I am culturing cells in a microfluidic device for several days. Before the experiment, I am performing a Nitrogen cavitation procedure in order to expose the intracellular compartments of the cells to the fluid flow. During Nitrogen cavitation (N2 bomb) you place the microfluidic device in a high pressure chamber and allow Nitrogen to fill the chamber. After a few minutes, you quickly release the pressure effectively breaking the cells apart.
Obviously there will be lots of bubbles in the device after the experiment. However, even if I get rid of them (using a vacuum chamber as suggested in another thread), more bubbles come out from what seems to be within the PDMS itself. PDMS is very permable to gases but it seems that even after 10 minutes the Nitrogen keeps seeping out from the PDMS. This effectively ruins the experiment as the bubbles destroy the cell remnants.
Any suggestions to stop this?
I don't have a solution for your problem, but some of the following comments may help:
1. PDMS is hydrophobic. Bubble problems are much worst when the surface is hydrophobic. Plasma treatment may render the PDMS hydrophilic, but the effect lasts some hours. You need days. There are other treatments, more complicated, to modify the PDMS surface properties. If you are interested look in the literature. Surface modification may also reduce the amount of gases that permeate the PDMS.
2. When you compress the fluid inside the microfluidic device with N2 a large amount of it will dissolve in the liquid. If you immerge your microfluidic device under water during the pressurization, maybe you can protect your liquid inside the device from N2 (it will take time for the N2 to diffuse to your liquid of interest).
3. Even liquid at 1 atm, before your experiment, will have gases dissolved so your vacuum treatment will remove those gases also. So, to minimize bubbles maybe you should try to use liquids that have been under vacuum before the experiment. Since your experiment takes days maybe you should also immerge your device on degassed water to avoid dissolving gases from the atmosphere.
4. You probably need a small amount of dissolved gases to break the cells, so 2 and 3 may not help.
5. Other possible solutions:
a) use traps to confine the cell debris and inject bubble free fluid
b) break the cells with other methods that do not require massive pressurization (e.g. ultrasonics)
I don't have a solution for your problem, but some of the following comments may help:
1. PDMS is hydrophobic. Bubble problems are much worst when the surface is hydrophobic. Plasma treatment may render the PDMS hydrophilic, but the effect lasts some hours. You need days. There are other treatments, more complicated, to modify the PDMS surface properties. If you are interested look in the literature. Surface modification may also reduce the amount of gases that permeate the PDMS.
2. When you compress the fluid inside the microfluidic device with N2 a large amount of it will dissolve in the liquid. If you immerge your microfluidic device under water during the pressurization, maybe you can protect your liquid inside the device from N2 (it will take time for the N2 to diffuse to your liquid of interest).
3. Even liquid at 1 atm, before your experiment, will have gases dissolved so your vacuum treatment will remove those gases also. So, to minimize bubbles maybe you should try to use liquids that have been under vacuum before the experiment. Since your experiment takes days maybe you should also immerge your device on degassed water to avoid dissolving gases from the atmosphere.
4. You probably need a small amount of dissolved gases to break the cells, so 2 and 3 may not help.
5. Other possible solutions:
a) use traps to confine the cell debris and inject bubble free fluid
b) break the cells with other methods that do not require massive pressurization (e.g. ultrasonics)
try to break the vaccum quickly , for example 5 min under vaccum and then you break the vacuum (p=1atm), under vacuum 5 min break the vacuum...etc
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