Cellular uptake mechanisms. Cells have various energy-dependent mechanisms for allowing extracellular cargo inside. Depending on the size of the cargo, processes such as caveolae mechanism are induced for small cargo, clathrin mechanism for intermediate cargo, and macropinocytosis for large cargo, are utilized by the cell. Because nanoparticles can vary in size ranging from 5 to 500 nm, different mechanisms are utilized by the cell to enable nanoparticle entry. (Reprinted with permission from Ref 17. Copyright 2014 American Chemical Society)