Microscope probes living cells at the nanoscale
Nov 18, 2011 2 comments
Researchers in the US and UK say they have invented a new microscopy technique for imaging live tissue with unprecedented speed and resolution. The technique involves using the tiny tip of an atomic force microscope to tap on a living cell and analysing the resulting vibrations to reveal the mechanical properties of cell tissue. The team says that the technique could have widespread applications in medicine. However, another expert in the field suggests that the group has not demonstrated the superiority of the technique to those already available.
Atomic force microscopy (AFM)is a standard technique for obtaining images of a surface at resolutions of a few nanometres. A needle sharpened to just one atom thick is mounted on the end of a flexible metal bar called a cantilever. The cantilever is vibrated, causing the needle to move up and down rapidly. The needle's tip is then brought so close to the surface that intermolecular forces between the two affect the movement of the needle. By scanning the cantilever across the sample, an image of the forces – and therefore the surface – is obtained.
One useful application of AFM is imaging living tissue. Whereas scanning tunnelling microscopy and electron microscopy usually require radical preparation of samples in ways that would kill a live organism, AFM can potentially work well with living cells because extreme preparation is not needed.
There are problems, however, including the fact that the interaction between soft tissue and the needle is more complex than when imaging a hard surface. One of the researchers, Arvind Raman of Purdue University, likens it to the difference between tapping a pen on a table top and on a cotton puffball. "Instead of tapping on it, you're going to be buried inside it and just moving up and down," he says.
Raman and colleagues at Purdue and the University of Oxford vibrate their AFM cantilever at a frequency of 7 kHz. They then bring it into gentle contact with the sample. Because of the squishiness of the soft tissue, it sinks under the force of the needle. The presence of the tissue interferes with the vibration of the needle so that, instead of containing just one pure frequency at 7 kHz, the cantilever vibrations now contain several harmonics of this frequency.
By analysing these harmonics, the researchers build up maps of the mechanical properties of different sorts of soft tissue, including bacteria and two types of cell. They claim to have enhanced the speed with which such properties can be mapped by a factor of 10–1000, a breakthrough that they say could have widespread applications in medicine, including watching cancers spreading and finding out how new drugs work.
One leading scientist in the field, however, believes that the research has a number of flaws, notably that other schemes already exist that are superior to the one described. The researcher, who has asked not to be named, argues that two commercially available atomic force microscopes can achieve greater speed and resolution in imaging soft tissue than a microscope using the method developed by the Purdue/Oxford collaboration.
Raman, however, feels that this criticism is unfair, because his group's technique is designed to be implemented on a standard atomic force microscope. "What we're talking about is using traditional AFM – where you don't have to buy a new microscope. You can take conventional AFM and up the speed of that," he says.
The anonymous critic also suggests that the needle's permanent contact with the tissue could cause problems with friction between the needle and the tissue sample, which could introduce artefacts into the images produced. In particular, the critic says that the researchers have not ruled this out by imaging a standard sample, such as a silicone gel, and checking their results against an image made using an accepted technique to check that they get the same results.
Raman accepts that this would have been a desirable check but argues that finding a standard sample soft enough to mimic the "puffball" properties of living tissue is not straightforward. "It's hard to find standard samples that you can use for validation," he says. "Having said that, we are currently looking at adapting the theory to allow for intermittent contact. That is an ongoing work."
The research is published in Nature Nanotechnology.
About the author
Tim Wogan is a science writer based in the UK