Poly (ethylene glycol) diacrylate polymerizes through a free radical based process, similar to acrylamide gel polymerization. In the case of acrylamide polymerization, free radicals are generated by TEMED/APS. In the case of photopolymerization, the free radicals are generated through a photoiniatiator – a chemical which generates free radicals upon exposure to UV light. If the UV light is masked, as seen on the far left, patterns can be generated. A series of spots may also be generated through serial exposures through a microscope objective. Finally, when combining this photopolymerization with optical trapping, we can position cells in arrays, then freeze them in the gel.
We tested different photoinitiators – some of which are more phototoxic than others. The Irgacure 651 generates a methyl free radical – which I speculate is more mobile and can do more damage than the large free radical generated by Darocur 1173. The sample on the left is an example of these two different gels, stained with LIVE/DEAD stain (green cells are alive and red are dead).
On the left is a movie of a spot of gel being polymerized by the objective illumination method. One of the useful things about photopolymerization is that spots can be gelled precisely in
relation to each other. In this way, different cells can be patterned near each other.
A major problem is that the cells do not stretch out in PEGDA as they do in other ECMs (e.g., Matrigel). Additionally, they can neither move or reproduce. The cells are alive, based on LIVE/DEAD staining, but their morphology differs from that of cells in a natural gel (Matrigel). It may be possible to modify the PEGDA (e.g., incorporating adhesive or degradable peptides) to resolve these issues.