Laser beam triggers a membrane breach.

The cell membrane serves as a tough, flexible barrier that protects a cell from its environment and holds in its components. Composed largely of fat molecules called lipids, this thin skin offers tremendous resistance to rupture and normally doesn't allow internal structures to pop out.

Researchers have now demonstrated that a low-power laser beam focused on an artificial structure that mimics a cell membrane can trigger the spontaneous ejection of material inside without permanently damaging either the membrane or the expelled object.

Physicists J. David Moroz and Philip Nelson of the University of Pennsylvania in Philadelphia and Roy Bar-Ziv and Elisha Moses of the Weizmann Institute of Science in Rehovot, Israel, report their findings in an article to be published in Physical Review Letters.

The technique is a new way of selectively disrupting membranes, the researchers say. The ability to induce expulsion "hints at the exciting practical possibility of transforming membrane structure when and where we wish to do so."

In water, lipid molecules spontaneously assemble themselves into closed, saclike structures, or vesicles, analogous to cells. One vesicle can form inside another.

Focused on a lipid membrane, a laser beam's electromagnetic field induces molecular changes that tighten the membrane, increasing internal pressure. The disturbance causes the system to act as a pump, pulling water in by osmosis and forcing internal material out. When a vesicle floating within the sac encounters the outer wall, it sticks. Within a few seconds, it begins to emerge, gradually passing through the outer membrane.