Microfluidic acoustic sawtooth metasurfaces for patterning and separation using traveling surface acoustic waves

Numerical modeling of ultrasound-triggered microneedle-mediated delivery of drug particles into bacterial biofilms

Ultrasonic microneedle patches, a class of ultrasound-driven transdermal drug delivery systems, are promising in addressing bacterial biofilms. This device has been proven to be more effective in treating Staphylococcus aureus biofilms than drug in free solution. However, there exists a notable gap in understanding how various excitation conditions and material parameters affect drug delivery efficiency. This study aims to fill this void by conducting an comprehensive multi-physics numerical analysis of ultrasonic microneedle patches, with the ultimate goal of enhancing drug delivery. First, we investigate the impact of various ultrasound frequencies on drug penetration depths. The findings reveal that local resonance can accelerate drug release within a shorter time window (first 1.5 h), whereas non-resonant frequencies enable more profound and prolonged diffusion. This information is crucial for medical professionals in selecting the most effective frequency for optimal drug administration. Furthermore, our investigation extends to the effects of applied voltage on temperature distribution, a critical aspect for ensuring medical safety during the application of these patches. Additionally, we examine how particles of different sizes respond to acoustic pressure and streaming fields, providing valuable insights for tailoring drug delivery strategies to specific therapeutic needs. Overall, our findings offer comprehensive guidelines for the effective use of ultrasonic microneedle patches, potentially shifting the paradigm in patient care and enhancing the overall quality of life.

Micro-Acoustic Holograms for Detachable Microfluidic Devices

Acoustic microfluidic devices have advantages for diagnostic applications, therapeutic solutions, and fundamental research due to their contactless operation, simple design, and biocompatibility. However, most acoustofluidic approaches are limited to forming simple and fixed acoustic patterns, or have limited resolution. In this study,a detachable microfluidic device is demonstrated employing miniature acoustic holograms to create reconfigurable, flexible, and high-resolution acoustic fields in microfluidic channels, where the introduction of a solid coupling layer makes these holograms easy to fabricate and integrate. The application of this method to generate flexible acoustic fields, including shapes, characters, and arbitrarily rotated patterns, within microfluidic channels, is demonstrated.

Cavity-agnostic acoustofluidic manipulations enabled by guided flexural waves on a membrane acoustic waveguide actuator

This article presents an in-depth exploration of the acoustofluidic capabilities of guided flexural waves (GFWs) generated by a membrane acoustic waveguide actuator (MAWA). By harnessing the potential of GFWs, cavity-agnostic advanced particle manipulation functions are achieved, unlocking new avenues for microfluidic systems and lab-on-a-chip development. The localized acoustofluidic effects of GFWs arising from the evanescent nature of the acoustic fields they induce inside a liquid medium are numerically investigated to highlight their unique and promising characteristics. Unlike traditional acoustofluidic technologies, the GFWs propagating on the MAWA's membrane waveguide allow for cavity-agnostic particle manipulation, irrespective of the resonant properties of the fluidic chamber. Moreover, the acoustofluidic functions enabled by the device depend on the flexural mode populating the active region of the membrane waveguide. Experimental demonstrations using two types of particles include in-sessile-droplet particle transport, mixing, and spatial separation based on particle diameter, along with streaming-induced counter-flow virtual channel generation in microfluidic PDMS channels. These experiments emphasize the versatility and potential applications of the MAWA as a microfluidic platform targeted at lab-on-a-chip development and showcase the MAWA's compatibility with existing microfluidic systems.

Optimizing coupling layer and superstrate thickness in attachable acoustofluidic devices

Superstrate-based acoustofluidic devices, where the fluidic elements are reversibly coupled to a transducer rather than bonded to it, offer advantages for cost, interchangeability and preventing contamination between samples. A variety of coupling materials can be used to transmit acoustic energies into attachable superstrates, though the dimensions and material composition of the system elements are not typically optimized. This work analyzes these coupling layers for bulk wavefront transmission, including water, ultrasound gel and polydimethylsiloxane (PDMS), as well as the material makeup and thickness of the superstrate component, which is commonly comprised of glass, quartz or silicon. Our results highlight the importance of coupling layer and superstrate dimensions, identifying frequencies and component thicknesses that maximize transmission efficiency. Our results indicate that superstrate thicknesses 0.55 times the acoustic wavelength result in maximal acoustic coupling. While various coupling layers and superstrate materials are capable of similar acoustic energy transmission, the inherent dimensional stability of the PDMS coupling layers, somewhat less common in superstrate work compared to liquid-based agents, presents advantages for practically maximizing acoustic efficiency.

Acoustofluidic patterning in glass capillaries using travelling acoustic waves based on thin film flexible platform

Surface acoustic wave (SAW) technology has been widely used to manipulate microparticles and biological species, based on acoustic radiation force (ARF) and drag force induced by acoustic streaming, either by standing SAWs (SSAWs) or travelling SAWs (TSAWs). These acoustofluidic patterning functions can be achieved within a polymer chamber or a glass capillary with various cross-sections positioned along the wave propagating paths. In this paper, we demonstrated that microparticles can be aligned, patterned, and concentrated within both circular and rectangular glass capillaries using TSAWs based on a piezoelectric thin film acoustic wave platform. The glass capillary was placed at different angles along with the interdigital transducer directions. We systematically investigated effects of tilting angles and wave characteristics using numerical simulations in both circular and square shaped capillaries, and the patterning mechanisms were discussed and compared with those agitated under the SSAWs. We then experimentally verified the particle patterns within different glass capillaries using thin film ZnO SAW devices on aluminum (Al) sheets. Results show that the propagating SAWs can generate acoustic pressures and patterns in the fluid due to the diffractive effects, drag forces and ARF, as functions of the SAW device's resonant frequency and tilting angle. We demonstrated potential applications using this multiplexing, integrated, and flexible thin film-based platform, including patterning particles (1) inside multiple and successively positioned circular tubes; (2) inside a solidified hydrogel in the glass capillary; and (3) by wrapping a flexible ZnO/Al SAW device around the glass capillary.

Acoustophoresis around an elastic scatterer in a standing wave field

Acoustofluidic systems often employ prefabricated acoustic scatterers that perturb the imposed acoustic field to realize the acoustophoresis of immersed microparticles. We present a numerical study to investigate the time-averaged streaming and radiation force fields around a scatterer. Based on the streaming and radiation force field, we obtain the trajectories of the immersed microparticles with varying sizes and identify a critical transition size at which the motion of immersed microparticles in the vicinity of a prefabricated scatterer shifts from being streaming dominated to radiation dominated. We consider a range of acoustic frequencies to reveal that the critical transition size decreases with increasing frequency; this result explains the choice of acoustic frequencies in previously reported experimental studies. We also examine the impact of scatterer material and fluid properties on the streaming and radiation force fields, as well as on the critical transition size. Our results demonstrate that the critical transition size decreases with an increase in acoustic contrast factor: a nondimensional quantity that depends on material properties of the scatterer and the fluid. Our results provide a pathway to realize radiation force based manipulation of small particles by increasing the acoustic contrast factor of the scatterer, lowering the kinematic viscosity of the fluid, and increasing the acoustic frequency.

Programmable Acoustic Holography using Medium-Sound-Speed Modulation

Acoustic holography offers the ability to generate designed acoustic fields to manipulate microscale objects. However, the static nature or large aperture sizes of 3D printed acoustic holographic phase plates limits the ability to rapidly alter generated fields. In this work， a programmable acoustic holography approach is demonstrated by which multiple discrete or continuously variable acoustic targets can be created. Here, the holographic phase plate encodes multiple images, where the desired field is produced by modifying the sound speed of an intervening fluid media. Its flexibility is demonstrated in generating various acoustic patterns, including continuous line segments, discrete letters and numbers, using this method as a sound speed indicator and fluid identification tool. This programmable acoustic holography approach has the advantages of generating reconfigurable and designed acoustic fields, with broad potential in microfluidics, cell/tissue engineering, real-time sensing, and medical ultrasound.

Sub-wavelength acoustic stencil for tailored micropatterning

Acoustofluidic devices are ideal for biomedical micromanipulation applications, with high biocompatibility and the ability to generate force gradients down to the scale of cells. However, complex and designed patterning at the microscale remains challenging. In this work we report an acoustofluidic approach to direct particles and cells within a structured surface in arbitrary configurations. Wells, trenches and cavities are embedded in this surface. Combined with a half-wavelength acoustic field, together these form an 'acoustic stencil' where arbitrary cell and particle arrangements can be reversibly generated. Here a bulk-wavemode lithium niobate resonator generates multiplexed parallel patterning via a multilayer resonant geometry, where cell-scale resolution is accomplished via structured sub-wavelength microfeatures. Uniquely, this permits simultaneous manipulation in a unidirectional, device-spanning single-node field across scalable ∼cm² areas in a microfluidic device. This approach is demonstrated via patterning of 5, 10 and 15 μm particles and 293-F cells in a variety of arrangements, where these activities are enabling for a range of cell studies and tissue engineering applications via the generation of highly complex and designed acoustic patterns at the microscale.

Heterogeneous tissue construction by on-demand bubble-assisted acoustic patterning

Highly heterogeneous structures are closely related to the realization of the tissue functions of living organisms. However, precisely controlling the assembly of heterogeneous structures is still a crucial challenge. This work presents an on-demand bubble-assisted acoustic method for active cell patterning to achieve high-precision heterogeneous structures. Active cell patterning is achieved by the combined effect of acoustic radiation forces and microstreaming around oscillating bubble arrays. On-demand bubble arrays allow flexible construction of cell patterns with a precision of up to 45 μm. As a typical example, the in vitro model of hepatic lobules, composed of patterned endothelial cells and hepatic parenchymal cells, was constructed and cultured for 5 days. The good performance of urea and albumin secretion, enzymatic activity and good proliferation of both cells prove the feasibility of this technique. Overall, this bubble-assisted acoustic approach provides a simple and efficient strategy for on-demand large-area tissue construction, with considerable potential for different tissue model fabrication.

Sound-Speed Modifying Acoustic Metasurfaces for Acoustic Holography

Acoustic metasurfaces offer unique capabilities to steer and direct acoustic fields, though these are generally composed of complex 3D structures, complicating their fabrication and applicability to higher frequencies. Here, an ultrathin metasurface approach is demonstrated, wherein planarized micropillars in a discretized phase array are utilized. This subwavelength metasurface is easily produced via a single-step etching process and is suitable for megahertz-scale applications. The flexibility of this approach is further demonstrated in the production of complex acoustic patterns via acoustic holography. This metasurface approach, with models used to predict their behavior, has broad potential in applications where robust, high-frequency acoustic manipulation is required, including microfluidics, cell/tissue engineering, and medical ultrasound.

Light-driven single-cell rotational adhesion frequency assay

The interaction between cell surface receptors and extracellular ligands is highly related to many physiological processes in living systems. Many techniques have been developed to measure the ligand-receptor binding kinetics at the single-cell level. However, few techniques can measure the physiologically relevant shear binding affinity over a single cell in the clinical environment. Here, we develop a new optical technique, termed single-cell rotational adhesion frequency assay (scRAFA), that mimics in vivo cell adhesion to achieve label-free determination of both homogeneous and heterogeneous binding kinetics of targeted cells at the subcellular level. Moreover, the scRAFA is also applicable to analyze the binding affinities on a single cell in native human biofluids. With its superior performance and general applicability, scRAFA is expected to find applications in study of the spatial organization of cell surface receptors and diagnosis of infectious diseases.

SUPPLEMENTARY INFORMATION

The online version contains supplementary material available at 10.1186/s43593-022-00020-4.

Enhanced Detection in Droplet Microfluidics by Acoustic Vortex Modulation of Particle Rings and Particle Clusters via Asymmetric Propagation of Surface Acoustic Waves

As a basis for biometric and chemical analysis, issues of how to dilute or concentrate substances such as particles or cells to specific concentrations have long been of interest to researchers. In this study, travelling surface acoustic wave (TSAW)-based devices with three frequencies (99.1, 48.8, 20.4 MHz) have been used to capture the suspended Polystyrene (PS) microspheres of various sizes (5, 20, 40 μm) in sessile droplets, which are controlled by acoustic field-induced fluid vortex (acoustic vortex) and aggregate into clusters or rings with particles. These phenomena can be explained by the interaction of three forces, which are drag force caused by ASF, ARF caused by Leaky-SAW and varying centrifugal force. Eventually, a novel approach of free transition between the particle ring and cluster was approached via modulating the acoustic amplitude of TSAW. By this method, multilayer particles agglomerate with 20 μm wrapped around 40 μm and 20 μm wrapped around 5 μm can be obtained, which provides the possibility to dilute or concentrate the particles to a specific concentration.