Biomaterials for MEMS

Vibration-Based Anti-Biofouling of Implants 207

For the former method, many fluorescence dyes with different emitting colors

are commercialized already and can be chosen to attach onto proteins before

or after adsorption. They usually are excited by a specific wavelength of light

and emit a specific longer wavelength of light, for example, Fluorescein IsoThio-

Cyanate (FITC) can be excited by blue light and emit green light. The fluorescence

photos of surfaces (PZT) adsorbed by BSA-FITC and the relation between intensity

and BSA concentration are shown in Fig. 9.2(a). The fluorescence intensity is

linearly depends on the quantity of adsorbed BSA. In addition, from Fig. 9.2(a),

the intensity is a function of BSA solution concentration and incubation time and

the function can be portrayed well by the Langmuir adsorption isotherm.

The EIS measures the impedance between two electrodes. The impedance (Z)

is like resistance, and follows the Ohm’s law; and is not like resistance because

that it depends on the frequency of signal and voltage and current of signal is

unnecessarily in phase. The measurement is usually a three electrodes setup:

working, counter, and reference electrodes. The surface of interest, such as protein

adsorption surface, is as working electrode, the much larger inert material, such

as platinum, is as counter electrode. Two plots, Nyquist and Bode plots, are used

to interpret the measured data.

46

Nyquist plot separates Z into real and imaginary

parts, resulted from the equivalent electrical circuit, can represent the parameters,

such as resistance and capacitance, between working and counter electrodes. Bode

plots shows Z or phase with the function of frequency so that reveals the frequency

information. The Nyquist plot of BSA adsorbed surface is shown in Fig. 9.2(b).

The inset is the equivalent circuit of the plot, where R

el

represents the electrolyte

resistance between working and reference electrodes, R

ct

and constant-phase el-

ement (CPE) are charge-transfer resistance and electric double layer capacitance

between electrolyte and working electrode. If there is protein adsorption on the

surface, the R

ct

and CPE will change. For example, as shown in Fig. 9.2(b), the R

ct

decreases 90% when the surface was incubated in BSA solution for 1 hour because

that BSA can help charge transfer from the electrolyte to the electrode. In addition,

the surface after vibration shows increasing resistance which means less proteins

adsorbed on the surface. The vibration on the attenuation of protein adsorption

will discuss in the later section.

9.3 INTERACTIONS BETWEEN PROTEINS AND SURFACE

In the scale of protein, some interactions between protein and surface are crit-

ical: van der Waals (VDW), electrostatic double-layer (EDL), and hydrophobic

interactions.

49,50

Sincealltheexperimentswereperformed on a piezoelectric plate,

all parameters related to the surface were those of piezoelectric plate. In this

chapter, we use PZT as the material.

Van der Waals force includes momentary attractions between molecules,

diatomic free elements, and individual atoms and other than those due to covalent

bonds or to the electrostatic interaction of ions with one another or with neutral

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208 P. Y. J. Yeh, J. N. Kizhakkedathu and M. Chiao



D





E



Figure 9.2. (a) BSA adsorption isotherms on a piezoelectric (PZT) surface incubated at

37

◦

C for 1 hour and 30 minutes. The fluorescence intensity was transfer from tri-color to

256 gray scale by Adobe Photoshop 6.0. The intensity is linearly to the quantity of adsorbed

BSA, and is a function, which follows the Langmuir isotherm, of BSA concentration. Two

inserted photos are fluorescence photos in pointed experimental condition. (b) Nyquist

plot of impedance between working electrodes (PZT) and platinum counter electrode. The

voltage data (in Z measurement) is corresponding to the reference voltage. The inset is the

corresponding equivalent circuit. Reproduced with the permission from Yeh et al. (2007).

For color reference, see page 274.

molecules. And it comes from the temporary fluctuating dipoles of molecules and

is correlated as the molecules come closer to originate an attractive force. There

is always a VDW force between two molecules or surfaces, and has a power-

law dependence on the separation between protein and surface. The VDW force,

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