7
(a) (b)
Figure 1.3: (a) Inertial force cell focusing chip [71]; and (b) DLD cell sorting chip [72].
1.2.2 OPTICAL METHODS
Optical tweezers utilize an optical gradient trap formed by a single beam to achieve capture and
movement of single cells, which forms a 3D optical potential well when the laser beam is focused
on the cell, thereby inducing optical pressure to capture the cell. By controlling the trajectory of the
optical tweezers, it is possible to accurately trap and move cells [73, 74] or perform cell alignment
patterning [75, 76]. Liberale et al. proposed a ber-based optical tweezer structure that achieves sta-
ble single-cell 3D capture by micro-prism beam deectors (Figure 1.4(a)) [77]. In general, only one
single cell can be operated at a time. In order to achieve multi-cell simultaneous operation, Werner
et al. proposed a microuidic chip with refractive multiple optical tweezers (Figure 1.4(b)). is
method was used to immobilize more than 200 yeast cells and perform patterning at a time [78].
Optical tweezers are also commonly used in cell sorting. According to the control method,
they can be divided into passive and active sorting methods. e passive sorting method uses a
xed-point optical tweezers to achieve cell sorting in the microuidic channel. Cells with dierent
refractive indices, sizes, or shapes will be subject to positional shifts in the light eld by dierent
sizes of optical forces (Figure 1.4(c)) [79]. e passive sorting methods rely on the dierences in
the intrinsic properties of the cells, the sensitivity to cell selection and the sorting eciency is low.
Active sorting methods are based on dierences in cell intrinsic properties and dierences in bio-
logical properties, such as cell sizes and uorescent label, using dynamic optical tweezers to classify
cells. Although the accuracy of active sorting is higher than that of passive sorting, it needs to be
combined with sensors to form a feedback control system (Figure 1.4(d)) [80].
1.2 SAMPLE MANIPULATION METHODS IN MICROFLUIDIC CHIPS
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