cartridge, and it's the single most common question we get.

When air is in there, pressing the plunger compresses the air before it moves any liquid. When you stop, that compressed air keeps expanding and pushes solution out afterward. Holding longer doesn't help, because time isn't the variable.

Purge it once after loading a fresh cartridge. Put a pen needle on, dial 10 to 20 units, hold the pen with the needle pointing straight up, and press until a clean drop appears at the tip. If nothing comes out, dial a few more units and press again. Once you get a drop with no sputtering, the air is out.

We use the vent needle method, and it needs no special equipment.

Insert the vent needle into the top of the cartridge. Add your solution alongside it. The vent needle lets displaced air escape so pressure doesn't build. Remove the vent needle, insert the cartridge into the pen, then purge the air as described above.

Cartridges hold 3 mL maximum. Don't overfill.

Yes. Pens ship without cartridges or needles. At minimum you need a 3 mL glass cartridge, pen needles, and a vent needle for filling.

Our kits bundle it together if you'd rather not assemble the list yourself.

All five take the same 3 mL cartridge and dial in 0.01 mL clicks, so dosing precision is identical across the line. What changes is capacity, body style, and how the cartridge loads.

V2 is our everyday pen and the one most people start with. Open spring end, so you can use it without removing the cap. 60 units.

V2 Pro is the same shape with twist-lock cartridge loading and a nicer finish. Our premium option. 60 units.

V1 is the classic metal pen with a cap and clip, and the only one that holds 80 units.

V3 is the V1's look with the V2's components. 60 units.

Disposable is ready out of the package for travel and short studies. One cartridge, start to finish.

Start with the V2 if you want the easiest handling. Go V2 Pro if it's going to see heavy use. Choose the V1 if capacity matters and you want 80 units. Pick the V3 if you like the classic look with modern internals.

We carry 31G x 8mm and 32G x 5mm. The 32G is finer, which means a smaller opening and slower flow. If your solution is on the thicker side, the 31G will deliver noticeably quicker.

Our pens accept universal pen needles in 28G to 33G.

Yes. The cartridge is the only part that holds solution, so swapping the cartridge swaps everything that touches it. Nothing carries over between them. Use a fresh pen needle each time.

If you're running two things at once, some people keep a pen for each simply to avoid swapping cartridges constantly. That's convenience, not necessity.

There's no limit on the reusable pens. That's the whole design. You replace the cartridge, not the pen, and one pen lasts hundreds of uses.

The Disposable is the exception. It takes one cartridge start to finish and isn't made to be reloaded.

Cartridges arrive in factory-sealed sterile packaging from the manufacturer. We don't independently test, validate, or guarantee sterility once shipped or handled.

Almost always it's the fill method rather than the cartridge. The two usual causes are a vent needle that was left out or removed too early, and a stopper that never seated properly during filling.

If the stopper looks tilted, or you see fluid pooling near the top of the cartridge, that's the tell. Email us and we'll work through it with you.

Most orders ship within one business day. Orders placed on weekends or holidays go out the next business day.

Transit is 3 to 6 business days within the US and 6 to 12 to Canada, on top of processing.

US standard is $8.95 and expedited is $15.00. Canada standard is $24.95 and expedited is $39.95.

Free shipping on orders over $125 to both the US and Canada. Free shipping uses standard service, and expedited is available as a paid upgrade even on qualifying orders.

Yes. Canadian orders may be subject to import duties, taxes and brokerage fees assessed at the border. Those aren't included in your order total and we have no control over them.

Delivery estimates for Canada don't include time spent clearing customs.

Usually not. Allow one to two business days for the first carrier scan. A package that hasn't scanned yet is typically in transit.

If tracking hasn't moved in more than five business days, email us and we'll open an inquiry with the carrier. If a package is confirmed lost, we reship at no cost by expedited service.

Unopened and unused items can be returned within 30 days for a refund of the product price, less shipping. Opened or used items aren't returnable for hygiene reasons.

Damaged or defective items must be reported within 7 days. Refunds process in 3 to 5 days once approved.

Six months on every pen. If it stops working correctly in that window, tell us and we'll take care of it.

No. Everything we sell is for research use only. Our pens and accessories are not medical devices, are not FDA cleared, and are not intended for human or animal therapeutic, diagnostic, or clinical use.

We're glad to answer any question about how a pen works mechanically. Questions about what to put in one are outside what we can help with.

Solid metal construction on all reusable models. We don't publish a specific alloy grade because we haven't independently verified it with our manufacturer, and we'd rather say nothing than claim something we can't stand behind.

3 mL glass cartridges, standard size, across every pen we sell. No 1.5 mL support on any model.