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Fiber Optic ‘Wrench’ Twists, Turns Tiny Particles

The fiber optic equivalent of the world’s smallest wrench, dubbed the fiber optic spanner, can precisely twist and turn microscale objects in any direction and along any axis without moving any optical component.

Developed at The University of Texas at Arlington, this new twist on controlling the incredibly small will allow scientists to manipulate single cells for cancer research, to twist and untwist individual strands of DNA, and to perform many other functions where microscopic precision is essential.

Fiber optically trapped and rotated human smooth muscle cell in the center of two transversely offset fibers (20 mW in each arm). The fiber optic equivalent of the world's smallest wrench was developed at UT Arlington and will allow will allow scientists to manipulate single cells for cancer research. Images courtesy of Optics Letters.

The ability to create rotation along any axis and in any direction is the distinguishing feature that sets this virtual tool apart from other optical tools. The innovation uses flexible optical fibers rather than stationary lasers, making it possible to position the fibers inside the human body, where they can manipulate and help study specific cells or potentially guide neurons in the spinal cord.

Rather than an actual physical device that wraps around a cell or other microscopic particle to apply rotational force, the spanner (the British term for a wrench) is created when two laser beams — emitted by a pair of optical fibers — strike opposite sides of the microscopic object, trapping and holding it in place. By slightly offsetting the fibers, the beams can impart a small twisting force, causing the object to rotate in place. It is possible to create rotation along any axis and in any direction, depending on the positioning of the fibers.

Individual photons impart a virtually imperceptible bit of force when they strike an object, but an intense laser beam 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,” said assistant professor of physics Samarendra Mohanty. The method is already used to perform optical tweezing, which moves objects backward and forward along a straight line.

Principle of a fiber optic spanner composed of transversely offset fibers. The device shows more control and versatility than optical tweezers.

“Optical tweezing is useful for biomedical and microfluidic research, but it lacks the control and versatility of our fiber optic spanner, especially when it comes to working deep inside,” Mohanty said.

The physicists rotated and shifted human smooth muscle cells without damaging them, demonstrating that the technique could have both clinical and laboratory use.

The spanner also could be used to rotate cells in a microfluidic analysis, to image them with tomography and then move them aside to allow the analysis of subsequent cells in the flow, or to rotate single cells to determine by their spin if they are normal or cancerous. Other possible applications include mixing or pumping the fluids in lab-on-a-chip devices or moving and rotating microspheres attached to the opposite ends of a DNA strand to stretch and uncoil the molecule, allowing it to be more efficiently sequenced.

Nonmedical macroscopic uses for the tool also are possible.

“I envision applications in the direct conversion of solar energy to mechanical energy, rotating large, macroscopic objects using this technique,” Mohanty said. This would “simulate an environment in which photons radiated from the sun could propel the reflective motors in solar sails, a promising future technology for deep-space travel.”

The findings were reported in Optics Letters

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