Harmonic acoustics for dynamic and selective particle manipulation.

Precise and selective manipulation of colloids and biological cells has long been motivated by applications in materials science, physics and the life sciences. Here we introduce our harmonic acoustics for a non-contact, dynamic, selective (HANDS) particle manipulation platform, which enables the reversible assembly of colloidal crystals or cells via the modulation of acoustic trapping positions with subwavelength resolution. We compose Fourier-synthesized harmonic waves to create soft acoustic lattices and colloidal crystals without using surface treatment or modifying their material properties. We have achieved active control of the lattice constant to dynamically modulate the interparticle distance in a high-throughput (>100 pairs), precise, selective and reversible manner. Furthermore, we apply this HANDS platform to quantify the intercellular adhesion forces among various cancer cell lines. Our biocompatible HANDS platform provides a highly versatile particle manipulation method that can handle soft matter and measure the interaction forces between living cells with high sensitivity.

Acoustofluidics-Assisted Fluorescence-SERS Bimodal Biosensors.

Acoustofluidics, the fusion of acoustics and microfluidic techniques, has recently seen increased research attention across multiple disciplines due in part to its capabilities in contactless, biocompatible, and precise manipulation of micro-/nano-objects. Herein, a bimodal signal amplification platform which relies on acoustofluidics-induced enrichment of nanoparticles is introduced. The dual-function biosensor can perform sensitive immunofluorescent or surface-enhanced Raman spectroscopy (SERS) detection. The platform functions by using surface acoustic waves to concentrate nanoparticles at either the center or perimeter of a glass capillary; the concentration location is adjusted simply by varying the input frequency. The immunofluorescence assay is achieved by concentrating fluorescent analytes and functionalized nanoparticles at the center of the microchannel, thereby improving the visibility of the fluorescent output. By modifying the inner wall of the glass capillary with plasmonic Ag nanoparticle-deposited ZnO nanorod arrays and focusing analytes toward the perimeter of the microchannel, SERS sensing using the same device setup is achieved. Nanosized exosomes are used as a proof-of-concept to validate the performance of the acoustofluidic bimodal biosensor. With its sample-enrichment functionality, bimodal sensing, short processing time, and minute sample consumption, the acoustofluidic chip holds great potential for the development of lab-on-a-chip based analysis systems in many real-world applications.

Acoustic Cell Separation Based on Density and Mechanical Properties.

Density and mechanical properties (e.g., compressibility or bulk modulus) are important cellular biophysical markers. As such, developing a method to separate cells directly based on these properties can benefit various applications including biological research, diagnosis, prognosis, and therapeutics. As a potential solution, surface acoustic wave (SAW)-based cell separation has demonstrated advantages in terms of biocompatibility and compact device size. However, most SAW-reliant cell separations are achieved using an entangled effect of density, various mechanical properties, and size. In this work, we demonstrate SAW-based separation of cells/particles based on their density and compressibility, irrespective of their sizes, by manipulating the acoustic properties of the fluidic medium. Using our platform, SAW-based separation is achieved by varying the dimensions of the microfluidic channels, the wavelengths of acoustic signals, and the properties of the fluid media. Our method was applied to separate paraformaldehyde-treated and fresh Hela cells based on differences in mechanical properties; a recovery rate of 85% for fixed cells was achieved. It was also applied to separate red blood cells (RBCs) and white blood cells (WBCs) which have different densities. A recovery rate of 80.5% for WBCs was achieved.

Acoustofluidic methods in cell analysis.

Cellular analysis is a central concept for both biology and medicine. Over the past two decades, acoustofluidic technologies, which marry acoustic waves with microfluidics, have significantly contributed to the development of innovative approaches for cellular analysis. Acoustofluidic technologies enable precise manipulations of cells and the fluids that confine them, and these capabilities have been utilized in many cell analysis applications. In this review article, we examine various applications where acoustofluidic methods have been implemented, including cell imaging, cell mechanotyping, circulating tumor cell phenotyping, sample preparation in clinics, and investigation of cell-cell interactions and cell-environment responses. We also provide our perspectives on the technological advantages, limitations, and potential future directions for this innovative field of methods.

Acoustic Actuation of in situ Fabricated Artificial Cilia.

We present on-chip acoustic actuation of in situ fabricated artificial cilia. Arrays of cilia structures are UV polymerized inside a microfluidic channel using a photocurable polyethylene glycol (PEG) polymer solution and photomasks. During polymerization, cilia structures are attached to a silane treated glass surface inside the microchannel. Then, the cilia structures are actuated using acoustic vibrations at 4.6 kHz generated by piezo transducers. As a demonstration of a practical application, DI water and fluorescein dye solutions are mixed inside a microfluidic channel. Using pulses of acoustic excitations, and locally fabricated cilia structures within a certain region of the microchannel, a waveform of mixing behavior is obtained. This result illustrates one potential application wherein researchers can achieve spatiotemporal control of biological microenvironments in cell stimulation studies. These acoustically actuated, in situ fabricated, cilia structures can be used in many on-chip applications in biological, chemical and engineering studies.

Acoustic Separation of Nanoparticles in Continuous Flow.

The separation of nanoscale particles based on their differences in size is an essential technique to the nanoscience and nanotechnology community. Here, nanoparticles are successfully separated in a continuous flow by using tilted-angle standing surface acoustic waves. The acoustic field deflects nanoparticles based on volume, and the fractionation of nanoparticles is optimized by tuning the cutoff parameters. The continuous separation of nanoparticlesis demonstrated with a ≈90% recovery rate. The acoustic nanoparticle separation method is versatile, non-invasive, and simple.

Acoustofluidic bacteria separation.

Bacterial separation from human blood samples can help with the identification of pathogenic bacteria for sepsis diagnosis. In this work, we report an acoustofluidic device for label-free bacterial separation from human blood samples. In particular, we exploit the acoustic radiation force generated from a tilted-angle standing surface acoustic wave (taSSAW) field to separate E. coli from human blood cells based on their size difference. Flow cytometry analysis of the E. coli separated from red blood cells (RBCs) shows a purity of more than 96%. Moreover, the label-free electrochemical detection of the separated E. coli displays reduced non-specific signals due to the removal of blood cells. Our acoustofluidic bacterial separation platform has advantages such as label-free separation, high biocompatibility, flexibility, low cost, miniaturization, automation, and ease of in-line integration. The platform can be incorporated with an on-chip sensor to realize a point-of-care (POC) sepsis diagnostic device.

Robot-assisted chirality-tunable acoustic vortex tweezers for contactless, multifunctional, 4-DOF object manipulation.

Robotic manipulation of small objects has shown great potential for engineering, biology, and chemistry research. However, existing robotic platforms have difficulty in achieving contactless, high-resolution, 4-degrees-of-freedom (4-DOF) manipulation of small objects, and noninvasive maneuvering of objects in regions shielded by tissue and bone barriers. Here, we present chirality-tunable acoustic vortex tweezers that can tune acoustic vortex chirality, transmit through biological barriers, trap single micro- to millimeter-sized objects, and control object rotation. Assisted by programmable robots, our acoustic systems further enable contactless, high-resolution translation of single objects. Our systems were demonstrated by tuning acoustic vortex chirality, controlling object rotation, and translating objects along arbitrary-shaped paths. Moreover, we used our systems to trap single objects in regions with tissue and skull barriers and translate an object inside a Y-shaped channel of a thick biomimetic phantom. In addition, we showed the function of ultrasound imaging-assisted acoustic manipulation by monitoring acoustic object manipulation via live ultrasound imaging.

Acoustofluidic black holes for multifunctional in-droplet particle manipulation.

Acoustic black holes offer superior capabilities for slowing down and trapping acoustic waves for various applications such as metastructures, energy harvesting, and vibration and noise control. However, no studies have considered the linear and nonlinear effects of acoustic black holes on micro/nanoparticles in fluids. This study presents acoustofluidic black holes (AFBHs) that leverage controlled interactions between AFBH-trapped acoustic wave energy and particles in droplets to enable versatile particle manipulation functionalities, such as translation, concentration, and patterning of particles. We investigated the AFBH-enabled wave energy trapping and wavelength shrinking effects, as well as the trapped wave energy-induced acoustic radiation forces on particles and acoustic streaming in droplets. This study not only fills the gap between the emerging fields of acoustofluidics and acoustic black holes but also leads to a class of AFBH-based in-droplet particle manipulation toolsets with great potential for many applications, such as biosensing, point-of-care testing, and drug screening.

Biomimetic apposition compound eye fabricated using microfluidic-assisted 3D printing.

After half a billion years of evolution, arthropods have developed sophisticated compound eyes with extraordinary visual capabilities that have inspired the development of artificial compound eyes. However, the limited 2D nature of most traditional fabrication techniques makes it challenging to directly replicate these natural systems. Here, we present a biomimetic apposition compound eye fabricated using a microfluidic-assisted 3D-printing technique. Each microlens is connected to the bottom planar surface of the eye via intracorporal, zero-crosstalk refractive-index-matched waveguides to mimic the rhabdoms of a natural eye. Full-colour wide-angle panoramic views and position tracking of a point source are realized by placing the fabricated eye directly on top of a commercial imaging sensor. As a biomimetic analogue to naturally occurring compound eyes, the eye's full-colour 3D to 2D mapping capability has the potential to enable a wide variety of applications from improving endoscopic imaging to enhancing machine vision for facilitating human-robot interactions.

Acoustohydrodynamic tweezers via spatial arrangement of streaming vortices.

Acoustics-based tweezers provide a unique toolset for contactless, label-free, and precise manipulation of bioparticles and bioanalytes. Most acoustic tweezers rely on acoustic radiation forces; however, the accompanying acoustic streaming often generates unpredictable effects due to its nonlinear nature and high sensitivity to the three-dimensional boundary conditions. Here, we demonstrate acoustohydrodynamic tweezers, which generate stable, symmetric pairs of vortices to create hydrodynamic traps for object manipulation. These stable vortices enable predictable control of a flow field, which translates into controlled motion of droplets or particles on the operating surface. We built a programmable droplet-handling platform to demonstrate the basic functions of planar-omnidirectional droplet transport, merging droplets, and in situ mixing via a sequential cascade of biochemical reactions. Our acoustohydrodynamic tweezers enables improved control of acoustic streaming and demonstrates a previously unidentified method for contact-free manipulation of bioanalytes and digitalized liquid handling based on a compact and scalable functional unit.

Acoustofluidic rotational tweezing enables high-speed contactless morphological phenotyping of zebrafish larvae.

Modern biomedical research and preclinical pharmaceutical development rely heavily on the phenotyping of small vertebrate models for various diseases prior to human testing. In this article, we demonstrate an acoustofluidic rotational tweezing platform that enables contactless, high-speed, 3D multispectral imaging and digital reconstruction of zebrafish larvae for quantitative phenotypic analysis. The acoustic-induced polarized vortex streaming achieves contactless and rapid (~1 s/rotation) rotation of zebrafish larvae. This enables multispectral imaging of the zebrafish body and internal organs from different viewing perspectives. Moreover, we develop a 3D reconstruction pipeline that yields accurate 3D models based on the multi-view images for quantitative evaluation of basic morphological characteristics and advanced combinations of metrics. With its contactless nature and advantages in speed and automation, our acoustofluidic rotational tweezing system has the potential to be a valuable asset in numerous fields, especially for developmental biology, small molecule screening in biochemistry, and pre-clinical drug development in pharmacology.

Fabrication of tunable, high-molecular-weight polymeric nanoparticles via ultrafast acoustofluidic micromixing.

High-molecular-weight polymeric nanoparticles are critical to increasing the loading efficacy and tuning the release profile of targeted molecules for medical diagnosis, imaging, and therapeutics. Although a number of microfluidic approaches have attained reproducible nanoparticle synthesis, it is still challenging to fabricate nanoparticles from high-molecular-weight polymers in a size and structure-controlled manner. In this work, an acoustofluidic platform is developed to synthesize size-tunable, high-molecular-weight (>45 kDa) poly(lactic-co-glycolic acid)-b-poly(ethylene glycol) (PLGA-PEG) nanoparticles without polymer aggregation by exploiting the characteristics of complete and ultrafast mixing. Moreover, the acoustofluidic approach achieves two features that have not been achieved by existing microfluidic approaches: (1) multi-step (≥2) sequential nanoprecipitation in a single device, and (2) synthesis of core-shell structured PLGA-PEG/lipid nanoparticles with high molecular weights. The developed platform expands microfluidic potential in nanomaterial synthesis, where high-molecular-weight polymers, multiple reagents, or sequential nanoprecipitations are needed.

Acoustofluidic centrifuge for nanoparticle enrichment and separation.

Liquid droplets have been studied for decades and have recently experienced renewed attention as a simplified model for numerous fascinating physical phenomena occurring on size scales from the cell nucleus to stellar black holes. Here, we present an acoustofluidic centrifugation technique that leverages an entanglement of acoustic wave actuation and the spin of a fluidic droplet to enable nanoparticle enrichment and separation. By combining acoustic streaming and droplet spinning, rapid (<1 min) nanoparticle concentration and size-based separation are achieved with a resolution sufficient to identify and isolate exosome subpopulations. The underlying physical mechanisms have been characterized both numerically and experimentally, and the ability to process biological samples (including DNA segments and exosome subpopulations) has been successfully demonstrated. Together, this acoustofluidic centrifuge overcomes existing limitations in the manipulation of nanoscale (<100 nm) bioparticles and can be valuable for various applications in the fields of biology, chemistry, engineering, material science, and medicine.

Acoustic Microfluidics.

Acoustic microfluidic devices are powerful tools that use sound waves to manipulate micro- or nanoscale objects or fluids in analytical chemistry and biomedicine. Their simple device designs, biocompatible and contactless operation, and label-free nature are all characteristics that make acoustic microfluidic devices ideal platforms for fundamental research, diagnostics, and therapeutics. Herein, we summarize the physical principles underlying acoustic microfluidics and review their applications, with particular emphasis on the manipulation of macromolecules, cells, particles, model organisms, and fluidic flows. We also present future goals of this technology in analytical chemistry and biomedical research, as well as challenges and opportunities.

A Cell-Phone-Based Acoustofluidic Platform for Quantitative Point-of-Care Testing.

Acoustofluidic methods, with advantages including simplicity of device design, biocompatible manipulation, and low power consumption, have been touted as promising tools for point-of-care (POC) testing. Here, we report a cell-phone-based acoustofluidic platform that uses acoustic radiation forces to enrich nanoscale analytes and red and green fluorescence nanoparticles (SiO2@R and G@SiO2) as probes for POC visual testing. Thus, the color signals from the fluorescent probes are enhanced, and colorimetric sensitivity is significantly improved. As a POC demonstration, the acoustofluidic platform is used to detect hemoglobin (Hb) from human blood, resulting in a rapid and straightforward measurement of normal blood Hb levels. Combining an acoustofluidic-based nanoparticle-concentration platform with cell-phone-based colorimetry, our method introduces a potential pathway toward practical POC testing.

Correction: Separating extracellular vesicles and lipoproteins via acoustofluidics.

Correction for 'Separating extracellular vesicles and lipoproteins via acoustofluidics' by Mengxi Wu et al., Lab Chip, 2019, 19, 1174-1182, DOI: .

Correction: Cell lysis via acoustically oscillating sharp edges.

Correction for 'Cell lysis via acoustically oscillating sharp edges' by Zeyu Wang et al., Lab Chip, 2019, 19, 4021-4032, DOI: .

Correction: An acoustofluidic device for efficient mixing over a wide range of flow rates.

Correction for 'An acoustofluidic device for efficient mixing over a wide range of flow rates' by Hunter Bachman et al., Lab Chip, 2020, 20, 1238-1248, DOI: .

Acoustofluidic multi-well plates for enrichment of micro/nano particles and cells.

Controllable enrichment of micro/nanoscale objects plays a significant role in many biomedical and biochemical applications, such as increasing the detection sensitivity of assays, or improving the structures of bio-engineered tissues. However, few techniques can perform concentrations of micro/nano objects in multi-well plates, a very common laboratory vessel. In this work, we develop an acoustofluidic multi-well plate, which adopts an array of simple, low-cost and commercially available ring-shaped piezoelectric transducers for rapid and robust enrichment of micro/nanoscale particles/cells in each well of the plate. The enrichment mechanism is validated and characterized through both numerical simulations and experiments. We observe that the ring-shaped piezoelectric transducer can generate circular standing flexural waves in the substrate of each well, and that the vibrations can induce acoustic streaming near the interface between the substrate and a fluid droplet placed within the well; this streaming can drive micro/nanoscale objects to the center of the droplet for enrichment. Moreover, the acoustofluidic multi-well plate can realize simultaneous and consistent enrichment of biological cells in each well of the plate. With merits such as simplicity, controllability, low cost, and excellent compatibility with other downstream analysis tools, the developed acoustofluidic multi-well plate could be a versatile tool for many applications such as micro/nano fabrication, self-assembly, biomedical/biochemical sensing, and tissue engineering.