Fig. 3.

In vivo multiphoton spectral images of topically applied ZnO NP on human skin. (a) Observations showing skin autofluorescence (green) with ZnO NP (red) on surface SC and in furrows versus the epidermis at 0, 4, and 24 h after application to inside forearm skin for volunteers with different racial skin types (recreated with permission from Ref. 46). (b) Effect of flexing on penetration through inside and outside wrist showing ZnO NP (red) on surface ($0\mu \mathrm{m}$) and in furrows with no ZnO NP present in epidermis ($30\mu \mathrm{m}$). Note the presence of motion-related artifacts in images of inside wrist.