Colorimetric assay for cellular activity in microcapsules

Cellular activity in microcapsules was determined by a simple colorimetric assay, based on the cellular transformation of a tetrazolium salt, 3-(4,5-dimethyl-thiazol-2-yl)-2,5- diphenyl-tetrazolium bromide, into an insoluble formazan which was quantified in a spectrophotometer. The results showed that when encapsulated Chinese hamster ovary fibroblasts were exposed to the tetrazolium salt containing tissue culture medium, the formazan crystals were formed inside the poly(hydroxyethyl methacrylate-methyl methacrylate) microcapsules; capsules containing no cells or dead cells formed no formazan. A detectable amount of formazan was readily obtained even from single capsules. Formazan production was dependent on the incubation time, but not on the amount of added reagent. Capsules from a high cell-density encapsulation (4 X 10(6) cells/ml) formed more formazan than capsules from a low cell-density (4 X 10(5) cells/ml) encapsulation, suggesting a positive correlation between the cell density and tetrazolium transformation in microcapsules. The tetrazolium assay indicated the maintenance of cellular activity but slow, if any, proliferation in microcapsules over a 2 wk testing period.