The vast majority of the hydrogels we use in our lab are PEGDA-polyethylene glycol diacrylate. PEGDA is biocompatible and easily makes hydrogels, but it’s also very expensive. The alternative is for a lab to make their own PEGDA, which is what I learned how to do this week. I asked Mohammed, a former student in our lab, for his help since he had done this many times. Outside of organic chemistry lab, this is the first synthesis I’ve done.
We started with polyethylene glycol (PEG). Basically, the synthesis attaches a new chemical arrangement (or functional group)-acrylate-to each end of a molecular of polyethylene glycol. This makes it functional for the crosslinking of hydrogels. We wanted an end average molecular weight of about 3400, so we started with PEG 3350.
We started by drying the PEG (to remove any acquired moisture from air exposure) by dissolving in organic solvent and using rotary evaporation. I think rotavaps are neat, so I’m going to talk about them. Basically, a liquid boils when its vapor pressure is equal to atmospheric pressure. To boil it, you can increase the heat and therefore the vapor pressure, or you can lower atmospheric pressure. That’s what a rotavap does. It uses vacuum to lower the pressure inside the flask, boiling off the solvent. The solvent vapor goes through the condenser, becomes a liquid again, and collects in a separate flask. As a bonus, the rotavap looks like a gun from a 50’s science fiction show:
The PEGDA synthesis is not exceedingly difficult, but the setup was time-consuming. There were ten pieces of glassware that had to be stacked on each other in three levels. Everything has to be well clamped into place and secure. This was probably the part that took the most time.
The reaction runs under noble gas to prevent any reactions with atmospheric oxygen, so the glassware has to be free of air leaks. Trying to get the right flow rate of the gas was also a challenge. The other hard part is working with nasty chemicals. Several chemicals we used smell a lot or were highly toxic-acryloyl chloride, methylene chloride, and diethyl ether. As is normally the case with such chemicals, we worked in the fume hood so our whole lab wasn’t exposed to the vapors. I’m also a big klutz, so I worry about spilling things or getting the chemicals on me.
The reaction ran overnight, and we precipitated the PEGDA this morning. I like watching precipitation reactions. Pour one liquid into another and poof! There’s a white solid. After precipitating, we filtered out the solvent and dried.
The final product is still drying, but from the appearance, the PEGDA looks pretty good. The yield wasn’t very high, but it’s not bad for a first try. We’ll confirm the success using IR spectroscopy and NMR spectroscopy.
Even in a week where my experiments don’t go well, if I make PEGDA, I can feel like I accomplished something!