Background: The development of inexpensive small flow cytometers is recognized as an important goal for many applications ranging from medical uses in developing countries for disease diagnosis to use as an analytical platform in support of homeland defense. Although hydrodynamic focusing is highly effective at particle positioning, the use of sheath fluid increases assay cost and reduces instrument utility for field and autonomous remote operations. Methods: This work presents the creation of a novel flow cell that uses ultrasonic acoustic energy to focus small particles to the center of a flowing stream for analysis by flow cytometry. Experiments using this flow cell are described wherein its efficacy is evaluated under flow cytometric conditions with fluorescent microspheres. Results: Preliminary laboratory experiments demonstrate acoustic focusing of flowing 10‐μm latex particles into a tight sample stream that is ∼40 μm in diameter. Prototype flow cytometer measurements using an acoustic‐focusing flow chamber demonstrated focusing of a microsphere sample to a central stream ∼40 μm in diameter, yielding a definite fluorescence peak for the microspheres as compared with a broad distribution for unfocused microspheres. Conclusions: The flow cell developed here uses acoustic focusing, which inherently concentrates the sample particles to the center of the sample stream. This method could eliminate the need for sheath fluid, and will enable increased interrogation times for enhanced sensitivity, while maintaining high particle‐analysis rates. The concentration effect will also enable the analysis of extremely dilute samples on the order of several particles per liter, at analysis rates of a few particles per second. Such features offer the possibility of a truly versatile low‐cost portable flow cytometer for field applications. © 2005 Wiley‐Liss, Inc.

Creation of inexpensive small-flow cytometers is important for applications ranging from disease diagnosis in resource-poor areas to use in distributed sensor networks. In conventional-flow cytometers, hydrodynamics focus particles to the center of a flow stream for analysis, which requires sheath fluid that increases consumable use and waste while dramatically reducing instrument portability. Here we have evaluated, using quantitative measurements of fluorescent microspheres and cells, the performance of a flow cytometer that uses acoustic energy to focus particles to the center of a flow stream. This evaluation demonstrated measurement precision for fluorescence and side scatter CVs for alignment microspheres of 2.54% and 7.7%, respectively. Particles bearing 7 x 10(3) fluorophores were well resolved in a background of 50 nM free fluorophore. The lower limit of detection was determined to be about 650 fluorescein molecules. Analysis of Chinese hamster cells on the system demonstrated that acoustic focusing had no effect on cellular viability. These results indicate that the ultrasonic flow cytometer has the necessary performance for most flow cytometry applications. Furthermore, through robust engineering approaches and the combination of acoustic focusing with low-cost light sources, detectors, and data acquisition systems, it will be possible to achieve a low-cost, truly portable flow cytometer.

Acoustic particle manipulation has many potential uses in flow cytometry and microfluidic array applications. Currently, most ultrasonic particle positioning devices utilize a quasi-one-dimensional geometry to set up the positioning field. A transducer fit with a quarter-wave matching layer, locally drives a cavity of width one-half wavelength. Particles within the cavity experience a time-averaged drift force that transports them to a nodal position. Present research investigates an acoustic particle-positioning device where the acoustic excitation is generated by the entire structure, as opposed to a localized transducer. The lowest-order structural modes of a long cylindrical glass tube driven by a piezoceramic with a line contact are tuned, via material properties and aspect ratio, to match resonant modes of the fluid-filled cavity. The cylindrical geometry eliminates the need for accurate alignment of a transducer/reflector system, in contrast to the case of planar or confocal fields. Experiments show that the lower energy density in the cavity, brought about through excitation of the whole cylindrical tube, results in reduced cavitation, convection, and thermal gradients. The effects of excitation and material parameters on concentration quality are theoretically evaluated, using two-dimensional elastodynamic equations describing the fluid-filled cylindrical shell with a line excitation.

Impulse response backscattering measurements are presented and interpreted for the scattering of obliquely incident plane waves by air-filled finite cylindrical shells immersed in water. The measurements were carried out to determine the conditions for significant enhancements of the backscattering by thick shells at large tilt angles. The shells investigated are made of stainless steel and are slender and have thickness to radius ratios of 7.6% and 16.3%. A broadband PVDF ͑polyvinylidene fluoride͒ sheet source is used to obtain the backscattering spectral magnitude as a function of the tilt angle ͑measured from broadside incidence͒ of the cylinder. Results are plotted as a function of frequency and angle. These plots reveal large backscattering enhancements associated with elastic excitations at high tilt angles, which extend to end-on incidence in the coincidence frequency region. Similar features are present in approximate calculations for finite cylindrical shells based on full elasticity theory and the Kirchhoff diffraction integral. One feature is identified as resulting from the axial ͑meridional ray͒ propagation of the supersonic a 0 leaky Lamb wave. A simple approximation is used to describe circumferential coupling loci in frequency-angle space for several surface waves. The resulting loci are used to identify enhancements due to the helical propagation of the subsonic a 0Ϫ Lamb wave. © 1998 Acoustical Society of America. ͓S0001-4966͑98͒04302-1͔PACS numbers: 43.30.Gv, 43.20.Fn ͓DLB͔ INTRODUCTIONRecent high-frequency sonar images of truncated cylindrical shells indicate that the visibility of the ends of the shell can be improved by an elastic response of the shell.1 The enhancements are associated with a category of lth class of leaky ray shown in Fig. 1. The enhancement occurs when the tilt angle ␥ of the cylinder is close to the leaky wave coupling angle l ϭsin Ϫ1 (c/c l ), where c l is the phase velocity of the leaky wave and c is the speed of sound in the surrounding water. This ray is referred to as a meridional ray 2 since it is propagated along the meridian defined by the direction of the incident wave vector and the cylinder's axis. An analysis shows that the backscattering enhancement is associated with the vanishing of Gaussian curvature of the wavefront backscattered in the direction of the receiver. The enhancements reported 1 were for tilts in the vicinity of 18°and 35°corre-sponding to the excitation of symmetric and antisymmetric (s 0 and a 0 ͒ generalizations of leaky Lamb waves on the stainless steel cylinder used in those experiments. The purpose of the present paper is to document the existence of high-frequency backscattering enhancements for tilted cylindrical shells relevant to larger values of the tilt ␥ and in some cases, extending to ␥ϭ90°͑i.e., end-on incidence͒.The method of our investigation concerns the global response in the frequency-angle domain rather than the spatial responses emphasized in Ref.1. There are several reasons for identifying such high-frequency enhancement...

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