24
2. THICK-ELECTRODE DEP FOR SINGLE-CELL 3D ROTATION
e application of thick electrodes in microuidic chips has demonstrated the potential of
thick-electrode DEP in microuidics. However, the fabrication process of existing thick-electrode
DEP chips is rather complicated, and the electrode structure is relatively simple, which limits the
application of the thick electrodes.
e basic thick-electrode DEP composed of two thick electrodes can generate DEP force
which can translate single cells. However, it is necessary to extend the basic two-electrode struc-
ture to multiple electrodes to achieve more complex manipulations, such as single-cell rotation. In
this work, a thick-electrode DEP chip for single-cell 3D rotation was proposed by using carbon
black-PDMS (C-PDMS, a mixture of nano-conductive carbon powder and PDMS) as electrode
material, which expands the application prospect of thick-electrode DEP in microuidics.
Single-cell 3D rotation means that cells suspended in solution can rotate about X/Y/Z-axis
and play an irreplaceable role in single-cell analysis. For example, when analyzing the biophysical
properties of cells, it is necessary to perform 3D surface imaging of cells [153], and even internal
structure scanning [154]. In order to obtain accurate imaging results, it is essential to rotate the
cells about more than one axis to obtain multi-dimensional image sequences, then reconstruct 3D
model. But 3D rotation is not as easy to implement; most mammalian cells are 10–100 µm in
diameter, and easily sink in solution. Although the current cell rotation can be achieved by various
methods, such as mechanical, optical, magnetic, acoustic, or electrical means, most of the methods
can only achieve in-plane rotation. Even for several existing 3D rotation methods, there is a prob-
lem that the rotation control is unstable.
Cells are polarized in a rotational electric eld and rotated by the torque generated by electric
eld. e electrical parameters of the cells such as cell membrane capacitance and cytoplasmic con-
ductivity of the cells can be measured by analyzing the rotation spectrum. However, at present most
electro-rotation methods using planar electrodes cannot achieve 3D rotation. And the rotation
speeds are dierent when cells are in dierent positions, which makes it impossible to accurately
measure electrical parameters of the cells.
is chapter presents an Armillary Sphere’ type single-cell 3D rotation chip. 3D rotation is
realized by the thick-electrode multi-electrode structure, and the electrical and physical properties
are measured based on 3D rotation.
2.2 PROGRESS IN CELL ROTATION MANIPULATION
Single-cell rotation has been widely used in biological operations such as cell injection, cell nuclear
extraction, and cell cloning. With the development of MEMS technology, there have been many
reports on the methods of cell rotation such as mechanical, acoustic, electric, optical, and magnetic.
1. Mechanical methods. Generally, a suction tube and an injection needle are used for
operation. First, the cells are adsorbed by the suction tube, and then the cells are ro-
25
tated by the needle. Chen et al. used this method to achieve rotation and enucleation
of bovine oocytes, as shown in Figure 2.2(a) [155]. However, the rotation eciency
of the mechanical methods is low, and a precise positioning manipulator is required
to accurately control the displacement of the glass needle. Furthermore, mechanical
methods are contact operation, which are easy to cause damage to the cells.
(a) (b)
Solid State Laser
λ = 1064 nm
Laser Diode
λ = 660 nm
Microchannel
Hologram to SL
M
IR Light
M/SLM
CCD1
CCD2 (out of focus)
Sample
Optical
Fiber
cde
b
Trapping Laser
Trapping Laser
Trapping Laser
x
x
y
y
z
z
Figure 2.2: (a) Mechanical method for single-cell rotation [155]; and (b) sperm cell rotation based on
the optical method [157], based on and used with permission from the Royal Society of Chemistry.
2. Optical methods. e methods use a laser beam to generate axial and gradient forces
for cell capture and precise control of cell movement [156]. However, it is dicult to
achieve cell rotation in one laser beam. Merola et al. proposed a single-cell rotation
microdevice based on an unstable Gaussian beam to achieve sperm cell rotation [157],
and 3D imaging of sperm by digital holography, as shown in Figure 2.2(b). But the
rotation method requires expensive optical systems with complex and precise controls.
3. Magnetic methods. e use of magnetic eld to control cells requires modication of
the cell surface with micro-magnetic beads and the external magnetic eld to achieve
cell movement [158, 159]. Elbez et al. used an external 3D magnetic coil to generate
a 3D magnetic eld to achieve 3D rotation of the cells [158], as shown in Figure
2.3(a). However, the external magnetic coil has a large volume, and the fabrication
are complicated.
4. Acoustic methods. e transducer vibrates the liquid in the microchannel and cre-
ates vortexes that allow single cells or even organisms (such as Caenorhabditis elegans)
to rotate [160, 161]. Tony Huang group developed a surface acoustic wave rotation
structure. As shown in Figure 2.3(b), the shapes of the vortexes were changed by
adjusting electric parameters of piezoelectric transducer. However, acoustic methods
2.2 PROGRESS IN CELL ROTATION MANIPULATION
26
2. THICK-ELECTRODE DEP FOR SINGLE-CELL 3D ROTATION
dont achieve precise and stable control of single cell rotation, and the states of rota-
tion depend on the shape of the bubbles and structures.
(a) (b)
Electromagnet
Magnetized Cell
Photodector
Iron Oxide
Nanoparticle
C. elegans
Figure 2.3: (a) e single-cell rotation based on magnetic method [158]; and (b) the rotation of C.
elegans and single cells based on acoustic methods [160].
5. Fluidic methods. Setting structures with specic shape in the microchannel produce
vortexes to make cells rotate [162]. e shape of structure can be either a microcol-
umn or a groove. Patrick Shelby et al. designed a horizontal groove structure that
produces horizontally rotational vortexes to achieve in-plane rotation of single cells
[163], as shown in Figure 2.4. Shetty et al. designed a vertical groove structure that
produces a vertically rotational vortex to achieve out-of-plane rotation of single cells
and 3D imaging of cells [164]. e uidic methods have low cost and simple oper-
ation, and dont require any pre-treatment on the cells, but the single-cell loading
eciency is low, and only 1D rotation can be achieved.
z
x
y
30 µm
30 µm
50 µm
55 µm
Figure 2.4: Single-cell rotation based on uid method; horizontal single-cell rotation using horizontal
concave microchannels [163], based on and used with permission from the Royal Society of Chemistry.
6. DEP methods. If cell is polarized under a rotational electric eld, it will be subjected
to DEP torque. A common device utilizes four planar electrodes to apply ac signal
with dierent phase shift on the four electrodes to form a rotational electric eld.
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