I’m writing an article for the EC’s Horizon magazine on efforts to track stem cells once they are put inside a body to repair something like a damaged liver. It’s a treatment fairly far into the future, but all the aspects of safety need investigating in the meantime. The biggest fear is that the stem cells might go elsewhere in the body and cause tumors, so it’s important to watch to see where they go.
Joan Comenge at the University of Liverpool is developing nano-sized gold rods to put inside these stem cells in their thousands, to act like bio-markers. A relatively new imaging technique called photoacoustic tomography can give incredibly high resolution images of what is happening inside the body. The stem cell tagging idea works thus: you irradiate the cells containing the nanorods with near infrared radiation. The body is pretty much transparent to this sort of radiation, but the nanorods aren’t: they absorb it like crazy and heat up to about a thousand degrees. Hot things vibrate, so they emit sound that an ultrasound device – a bit like the devices used for imaging a baby in utero – can pick up. Scientists hope to be able to reconstruct the acoustic signals into an image of the cells containing these nanorods and where they are situated.
The techniques are in their early stages, and haven’t yet been tested in animals. There are all sorts of details to unpick, such as what coating is best on the nanorods to prevent them from interacting too much with the cells, and what happens to the nanorods once the cells die. And the biggest problem of all seems penetration – the imaging technique only works to a depth of about 5 centimetres, so imaging tagged stem cells isn’t yet workable in humans. But the idea is there.