Bullet jet as a tool for soft matter piercing and needle-free liquid injection.

The collapse of a laser-induced vapor bubble near a solid boundary usually ends in a liquid jet. When the boundary is from a soft material the jetting may pierce the liquid-solid interface and result in the injection of liquid into it. A particular impulsive jet flow can be generated when a laser pulse is focused just below the free surface of a thin liquid layer covering a gelatin sample used as a surrogate of biological tissue. Here, a downwards jet forms from a liquid splash at the free surface and then penetrates through the liquid layer into the soft boundary. In the present manuscript we report on the use of this novel jet, termed "bullet" jet, to pierce soft materials and we explore its potential to become an optical needle-free injection platform. The dynamics and depth of the injection is studied as a function of the elasticity of the solid and the liquid properties. Injections of up to 4 mm deep into 4 %w/w gelatin within 0.5 ms are observed. The advantages of the bullet jet over other kinds of impulsively generated jets with lasers are discussed.