61
crochannel can be easily integrated into the thick electrodes, thereby achieving stable
single-cell capture, eective stretching, and mechanical property measurement.
3.3.2 PRINCIPLE OF OPTICAL STRETCHER
e change in light momentum caused by light irradiation on a cell [194, 195] produces axial and
gradient forces. e axial force is caused by the collision of photons on the cell, along the direction
of propagation of the beam; and the gradient force is caused by the intensity of the light eld, and
the direction of the light intensity is greatest along the vertical direction of light propagation.
e gradient and axial forces exerted on the cell are related to the laser wavelength and
cell size. When the cell radius r is much smaller than the laser wavelength, the gradient and axial
forces conform to the Rayleigh model [196], and when cell radius r is much larger than the laser
wavelength, the forces conform to the Mie model [197, 198]. For most cells of interest, the cell size
corresponds to the Mie model relative to the laser wavelength.
As light enters a cell, the light gains momentum so that the surface gains momentum in the
opposite direction. Similarly, the light loses momentum upon leaving the cell so that the opposite
surface gains momentum in the direction of the light propagation. e reection of light on either
surface also leads to momentum transfer on both surfaces in the direction of light propagation. For
an incident beam, it can be decomposed into multiple beams. Figure 3.3 shows one of the beams
with power P
1
incident on the cell.
Fiber
P
1
R
P
1
O
α
1
n
0
n
1
α
2
θ
P
1
T
2
P
1
T
2
R
Figure 3.3: Reection and refraction of the incident light on the cell.
e beam will be reected and refracted when it enters the cell, and its reection coecient
R and refractive coecient T can be expressed as
R =
1
[
sin
2
(α
1
‒α
2
)
+
tan
2
α
1
‒α
2
)
], (3-1)
3.3 ELECTROROTATION CHIP FUNCTION EXPANSION
2 (sin
2
(α
1
+α
2
) tan
2
(α
1
+α
2
)
62
3. OPTO-ELECTRONIC INTEGRATION OF THICK-ELECTRODE DEP MICROFLUIDIC CHIP
T = 1 - R, (3-2)
where α
1
is the angle of incidence and α
2
is the angle of refraction.
e Mie model gives the axial force F
scat
and the gradient force Fgrad [199]:
F
scat
=
n
0
P
1
1 + Rcos2α
1
‒
T
2
[cos(2α
1
‒ 2α
2
)] + Rcos2α
1
) (3-3)
F
grad
=
n
0
P
1
Rsin2α
1
‒
T
2
[sin(2α
1
‒ 2α
2
)] + Rcos2α
1
), (3-4)
where n
0
is the refractive index of the medium, P
1
is the optical power of the incident beam, c is
the speed of light in the vacuum, and n
1
is the refractive index of the cell.
In 2000, Guck et al. rst proposed a dual-ber optical stretcher [195]. Figure 3.4 shows the
schematic diagram of the dual-ber optical stretcher. e dual beams of the oppositely propagated
laser beam can capture and stretch single cells. According to the principle of conservation of light
momentum, when the laser is incident on the cell, the front and back surfaces of the cell are sub-
jected to an outward force, and the direction is perpendicular to the cell surface. When the cells are
in the middle of the optical bers, the force equilibrium can be achieved in the axial direction and
the normal direction to achieve single-cell capture, and then the single cell can be stretched and
deformed by increasing the optical power.
F
grad
P
1
= 200m
W
P
2
> P
1
r
0
r
0
+ ∆r
F
scat
(a)
(b)
(c)
Figure 3.4: e working principle of optical stretcher: (a) the cell are subjected to axial and gradient
forces when it osets from the ber axis; and (b) the cell is trapped; (c) the cell is stretched when opti-
cal power is increased.
c
1+R
2
+ 2Rcos2α
2
c
1+R
2
+ 2Rcos2α
2
..................Content has been hidden....................
You can't read the all page of ebook, please click here login for view all page.