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Tiny hands haul lilliputian loads

BioPhotonics
Feb 2007
Michael A. Greenwood

Researchers at the University of Glasgow in the UK are trying to give their colleagues in the scientific community a helping hand — a very, very, very small helping hand.

They have developed an optical trapping technique that relies on holography and allows the user to manipulate in real time micron-size particles into almost any possible configuration. Head of the group, Miles Padgett, a professor of optics, said the ability to control the position of micro- and nanoscale particles has many applications in current and future biomedical research.

Although optical tweezers have been a laboratory tool for years, their effectiveness is sometimes limited by their reliance on the gradient force generated by the interaction of transparent objects with focused laser light. As a result, trapping opaque particles can be difficult.

With the holographic optical tweezers technique, a laser beam is split into multiple traps with a spatial light modulator. The user’s fingertips are mapped to the position of silica beads captured in the optical traps, creating, in essence, a versatile microhand that directly corresponds to the researcher’s finger movements. The setup enables fine-tuning of the arrangement of particles being studied. The researcher can nudge an individual particle or grasp multiple particles and create complex shapes — even 3-D geometric patterns. Multiple optical traps can be controlled independently or simultaneously.

BNHand_Fig-2_HAND-4.jpg

A camera images the coordinates of white beads attached to fingertips. The position of each fingertip is mapped to the position of optical traps, providing direct, visually controlled manipulation of microscale objects. Reprinted with permission of Optics Express.

During experiments, the researchers defined the position of several traps and mapped a hologram onto an 8-bit gray-scale image. This was relayed onto a HoloEye spatial light modulator via a video card interface. A desktop computer calculated and displayed a 512 × 512-pixel hologram at 8 fps, adequate for real-time interface.

The optical tweezers were positioned using an inverted Zeiss microscope with a 1.3-NA, 100× objective lens. The diode-pumped, frequency-doubled solid-state trapping laser emitted up to 3 W of 515-nm light. A web camera positioned above the work area imaged the X- and Y- positions of white beads attached to the fingers of black gloves worn by the user. The Z-position was inferred from the beads’ size in the image. The positions were then scaled and fed into a hologram calculation algorithm to produce optical traps at corresponding positions within the 3-D space accessible to the traps.

Fingertip positions were mapped to the position of each trapped bead within the tweezers. To improve the system’s stability, researchers set the maximum speed limit of the traps at 5 μm/s. If the fingers moved too rapidly, the trap positions froze. To regain control, the fingers had to be repositioned on the traps. Researchers monitored the corresponding motions of the fingertips and the optical traps on a split-screen video.

BNHand_Fig-1_hands2.jpg
The user’s fingers (left) control the movement of a microhand based on optical tweezers (right) to manipulate a red blood cell.

They explained that the microhand interface enhances the range of applications for optical trapping. But the holographic technique still has a few glitches to be worked out. Trapping multiple particles at various axial positions can be difficult. Directly trapping biological samples, such as cells, also is challenging. The powerful illumination involved can harm the cell, and the low contrast in the refractive index between the cell and the surrounding fluid creates a weak trapping force. The algorithms necessary for hologram design also can be very challenging to create.

Optics Express, Dec. 11, 2006, pp. 12497-12502.


GLOSSARY
holography
The optical recording of the object wave formed by the resulting interference pattern of two mutually coherent component light beams. In the holographic process, a coherent beam first is split into two component beams, one of which irradiates the object, the second of which irradiates a recording medium. The diffraction or scattering of the first wave by the object forms the object wave that proceeds to and interferes with the second coherent beam, or reference wave at the medium. The resulting...
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