Picosecond Ultrasonics Probe Human Cells

French researchers have employed high-frequency sound waves to examine the viscosity and stiffness of the nuclei of individual human cells. The scientists foresee that the probe could ultimately help resolve questions such as how cells bind to medical implants and why healthy cells become cancerous.

“We have developed a new noncontact, noninvasive tool to measure the mechanical properties of cells at the sub-cell scale,” stated Bertrand Audoin, a professor in the mechanics laboratory at the University of Bordeaux (France). “This can be useful to follow cell activity or identify cell disease.” The research was presented at the 57th annual meeting of the Biophysical Society (BPS), held February 2-6, 2013, in Philadelphia (PA, USA).

The technology that was used, called picosecond ultrasonics, was first applied in the electronics industry in the mid-1980s as a way to gauge the thickness of semiconductor chip layers. Prof. Audoin and his colleagues, in collaboration with a research group in biomaterials led by Marie-Christine Durrieu from the Institute of Chemistry & Biology of Membranes & Nano-objects at Bordeaux University, modified picosecond ultrasonics to research living cells. They grew cells on a metal plate and then flashed the cell-metal interface with an ultra-short laser pulse to generate high-frequency sound waves. Another laser measured how the sound pulse propagated through the cells, providing the investigators with insights into the mechanical characteristics of the individual cell components.

“The higher the frequency of sound you create, the smaller the wavelength, which means the smaller the objects you can probe,” stated Prof. Audoin. “We use gigahertz waves, so we can probe objects on the order of a hundred nanometers.” For comparison, a cell’s nucleus is about 10,000-nm wide.

The scientists faced hurdles in applying picosecond ultrasonics to study biologic systems. One challenge was the fluid-like substance characteristics of the cell. “The light scattering process we use to detect the mechanical properties of the cell is much weaker than for solids,” said Prof. Audoin. “We had to improve the signal to noise ratio without using a high-powered laser that would damage the cell.” The scientists also faced the challenge of natural cell variation. “If you probe silicon, you do it once, and it’s finished,” noted Prof. Audoin. “If you probe the nucleus you have to do it hundreds of times and look at the statistics.”

The investigators developed techniques to overcome these challenges by testing their techniques on polymer capsules and plant cells before moving on to human cells. In the coming years, the team envisions studying cancer cells with sound. “A cancerous tissue is stiffer than a healthy tissue,” noted Prof. Audoin. “If you can measure the rigidity of the cells while you provide different drugs, you can test if you are able to stop the cancer at the cell scale.”

Related Links:
University of Bordeaux