Emergency backups: a pen that can use PumpCart insulin

When travelling I used to take insulin containers (vials/cartridges) plus pump reservoirs, and fill them as needed. The YpsoPump has changed my habits. Its glass “PumpCart”-format reservoirs are glass, and approved for holding insulin a lot longer than the plastic reservoirs of other pumps. In some countries NovoRapid and Fiasp are sold in pre-filled PumpCart format, but here in Australia we use the Ypsomed reservoir kits to fill blank reservoirs with whatever insulin we’re using.

I soon got into the habit of filling almost a month’s worth of reservoirs at once, putting them in the provided black storage container, and back into the fridge. Then topping it up later in the month. This matches the approved usage of the reservoirs.

Over the years I have travelled a lot, and am careful about taking enough gear to have backups for my backups. But with COVID/etc I’ve only recently started travelling again. Now I find myself taking all my insulin for the trip pre-filled into YpsoPump reservoirs. I do take the labels for security/Customs purposes. Plus a few blank reservoirs thrown in for an emergency where I might need to refill from new supplies.

What if my pump fails?

With the old setup I could (instead of filling new pump reservoirs) put the insulin into a pen or use syringes for manual backup if needed. I have tended to get my insulin in pen cartridges to make this easy. But with my insulin packaged into PumpCarts, my main backup option seemed to be to use syringes.

But I now have an insulin pen that I can drop YpsoPump reservoirs into in case I need to use that insulin outside the pump.

PumpCarts into a pen!

This idea was based on the fact that the PumpCart and Novo Penfill formats only differ in the length of the tube. The Penfill cartridges have a plastic threaded cap pushed onto the end to interface with the pen and needle. Even the disposable FlexPen and FlexTouch pens use the same cartridges (just without the threaded cap). An example of this can be seen in my article about converting Fiasp FlexTouch pens to Penfill format.

The PumpCarts are simply 22 mm shorter. Incidentally, the glass cartridge inside the 1.5 mL FlexTouch pens (for example the 0.25/0.5 mg Ozempic pens) happens to also be the same size as PumpCart. So the same design shows up in a few places in the Novo ecosystem.

So what I’ve made is a way of mounting PumpCart reservoirs in a NovoPen.

By pushing the threaded cap onto the end of a PumpCart reservoir, that solves only part of the problem. The other end of the Penfill reservoir is designed to push down on a release which allows the pen plunger to move forward. The solution to that was conceptually simple, but needed a bit of crafting.

A spacer tube sits below the PumpCart and pushes down on that release. The tube needs to fit within the pen’s cartridge holder, and have enough space inside to allow the head of the plunger to pass through unimpeded.

Making the adapter

The outside diameter of the Penfill (and PumpCart) is 10.9 mm, and the internal diameter is 9 mm. The internal diameter is 8.68 mm.

The tube I’m using is 22 mm long, with an external diameter of 11 mm, and an internal diameter of 8.7 mm. That’s just wide enough to allow the NovoPen’s plunger to pass. I looked at 3D-printing new tubes, but matching those tight tolerances was going to be a challenge, and I found a quicker way to make a prototype. Instead I cut a section out of the barrel of a Pilot FriXion Clicker pen.

I was able to cut two 22 mm sections from the barrel, and discarded the rest. The internal diameter wasn’t perfectly constant, but a light ream of one tube opened it enough for the plunger to pass. I did have to trim them very carefully.
It’s not a high-tech solution, but it works very well! At least the refill can be used in another pen, and I’ve now got two adapters which is more than enough.

Dosing accuracy

Because the internal diameter of the PumpCart and Penfill cartridges are identical, the accuracy of dispensed doses should also be the same. A few other PumpCart-compatible pens do exist, just not so much in Australia.

Using the NovoPen Echo I get 0.5 U dosing (instead of the 1 U increments of the other NovoPen models).

How is this better than syringes?

A stack of pen needles takes up a lot less space than a stack of syringes. And when travelling, space is usually at a premium! Also I find a pen much more convenient and precise to use than syringes.

Now in my travel kit is a NovoPen Echo. With a spacer tube and a Penfill cap hiding away inside it waiting for a PumpCart. Just in case of emergency!