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Optical tweezers

Optical tweezers do the same thing as regular tweezers — pick up small objects and manipulate them. However, optical tweezers work at a much smaller scale and use light to capture and move objects. They use light in the form of a high-powered laser beam to hold and manipulate microscopically small objects such as biological molecules or even living cells.   First demonstrated over 20 years ago, optical tweezers have become an established tool in research fields ranging from biophysics to cell biology.


How does it work? When light passes through an object, the light refracts, that is, it changes direction. It does this as it enters the object, as it passes from one substance to another inside, and as it exits. Photons, the quanta of light, carry momentum, and the light’s momentum is changed by being bent as it passes through the object. To conserve the total momentum, the object itself acquires momentum equal to that lost by the photons, and this momentum can be used to move the object into a trap in the optical system.


To make a trap, a laser beam is set up with an intensity that diminishes moving out from the center of the beam, Lenses bring the beam to a focus, the point of maximum light intensity, thereby creating a spot.  In this spot, a particle with dimensions on the order of microns will experience a force due to the transfer of momentum from the scattering of photons.


Optical tweezers can also make accurate measurements of the tiny, sub-picoNewton forces exerted on the trapped objects. This allows researchers to study the diffusion dynamics (or Brownian motion) of an object in a solvent — a property that can play a key role in the function of many biological molecules. Optical tweezers can also be used to micromanipulate an object using well-controlled forces.


This technique has become an important tool in a wide range of fields such as bioengineering, material science, and physics due to its ability to hold and manipulate particles and to measure forces in the femtonewton and piconewton ranges.


Optical Tweezer Technology Breakthrough Overcomes Dangers of Heat, reported in June 2021

However, optical tweezers do have flaws. The prolonged interaction with the laser beam can alter molecules and particles or damage them with excessive heat. Researchers at The University of Texas at Austin have created a new version of optical tweezer technology that fixes this problem, a development that could open the already highly regarded tools to new types of research and simplify processes for using them today.


The breakthrough that avoids this problem of overheating comes out of a combination of two concepts: the use of a substrate composed of materials that are cooled when a light is shined on them (in this case, a laser); and a concept called thermophoresis, a phenomenon in which mobile particles will commonly gravitate toward a cooler environment. The cooler materials attract particles, making them easier to isolate, while also protecting them from overheating. By solving the heat problem, optical tweezers could become more widely used to study biomolecules, DNA, diseases and more.


“Optical tweezers have many advantages, but they are limited because whenever the light captures objects, they heat up,” said Yuebing Zheng, the corresponding author of a new paper published in Science Advances and an associate professor in the Walker Department of Mechanical Engineering. “Our tool addresses this critical challenge; instead of heating the trapped objects, we have them controlled at a lower temperature.”


Analyzing DNA is a common use of optical tweezers. But doing so requires attaching nano-sized glass beads to the particles. Then to move the particles, the laser is shined on the beads, not the particles themselves, because the DNA would be damaged by the heating effect of the light.


“When you are forced to add more steps to the process, you increase uncertainty because now you have introduced something else into the biological system that may impact it,” Zheng said. This new and improved version of optical tweezers eliminates these extra steps. The team’s next steps include developing autonomous control systems, making them easier for people without specialized training to use and extending the tweezers’ capabilities to handle biological fluids such as blood and urine. And they are working to commercialize the discovery.


Zheng and his team have much variety in their research, but it all centers on light and how it interacts with materials. Because of this focus on light, he has closely followed, and used, optical tweezers in his research. The researchers were familiar with thermophoresis and hoped they could trigger it with cooler materials, which would actually draw particles to the laser to simplify analysis.


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