The ability to manipulate single cells more precisely is crucial for many applications in biotechnology. Huang et al. report a method to do this using two counter-propagating optical fibers on a microfluidic chip. They show that a variety of single cell manipulation modes are possible with this type of device, including cell capture, stretching, translation and rotation.

Previous techniques for single cell manipulation include mechanical, magnetic, electrical, acoustic and optical methods, such as optical tweezers. The technique proposed by the authors uses two optical fibers, each 125 microns in diameter, embedded within a microchannel inside a small cell flow device.

Light from a 980 nm laser was evenly split into two beams and directed into the fibers. This wavelength was chosen because it is poorly absorbed by water, decreasing the possibility of heating and thermal damage to the cells.

By tuning the light alignment in each fiber, the authors were able to trap, move, stretch, or even spin the cells. For example, when the fibers are misaligned, the light beams create a torque on the cells, causing them to rotate.

“Generally, the change of light momentum caused by light irradiating the cell will induce axial and gradient forces,” said author Wenhui Wang. The axial force is generated by photons scattering off of the cell along the propagation axis of the light, while the gradient force is caused by uneven light field intensity and is perpendicular to the direction of light propagation.

In addition to manipulating cells, the technique may also be used for the characterization of certain physical properties of the cells. “Taking advantage of out-of-plane rotation, the 3D morphology of single cells can be reconstructed,” said Wang.

Source: “Dual-fiber microfluidic chip for multimodal manipulation of single cells,” by Liang Huang, Yongxiang Feng, Fei Liang, Peng Zhao, and Wenhui Wang, Biomicrofluidics (2021) The article can be accessed at https://doi.org/10.1063/5.0039087.