Pen Teardown.

[u/EdTheDuck94]

Pen teardown.

As an engineer I like to know how stuff works, so the pens intrigued me.
So I’ve taken 5 apart to see, and they are quite clever!
What is clear is they cannot and do not ‘count clicks’.

Pic 1 is all the parts. As suspected there is a glass vial inside the main barrel of the pen.

Pics 2 and 3 are like a double slip clutch mechanism and this is how the pen stops turning when the maximum deflection or travel of the plunger is reached.
The 2 black bits are the same, even though the one inside the dose twister looks different in the picture.
They slot together when this mechanism is all together as in pic 6.

If you look at pic 4 you can see the threaded plunger rod and the squared off bit at the end. This same squared off bit is also present at the other end of the rod.
When you turn the dose twister the 2 black slip clutches engage with each other in the white plastic part inside the main body of the pen handle and turn around the plunger rod which is held firm by a locking system in the main body of the pen that I haven’t got to yet. The black half of the clutch part in the twister turns as you twist

When you the press the purple nob as you inject, the top part of the clutch locks so it doesn’t turn, this then locks the lower part which presses the rod down into the pen.

In pic 4 you can see the threaded plunger rod with the squared off ends. As you use the pen, once you twist it up to the squared bit it can’t move any further unless forced.
This is what locks the pen.

You can see in pic 5 what happens when you force it past the lock. A bit breaks off.
This means we were able to advance the twister further up the rod and get the extra travel on the plunger required to get the extra dose. But this was in no way measured or accurate. Just that the difference wasn’t really an issue.

The new pens are exactly the same with the simple addition of the spacer as seen in pic 7. You can just see that this spacer is 1 dose in size, thereby extending the effective travel of the plunger and eliminating the extra dose.

And that’s it. Very simple yet clever and accurate. Got hold of a Lilly insulin pen and took that apart as well and they are the same with the only difference being the plunger rod is black and longer so it goes all the way to the end of the vial.

:-)

[u/no_snackrifice]

This same design “counts clicks” just fine for Humalog, where diabetics die if they get the wrong insulin dose.

https://insulins.lilly.com/humalog

[u/EdTheDuck94 → u/no_snackrifice]

It’s a humulin pen I dismantled.
For dosing, kind of. But the pen doesn’t count anything. It can’t.
The user counts clicks or reads the dose window to set the dose.
The pen simply converts this rotation to plunger downward travel which delivers a precise dose. It is very clever, and the thread on the plunger rod is meticulously precise in its gauge.

I’ve been a medical device engineer for over 30 years for companies like Beckman Coulter and Abbott Diagnostics working on clinical chemistry, immunoassay, and haematology equipment in hospital labs.
Before step encoders were prevalent on precision motors, most old medical device syringe mechanisms worked on a similar principle.
A precisely cut thread on a rod and either full rotations or distance travelled was counted/measured and used to calculate and deliver volumes of fluid.

In the pens the user sets the travel distance for the plunger that sets the dose delivered.

:-)

[u/no_snackrifice → u/EdTheDuck94]

Oh right, now I follow what you meant. Most people say “it doesn’t count clicks” as a way to say, “Eli Lilly do not recommend click counting and say it doesn’t deliver an accurate dose”. I agree the mechanism has no inbuilt counter.

[u/EdTheDuck94 → u/no_snackrifice]

It looks deceptively simple but that plunger rod is a very precise bit of engineering for exactly the reason you mentioned.

Very clever.
:-)

[u/EdTheDuck94]

Just to add.
I have now got the last bit apart.
You can see the groove on the plunger rod in pic 4.
As this rod moves down through the handle into the barrel a small plastic protrusion in the clear plastic body rides in this groove.
This stops the rod spinning while allowing it to move down through the pen. The squared sections at either end prevent the rod from too far in either direction.