Investigation into the Effect of Acoustic Radiation Force and Acoustic Streaming on Particle Patterning in Acoustic Standing Wave Fields

Acoustic standing waves have been widely used in trapping, patterning, and manipulating particles, whereas one barrier remains: the lack of understanding of force conditions on particles which mainly include acoustic radiation force (ARF) and acoustic streaming (AS). In this paper, force conditions on micrometer size polystyrene microspheres in acoustic standing wave fields were investigated. The COMSOL® Mutiphysics particle tracing module was used to numerically simulate force conditions on various particles as a function of time. The velocity of particle movement was experimentally measured using particle imaging velocimetry (PIV). Through experimental and numerical simulation, the functions of ARF and AS in trapping and patterning were analyzed. It is shown that ARF is dominant in trapping and patterning large particles while the impact of AS increases rapidly with decreasing particle size. The combination of using both ARF and AS for medium size particles can obtain different patterns with only using ARF. Findings of the present study will aid the design of acoustic-driven microfluidic devices to increase the diversity of particle patterning

Three-dimensional heating and patterning dynamics of particles in microscale acoustic tweezers

oai:www.db-thueringen.de:dbt_mods_00053576Acoustic tweezers facilitate a noninvasive, contactless, and label-free method for the precise manipulation of micro objects, including biological cells. Although cells are exposed to mechanical and thermal stress, acoustic tweezers are usually considered as biocompatible. Here, we present a holistic experimental approach to reveal the correlation between acoustic fields, acoustophoretic motion and heating effects of particles induced by an acoustic tweezer setup. The system is based on surface acoustic waves and was characterized by applying laser Doppler vibrometry, astigmatism particle tracking velocimetry and luminescence lifetime imaging. In situ measurements with high spatial and temporal resolution reveal a three-dimensional particle patterning coinciding with the experimentally assisted numerical result of the acoustic radiation force distribution. In addition, a considerable and rapid heating up to 55 °C depending on specific parameters was observed. Although these temperatures may be harmful to living cells, counter-measures can be found as the time scales of patterning and heating are shown to be different

Shear Induced Fiber Alignment and Acoustic Nanoparticle Micropatterning during Stereolithography

The stereolithograpy method, which consists of a light source to polymerize the liquid photocurable resin, can produce structures with complex shapes. Most of the produced structures are unreinforced neat pieces. The addition of reinforcement, such as fibers and particles are regularly utilized to improve mechanical properties and electrical conductivity of the printed parts. Added fibers might be chosen as short or continuous fibers and the properties of the reinforced composite materials can be significantly improved by aligning the fibers in preferred directions. The first aim of this dissertation is to enhance the tensile and flexural strengths of the 3d printed composites by using shear induced alignment of short fibers. The second aim is to print parts with conductive embedded microstructures by utilizing acoustic patterning of conductive particles. Both aims are utilized during the stereolithography process.A lateral oscillation mechanism, which is inspired by large amplitude oscillatory shear test, was designed to generate shear flow. The alignment method, which combines the lateral oscillation mechanism with 3d printed wall patterns, is developed to utilized shear flow to align the fibers in the patterned wall direction. Shear rate amplitude, fiber concentration, and patterned wall angle were considered as parameters during this study.The stereolithography device incorporated with oscillation mechanism was utilized to produce short fiber reinforced ceramic composites and short nanofiber reinforced polymer composites. Nickel coated short carbon fibers, alumina and silica short fibers were used to reinforce the ceramic matrix with different fiber contents. The printed walls were demonstrated to align the short fibers parallel to the wall which was different from the oscillation direction up to 45°. The flexural strength of the ceramic matrix was improved with the addition and alignment of the short fibers. The alumina nanofibers were used as reinforcement in the photocurable polymer resin. The alumina nanofibers were treated with a silane coupling agent to improve interfacial bond between alumina fibers and polymer resin matrix. The aligned specimen demonstrated improvement in tensile strength with increasing nanowire content and their alignment.A hexagon shaped acoustic tweezer was incorporated into the stereolithography device to pattern conductive micro- and nanoparticles. This new approach for particle microstructuring via acoustic aligning during the stereolithography was used to produce embedded conductive microstructures in 3d printed parts. The acoustic tweezer was used to pattern the conductive particles into horizontal, 60°, and 120° parallel striped lines. The influence of the particle percentage content onto the electrical resistivity and thickness of the patterned lines were also investigated for different materials such as copper, magnetite, and carbon fiber. The copper patterns show less resistance to electrical currents compare to magnetite and carbon nanofiber patterns. Additionally, the influence of the particle concentration to the height of the pattern was studied and the data was utilized to achieve conductivity along z-axis. Later, this approach was used to fabricate examples of embedded conductive complex 3D microstructures

Numerical and experimental study of interactions between surface acoustic waves, fluids and particles in acoustofluidic systems

Acoustofluidics refers to the multidisciplinary field of investigating the integration of acoustics with microfluidics which can be used to develop tools to manipulate and control microfluids and particles. Acoustofluidic technologies present numerous advantages including small size, simple design, low cost, reliability, efficiency, and as a result can be adapted to various applications. Moreover, for biomedical applications, acoustofluidics offer benefits such as non-invasive and contact-free manipulation with high biocompatibility, conserving cell viability and proliferation. With these advantages, acoustofluidics present a great potential to be utilised in many clinical and biomedical applications such as lab-on-chip, organ-on-chip, and controllable drug delivery platforms. Most of the current studies on acoustofluidics are based on an experimental investigation which is essential for testing a hypothesis scientifically and to ensure that the acoustofluidic system functions properly. Experimental methods can be used for optimising acoustofluidic systems or evaluating a new design based on trial-and-error approaches. However, experimental acoustofluidcs could be costly and time-consuming. Computational modelling can provide detailed information of the underlying physics of the complex acoustofluidic systems in a more cost and time-effective manner. These details, which can be useful for adapting acoustofluidic systems in practical devices, are sometimes hard or even impossible to obtain through experimental work. In this thesis, computational models are utilised in order to investigate the behaviour of fluids and particles in novel acoustofluidic platforms including flexible acoustofluidics and capillary bridge channels. The models are first validated using the experimental results reported in the literature and then, they are used to analyse the behaviour and to understand the underlying physics of novel acoustofluidic platforms such as flexible thin film surface acoustic wave devices and capillary bridges for the purpose of particle manipulation. Typically, most acoustofluidic systems presented in existing literature are designed using rigid piezoelectric materials to generate acoustic fields. These rigid piezoelectric materials are generally brittle, fragile, and prone to breaking when applied with higher powers. In this thesis, flexible thin film surface acoustic wave devices with metal substrates are utilised for particle and cell manipulations, and to study their acoustofluidic behaviour in different conformations obtained through bending and to investigate the effects of bending curvatures on microparticle’s manipulation inside a microchamber. These flexible thin film devices present advantages including high wave speed and reasonable electro-mechanical properties for the flexible thin film devices. Additionally, for continuous flow applications to enable the fluid flow, microchannels are typically fabricated with solid materials and in cleanroom environment using complicated and time-consuming processes. This thesis presents the idea to integrate flexible thin film surface acoustic wave devices with continuous flow wall-less microfluidic platforms designed using capillary bridges. Using capillary bridge channels simplifies the production of microchannels while decreasing the fabrication cost and time. This new platform which comprises of flexible thin film surface acoustic wave devices with metal substrates and capillary bridge channels is utilised for particle and cell manipulation and is investigated in detail through both computational modelling and experimental study. These novel acoustofluidic platforms can offer potential applications in flexible microfluidics, bio-inspired and body conforming wearable devices, and wearable point-of-care applications. The significant contributions of this thesis can be summarised as follows: 1. For the first time, flexible thin film surface acoustic wave devices with metal substrates are utilised for the purpose of particle and cell manipulation. Through both experimental study and computational modelling, the effects of various vibration modes, different bending curvatures, and twisting geometries are investigated. It was presented that flexible surface acoustic wave devices bent in concave/convex geometries produce particle patterns converged with a slope towards/ diverged with a slope away from the centre of the curvature of the geometry. 2. Glass microtubes (with both rectangular and circular cross-sections) are integrated with flexible thin film surface acoustic wave devices for the purpose of particle and cell manipulation with and without fluid flow. The effects of different microtube cross-sections, microtube inclination angle regarding the electrodes of the surface acoustic wave device, and different fluid flow rates on particle patterning are systematically investigated. For rectangular microtubes placed at an angle relative to the electrodes, particle pattern lines were parallel to the tube walls. For circular microtubes, different particle patterns were observed which were dependent on their positions along the tube’s height. In the bottom/middle height of the tube, the particle pattern lines were parallel to the tube direction due to the acoustic wave propagation into the water and formation of a standing wave along the direction of the circular tube/ perpendicular to the tube direction as the standing wave propagated around the circular cross-section of the tube perpendicular to the tube direction. 3. For the first time, capillary bridge channels are integrated with flexible thin film surface acoustic wave devices with metal substrates to develop a continuous flow acoustofluidic setup for particle and cell manipulations. Through both experimental work and three-dimensional numerical modelling, the effects of different frequencies, channel geometries, particle properties, and flow rates are investigated. It was shown that the particles were aligned on the pressure node lines of the acoustic pressure field and parallel to the air-water walls of the capillary bridge channels due to the combined effects of the acoustic wave field inside the water channel and the fluid flow. 4. The effects of acoustic streaming on fluid and microparticles in a microchannel flow are investigated through both experimental studies and three-dimensional numerical modelling. Two different modelling approaches are compared: 1st Approach: The whole acoustic field coupled to the flow field is simulated and the acoustic streaming force is calculated using the first order acoustic density and velocity which predicted the acoustofluidic system more accurately. 2nd Approach: The acoustic streaming is modelled by assuming the velocity of a one-dimensional attenuating surface acoustic wave and using the acoustic streaming force formula which is more efficient in terms of computational cost and time while still presenting results with reasonable accuracy

Thick film PZT transducer arrays for particle manipulation

This paper reports the fabrication and evaluation of a two-dimensional thick film PZT ultrasonic transducer array operating at about 7.5 MHz for particle manipulation. All layers on the array are screen-printed and sintered on an Al2O3 substrate without further processes or patterning. The measured dielectric constant of the PZT is 2250 ± 100, and the dielectric loss is 0.09 ± 0.005 at 10 kHz. Finite element analysis has been used to predict the behaviour of the array and impedance spectroscopy and laser vibrometry have been used to characterise its performance. The measured deflection of a single activate element is on the order of tens of nanometres with 20 Vpp input. Particle manipulation experiments have been performed by coupling the thick film array to a capillary containing polystyrene microspheres in water

Investigating the motility of Dictyostelium discodeum using high frequency ultrasound as a method of manipulation

Cell motility is an essential process in the development of all organisms. The earliest stages of embryonic development involve massive reconfigurations of groups of cells to form the early body structures. Embryos are very complex systems, and therefore to investigate the molecular and cellular basis of development a simpler genetically tractable model system is used. The social amoeba Dictyostelium Discoideum is known to chemotax up a chemical gradient. From previous work, it is clear that cells generate forces in the nN range. This is above the limit of optical tweezers and therefore we are investigating the use of acoustic tweezers instead. In this paper, we present recent progress of the investigation in to the use of acoustic tweezers for the characterisation of cell motility and forces. We will describe the design, modelling and fabrication of several devices. All devices use high frequency (>15MHz) ultrasound to exert a force on the cells to position and/or stall them. Also, each device is designed to be suitable for the life-sciences laboratory where form-factor and sterility is concerned. A transducer (LiNo) operating at 24 MHz excites resonant acoustic modes in a rectangular glass capillary (100um by 2mm). This device is used to alter the directionality of the motile cells inside the fluid filled capillary. A quarter-ring PZT26 transducer operating at 20.5MHz is shown to be useful for manipulating cells using axial acoustic radiation forces. This device is used to exert a force on cells and shown to pull them away from a coverslip. The presented devices show promise for the manipulation of cells in suspension. Currently the forces produced are below that required for adherent cells; the reasons for this are discussed. We also report on other issues that arise when using acoustic waves for manipulating biological samples such as streaming and heating

Modeling and analysis of the two-dimensional axisymmetric acoustofluidic fields in the probe-type and substrate-type ultrasonic micro/nano manipulation systems

© 2019 by the authors. The probe-type and substrate-type ultrasonicmicro/nanomanipulation systems have proven to be two kinds of powerful tools for manipulating micro/nanoscale materials. Numerical simulations of acoustofluidic fields in these two kinds of systems can not only be used to explain and analyze the physical mechanisms of experimental phenomena, but also provide guidelines for optimization of device parameters and working conditions. However, in-depth quantitative study and analysis of acoustofluidic fields in the two ultrasonic micro/nano manipulation systems have scarcely been reported. In this paper, based on the finite element method (FEM), we numerically investigated the two-dimensional (2D) axisymmetric acoustofluidic fields in the probe-type and substrate-type ultrasonic micro/nano manipulation systems by the perturbation method (PM) and Reynolds stress method (RSM), respectively. Through comparing the simulation results computed by the two methods and the experimental verifications, the feasibility and reasonability of the two methods in simulating the acoustofluidic fields in these two ultrasonic micro/nano manipulation systems have been validated. Moreover, the effects of device parameters and working conditions on the acoustofluidic fields are clarified by the simulation results and qualitatively verified by the experiments

Surface acoustic waves induced micropatterning of cells in gelatin methacryloyl (GelMA) hydrogels

Acoustic force patterning is an emerging technology that provides a platform to control the spatial location of cells in a rapid, accurate, yet contactless manner. However, very few studies have been reported on the usage of acoustic force patterning for the rapid arrangement of biological objects, such as cells, in a three-dimensional (3D) environment. In this study, we report on a bio-acoustic force patterning technique, which uses surface acoustic waves (SAWs) for the rapid arrangement of cells within an extracellular matrix-based hydrogel such as gelatin methacryloyl (GelMA). A proof-of-principle was achieved through both simulations and experiments based on the in-house fabricated piezoelectric SAW transducers, which enabled us to explore the effects of various parameters on the performance of the built construct. The SAWs were applied in a fashion that generated standing SAWs (SSAWs) on the substrate, the energy of which subsequently was transferred into the gel, creating a rapid, and contactless alignment of the cells (<10 s, based on the experimental conditions). Following ultraviolet radiation induced photo-crosslinking of the cell encapsulated GelMA pre-polymer solution, the patterned cardiac cells readily spread after alignment in the GelMA hydrogel and demonstrated beating activity in 5–7 days. The described acoustic force assembly method can be utilized not only to control the spatial distribution of the cells inside a 3D construct, but can also preserve the viability and functionality of the patterned cells (e.g. beating rates of cardiac cells). This platform can be potentially employed in a diverse range of applications, whether it is for tissue engineering, in vitro cell studies, or creating 3D biomimetic tissue structures

Particle Enrichment in Longitudinal Standing Bulk Acoustic Wave Microfluidics

Separation, isolation, and enrichment of targeted nano- and microparticles are critical to a variety of biomedical applications from clinical research (development of therapeutics and diagnostics) to fundamental investigations that require concentration of specific cells from culture, separation of target species from heterogenous mixtures, or controlled perturbation of cells and microorganisms to determine their response to stimuli. Numerous techniques are available for bench-scale and medical settings; however, these traditional approaches are often labor intensive, time-consuming, costly, and/or require modification of the target. Efficiency and specificity are also lacking. Recently, techniques that exploit the similar scales of microfluidic technologies and the intrinsic properties of cells have allowed for increased automation, reduced reagent waste, and decreased cost, as well as improved performance. So-called lab-on-a-chip (LOC) approaches enable rapid fabrication and optimization of small-scale, low-volume microchannels capable of high performance enrichment and separation owing to precise control of the forces driving the manipulation. Depending on the physics underlying a particular method, devices are classified as optical, hydrodynamic, dielectrophoretic, magnetic, or acoustic. Acoustics, and specifically ultrasound, permits noncontact cell separation and retention, which reduces the potential for undesirable surface interactions and physical stress on sensitive biological samples. Typically, separation is achieved by pinning a standing wave perpendicular (conventional lateral acoustophoresis) or parallel (longitudinal acoustic trapping) to the direction of flow. In this thesis, we report a novel longitudinal standing bulk acoustic wave (LSBAW) microfluidic channel that incorporates pairs of pillar arrays oriented perpendicular to the inflow direction. The pillar arrays act as ‘pseudo walls’ that locally amplify the pressure in the enrichment zone, which can be tuned to overcome the drag force for particles of size greater than a critical diameter. Thus, these particles are preferentially retained within the nodes of the local pressure field. In our study, model predictions were used to guide experimental trapping of particles in microchannels with two pillar configurations. We created six different microfluidic channels with varying inlet/outlet geometries, widths, and pillar shapes. Model results showed pressure field amplification caused by the ‘pseudo walls’ bounding the enrichment zone of each design. We also demonstrated trapping of polystyrene beads (5 μm and 20 μm) and 10 μm fluorescent hollow glass spheres during actuation at various predicted half-wave resonances of these devices. Certain channel architectures achieved acoustic field amplification suitable for particle trapping at flow rates up to ~20 μL/min. In addition, the simulated pressure fields (eigenmodes) were consistent with experimentally observed mode shapes, which validated our modeling approach. Computational and experimental results suggest that LSBAW pillar geometries and flow parameters can be tuned to achieve enhanced enrichment of targeted particles in a predefined region