Fig. 1.

Neutral Red staining and cytotoxicity assay. Neutral red staining of human THP-1 macrophages visualised by light microscopy: control group (A) and Co group at 100 ​μg/mL (B). White arrows indicate representative living cells and black arrows indicate representative dead cells. Bar ​= ​100 ​μm. Cytotoxicity of different concentrations of Cr, Ti, Co, and Cr/Ti/Co on human THP-1 macrophages is shown in (C). The X-axis indicates different concentrations of added metal nanoparticles; Y-axis indicates the optical density (OD) of neutral red at an absorbance at 540 ​nm visible light. Asterisk (∗) indicates comparison against the control group (∗p ​< ​0.05, ∗∗p ​< ​0.01, ∗∗∗p ​< ​0.01); hash key (#) indicates comparison against the Mix group (#p ​< ​0.05, ##p ​< ​0.01, ###p ​< ​0.001).