Is a step or two closer to reality. Claus-Dieter Ohl and Roy Ikink (University of Twente, the Netherlands) found that tiny bubbles (7–55 µm across) floating in water develop needlelike tips when exposed to acoustic shock waves. The effect was seen with pressure amplitudes of 11–21 MPa, much smaller than needed, for example, to break up kidney stones. The tips formed in the direction of the propagating shock, when the liquid surrounding each bubble was accelerated through the bubble’s center and pierced the opposing bubble wall, much like the jet from a syringe. The researchers suggest that, if the tip can actually penetrate a cell’s membrane, then drug-coated bubbles could be used for in vivo local drug delivery. Separately, Phillippe Marmottant and Sascha Hilgenfeldt (also at Twente) have experimented with bubbles attached to a substrate. They showed that very gentle bubble oscillations—in an acoustic field of only 0.01 MPa—can...

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