Generation and control of underwater microshock waves and microbubbles by a femtosecond pulse laser

Abstract

This paper describes the generation and control of underwater microshock waves and microbubbles by a femtosecond pulse laser for regenerative medicine applications. To achieve local stimulation of individual cells in this field, it is essential to generate and control microshock waves at the same scale as cells. Consequently, the use of femtosecond pulse lasers has been suggested by researchers due to their noninvasive nature when generating microshock waves. However, the characteristics and control methods of microshock waves and microbubbles have not been sufficiently investigated. In this research, the laser-induced microshock waves were generated by a femtosecond laser with a pulse duration of 260 fs and a pulse energy of \(2.1\,\upmu \hbox {J}\). First, pressure measurements of the shock waves were carried out, and their overpressure was found to exceed 0.3 MPa at a distance of \(300\,\upmu \hbox {m}\) from the laser focal area. Second, the generation and behavior of microbubbles were successfully observed by optical measurements. A single bubble was generated when the femtosecond pulse laser was focused into water, and it subsequently expanded and contracted according to the Rayleigh–Plesset equation. In addition, its initial behavior was observed, and a comparison between optical measurements and high-speed images revealed that the shock waves were generated 200 ns after the laser has focused.