I made a cartridge for a Tresiba insulin pen that will fit a Bic ink pen. I am a diabetic and have a bunch of friends that are, and thought this would be a fun thing for us to use.

To disassemble the pen, rock the area surrounding the insulin cartridge back and forth until the two pieces separate. You can then remove the glass insulin cartridge. From there, you can either cut down the ink pen (Warning: this can get messy), or you can remove some of the innards from the dial and insert the full pen through it. The dial and button will still work, which makes a great fidget.