I used agar based hydrogel for encapsulation of saos2 cell. Now, I want to make MTT assay and LDH assay of cells within hydrogel. I used DMSO for formazan crystals diffusing but, it did not useful.
Isopropyl alcohol (I'd try 70% to start) will dissolve the formazan crystals and allow you to use a plate reader to measure absorbance at 585nm. I'd recommend running the plate before adding MTT to the cells so you can get a baseline absorbance for each well, as the hydrogel thickness will likely affect absorption. If the hydrogel is too optically dense, you could add excess IPA to each well and then extract ~50-100uL for analysis.