**3. Manipulation of single cell and multiple cells by OFTs**

OFTs can serve as a powerful tool for the trapping and manipulation of cells. Using *Escherichia coli* as an example, both single and multiple motile bacteria have been trapped and manipulated in a non-contact manner [15]. **Figure 2a** shows the experimental schematic for non-contact trapping of *E. coli* using OFTs. In this scenario, a laser beam at a wavelength of 980 nm was launched into the OFTs. A *E. coli* bacterium that was randomly swimming in the suspension was then trapped by the OFTs. The trapping was a non-contact trapping, and the bacterium was in the trapping position with several microns to the tip of the OFTs. During the trapping, the highly active bacterium was struggling around the trapping region. **Figure 2b**–**d** shows the detailed process for the trapping and struggling dynamics. The bacterium was trapped by the OFTs in a non-contact manner. However, due to the motility, the trapped bacterium was struggling after trapping. This phenomenon provides a new method for the studying of bacteria dynamics using OFTs.

In addition to the trapping and manipulation of single cells, OFTs can also be used for the trapping and assembly of multiple cells. For example, **Figure 3a** shows a schematic for the trapping and assembly of multiple *E. coli* cells in a microfluidic channel using OFTs [16]. Light output from the OFTs can trap the *E. coli* bacteria delivered by microfluidics. After a single bacterium was trapped, light can further propagate along the cell, and can be used for the trapping of other bacteria. Therefore, multiple bacteria can be trapped and assembled into cell chains with different lengths. To show the multiple trapping capability, **Figure 3b** shows the simulated light propagation along multiple cells. It can be seen that, light can propagate along the trapped cells, and the exerted optical force can be used for further trapping of other bacteria (**Figure 3c**). To experimentally demonstrate stable trapping and connecting of multiple *E. coli* cells with highly organized orientation, i.e., realization and retaining of *E. coli* cell–cell contact, the 980-nm wavelength laser with an optical power was launched into the fiber probe. **Figure 3d** shows the trapped multiple cells and formed cell chains with different numbers of cells at different input optical powers. By moving the fiber probe, the assembled cell chains can further be flexibly manipulated.

#### **Figure 2.**

*Optical trapping of a single bacterium using OFTs [15]. (a) Schematic illustration of the non-contact optical trapping of a single bacterium and the struggling dynamics. (b) Optical microscope images of the trapping and struggling process of a single bacterium.*

#### **Figure 3.**

*Optical trapping of multiple cells using OFTs [16]. (a) Schematic of multiple E. coli trapping using OFTs. A laser at 980 nm wavelength was launched into the fiber probe which was placed in a microfluidic channel with a flowing suspension of E. coli cells. Multiple E. coli cells were trapped and connected orderly at the tip of the fiber probe. (b) Simulated light propagation along multiple bacteria. (c) Simulated light distribution along the assembled cell chains. (d) Calculated optical trapping force exerted on the last cell of each cell chain and the trapping potential.*
