Holographic Technique Traps Irregularly Shaped Microscopic Objects

An optical manipulation technique that can securely control the position, orientation and shape of non-spherical microscopic samples, such as living cells, could have direct applications in biophotonics and soft matter physics.

Most experiments using optical tweezers have focused on trapping spherical particles, where the optical forces and responding motion of microspheres can be easily predicted based on physical principles. However, when this method is used for trapping objects with complicated shapes, it can induce unstable motion in the particles and provide only limited control of the particles’ orientation.

Researchers at the Korea Advanced Institute of Science and Technology (KAIST) achieved real-time optical control of arbitrarily shaped particles by combining the wavefront shaping of a trapping beam with measurements of the 3D refractive index distribution of samples. The technique uses a 3D holographic microscope to measure 3D structures of an object in real time.

This is a concept of optical manipulation techniques. Courtesy of KAIST.

Based on the measured 3D shape of the object, the shape of light that is needed to control it can be precisely calculated. When the shape of light is the same as the shape of the object, the energy of the object is minimized, which allows for the stable trapping of the object.

The team engineered the 3D light field distribution of a trapping beam based on the 3D refractive index map of samples to generate a light mold, which was used to manipulate colloidal and biological samples with arbitrary orientations and/or shapes.

This image shows the experimental setup. Courtesy of KAIST.

By controlling the shape, position and direction of light, the team was able to control the 3D motion of the object and induce the desired shape in it. Because their approach is similar to generating a mold for casting a statue, the researchers called the technique tomographic mold for optical trapping (TOMOTRAP).

The team trapped individual human red blood cells, stabilizing and rotating them to the desired orientations, including folding them in an L-shape and assembling two red blood cells together to form a new structure. In addition, the team demonstrated that colon cancer cells with a complex structure could be stably trapped and rotated at desired orientations.

According to the team, the TOMOTRAP technique provides stable control of the orientation and assembly of arbitrarily shaped particles without knowing a priori information about the sample geometry.

Professor YongKeun Park said, “Our technique has the advantage of controlling the 3D motion of complex-shaped objects without knowing prior information about their shape and optical characteristics, and can be applied in various fields including physics, optics, nanotechnology and medical science.”

Researcher Kyoohyun Kim said, “This approach can also be applied to real-time monitoring of surgical prognosis of cellular-level surgeries for capturing and deforming cells as well as subcellular organelles.”

The research was published in Nature Communications (doi:10.1038/ncomms15340).