Insulin Reservoir (Major Outstanding Issue - help needed)

If you haven't established already then I can tell you here that this part of the project is causing me the most difficulty. I haven't been able to find an off-the-shelf product, and what I have come up with is not really satisfactory. I may describe what I am presently using in more detail, but today (6 July 2009) I have come up with something that may work, after re-reading a thesis I found on the internet.

My idea is to use a coil of tubing, specifically the tubing that Bartels have supplied to me for use with their micropump. I've only just come up with this idea so I'm going to have to test it, but the general idea is that the coil of tubing will hold the insulin. The insulin will be held in place by surface tension and by the same principle that holds liquid in a straw when you put your finger over the end, with the open end pointing down. I'm also wondering if coiling the tubing will help cancel out the effects of gravity pushing the insulin out of the tubing as insulin on one side of the coil will be subject to the same force of gravity as insulin on the other side of the coil but in opposite direction.

The coil of tubing will be connected to the micropump at one end, but the other end will be left open. Once primed (i.e. the tube coil is filled and connected to the pump) air shouldn't be able to get in as the insulin is not moving in the tube due to surface tension with the sides. So as the micropump pumps it draws insulin through the tube and air just enters at the other end.

This will also make refilling and priming easy. The user will inject insulin into the open end of the tube-reservoir using an insulin pen. The force should be able to overcome the back pressure of the pump and check valve, so the insulin reservoir will be full and the pump primed once insulin is pushed out of the end connected to the infusion set.

On the 7 August 2009 I tested this idea and it appears to work well. The test conditions were as follows.

Length of tubing: 100cm

Coil diameter: 8cm (loosly coiled)

Insulin used: Humalog

Forward leakage (out of insulin set): None observed

Reverse leakage (out of the coil): None observered

Next steps: Tigher coiling. Less tubing.

High-Level Requirements

Here are some rough high-level requirements I've come up with.

  1. The volume of insulin that the reservoir must hold will be very small; in the order of 200mm3 (2ml).
  2. The size of the reservoir must be small as the final i-Pump must be wearable. Its depth should be no greater than 5mm. The size should be in the order of 10mm square or diameter. Note, I'm thinking of the reservoir as a thick coin when I state these dimensions.
  3. I think that the reservoir should maintain a constant pressure on the insulin contained within, and this pressure must be not more than the Bartels micropump (and standard Bartels check valve) can sustain. Although I am prepared to consider other proposals that ensure no air gets into the system.
  4. Obviously the reservoir must be sterile.
  5. The pump must be refillable using a standard insulin pen (such as NovoPen from Novo Nordisk) and 5mm needle.