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Biosensors typically operate in liquid media for detection of biomarkers and suffer from fouling resulting from nonspecific binding of protein molecules to the device surface. In the current work, using a coupled field finite element fluid-structure interaction simulation, we have identified that fluid motion induced by high intensity sound waves, such as those propagating in these sensors, can lead to the efficient removal of the nonspecifically bound proteins thereby eliminating sensor fouling. We present a computational analysis of the acoustic-streaming phenomenon induced biofouling elimination by surface acoustic-waves (SAWs) propagating on a lithium niobate piezoelectric crystal. The transient solutions generated from the developed coupled field fluid solid interaction model are utilized to predict trends in acoustic-streaming induced forces for varying design parameters such as voltage intensity, device frequency, fluid viscosity, and density. We utilize these model predictions to compute the various interaction forces involved and thereby identify the possible mechanisms for removal of nonspecifically-bound proteins. For the range of sensor operating conditions simulated, our study indicates that the SAW motion acts as a body force to overcome the adhesive forces of the fouling proteins to the device surface whereas the acoustic-streaming induced hydrodynamic forces prevent their reattachment. The streaming velocity fields computed using the finite element models in conjunction with the proposed particle removal mechanism were used to identify the optimum conditions that lead to improved removal efficiency. We show that it is possible to tune operational parameters such as device frequency and input voltage to achieve effective elimination of biofouling proteins in typical biosensing media. Our simulation results agree well with previously reported experimental observations. The findings of this work have significant implications in designing reusable, selective, and highly sensitive biosensors. [ABSTRACT FROM AUTHOR]

Fluid motion induced on the surface of 100 MHz focused surface acoustic wave (F-SAW) devices with concentric interdigital transducers (IDTs) based on Y-cut Z-propagating LiNbO3 substrate was investigated using three-dimensional bidirectionally coupled finite element fluid-structure interaction models. Acoustic streaming velocity fields and induced forces for the F-SAW device are compared with those for a SAW device with uniform IDTs (conventional SAW). Both, qualitative and quantitative differences in the simulation derived functional parameters, such as device displacements amplitudes, fluid velocity, and streaming forces, are observed between the F-SAW and conventional SAW device. While the conventional SAW shows maximum fluid recirculation near input IDTs, the region of maximum recirculation is concentrated near the focal point of the F-SAW device. Our simulation results also indicate acoustic energy focusing by the F-SAW device leading to maximized device surface displacements, fluid velocity, and streaming forces near the focal point located in the center of the delay path, in contrast to the conventional SAW exhibiting maximized values of these parameters near the input IDTs. Significant enhancement in acoustic streaming is observed in the F-SAW device when compared to the conventional ones; the increase in streaming velocities was computed to be 352% and 216% for tangential velocities in propagation and transverse directions, respectively, and 353% for the normal velocity. Consequently, the induced streaming force for F-SAW is 480% larger than that for conventional SAW. In biosensing applications, this allows for the removal of smaller submicron sized particles by F-SAW which are otherwise difficult to remove using the conventional SAW. The F-SAW presents an order of magnitude reduction in the smallest removable particle size compared to the conventional device. Our results indicate that the acoustic energy focusing and streaming enhancement brought about by the F-SAW device manifests itself as enhanced biofouling removal efficiency of F-SAW throughout the device delay path compared to the conventional device, thereby providing enhanced device sensitivity, selectivity, and reusability. Furthermore, contrary to the conventional SAW in which the smallest particle is removable near the input IDTs, the F-SAW device removes the smallest particle near the device focal point. The results of this work are shown to have significant implications in typical biosensing and microfluidic applications. In a broader context, the results of the present study demonstrate a technique of enhancing streaming induced flows, which is of great importance to contemporary problems involving microfluidic and sensing applications of piezoelectric devices. [ABSTRACT FROM AUTHOR]

Focused interdigital transducers (F-IDTs) patterned on surfaces of piezoelectric substrates can be used to generate surface acoustic waves (SAW) with high intensity and high beam-width compression ratio. A three dimensional coupled field finite element model of a focused SAW (F-SAW) device with interdigital transducers shaped as concentric circular arcs based on a YZ LiNbO3 substrate is developed in this study. This model was utilized to investigate the effect of geometric shape of transducers on the focusing properties of F-IDTs to identify the optimal design for potential microfluidic applications. The transducer design parameters investigated in the current study include number of finger pairs, degree of arc, geometric focal length, and wavelength of F-SAW. The transient response of the device on application of impulse and ac electrical inputs at the transmitting FIDT fingers were utilized to deduce the device frequency response and propagation characteristics of F-SAWs, respectively. The influence of applied input voltage on the propagation characteristics is also investigated. The insertion loss calculated for the various F-IDT designs was used to identify the optimal transducer configuration for sensing and microfluidic applications. The focusing properties as well as the wave propagation characteristics for the various F-IDT designs were evaluated in terms of the amplitude field and displacement contours generated in regions close to and at the focal point. Comparison with a conventional SAW device operating at megahertz frequency range and uniform IDT design is also made. Our study indicates that the focusing property of the device is significantly influenced by the geometric shape of the F-IDTs. The streaming phenomenon induced by F-SAW propagation, when in contact with a fluid medium, is discussed in detail. The simulated amplitude fields generated using ac analysis for the various designs in conjunction with wave propagation parameters derived using perturbational techniques such as Campbell–Jones are utilized to calculate the streaming forces and velocities based on successive approximation technique applied to Navier–Stokes equation (Nyborg’s theory). The maximum streaming force and velocity are obtained at the focal point of the F-SAW device. The magnitude of the generated streaming force and induced streaming velocity are strongly influenced by the transducer configurations. Based on the simulation results of this study, we provide guidelines for designing various F-IDTs to suite desired applications. F-SAW devices operating with higher applied input voltages and at higher frequencies, with optimal geometric length and larger degree of arc, are best suited for actuation and fluid microtransport. [ABSTRACT FROM AUTHOR]