Another challenge is the generally poor ability of active articles to reach the interior of cells, although active internalization and subsequent manipulation of magnetic and acoustic nanomotors in living cells have been demonstrated . From the blood vessels, the particles will need to go through the epithelial tissue and extracellular matrix before reaching and penetrating the cell membrane. It is not clear that the current nanomotors are powerful enough to cross these biological barriers that impede passive nanoparticles, although there has been significant progress made with magnetically, acoustically, and chemically driven micro/nanomotors to move through relatively dense biological environments . With further engineering of the materials and geometry, coupled with more sophisticated instrumentation, we envision the "fantastic voyage" through dense organs may soon be realized.