Scientists measure single cells

Previously, such precise measurements of living cells were impossible because any sample would need to be dried - a process that kills the cells.

Samples as light as one thousandth of a millionth of a millionth of a gram (one femtogram) can now be weighed while they remain in fluid.

The work by US researchers is published in the journal Nature.

"We turned the problem inside out"
Thomas Burg

Thomas Burg, an author of the study from the department of biological engineering at the Massachusetts Institute of Technology (MIT), said: "The most precise mass measurements done today, down to the zeptogram (one thousandth of a billionth of a billionth of a gram) [in weight], require that objects are weighed in a vacuum.

"But so far, we haven't been able to do this for living biological samples."

So the team, led by Dr Scott Manalis, an associate professor of biological and mechanical engineering at MIT, decided to re-examine these ultra-accurate systems - called micromechanical resonators - to see if they could be modified to measure materials in fluid.

Traditional micromechanical resonators work by attaching a sample to a tiny solid silicon slab, called a resonator, within a vacuum and making it vibrate. Because objects of different mass vibrate at different frequencies, their mass can then be calculated.

But if fluid is then added to the vacuum, the sensitivity of the measurement is diminished, Dr Burg explained.

Diagnostics

"So, we turned the problem inside out," he said.

"We decided to make a hollow resonator, within which you could put the fluid sample. The resonator is still surrounded by a vacuum, but the fluid is inside of it. So you 'ping' it, it vibrates, and you can then look at the frequency to determine the mass."

The researchers say this means that living cells and other samples that need to be kept in fluid can be measured to a much higher degree of accuracy than was possible before.

They hope to develop the technique further so even lighter objects can be weighed, and in the future it may have a number of applications in cell biology, such as seeing how the mass of a cell changes as it goes through cell division.

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