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Dec 4, 2012

Tiny Wrench Untwists DNA

  • Researchers at The University of Texas at Arlington say they’ve created the fiber-optic equivalent of the world’s smallest wrench. The team claims this virtual tool can precisely twist and turn the tiniest of particles, giving scientists the ability to skillfully manipulate single cells for cancer research, twist and untwist individual strands of DNA, and perform many other functions where microscopic precision is essential.

    The technique can spin or twist microscale objects in any direction and along any axis without moving any optical component, according to the scientists. It’s able to do this because it uses flexible optical fibers rather than stationary lasers to do the work. The optical fibers can be positioned inside the human body, where they can manipulate and help study specific cells or potentially guide neurons in the spinal cord.

    This fiber-optic spanner, as the team is calling it (spanner is the British term for wrench), is created when two beams of laser light—emitted by a pair of optical fibers—strike opposite sides of a microscopic object. Individual photons impart a virtually imperceptible bit of force when they strike an object, but an intense beam of laser light can create just enough power to gently rotate microscopic particles.

    “When photons of light strike and then get reflected back from an object, they give it a small push from an effect called scattering forces,” explains Samarendra Mohanty, assistant professor of physics at The University of Texas at Arlington and lead author of the study. This technique is already used to perform optical “tweezing”, which can move an object forward and backward along a straight line. “Optical tweezing is useful for biomedical and microfluidic research,” said Mohanty. “But it lacks the control and versatility of our fiber optic spanner, especially when it comes to working deep inside.”

    In the team’s new optical spanner, the optical fibers use laser beams to first trap an object and then hold it in place. By slightly offsetting the optical fibers, the beams are able to impart a small twisting force, which causes the object to rotate in place. Depending on the positioning of the fibers, it is possible to create rotation along any axis and in any direction. This can greatly enhance researchers’ ability to study and image cells and groups of cells for biological research and medical analysis.

    In their research, Mohanty and graduate student Bryan Black used their new technique to rotate and shift human smooth muscle cells without damaging them. The spanner could rotate cells in a microfluidic analysis, image them with tomography, and then move them aside to allow the analysis of subsequent cells in the flow.

    The team sees much potential in their new technique. They say it could be used to rotate single cells to determine by their spin if they are normal or cancerous. It could also help examine embryos to aid in in vitro fertilization. It could mix or pump the fluids in lab-on-a-chip devices, or move and rotate microspheres attached to the opposite ends of a DNA strand to stretch and uncoil the molecule, allowing it to be sequenced more efficiently.

    The authors describe their new technique in a paper called “Fiber-Optic Spanner”, published online in the December 15 issue of Optics Letters.


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