Biomaterials for MEMS

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 120∼900 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

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