First 4-Hour HD with 75-nm NPN
I assembled two 2-chip dialyzers using the thicker, 75-nm NPN membrane chips (10 mm long multichannel, 22 mm x 24 mm). The dialyzers were pumped full of IPA, to fill with fewer bubbles, then infused with PBS for at least one hour. Unfortunately both of these either broke when hooked up to the animal, or were broken before hand. The dialyzers were disassembled to retrieve the non-broken membranes and reassembled in a third. This one burned down, fell over and sunk into the swamp. That’s how it felt anyway. This dialyser also broke when blood flowed through it. I rescued the one remaining membrane and using a fresh membrane assembled a fourth dialyzer. This one filled with blood and remained intact for two hours of dialysis then broke.
Some of the first three membranes that broke could have been due to mishandling, valves turned the wrong way, or the pumped turned on in the wrong direction, but certainly this last one broke while pumping and without human intervention. I do have the three blood samples, initial, hour 1, and hour 2, and will post those results when I analyze them.
I have some of the broken membranes and then can be examined under a microscope or SEM. Perhaps Josh would like to take a look. I’m not sure I would be able to detect anything unusual.
As you can imagine I am now running a bit short on chips, but will employ the PDMS patch techniques that Josh and I investigated previously.
I may also need to implement a means of detecting membrane breakage before blood is introduced, such as adding microbeads to the dialysate side and looking for them on the blood side. Assuming no ruptures, these could be rinsed from the dialysate side prior to the animal experiment.
I’m confident that this week we can use up fewer chips getting to the dialysis, but I’m not at all sure what is causing the breakage at this point.
I am considering using the LabSmith pressure sensors in the animal experiment, to record the inlet/outlet pressures for the duration of the dialysis. I can set aside the last animal of five for this purpose.
So, in summary, bad news on the robustness front.
The idea of adding sensors to monitor device pressures during dialysis is a good one. I don’t understand membranes breaking before the animal hook-up. I thought we had gotten past that. Provide more detail here please.
We should also think about getting benchtop blood experiments going. Blood might be causing an instability that PBS and protein solutions are not simulation.
The membranes are (apparently) intact before the hookup to the animal, but the hookup and wetting is troublesome. Swapping tubing for hookup to animal and dialysate introduces bubbles that have to be flushed out, and often times the fluid chooses one of the chips (or one section of one of the chips) to flow over preferentially, creating weird flow patterns and probably undesirable pressures or shear forces.
Nonetheless, my intuition is that these membranes should be more robust than they are. Nitride, especially such thick nitride, should be able to handle what we’re giving it. I agree with the thinking that blood itself (perhaps even simply due to increased viscosity) is a likely culprit.
I would also want to try the animal experiments with a simplified design. Two chips is good from the third floor of Goergen, and it’s not so bad in the basement, but across the street it’s just not behaving itself. My experience suggests that we’d be better off with a design in which the flow doesn’t have the opportunity to branch off to avoid stagnant regions. I can describe this more clearly with a whiteboard at NRG.