Reetu Singh

Acoustic streaming induced elimination of nonspecifically bound proteins from a surface acoustic wave biosensor: Mechanism prediction using fluid-structure interaction models

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 vario...

Enhanced surface acoustic wave biosensor performance via delay path modifications in mutually interacting multidirectional transducer configuration: A computational study

Multidirectional interdigital transducers (IDTs) combined with delay path modifications for surface acoustic wave (SAW) sensors in a Langasite substrate are shown to positively and significantly impact power consumption, device sensitivity, and biofouling elimination capability. Simulated devices have mutually interacting orthogonal IDTs and microcavities of square cross sections of side λ/2, and of different depths located in the middle of the delay path. A combined orthogonal IDT-polystyrene filled microcavities device (dimensions λ/2×λ/2×λ/2), with constructive wave interference and enhanced SAW entrapment in the delay region, is shown to be most efficient and reduces insertion loss by 23.6 dB, generates two orders of magnitude larger streaming forces, and exhibits velocity sensitivity 100% larger than that of a simulated standard SAW sensor with unidirectional IDTs along the (0, 22, 90) direction.

Orthogonal surface acoustic wave device based on langasite for simultaneous biosensing and biofouling removal

We report a combined three-dimensional structural and fluid structure interaction finite element study of an orthogonal surface acoustic wave (SAW) device based on langasite (LGS). Our simulation results indicate that simultaneous sensing and nonspecifically bound protein removal can be achieved through the use of multidirectional transducers on a single piezoelectric device. We find that the (0, 22, 90) Euler direction on the LGS-based device is suitable for biosensing via propagation of pure shear-horizontal waves, whereas the (0, 22, 0) direction allows for acoustic streaming induced biofouling removal through the propagation of mixed mode waves with prominent surface normal component. This study reveals the possibility of integrating sensing and biofouling removal functions on a single SAW device, thereby enhancing sensor performance.

Enhancement of acoustic streaming induced flow on a focused surface acoustic wave device: Implications for biosensing and microfluidics

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 str...

Computational design of quartz crystal nanobalance for uniform sensitivity distribution

The mass sensitivity (> 1 micrograms) of current commercial analytical instruments, such as thermal gravimetric analyzers, severely limits their utility for measurement of valuable but poorly soluble materials such as synthetic proteins or DNA fragments. A quartz crystal microbalance (QCM), based on a transverse shear mode piezoelectric crystal operating at high frequencies, is gaining popularity in chemical and bio-sensing applications due to higher mass sensitivities as compared to the traditional analyzers and lesser sensitivity to vibrations. However, these devices suffer from non-uniformity of sensitivity distribution along the sensor surface thereby limiting their use for the determination of mass. Overcoming this limitation would lead to the development of a robust sensor with improved mass sensitivities and reduced sensitivity to vibrations, as compared to the currently available microbalances. The sensitivity profile can be influenced by a number of factors the electrode design and surface properties of the crystal. In the current work, we develop a finite element (FE) model of the QCM to investigate the mass sensitivity and its radial distribution on the sensor surface for various electrode designs. Such a model will aid in the development of versatile nano-balances with a uniform sensitivity distribution.

Design of mutually interacting multi-directional transducer configurations on a surface acoustic wave device for enhanced biosensing

Transducers used in biosensing applications are plagued by biofouling, which refers to the binding of nonspecific 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. Multi-directional transducers have the capability of achieving the dual objectives of biosensing and non-specifically bound protein removal for improved sensor performance. Also, focused interdigital transducers (IDTs) have the potential for acoustic energy focusing, thereby increasing the intensity of acoustic streaming. We have identified that various crystallographic orientation allow the propagation of different modes thereby rendering them suitable for different applications. For example, in Langasite, shear horizontal modes propagate along (0, 22, 90) Euler direction while mixed modes with prominent surface normal component are obtained along (0, 22, 0) direction. Thus, the (0, 22, 90) and (0, 22, 0) directions are suitable for biosensing and is suited for removal of NSB founding proteins from device surface. In this work, we investigate a Langasite based biosensor with a mutually interacting multidirectional IDT configuration along the two identified Euler directions for enhanced biosensor performance. Uniform IDTs (U-IDTs) are employed in the (0, 22, 90) direction while focused IDTs (F-IDTs) are placed 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. Thus, the Langasite based device with mutually interaction U-IDTs and F-IDTs represents a significant enhancement over the conventional SAW device having uniform IDTs and is better suited for biosensing applications. 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.

Enhancement in ultrasonic micro-transport using focused inter-digital transducers in a surface acoustic wave device: Fluid-structure interaction study

Biosensing and microfluidic applications of surface acoustic wave (SAW) devices, involving micro-transport, rely on acoustic streaming resulting from high intensity sound waves interacting with the fluid medium. In this work, we investigate the enhancement in the efficiency of acoustic streaming via the use of interdigital transducer (IDT) modifications, viz. focused IDTs (F-IDTs). We have developed, for the first time, a three dimensional bi-directionally coupled fluid-structure interaction finite element model of a focused SAW (F-SAW) device with F-IDTs based on concentric wave surfaces and subject to liquid loading. The simulated device displacement profiles indicate focusing and enhancement of surface displacement amplitudes, instantaneous fluid velocities, and streaming velocities in a F-SAW device compared to a conventional SAW device having uniform IDTs with a similar size, finger periodicity and applied input voltage. Furthermore, the F-SAW device brings about focusing of acoustic energy near the center of the device, thereby enhancing the device displacements and fluid velocities in the center of the delay path in contrast to the conventional SAW in which these quantities decay on moving away from input IDTs towards the device center. Thus, our results indicate enhancement in acoustic streaming induced flow in F-SAW devices compared to conventional SAW. The results have a general applicability for various biosensing and microfluidic actuation applications that rely on the acoustic streaming phenomenon.

A novel three dimensional fluid-structure interaction finite element model of wave propagation in SAW device: Application to biosensing & microfluidics

The key issues related to biosensor technology include selectivity, sensitivity, response and recovery times, and detection limit; most of these limitations stem from biofouling resulting from the binding of undesirable moieties such as non-specific proteins to the sensor surface. Thus, removal of non-specifically bound (NSB) proteins remains a significant challenge in biosensing applications. Operation of biosensors in liquid media necessitates an investigation of the fluid-device interaction to understand the mechanisms of biofouling elimination. In this study, we report for the first time, a fully coupled three dimensional transient finite element fluid-solid interaction (FSI) model of the SAW device subject to liquid loading to investigate the streaming velocity fields and forces induced by SAW device. Our simulation results suggest that the SAW-fluid interaction creates a pressure gradient in the direction of acoustic wave propagation in the fluid, leading to an acoustically driven streaming phenomenon known as SAW streaming which can be used for removal of non-specifically bound (NSB) proteins. Computed velocity fields indicate that the normal component of fluid velocity is smaller than the tangential component along the propagation direction. Thus, the SAW induced drag force, arising from the tangential component of fluid velocity and leading to particle advection is an important mechanism in biofouling removal from the SAW device surface and the normal component would prevent the reattachment of the particles to the device surface. Apart from microfluidic applications, 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.

Influence of non-newtonian fluid dynamics on SAW induced acoustic streaming in view of biological applications

Surface acoustic wave (SAW) devices are finding increasing use in medical diagnostic applications, such as detection of specific proteins in bodily fluids for detection of pathologies. These devices can also be used in Lab-On-a-Chip devices for biological applications that utilize micro-fluidics for detection, transport, mixing, and biological assays. In applications aimed at biological sensing, the sensing medium such as blood exhibits a non-Newtonian behavior. In biosensing applications of SAW devices, SAW induced acoustic streaming which refers to fluid motion induced by high frequency sound waves, is an important phenomenon that can be used for the removal of non-specifically bound proteins from the device surface. Acoustic streaming also finds use in a wide variety of other applications such as detection of ovarian cysts and detection of blood clotting via ultrasound and convective transport in microfluidic applications of SAW devices. This work reports on the influence of non-Newtonian fluid dynamics on the acoustic streaming and fluid velocity profiles in SAW devices, using a computational fluid-structure interaction finite element model.

Engineering picogram level detection using high frequency surface acoustic wave chemical and biological sensors based on multilayered Diamond/AlN/LiNbO 3 substrates

Operating SAW devices in the GHz frequency range can enable detection of single molecules by imparting high sensitivities and low detection limits. In the present work, we used 3-D coupled field structural as well as fluid-solid interaction finite element models to study the acoustic wave propagation characteristics of diamond/AlN/LiNbO3 multi-layered piezoelectric surface acoustic wave devices under the influence of fluid loading for applications in chemical and biological sensing. These devices were studied as a method to increase device frequency and sensitivity, and maintain standard fabrication procedures. The operating frequency of SAW devices is directly proportional to the substrates acoustic wave velocity; hence the highest acoustic wave velocity material (diamond) is needed for fabrication of MEMS GHz frequency devices. The aluminum nitride piezoelectric layer also has a very high acoustic wave velocity and a fairly large piezoelectric coupling coefficient along its c-axis, in comparison to other piezoelectric materials. Although recent experimental investigations have realized GHz frequency devices based on such multilayered substrates, very little is known about the acoustic wave propagation characteristics in these devices. Identifying the optimum configuration and thickness of the various layers involved still represents a challenge, which is addressed in this work.