Changes in the masses of microscopic living cells have been measured using a tiny vibrating cantilever. By monitoring the resonant frequency of the cantilever, researchers in Switzerland and the UK could detect changes in mass as small as 1%. ETH Zurich’s Daniel Müller, says that this new ability to measure cell mass introduces "a new parameter into biology", which the team has already used to made new discoveries about cells' behaviour. The researchers believe the technique could have a wide range of applications including stem-cell biology, drug discovery and even cancer research.

Tricky measurement

Measuring the size of living cells has been integral to biology for decades, but accurately tracking their mass is much trickier. For over 50 years, the basis of cell analysis has been flow cytometry. This determines the size of cells by measuring the changes in the electrical resistance and/or the optical properties of a solution of cells when it passes through a narrow tube.


“It's like trying to characterize the behaviour of a Swiss cow on the moon”
Daniel Müller, ETH Zurich

"These devices are very powerful," says David Martínez Martín, also of ETH Zurich. "Doctors use them to do a blood analysis to tell you the size of your red blood cells and see, for example, whether you have anaemia."

The technique has several limitations, however. It measures volume but not mass, so changes in density are undetectable. Also, it cannot study changes in specific cells on short time scales. Finally, cells may behave differently if extracted from tissue and placed in solution. "It's like trying to characterize the behaviour of a Swiss cow on the moon," says Müller. "It's just not a native environment."

Small masses can be measured using a tiny cantilever like that used in a scanning probe microscope. When a mass is attached to the free end, the cantilever's resonant frequency drops and this drop can be measured. Similar principles have already been used to measure the mechanical properties of living cells by using piezoelectric materials to drive the cantilever or simply using its natural thermal oscillations. The noise in these oscillations, however, compromises the cantilever's sensitivity to tiny changes in cell mass.

Müller and colleagues attach single mammalian cells or small cell clusters to a cantilever, which is then set oscillating by a laser beam that is modulated at the cantilever's resonant frequency. A second laser is used to measure the actual frequency of oscillation and an electronic feedback loop adjusts the modulation frequency to ensure that the cantilever is always driven on resonance.

Cool running

This presented a challenge to the team, explains Martínez Martín, because they had to excite sufficient oscillations of the cantilever to measure changes in its resonant frequency without heating it up and destroying the cells. "Most other physicists said it would not work," he says. Detecting oscillation amplitudes as small as 0.1 nm allowed the researchers to use microwatt laser powers and thereby keep the temperature of the cantilever within 0.1°C of their desired temperature for days.

The team could monitor changes of around 15 pg (approximately 1–4% of a cell's mass), with a time resolution of 10 ms. They noticed two distinct sets of oscillations – one with period about 2 s and one with period around 18 s – neither of which had been seen before. The researchers found the oscillations were suppressed when they disrupted cellular energy and water exchange, so they concluded these processes were responsible for the cyclic changes in mass.

The team also studied the cells' response to viral infection. Over 40 h, healthy cells were seen to increase in mass as they grew and divided. The mass of cells infected with a virus, however, did not increase. This was unexpected as an infected cell continuously produces new virus particles that then burst out from inside it. "Most people would have said 'Of course a cell grows if it produces viruses', although there was no real data until now," says Müller.

Cell regulation

The researchers believe the technique could find numerous applications: "We've got an enormous response from biologists," says Müller adding that it will allow scientists to study how cells regulate their masses and volumes. Crucially, the technique could reveal how this regulation is disrupted by disease.

"Within cell biology, mass is not something that you regularly see measured and reported," says Thomas Burg of the Max Planck Institute for Biophysical Chemistry in Göttingen, Germany, who was not involved in the research. "I think this [work] will significantly contribute to raising awareness that mass measurements can reveal interesting phenomena in cells and to raising new questions and hypotheses around mass, which will lead to new insights into how cells develop, live and grow."

The device is described in Nature.