(190g) Integrated Focused and Conventional Transducer Configuration On a Single Piezoelectric Device for Simultaneous Biosensing and Biofouling Removal

Summary Transducers used in biosensing applications are plagued by biofouling, which refers to the binding of non-specific proteins to the device surface resulting in a compromise of the device sensitivity and selectivity. Acoustic streaming, resulting from high intensity sound waves, has the potential to address the issue of biofouling elimination in biosensors [1]. Focused interdigital transducers (IDTs) have the potential for acoustic energy focusing, thereby increasing the intensity of acoustic streaming [2]. We have identified that in a Langasite substrate, the (0, 22, 90) Euler direction is suited for biosensing via the propagation of predominantly shear horizontal waves and the (0, 22, 0) direction allows for biofouling removal via mixed mode propagation with prominent surface normal component. To design a better biosensor, we develop a finite element model of a surface acoustic wave device with a multidirectional mutually interacting IDT configuration: uniform IDTs (U-IDTs) along (0, 22, 90) direction and focused IDTs (F-IDTs) along the (0, 22, 0) direction. The enhancement in sensor performance was analyzed in terms of device sensitivity and acoustic streaming force. Our results indicate that the streaming force and the sensitivity for the device with the mutually interacting U-IDTs/F-IDTs are significantly higher when compared to uniform unidirectional IDTs. This work broadly applies to all transducers used for biological species sensing that suffer from fouling and non-specific binding of protein molecules to the device surface. Computational details A three dimensional finite element model of a SAW device based on Langasite substrate, with U-IDTs/F-IDTs was developed. The dimensions of the piezoelectric substrate were 1600μm width x 1600μm propagation length x 200μm depth. The device was modeled with two port delay line consisting of two sets of IDTs along each of the two delay paths: the input IDTs and the output IDTs. The fingers were defined with periodicity of 40 μm and aperture width of 200 μm along (0, 22, 90) direction. Along the (0, 22, 0) direction, aperture width of the fingers varies depending on their radial distance from device center 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. To optimize on the computation time while capturing the dynamics accurately, highest mesh densities were ensured near the device surface and the middle of the substrate. A total of 388, 893 nodes and 266, 762 elements were generated. An impulse response analysis was performed during 190 ns by applying an impulse voltage of 100 V at the input IDT and employing a time step of 0.95 ns to deduce the central frequency of the device in each direction. The central frequencies were computed as 68 MHz and 64 MHz along the (0, 22, 90) and (0, 22, 0) direction, respectively. Subsequently, an AC analysis was carried out, by applying a peak voltage of 2.5 V to the input IDTs in each of the two Euler directions and employing a time step of 1 ns, to investigate the nature of waves propagating in the two directions. The device sensitivity and acoustic streaming force were computed and compared with devices having uniform unidirectional IDTs in the respective directions. Results Our previous experimental studies have demonstrated acoustic streaming can be effectively used to achieve removal of non-specifically bound (NSB) proteins from the device surface (Fig. 1), thereby enhancing device performance. We present the first report on mutually interacting U-IDTs/F-IDTs on a Langasite substrate to enhance device performance in biosensing applications by increasing the intensity of acoustic streaming forces for efficient biofouling removal and amplify the displacement/surface acoustic energy for increased device sensitivity. Our simulation studies indicate the propagation of pure shear horizontal wave along the (0, 22, 90) direction, which allow for biosensing. Focused IDTs along the (0, 22, 0) direction lead to the focusing of waves (Fig. 2) thereby maximizing the device displacements near the focal point. Further, the use of focused IDTs along (0, 22, 0) direction allows for amplification of the prominent surface normal component in the propagating mixed mode wave. This amplification of the surface normal component leads to a 266% increase in the amplitude of streaming force (4.2*105 N/m2 for U-IDTs/F-IDTs design vs. 1.2*105 N/m2 for standard uniform IDTs). In addition, the constructive interference of propagating waves in the delay path leads to maximized energy entrapment and displacements on the device surface, thereby imparting 32.4% higher sensitivity to the device with U-IDTs/FIDTs (15.3 Hz-cm2/ng) compared to the standard unidirectional IDT design (11.56 Hz-cm2/ng). Our simulation results indicate that the use of mutually interacting U-IDTs/F-IDTs on a Langasite substrate leads to an efficient sensor with increased device sensitivity and intensity of acoustic streaming compared to a device with uniform unidirectional IDTs. Results will be presented at the AIChE 2009 meeting.