Electroactive Polymer Based Micro-ElectroMechanical System as Biosensor Platform
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Electroactive Polymer Based Micro-ElectroMechanical System as Biosensor Platform Zhimin Li, Suiqiong Li and Z.-Y. Cheng Materials Research and Education Center, Auburn University, Auburn, AL36849, USA ABSTRACT A micro-electromechanical diaphragm (MEMD) as micro-sensor platform is introduced. The performance of a MEMD is compared with that of a microcantilever (MC). It is theoretically found that the sensitivity of a MEMD is about 50 times higher than that of a MC. The measured resonance frequencies in air proved the validity of the MEMD design. More importantly, the quality merit factor (Q value) of MEMD is higher than that of MC. The damping effect of liquid medium on a MEMD is much smaller than on a MC. It is experimentally demonstrated that the MEMD works well in both air and liquid. INTRODUCTION Biosensors for detecting and quantifying the presence of a small amount of biological threat agents in a real-time manner are urgently needed for a wide range of applications. Different types of sensor platforms are used and/or are being developed for use as biosensors. Among the platforms, acoustic wave (AW) devices attract a great deal of attention [1-3]. Many AW devices, such as surface acoustic wave (SAW), shear horizontal wave (SHW), flexural plate wave (FPW) and microcantilever (MC) have been studied. Since MC has many advantages over other devices, such as compact size, easy integration with analysis circuit and high mass detection sensitivity, it is considered as the state-of-art AW platform. Therefore, MCs and other MEMS – based AW devices as sensor platforms are very attractive for current and future technologies [4-9]. In order to compare different AW devices as sensor platforms, a normalized mass sensitivity df , was introduced [4,8,10]. The mass sensitivity is defined as the resonance (Sm), S m = − dm frequency shift due to unit mass load. A higher value of Sm means that the device is more sensitive. The sensitivity of MC with mass load m at the tip of MC was obtained as [6,8,11]: df 1 0.592 1 E E = Sm = − = = 0.592n 4 (1) 2 3 n A 3ρ 3 dm L 3ρ where E and ρ are the Young’s modulus and density, A=L×w is the (active) area of the MC, L and w are the length and width of the MC, respectively, and n = L / w > 1 . The sensitivity of MC with uniform mass loading over the active area is: f df 0.14 1 E E 1 = Sm = − = 0 = 0.14n 4 (2) 3 2 n A 3ρ 3 L 3ρ dm 2M In this paper, a design of acoustic wave device as sensor platform, in which a microdiaphragm made of piezoelectric polymer employed, is presented. This diaphragm is named as Micro- ElectroMechanical Diaphragm (MEMD). The configuration of a diaphragm is shown in figure 1. With diameter φ of a clamped circular area of material in plate shape and thickness t, the natural fundamental frequency can be written as [12]:
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λ2t ⎡ E f0 = 2 ⎢ πφ ⎢⎣ 3ρ 1 − υ 2
(
12
⎤ ⎥ ⎥⎦
( λ2 = 10.22 --- first vibration mode and no mass load)
)
(3)
where υ is the Poisson’s ratio of the film and λ is a dimensionless frequency parameter dependent on the vibration mode a
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