Majorana fermions and rat brains

An over-capacity crowd greeted Leo Kouwenhoven’s talk on Majorana fermions

By Margaret Harris

The hottest talk of the APS March Meeting so far took place yesterday, when Leo Kouwenhoven revealed that his group at TU Delft in the Netherlands may have observed Majorana fermions in one-dimensional nanowires.

Majorana fermions have a curious property – they are their own antiparticles – and particle physicists have been looking for fundamental Majorana fermions for decades. A few years ago, condensed-matter physicists got in on the act too, seeking evidence of Majorana-like behaviour in fermionic quasiparticles such as those formed by electrons in superconductors. But so far, no-one has ever found conclusive evidence that such particles exist – so if this nanowire result holds up, it would be quite the coup for Kouwenhoven and his group.

Unfortunately, Kouwenhoven’s talk was so popular that the crowd overflowed into the hallway outside, and with conference centre staff talking anxiously about fire regulations, it proved impossible for me to squeeze in (Eugenie Reich of Nature was luckier – you can read her summary here. So instead, I headed to the room next door, where Krastan Blagoev of the US National Science Foundation was delivering an inspiring talk on the kinetics of metastatic cancer.

After making a number of refreshingly frank comments about drug companies (“they only want the strongest, healthiest patients for their trials” was among the milder ones), Blagoev concluded by observing that despite years of research and billions of dollars, there is still “no unifying theory of how this disease progresses” and that physicists could make a significant impact on the field.

What followed was a string of excellent talks from physicists who are beginning to do just that. First up was Christian Kunert, a theoretical physicist working in a tumour biology group at the Massachusetts General Hospital. He showed off some simulations of fluid flow through the lymphatic system, which is implicated in the spread of secondary cancers, or metastases, around the body. Later, Louis McLane explained how he and his colleagues at the Georgia Institute of Technology are probing the protein coats of cells. Their experimental set-up employs an optical trap to slide a tiny non-stick bead in and out of a cell coating, allowing them to build up a detailed profile of the forces acting on the bead when it is at equilibrium inside the coating. Certain mechanical and structural changes in this cell coating are thought to be associated with accelerated tumour growth and metastases, so it’s useful to know more about its properties.

My two favourite talks of the session, though, were by Igor Sokolov and Muna Aryal. Sokolov, a physicist at Clarkson University in New York, is studying the emergence of fractal characteristics in the surfaces of malignant cells. It’s been known for some time that the surfaces of cancer cells exhibit fractal behaviour, while healthy cell surfaces do not. Now, however, Sokolov and his group are trying to turn this observation into a diagnostic tool. At the moment, they are working on cervical cancer cells, but after the session Sokolov told me that other types of cancer may end up being more important, since they are harder to diagnose at early stages.

Aryal’s talk was probably the most unusual of the bunch, at least in the context of a physics conference. She and her colleagues at Harvard Medical School are exploring a novel method of breaking the so-called “blood–brain barrier” that prevents chemotherapy drugs from reaching brain tumours. By applying focused, 10 ms bursts of ultrasound to the brains of tumour-ridden rats, they have shown that they can temporarily break down this barrier, opening a “window” of a few hours in which drugs can get through.

The results of their studies were promising: rats treated with a combination of focused ultrasound and conventional chemotherapy lived, on average, significantly longer (34.5 days) than those who received just chemotherapy (21 days), just ultrasound (18.5 days), or no treatment at all (16 days). In several cases, the tumours in rats treated with the combination therapy practically disappeared, decreasing visibly in size for up to 98 days.

It’s not every day that I see images of diseased rat brains in physics talks, but on the evidence of yesterday afternoon’s session, I could definitely get used to it. The physics of cancer may not have the cachet of Majorana fermions quite yet, but it’s clearly a field that’s going places.