Figure 1: Metabolic activity (measured by the MTS assay) in primary human liver stellate cells exposed to four types of ZnO nanoparticle or dissolved ZnSO4 at concentrations of 0-80 μg/mL. The signals at 24 h from cells, treated in quadruplicate for each concentration, were averaged and expressed as a percentage of signals from time‑matched untreated cells set as 100%. DMSO (5%) was used as a positive control for the assay.