High-speed imaging of gas-bubble formation during femtosecond-laser cell optoporation

Femtosecond-laser pulses can create transient holes in the membrane of a cell, making it briefly permeable to genetic macromolecules. This is a highly effective method for cell transfection and reprogramming. For sufficiently high irradiance values, the laser radiation leads to plasma formation through multiphoton ionization and a subsequent formation of gas-filled bubbles which are shortly visible after the irradiation. While this bubble formation is well known, the underlying microscopic processes, the optimal bubble size and duration which indicate the transient hole formation are less clear. The correspondence between bubble formation and successful optoporation is further complicated by the fact that the formation greatly depends on the irradiated cell position and laser irradiation parameters (power, exposure time). We have investigated the formation of bubbles resulting from short pulse irradiation with two commercial Ti:sapphire lasers using a high-speed camera. Higher laser powers and longer exposure times yielded bigger bubbles which took longer to collapse. Additionally, a correlation between the bubble characteristics and the cell's metabolism and post-optoporation viability was found. These results can help to optimize the laser parameters for efficient optoporation and high post-treatment cell viability as well as to shine light on the microscopic excitation processes behind the bubble formation.