20
18
16
14
N)
ce (p
12
For
10
apping
8
Tr
6
4
2
0
0
200
400
600
800
1000 1200 1400 1600 1800
Power (mW)
FIGURE 8.4
Optical force calibration versus various laser powers by trapping a polystyrene bead with
radius of 1.5 μm at the separation depth h = 5 μm from the coverslip.
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Biologically Inspired Robotics
The force calibration results are shown in Figure 8.4 over a range of laser
power. At each level of laser power, five separate measurements were conducted and the results were averaged. All data can be fitted by a straight line,
which is consistent with the results reported by Svoboda and Block (1994)
and Mills et al. (2004).
8.3.4 Robotic Manipulation of Microbeads
Robotic manipulation of biological cells has been reported in mechanical contacts (Huang et al. 2009a, 2009b; Li, Zong, and Bi 2001; Sun and Mills 2002;
Wejinya, Shen, and Xi 2008; Xie et al. 2009). In many noninvasive cell manipulation applications (Arai et al. 2001; Gu, Kuriakose, and Gan 2007), cells were held
and manipulated by laser traps directly. In this study, microbeads were attached
to the cell surface, serving as handles to stretch the RBCs. The laser beam was
focused on the attached bead instead of the cell to minimize the potential
optical damage. Microbeads were manipulated by optical traps directly in
RBC stretching experiments. Here, we first demonstrate the efficiency of bead
manipulation by optical tweezers. Figure 8.5 illustrates the manipulation process. When the moving velocity of the motorized stage is lower than the critical
value calculated from Equation (8.6), the trap holds the bead tightly, as shown
in Figures 8.5a–c. As the velocity increases beyond this critical value, the bead
escapes the trap, as shown in see Figure 8.5d. During cell manipulation, the
centroids of microbeads can be obtained through image processing. Because