Dear Anil,

I have made the general experience that various monomers, for example, N-isopropylacrylamide (NIPA) or PEDGA as in your case, polymerize more easily in bulk than in droplets.  Even more, there seems to be a dependence on the droplet size: the smaller the drops, the more difficult the monomer polymerization inside them.  I can think about different reasons for this, and I prefer not to speculate about it here, but I'm happy to share my very simple practical solution to this challenge with you: just use more initiator and (if possible in view of your desired application) a higher monomer concentration.  It's as simple as that.  I have experienced for NIPA monomer that, if a certain amount of initiator works fine in bulk, the 5-to-10-fold concentration must be used in microfluidics, with more initiator at smaller droplet size.

Greetings from Berlin,

Sebastian Seiffert

Which photoinitiator do you use? At that wavelength 2-Hydroxy-2-methylpropiophenone is quite effective.

I am using Irgacure 2959 as photoinitiator. As it is mild and cause less damage to cells incorporated in gel. The gel is polymerizing in bulk (in gel casting dye between glass slides) but not in microdroplet size. I am using 3% SPAN 80 in oil phase for stabilizing the droplets produced.

Dear Anil,

I have made the general experience that various monomers, for example, N-isopropylacrylamide (NIPA) or PEDGA as in your case, polymerize more easily in bulk than in droplets.  Even more, there seems to be a dependence on the droplet size: the smaller the drops, the more difficult the monomer polymerization inside them.  I can think about different reasons for this, and I prefer not to speculate about it here, but I'm happy to share my very simple practical solution to this challenge with you: just use more initiator and (if possible in view of your desired application) a higher monomer concentration.  It's as simple as that.  I have experienced for NIPA monomer that, if a certain amount of initiator works fine in bulk, the 5-to-10-fold concentration must be used in microfluidics, with more initiator at smaller droplet size.

Greetings from Berlin,

Sebastian Seiffert

I2959 is not well suited for curing at 365nm as it shows better absorption at lower wavelengths. In my experience a main reason for incomplete curing in microstructures is due to oxygen inhibition (e.g. if using pdms). Curing in a nitrogrn environment helps. Further, incomplete curing is adverse for cell viability because of remaining cytotoxic acrylate monomers. Therefore, using a photoinitiator system with hiqh quantum yield is advantagous. 0,1% HMPP has worked fine for me. For sensitive cells also lithium phenyl-2,4,6-trimethylbenzoylphosphinate (LAP) should work.

I will try to do the polymerization in inert environment. Till now I was degassing by purging with Nitrogen and high vacuum. May be overall inert atmosphere around droplets will help. If it will not work I will use LAP. Increasing the photoinitiator concentration may cause fast gelling but it can also cause excessive cell death, that's why I cannot use a high concentration of Irgacure 2959. Also the pore size in the hydrogel is important for cells to grow, increasing the monomer concentration will increase the crosslinking density and will reduce the pore size.

Thank you both of you for your valuable suggestion for this problem. I will update about the outcome of this experiment very soon thank you once again. 

Finally I got the polymerized microgels. Purging of the paraffin oil with nitrogen for 15 minutes which I was using as continuous phase helped. The microgels polymerised within 2 minutes. It seems the oxygen present in the oil was diffusing significantly inside the microdroplet when the droplet size is small, as it increases the surface area to volume ratio. I had used high vacuum to remove dissolved oxygen from the oil phase earlier, but it was not enough. Thank you Dietmar Puchberger-Enengl and Sebastian Seiffert for helping me. 

Regards

Anil Chandra

I am now facing the same problem with polyacrylamide. My problem is that after I transform the good shape microgel from organic phase to water phase, they melt into a big piece of hydrogel. For me the organic phase I tried mineral oil and hexane. And the photoinitiator I use Irgacure 2100. I want to ask, how can you make Irgacure 2959, which is white powder to evenly distributed in the water phase because they don't feel very soluble?

Thank you very much! 

In my case I ultrasonicated the solution containing Irgacure 2959, and it worked for me. You can also first prepare a stock solution of photoinitiator in ethanol and use it in aqueous solution for getting the desired concentration.

Thank you very much!

I am facing the issue of not being able to get peg diacrylate 6000 (10%) polymerized in the presence of 2- Hydroxy- 2- methylpropiophenone. We make around few micron size PEG gels.  Can you guide me if removal of MEHQ from the main stock of PegDA is absolutely essential,  I am suspecting that presence of MEHQ is not letting the photopolymerization to proceed. Or can you think of any other reason.