Hi! I tried to assemble HOB autotrophic bioreactor today. I wonder whether the direction of filter disturb the aseptic level. We need to maintain the inner pressure through a tube connecting to the external environment, but not sure whether this setting can avoid contamination.
The bottle in your photograph looks like the product bottle, is that correct? [edit: of course it is, you said ‘product bottle’ in the post’s title] Female luerlock inlet, two ports blocked by a single piece of tubing going from one to the other. Then a short outlet tube with male luer lock going into the inlet side of the outlet filter.
I think you’re going to struggle to maintain positive pressure in the product vessel this way - if that’s what you want to do.
The peristaltic pumps on the inlet and outlet to the pioreactor will always have two points pinched, so the generation of gas from the electrodes, and the CO2 inlet (particularly with continuous sparging) should ensure positive pressure in the vial.
If you want to maintain positive pressure in your product vessel, then the easiest way may be to connect an air pump running through a second filter going into the product vessel. I haven’t tested this yet, but I don’t think that would give any backflow into the pioreactor vessel, as long as the peristaltic is operating correctly.
Does that make sense?
I thought the point of having filters on the outlets is so that we don’t need to maintain positive pressure?
I don’t think the PTFE syringe filters are directional, no.
Good aseptic practice, with positive pressure, reduces the risk of contamination. Luer lock fittings aren’t infallible, so operating under positive pressure encourages any leaks to flow outwards rather than inwards.
I thought the PTFE syringe filters were directional. The mechanics certainly are, the female luer lock side connects to the high pressure syringe, male luer slip is used on the low pressure side. Most membrane filters I have worked on are asymmetric hence have directionality to the membrane: a thin working surface rated to your specified pore size with a coarser thicker section to provide mechanical support. There’s quite an art to hand casting them.
Having to add air pumps sounds a bit overkill, no?
I’ve had a look at the datasheet for the Exapure filters, and they do indeed have a flow direction:
In the photo above the outlet is connected to the female luer lock, so that seems correct? That said, we’re using them as vent filters and not pushing liquid through at high pressure, so still not entirely convinced that directionality really matters?
If you need to ensure sterility in your product bottle one air pump between a series of AEPs seems a pretty sensible approach to me. I wasn’t aware that we really needed to ensure sterility there though. I would imagine it would be safer to take samples for genomic analysis, etc. directly from the vial. I think that’s common practice with Pioreactor experiments which is why Cam labeled what I call the product pump ‘waste’.
Directionality matters with any asymmetric membrane because you have the force of whatever is being pushed through the membrane on the thin filtration surface. That then pushes onto the backing support which gives it physical strength. If you go the other way, there’s a risk that the membrane delaminates and passes.
Extrapolating to the wider world of membrane filtration (not relevant here) you also get irreversible fouling if you collect retentate in the support, as it can’t be removed by backwashing, side washing, etc.