**4. Assembly of cell-based biophotonic waveguides by OFTs**

Based on the multiple cell trapping capability of OFTs, direct formation of biophotonic waveguides with *E. coli* were reported [17]. By launching a laser of 980 nm wavelength into the OFTs, multiple *E. coli* were trapped and connected together with highly ordered organizations, forming biophotonic waveguides with different lengths (**Figure 4a**). By coupling a visible laser beam into the formed biophotonic waveguides, light propagation along these biophotonic waveguides can be directed observed as indicated by the red-light spots at the end of the waveguides (**Figure 4b**). The light propagation loss along the formed waveguides can be measured using an optical power meter by coupling another tapered optical fiber at the end of the formed biophotonic waveguide. As shown in **Figure 4c** and **d**, the measured propagation loss was measured to be 0.23 dB/μm.

In addition to the linear biophotonic waveguides, using OFTs, branched photonic probes can also been assembled. For example, **Figure 5** shows the assembled branched photonic probes with *E. coli* bacteria [18]. By designing a specially segmented tapered optical fiber, light output from the fiber can be divided into three individual beams, and *E. coli* bacteria can be trapped by the individual beams, further forming into branched biophotonic probes with different lengths (**Figure 5**). These branched photonic probes show strong stability, and can be used for further applications. By moving the OFTs, the formed biophotonic probes can be flexibly manipulated to different designated positions for further applications. These results show that the OFTs offer a seamless interface between optical and biological worlds for biophotonic probes formation with natural

#### **Figure 4.**

*Biophotonic waveguides formation [17]. (a) Optical microscope images of formed bio-waveguides (bio-WGs) with different lengths. (b) Light propagation observation along the formed biophotonic waveguides. (c) Normalized optical power measured at the end of each waveguides. (d) Measured optical loss of the waveguides.*

#### **Figure 5.**

*Optical assembly of branched biophotonic structures [18]. (a, b) Assembly of two-branch structures. (c) Assembly of three-branch structures.*

materials, and provides a new opportunity for direct sensing and detection of biological signal and information in biocompatible microenvironments.
