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New method offers alternative to optical trapping

Apr 2006
Gary Boas, News Editor,

Researchers use optical trapping to answer a variety of biological questions. However, the technique requires optical systems with high numerical apertures and short working distances and is applicable only in shallow samples of fluid with low scattering and absorption.

Now scientists with the National Research Council of Canada in Ottawa and at the University of Western Ontario in London have shown that they can overcome these limitations by performing soft trapping and manipulation of cells with a microbubble formed at the end of an optical fiber immersed in liquid.

Researchers have demonstrated a method for trapping and manipulating cells using a microbubble formed at the end of an optical fiber in a modified near-scanning optical microscopy probe. The technique enables trapping inside large volumes of biologically relevant fluids, in which high scattering and absorption typically hinder optical tweezing. Courtesy of Mamadou Diop.

Their work grew out of an earlier study in which they created a long-lifetime microbubble with continuous-wave laser radiation, demonstrating that they could move it easily and quickly in three dimensions to pick up one or more microparticles. They noted that the bubble deformed locally as it lifted particles from a surface, allowing for a very gentle trapping mechanism, according to researcher Mamadou Diop.

The investigators reasoned that they might be able to remove individual living cells from a glass coverslip without damaging them. Trapped on the surface of the microbubble, these cells could be manipulated, imaged, probed and released.

To demonstrate the potential of the technique, they used it to trap vigorously moving swine sperm cells as well as human embryonic kidney cells. They generated the microbubble using a laser made by Lightwave Electronics Corp. of Mountain View, Calif. (now JDSU Corp.), emitting at 1320 nm. A microscope objective with a numerical aperture of 0.32 coupled the light into the core of a single-mode optical fiber made by Fibercore Ltd. of Southampton, UK. The opposite end of the fiber — with a chemically etched and platinum-coated conical tip with a hollow reservoir — was placed in the medium in which the bubble was to be generated. The fiber itself was fixed in a holder mounted on a stage that could be tilted and moved in three dimensions with micron precision.

The method trapped and manipulated vigorously moving swine sperm cells without damaging them. The instrumentation permitted the researchers to manipulate the cells in three dimensions with micron precision and to image and probe them.

The scientists demonstrated trapping and manipulation of cells by moving the stage or by moving the glass slide containing the medium. They imaged the fiber tip region using an overhead microscope from Nikon Instruments of Tokyo and either a digital camera from Nikon or a high-performance monochrome CCD camera made by Cooke Corp. of Auburn Hills, Mich.

The setup proved easier to use than the sophisticated high-numerical-aperture systems typically required for optical trapping. In addition, the microbubble enabled trapping of cells in biologically relevant fluids, including fluids with high scattering and absorption.

“This is difficult to do [with conventional optical trapping methods], since the focused optical beam must propagate through the fluid,” Diop said, “whereas, for the microbubble, the laser radiation is delivered to the microtip from inside the fiber.” The latter approach, therefore, allows trapping inside large volumes of fluids, well beyond the few hundred microns that can be achieved using a highly focused beam.

There are some disadvantages. For example, the method offers poorer trapping accuracy than conventional techniques. In principle, the microbubble can trap single submicron particles, Diop said. However, it would be difficult to do so if the particle’s nearest neighbors were less than a few microns away. Also, it cannot trap biological material inside a cell.

The researchers plan to use the technique to study the pulmonary surfactant films that form in the lungs. Deficiency of such a film in premature infant lungs can lead to alveolar collapse, which could be life-threatening.

“The mechanism promoting the transformation of lung surfactant particles into surface-activated components is poorly understood,” Diop said. “The miniaturized and controllable liquid-air interface represented by the microbubble offers an attractive tool to study these mechanisms in order to have a better understanding of pulmonary surfactant formation.

Biophysical Journal BioFast, published online Feb. 24, 2006; doi: 10.1529/biophysj.105.075614.


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