(683d) Finite Element Model of Surface Acoustic Wave Biosensor Based on Langasite

Surface acoustic wave sensors detect chemical and biological species by monitoring the shifts in frequency of surface acoustic waves generated on piezoelectric substrates. These devices are conveniently small, relatively inexpensive and quite sensitive. Considerable attention has been focused on the development of response models to understand the characteristics of surface acoustic waves generated in SAW devices. Most of the analytical techniques require simplification of second order effects such as backscattering, charge distribution, diffraction and mechanical loading. Our previous investigations involved development of structural models for SAW gas sensors based on LiNbO3 substrate.

In this work, we report the development of a finite model of an orthogonal SAW biosensor based on a Langasite substrate to achieve the multiple objective of simultaneous biosensing and removal of non-specifically bound proteins. (0, 22, 90) and (0, 22, 0) Euler directions were investigated. In the current work, a SAW device based on Langasite with a liquid loading was modeled to gain insights into the acoustic streaming phenomenon (Fig. 1). The dimensions of the piezoelectric substrate were 120μm width x 800μm propagation length x 300μm depth was simulated to gain insights into the acoustic streaming in SAW devices. Two IDT finger pairs in each port were defined at the surface of the Langasite substrate. The fingers were defined with periodicity of 40 μm and aperture width of 200 μm. The IDT fingers were modeled as mass-less conductors and represented by a set of nodes coupled by voltage degrees of freedom (DOF). The model was meshed with tetragonal solid elements with four degrees of freedom, three of them being the three translations and the fourth being the voltage.

The center frequency of the device calculated using an impulse response analysis for the current model was 68 MHz. Therefore, an AC analysis was carried out with a peak voltage of 2.5 V and frequency of 68 MHz applied to the transmitter IDT fingers. The propagation characteristics of waves in the different Euler directions were studied and will be presented in this work. The results indicate that pure shear horizontal (SH) mode propagates along the (0, 22, 90) direction making it suitable for biosensing. Our work also suggests that the (0, 22, 90) direction possesses shallow penetration depth, thereby making it suited for liquid sensing applications, such as those in biosensing in bodily fluids. In addition, (0, 22, 0) direction shows the presence of mixed modes. Despite the wave being mixed mode, surface normal component is the dominant component in the orthogonal (0, 22, 0) direction; this can be utilized for acoustic streaming. The acoustic streaming forces for wave propagation along (0, 22, 0) direction were computed using Campbell Jones method in conjunction with the FE model. Thus, the novel biosensor based on a Langasite substrate can be used for simultaneous sensing and NSB removal.