In the realm of science fiction, the idea of tiny nanorobots that can enter the human body and seek out and destroy unwanted elements has been prevalent. This idea has been made a reality by a research team led by Bin Kang and Yaodong Dai at Nanjing University and the Georgia Institute of Technology [Kang, et al., Small (2009), doi: 10.1002/smll.200801820]. They have used what is called the photoacoustic effect of carbon nanotubes (CNTs) to make what are essentially nanogrenades that can target and destroy cancer cells. Says Professor Dai, “Under the irradiation of a Q-switched millisecond pulsed laser, the acoustic wave generated in a nanotube solution is so strong that it can trigger a micro-explosion at the nanoscale. So we had a good idea: if the nanotube in solution can explode at the nanoscale like a bomb, why don't we try to use this nanobomb to kill cancer?”

To make these weapons of minute destruction, they start with carbon nanotubes that are functionalized with folate acid, which causes them to be selectively absorbed into cancer cells which have an overabundance of folate receptor. Once inside the cancer cell, the nanotube grenade is “detonated” using a short millisecond pulse of energy from a 1054 nm laser. The nanotubes absorb very strongly in the near infrared range, while most biological species are transparent in this region (control studies showed cells without nanotubes are undamaged by the laser). The nanotubes absorb the laser energy, causing severe local heating and non-linear transfer of heat into the surrounding cell, a process that generates a shock wave that reaches upwards of 100 MPa in magnitude, and destroys the cell in a matter of milliseconds. Using this technique, they showed that 85% of cells with nanotube uptake were destroyed within 20 seconds, while 90% of cells without the nanotubes survived. Professor Dai claims that this photoacoustic method of cell destruction using carbon nanotubes is superior to previous photothermal methods because it uses hundreds of times less energy, and does not cause excessive heating, which could lead to damage of the surrounding tissue. Professor Dai says that the next step in this research will be to attempt this cancer treatment on mice. If that works they “will try bigger animals such as rabbits or dogs.” They warn that eventual human clinical treatments using this technique have “a long-long way to go”.