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This Letter reports a new, to the best of our knowledge, photoacoustic excitation method for evaluating the shear viscoelastic properties of soft tissues. By illuminating the target surface with an annular pulsed laser beam, circularly converging surface acoustic waves (SAWs) are generated, focused, and detected at the center of the annular beam. The shear elasticity and shear viscosity of the target are extracted from the dispersive phase velocity of the SAWs based on the Kelvin–Voigt model and nonlinear regression fitting. Agar phantoms with different concentrations, and animal liver and fat tissue samples have successfully been characterized. Different from previous methods, the self-focusing of the converging SAWs allows sufficient SNR to be obtained even with low pulsed laser energy density, which makes this approach well compatible with soft tissues under bothex vivoandin vivotesting conditions.
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This Letter reports ring-shaped photoacoustic (PA) tweezers that are capable of manipulating single or multiple micron-sized particles. By illuminating a thin layer of an optically absorptive liquid medium with a focused annular pulsed laser beam and a higher pulse repetition rate (e.g., 800 Hz), both acoustic radiation force and instantaneous vaporization repulsion are generated within a certain distance of the illumination region. This makes it possible to conduct continuous and versatile locomotion of single or multiple microparticles. In this Letter, interactions between two or more particles are demonstrated, such as separation, attachment, and grouping of microparticles. The PA tweezers combine some of the advantages of conventional optical and acoustic tweezers and are expected to be a useful alternative approach for the manipulation of microscale objects.more » « less
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Oraevsky, Alexander A.; Wang, Lihong V. (Ed.)
