Probing single-cell mechanics with picosecond ultrasonics.

The mechanical properties of cells play a key role in several fundamental biological processes, such as migration, proliferation, differentiation and tissue morphogenesis. The complexity of the inner cell composition and the intricate meshwork formed by transmembrane cell-substrate interactions demands a non-invasive technique to probe cell mechanics and cell adhesion at a subcell scale. In this paper we review the use of laser-generated GHz acoustic waves--a technique called picosecond ultrasonics (PU)--to probe the mechanical properties of single cells. We first describe applications to vegetal cells and biomimetic systems. We show how these systems can be used as simple models to understand more complex animal cells. We then present an opto-acoustic bio-transducer designed for in vivo measurements in physiological conditions. We illustrate the use of this transducer through the simultaneous probing of the density and compressibility of Allium cepa cells. Finally, we demonstrate that this technique can quantify animal-cell adhesion on metallic surfaces by analyzing the acoustic pulses reflected off the cell-metal interface. This innovative approach allows investigating quantitatively cell mechanics without fluorescent labels or mechanical contact to the cell.

Interferometric detection in picosecond ultrasonics for nondestructive testing of submicrometric opaque multilayered samples: TiN/AlCu/TiN/Ti/Si.

An experimental investigation of nanometric thin films by a picosecond ultrasonic technique is presented. A photoelastic model is used with an interferometric device, combined with ultrafast optical pump and probe setup, to measure the thicknesses of submicrometric layers made of TiN, Ti, and AlCu deposited on silicon (Si) wafers. The results are in good agreement with ellipsometry measurements showing that the picosecond ultrasonic technique can give accurate results even when the reflectance signal is very low. Additional important results are first, that the adhesion of the TiN surface film is probed by processing both the frequency and the damping of the oscillation of a resonance acoustic mode; and second, the presence of a thin buried TiN layer under an opaque AlCu film is highlighted by the interferometric setup.