63
e dual-ber optical stretcher has the following advantages: (1) the beam is unfocused,
so the power on the cell is not high, and the damage to the cell is negligible; (2) the capture and
stretching of cells can be easily achieved; and (3) the dual-ber optical stretcher is low in cost, and
the operation is simple and easy to combine with microuidic technologies. However, in order to
achieve stable capture, the optical stretcher must use single-mode ber, while the single-mode ber
has a core diameter of only 9 µm, which requires accurate alignment of the bers. is imposes
some diculty in chip fabrication.
3.3.3 STEPSTRESS ANALYSIS OF CELL MECHANICAL PROPERTIES
e mechanical properties of the cells can be measured by step-stress tests on single cells [200].
∆r is the deformation of the cells along the optical axis, and r is the original diameter of the cell.
Figure 3.5 is a schematic diagram of the step-stress response curve of a cell.
Trap
Time t
1
Deformation γ
t
TrapStretch
Figure 3.5: Step stress response curve of cells.
Cell mechanical properties can be tted by the following formula describing the response
curve:
γ(t) = σ
0
b
1
–
a
2
1 – exp –
a
1
t +
σ
0
t, (3-5)
where a
1
, a
2
, and b
1
are parameters of the tted curve. σ
0
is the maximum stress applied to the cell
along the beam:
σ
0
=
n
med
I
0
(2 – R
ref
+ R
ref
2
)
n
cell
– 1, (3-6)
where c is the velocity of light in vacuum, n
med
is the refractive index of the medium (normally
n
med
≥ 1.335), n
cell
is the refractive index of the cell, and R is the amount of light reected at the
interface between the medium and the cell, which can be expressed as
a
1
a
1
2
a
2
a
1
c
n
med
3.3 ELECTROROTATION CHIP FUNCTION EXPANSION
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