Watching bubbles puncture a cell membrane

During the collapse of a cavitation bubble, the gas inside it can reach temperatures of more than 15 000 K—as hot as the surface of a star—and the energy can be released in the form of shock waves, heat, light, turbulent vortices, and high-speed jets of fluid. For decades, medical researchers have worked to harness that energy for therapeutic applications, such as the disintegration of cancerous tumors using focused ultrasound (see the article by Gail ter Haar in PHYSICS TODAY, December 2001, page 29 ) and the delivery of drugs or genes into living cells (see the article by Detlef Lohse in PHYSICS TODAY, February 2003, page 36 ). Although studies have demonstrated that microbubbles can rupture nearby cells, control over the bubble–cell interaction has remained difficult. Duke University researchers led by Pei Zhong have now demonstrated an approach to puncturing a cell’s membrane that entails carefully manipulating the fluid dynamics around it. The high-speed sequence of photographs captures the process: Two laser pulses, offset in space by 40 µm and time by 4 µs, create two bubbles (B₁ and B₂) that act in concert. The rapid expansion of the second bubble causes the collapse of the first bubble by pressing against it; the interaction deforms the shape of both. The bubbles’ asymmetric collapse gives rise to two localized microjets—one toward the cell between 6 and 7 µs, one away from it 2 µs later. The researchers can control the microjets’ impact by adjusting the bubbles’ position, spacing, and orientation relative to the cell. (G. N. Sankin, F. Yuan, P. Zhong, Phys. Rev. Lett., in press.)—R. Mark Wilson