(241d) Orthogonal SAW Device Based On Langasite for Simultaneous Biosensing and Biofouling Removal: A Fluid Structure Interaction Study

Summary: Multi-directional transducers have the capability of achieving the dual objectives of biosensing and non-specifically bound protein removal for improved sensor performance. Understanding the acoustic wave propagation and the sensing mechanism in SAW biosensors involving fluid interactions with complex multi-directional transducers represents a significant challenge. Till date, attempts to address this have relied on simplified numerical and analytical models or perturbation theories which neglect the mechanical properties of the fluid and treat the leaky wave as a first-order perturbation on the non-leaky wave associated with surface-wave propagation1. We present the first report on a three dimensional fluid-structure interaction of SAW sensor utilizing a multi-directional transducer based on a Langasite substrate. Numerical solutions are obtained by sequentially solving the generalized Navier-Stokes equation and acousto-electric equations for the solid motion2. Precise knowledge of the induced streaming phenomenon due to the developing flow fields and wave propagation characteristics arising from the transient fluid-piezoelectric interaction is provided by the developed 3-D models. These are utilized to compute streaming forces for predicting mechanism of biofouling elimination in acoustic devices. The results are of tremendous significance for not only improving the device design, but also for understanding biosensing mechanisms in multi-directional acoustic wave devices as well as actuation mechanisms in potential microfluidic applications of these devices. Computational details: In this work, a novel biosensor based on a Langasite substrate is investigated to analyze its suitability for biosensing applications, using combined structural and fluid-solid interaction finite element models. The fingers were defined with periodicity of 40 μm along both the propagation directions. Quadrilateral coupled field solid elements were used to model the solid piezoelectric domain. Impulse and AC analysis was carried out using structural models to determine the device frequency response and wave propagation characteristics. Additionally, coupled field fluid-structure interaction (FSI) model was developed to study surface acoustic wave interaction with fluid loading. Specifically, finite element models of a SAW device based on a micron sized LGS (0, 22, 0) piezoelectric substrate in contact with a liquid loading were developed and solved to gain insights into the acoustic streaming phenomenon in SAW devices. Fluid domain was modeled using the Navier Stokes' equation; the arbitrary Lagrangian Eulerian approach was employed to handle the mesh distortions arising from the motion of the solid substrate. The fluid was modeled as incompressible, viscous, and Newtonian. The fluid-solid coupling was established by maintaining stress and displacement continuity at the fluid-structure interface. The fluid mesh was continuously updated as the piezoelectric substrate undergoes deformation. A transient analysis was carried out by applying a time varying voltage to the transmitter IDT fingers. Results and discussion: We present the first report on a 3-D fluid-structure interaction (FSI) model of SAW sensor utilizing a multi-directional transducer based on a LGS substrate to investigate the streaming velocity fields and forces induced by SAW device. Figure 1 shows the schematic of the transducer configuration in the simulated SAW device, with IDT finger pairs defined on the surface for each port along Euler directions (0, 22, 0) and (0, 22, 90). The center frequency of the device calculated using an impulse response analysis 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. Our results indicate that pure shear horizontal (SH) mode propagates along the (0, 22, 90) direction making it suitable for biosensing (Fig. 2). 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 (Fig. 3). 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. Further, a fluid-structure interaction model for the device with transducer along (0, 22, 0) direction was developed and solved (Fig.4). The fluid velocity profiles obtained as a result of the fluid-piezoelectric device interaction indicate fluid recirculation in the regions close to fluid-device surface. Fluid motion, observed near the device surface, exerts stresses on the fluid-structure interfacial boundary which in turn induces the removal of loosely and non-specifically bound proteins from the device surface. Whereas the tangential components of the fluid velocity exert viscous shear stress creating a drag force, the normal component exerts a lift force. Our 3D FSI model results indicate that the tangential velocity component along the propagation direction is predominant suggesting that the drag force is significantly greater than the lift force and therefore, the particle advection induced by the former is the dominant mechanism of bio-fouling removal for the SAW sensor based on LGS. Our simulation results indicate that simultaneous sensing and non-specifically bound protein removal can be achieved through the use of multidirectional transducers on single piezoelectric device. Details will be presented at the AIChE 2009 meeting. References 1. J. J. Campbell and W. R. Jones, IEEE Transactions on Sonics and Ultrasonics 17, 71 (1970). 2. S. K. R. S. Sankaranarayanan, S. Cular, V. R. Bhethanabotla, et al., Physical Review E. 77, 066308 (2008)