Probing cell mechanics with femtosecond laser pulses

We use femtosecond laser pulses to selectively disrupt the cytoskeleton of a living cell and probe its mechanical properties. Our nanosurgery setup is based on a home-built fluorescence microscope with an integrated femtosecond laser. We severed single actin bundles inside live cells to probe the local dynamics of the cytoskeleton and correlate it to global changes in cell shape. Simultaneous cutting and imaging allows us to study immediate cellular response with several hundred-nanometer spatial and less than 500-ms time resolution. The targeted actin bundle retracts rapidly after laser cutting due to prestressed tension stored in the actin filaments. We show that actin bundles in living cells can be modeled as viscoelastic elements. Using this nanosurgery technique we can further understand and model the stress distribution in the actin filaments and elucidate its contribution to cell shape and function. We also studied the threshold pulse energy for nanosurgery as a function of the repetition rate of the laser. Previous work on femtosecond laser micromachining in glasses shows that at high laser repetition rates there is a marked accumulation effect that defines the morphology of the structure written in the glass. An equivalent effect is observed in biological cells, where pulse energy threshold and irradiation time necessary for nanosurgery is dependent on the laser repetition rate.