216 P. Y. J. Yeh, J. N. Kizhakkedathu and M. Chiao
BSA adsorption. So under the AC application, the antifouling mechanism is
two steps, firstly, the BSA is desorbed by the surface charges, and secondly, the
shear stress prevents the re-adsorption of BSA when the surface charges becomes
positive because of the AC application.
9.5 MICROFABRICATION
The vibration-based antifouling mechanism is applied on the micromachined
membrane.
63
The scheme of setup is shown in Fig. 9.6(a). The device starts from
the SOI wafer (Si(2 μm)-SiO
2
(1 μm)-Si(400μm)) as substrate. The process is briefly
described as following. Firstly, SOI wafer is thermal oxidized to grow SiO
2
around
SOI wafer as protection layer. The cavity (2000 μm × 500 μm) within the substrate
is anisotropic etched from the 400 μm silicon side by 20% Tetra Methyl Ammonium
Hydroxide (TMAH) at 85
C. During the etching, areas except cavity opening are
protected by SiO
2
.TheTMAHetchingstopsonSiO
2
of SOI wafer. After TMAH
etching, the protective thermal SiO
2
was removed and the silicon/SiO
2
membrane
is then remained. The diced PZT plate (3000 μm × 1000 μm × 500 μm) is attached
on the silicon/SiO
2
membrane by silver epoxy. The SEM photos of the front
view and back view of the device are shown in Fig. 9.6(b) and 9.6(c), respectively.
The cavity in Fig. 9.6(c) can be filled with protein solution for protein adsorption
and PBS for washing, only back side of the membrane is contacting the solution.
During the vibration and protein incubation, the plastic cover is shed on the cavity.
Figure 9.6. (a) The experimental setup (b) Front side view of a micromachined PZT plate
(3000 μm × 1000 μm × 500 μm) on membrane with an electrical wire bonded onto the plate
surface(c)ThebacksideviewoftheSi/SiO
2
membrane. In the following experiments, the
proteins were adsorbed onto the SiO
2
side of the Si/SiO
2
membrane. Reproduced with the
permission from Yeh et al. (2008).
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Vibration-Based Anti-Biofouling of Implants 217
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Figure 9.7. (a) The simulation and experimental vibration spectrum of the PZT
plate/siliconmembrane. The vibration at 308, 320, 500, and 575 kHz correspond to 1.5,
2.5, 3.5 wavelength bending mode, and longitudinal mode, respectively. (b) The simulation
and experimental vibration amplitude distribution across the Si/SiO
2
membrane along the
membrane center line. The x axis is given as the distance from the center of the membrane
to the left periphery. The half length of the membrane is 1000 μm. Reproduced with the
permission from Yeh et al. (2008). For color reference, see page 274.
The vibration of membrane is measured by LDV and simulated by finite
element software (ANSYS 8.0). Fig. 9.7 (a) shows the vibration spectrum at the
center of a Si/SiO
2
membrane, with cavity filled with PBS solution. The PZT was
drivenbya10V
pp
sinusoid waveform from 120900 kHz to find the resonance fre-
quency. The observed resonance frequency with largest vibration amplitude is 308
kHz, and the out-of-plane vibration amplitude at 308 kHz is 105 nm. Simulation
result matches well with the experiment (other minor resonance frequencies and
major resonance frequency, which is(269 kHz, compared to measured 308 kHz).
Figure 9.7(b) shows the displacement at different locations of the membrane at the
major resonance frequency (269 kHz). The displacement is defined as the distance
measured from the center of the membrane to left periphery. Both simulation
and experiment show that the vibration of membrane is in a FPW mode with
the wavelength of 4000 μm at major resonance frequency. The vibration at 308,
320, 500, and 575 kHz in Fig. 9.7(a) correspond to 1.5, 2.5, 3.5 wavelength bending
mode, and longitudinal mode, respectively.
The FPW mode happens when the thickness of plate is much less than the
wavelength of FPW.
64,65
If contacted with fluid, the advantage of the FPW mode
is that the phase velocity is lower than the wave velocity in fluid. Hence, the
energy can be restricted within an so-called evanescent decay length and can be
transferred to generate acoustic flow near the surfaces.
The attenuation of proteins adsorption by the membrane is tested on BSA, IgG,
and blood plasma. Figure 9.8 shows the results of relative protein desorption due
to vibration. About 57 ± 10% of BSA and 47 ± 13% of IgG were removed from
the membrane surface after the vibration was applied (10 V
pp
at major resonance
frequency). This shows the same mechanism works on both PZT plate and
SO13997_text.indd 225SO13997_text.indd 225 26/01/2011 3:50 PM26/01/2011 3:50 PM

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