Examples of varying fiber numbers for MWCNT structures. Left, enhanced darkfield image (1 day post-exposure) shows examples of MWCNT structures containing 1, 2, 3, 4 and >4 fibers per structure. MWCNT fibers (and most nanoparticles in general) are bright white in enhanced darkfield images due to scattering of light while tissue is dark and airspaces are black. FESEM micrograph on right shows an example of a singlet MWCNT penetration of the alveolar epithelium. Arrow in the FESEM indicates the end of the singlet that is in the airspace where partial shells of the MWCNT are visible. FESEM is from 168 days post-exposure.