Biomedical Engineering Reference
In-Depth Information
Fig. 4.19
Drug delivery
biochip based on microfluidic
devices
micro/nanosensors
microfluidic
channel
reservoir
micropump
electronic control
drug delivery
biochip
pressure of 310 hPa in the forward direction and 70 hPa in the reverse direction,
with corresponding flow rates of 850 and 200lmin
1
, respectively (
Zengerle
et al. 1995
). The pump diaphragm, with dimensions of 7
7
2 mm
3
, is actuated by
square waves with amplitudes of 150-200 V and frequencies starting from 0.1 Hz
up to a few 100 Hz. These square waves are applied between the diaphragm and the
counter electrode. The total power consumption is less than 1 mW. The valve flow
is quasistatic when the actuation frequency is lower than the flap, and in this case,
the valve opening and the fluid flow are given by
x.t/
D
DSp
fluid
.t/;
(4.6)
F.t/
D
4
lD
Œ.2p
fluid
/
3
=
1=2
;
(4.7)
where D is the damping constant of the valve, is the fluidic density, p
fluid
is the
pressure in the fluid,
Š
0:5
1, l is the length of the valve, and S is the area of
the valve orifice. The pump is depicted schematically in Fig.
4.20
.
The example above is one among many micropumps based on piezoelectric,
thermopneumatic, or shape memory alloy effects, magnetohydrodynamic, chemical,
or osmotic effects. Another interesting example due to the low actuation voltages
applied is the electrowetting pump (
Tsai and Sue 2007
). The electrowetting effect
deals with the control of the surface tension T
ls
between two different materials
(solid/liquid or liquid/liquid) by an applied electrostatic force. The Lippmann-
Young equation describes the relation between the contact angle , the liquid-solid
interface tension T
ls
, and the capacitance C of the dielectric layer:
cos
D
cos
0
C
.1=T
ls
/
CV
2
=2
(4.8)
where
0
is the equilibrium contact angle.
The continuous electrowetting effect is used to tune the surface tension between
two immiscible liquids. For example, a mercury droplet surrounded by an electrolyte
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