Acoustofluidic Separation of Proteins Using Aptamer-Functionalized Microparticles

Acoustic Waves Coupling with Polydimethylsiloxane in Reconfigurable Acoustofluidic Platform

Acoustofluidics is a promising technology that leverages acoustic waves for precise manipulation of micro/nano-scale flows and suspended objects within microchannels. Despite many advantages, the practical applicability of conventional acoustofluidic platforms is limited by irreversible bonding between the piezoelectric actuator and the microfluidic chip. Recently, reconfigurable acoustofluidic platforms are enabled by reversible bonding between the reusable actuator and the replaceable polydimethylsiloxane (PDMS) microfluidic chip by incorporating a PDMS membrane for sealing the microchannel and coupling the acoustic waves with the fluid inside. However, a quantitative guideline for selecting a suitable PDMS membrane for various acoustofluidic applications is still missing. Here, a design rule for reconfigurable acoustofluidic platforms is explored based on a thorough investigation of the PDMS thickness effect on acoustofluidic phenomena: acousto-thermal heating (ATH), acoustic radiation force (ARF), and acoustic streaming flow (ASF). These findings suggest that the relative thickness of the PDMS membrane (t) for acoustic wavelength (λPDMS) determines the wave attenuation in the PDMS and the acoustofluidic phenomena. For t/λPDMS ≈ O(1), the transmission of acoustic waves through the membrane leads to the ARF and ASF phenomena, whereas, for t/λPDMS ≈ O(10), the acoustic waves are entirely absorbed within the membrane, resulting in the ATH phenomenon.

Acoustofluidic precise manipulation: Recent advances in applications for micro/nano bioparticles

Acoustofluidic technologies that integrate acoustic waves and microfluidic chips have been widely used in bioparticle manipulation. As a representative technology, acoustic tweezers have attracted significant attention due to their simple manufacturing, contact-free operation, and low energy consumption. Recently, acoustic tweezers have enabled the efficient and smart manipulation of biotargets with sizes covering millimeters (such as zebrafish) and nanometers (such as DNA). In addition to acoustic tweezers, other related acoustofluidic chips including acoustic separating, mixing, enriching, and transporting chips, have also emerged to be powerful platforms to manipulate micro/nano bioparticles (cells in blood, extracellular vesicles, liposomes, and so on). Accordingly, some interesting applications were also developed, such as smart sensing. In this review, we firstly introduce the principles of acoustic tweezers and various related technologies. Second, we compare and summarize recent applications of acoustofluidics in bioparticle manipulation and sensing. Finally, we outlook the future development direction from the perspectives such as device design and interdisciplinary.

Recent Advances in Acoustofluidics for Point-of-Care Testing

Point-of-care testing (POCT) has played important role in clinical diagnostics, environmental assessment, chemical and biological analyses, and food and chemical processing due to its faster turnaround compared to laboratory testing. Dedicated manipulations of solutions or particles are generally required to develop POCT technologies that achieve a "sample-in-answer-out" operation. With the development of micro- and nanotechnology, many tools have been developed for sample preparation, on-site analysis and solution manipulations (mixing, pumping, valving, etc.). Among these approaches, the use of acoustic waves to manipulate fluids and particles (named acoustofluidics) has been applied in many researches. This review focuses on the recent developments in acoustofluidics for POCT. It starts with the fundamentals of different acoustic manipulation techniques and then lists some of representative examples to highlight each method in practical POC applications. Looking toward the future, a compact, portable, highly integrated, low power, and biocompatible technique is anticipated to simultaneously achieve precise manipulation of small targets and multimodal manipulation in POC applications.

The impact of an ultrasonic standing wave on the sorption behavior of proteins: Investigation of the role of acoustically induced non-spherical bubble oscillations

HYPOTHESIS

Protein molecules adsorb on the air/liquid interface due to possessing a hydrophobic side. A full surface coverage is important in many processes such as in protein harvesting by foam fractionation. The adsorption of proteins in low concentration solutions is preceded by a relatively long time lag known as the induction period. This has been attributed to the formation of an adsorbed monolayer, which relies on the reorientation of the protein molecules. The reduction of the induction period can significantly facilitate the sorption process to reach full protein coverage. For this purpose acoustically induced non-spherical bubble oscillations can aid in the formation of the monolayer and enhance the sorption process.

EXPERIMENT

In this study, low frequency ultrasound was used to induce non-spherical oscillations on an air bubble attached to a capillary. Profile analysis tensiometry was deployed to examine the effect of these non-spherical oscillations on the sorption dynamics of different proteins.

FINDINGS

We observed that during the initial stages of adsorption, when the bubble surface is almost empty, non-spherical oscillations occur, which were found to significantly expedite the adsorption process. However, during later stages of the adsorption process, despite the continued presence of several sonication phenomena such as the primary radiation force and acoustic streaming, no change in adsorption behavior of the proteins could be noted. The occurrence, duration, and intensity of the non-spherical bubble oscillations appeared to be the sole contributing factors for the change of the sorption dynamics of proteins.

Acoustofluidic separation of prolate and spherical micro-objects

Most microfluidic separation techniques rely largely on object size as a separation marker. The ability to separate micro-objects based on their shape is crucial in various biomedical and chemical assays. Here, we develop an on-demand, label-free acoustofluidic method to separate prolate ellipsoids from spherical microparticles based on traveling surface acoustic wave-induced acoustic radiation force and torque. The freely rotating non-spherical micro-objects were aligned under the progressive acoustic field by the counterrotating radiation torque, and the major axis of the prolate ellipsoids was parallel to the progressive wave propagation. The specific alignment of the ellipsoidal particles resulted in a reduction in the cross-sectional area perpendicular to the wave propagation. As a consequence, the acoustic backscattering decreased, resulting in a decreased magnitude of the radiation force. Through the variation in radiation force, which depended on the micro-object morphology enabled the acoustofluidic shape-based separation. We conducted numerical simulations for the wave scattering of spherical and prolate objects to elucidate the working mechanism underlying the proposed method. A series of experiments with polystyrene microspheres, prolate ellipsoids, and peanut-shaped microparticles were performed for validation. Through quantitative analysis of the separation efficiency, we confirmed the high purity and high recovery rate of the proposed acoustofluidic shape-based separation of micro-objects. As a bioparticle, we utilize Thalassiosira eccentrica to perform shape-based separation, as the species has a variety of potential applications in drug delivery, biosensing, nanofabrication, bioencapsulation and immunoisolation.

A review of acoustofluidic separation of bioparticles

Acoustofluidics is an emerging interdisciplinary research field that involves the integration of acoustics and microfluidics to address challenges in various scientific areas. This technology has proven to be a powerful tool for separating biological targets from complex fluids due to its label-free, biocompatible, and contact-free nature. Considering a careful designing process and tuning the acoustic field particles can be separated with high yield. Recently the advancement of acoustofluidics led to the development of point-of-care devices for separations of micro particles which address many of the limitations of conventional separation tools. This review article discusses the working principles and different approaches of acoustofluidic separation and provides a synopsis of its traditional and emerging applications, including the theory and mechanism of acoustofluidic separation, blood component separation, cell washing, fluorescence-activated cell sorting, circulating tumor cell isolation, and exosome isolation. The technology offers great potential for solving clinical problems and advancing scientific research.

Low-noise fluorescent detection of cardiac troponin I in human serum based on surface acoustic wave separation

Acute myocardial infarction (AMI) is a life-threatening disease when sudden blockage of coronary artery occurs. As the most specific biomarker, cardiac troponin I (cTnI) is usually checked separately to diagnose or eliminate AMI, and achieving the accurate detection of cTnI is of great significance to patients' life and health. Compared with other methods, fluorescent detection has the advantages of simple operation, high sensitivity and wide applicability. However, due to the strong fluorescence interference of biological molecules in body fluids, it is often difficult to obtain high sensitivity. In order to solve this problem, in this study, surface acoustic wave separation is designed to purify the target to achieve more sensitive detection performance of fluorescent detection. Specifically, the interference of background noise is almost completely removed on a microfluidic chip by isolating microbeads through acoustic radiation force, on which the biomarkers are captured by the immobilized detection probe. And then, the concentration of cTnI in human serum is detected by the fluorescence intensity change of the isolated functionalized beads. By this way, the detection limit of our biosensor calculated by 3σ/K method is 44 pg/mL and 0.34 ng/mL in PBS buffer and human serum respectively. Finally, the reliability of this method has been validated by comparison with clinical tests from the nephelometric analyzer in hospital.

An ultra-compact acoustofluidic device based on the narrow-path travelling surface acoustic wave (np-TSAW) for label-free isolation of living circulating tumor cells

Obtaining highly purified intact living cells from complex environments has been a challenge, such as the isolation of circulating tumor cells (CTCs) from blood. In this work, we demonstrated an acoustic-based ultra-compact device for cell sorting, with a chip size of less than 2 × 1.5 cm². This single actuator device allows non-invasive and label-free isolation of living cells, offering greater flexibility and applicability. The device performance was optimized with different-sized polystyrene (PS) particles and blood cells spiked with cancer cells. Using the narrow-path travelling surface acoustic wave (np-TSAW), precise isolation of 10 μm particles from a complex mixture of particles (5, 10, 20 μm) and separation of 8 μm and 10 μm particles was achieved. The purified collection of 10 μm particles with high separation efficiency (98.75%) and high purity (98.1%) was achieved by optimizing the input voltage. Further, we investigated the isolation and purification of CTCs (MCF-7, human breast cancer cells) from blood cells with isolation efficiency exceeding 98% and purity reaching 93%. Viabilities of the CTCs harvested from target-outlet were all higher than 97% after culturing for 24, 48, and 72 h, showing good proliferation ability. This novel ultra-miniaturized microfluidic chip demonstrates the ability to sorting cells with high-purity and label-free, providing an attractive miniaturized system alternative to traditional sorting methods.

Acoustofluidic separation of proteins from platelets in human blood plasma using aptamer-functionalized microparticles

Microfluidic liquid biopsy has emerged as a promising clinical assay for early diagnosis. Herein, we propose acoustofluidic separation of biomarker proteins from platelets in plasma using aptamer-functionalized microparticles. As model proteins, C-reactive protein and thrombin were spiked in human platelet-rich plasma. The target proteins were selectively conjugated with their corresponding aptamer-functionalized microparticles of different sizes, and the particle complexes served as a mobile carrier for the conjugated proteins. The proposed acoustofluidic device was composed of an interdigital transducer (IDT) patterned on a piezoelectric substrate and a disposable polydimethylsiloxane (PDMS) microfluidic chip. The PDMS chip was placed in a tilted arrangement with the IDT to utilize both vertical and horizontal components of surface acoustic wave-induced acoustic radiation force (ARF) for multiplexed assay at high-throughput. The two different-sized particles experienced the ARF at different magnitudes and were separated from platelets in plasma. The IDT on the piezoelectric substrate could be reusable, while the microfluidic chip can be replaceable for repeated assays. The sample processing throughput with the separation efficiency >95% has been improved such that the volumetric flow rate and flow velocity were 1.6 ml/h and 37 mm/s, respectively. For the prevention of platelet activation and protein adsorption to the microchannel, polyethylene oxide solution was introduced as sheath flows and coating on to the walls. We conducted scanning electron microscopy, x-ray photoemission spectroscopy , and sodium dodecyl sulfate- analysis before and after the separation to confirm the protein capture and separation. We expect that the proposed approach will provide new prospects for particle-based liquid biopsy using blood.

Microsecond cell triple-sorting enabled by multiple pulse irradiation of femtosecond laser

Femtosecond-laser-assisted cell manipulation, as one of the high throughput cell sorting techniques, is tailored for single-step multiple sorting based on controllable impulsive force. In this paper, femtosecond laser pulses are focused within a pocket structure and they induce an impulse force acting on the flowing objects. The impulsive force is shown to be controllable by a new method to adjust the femtosecond pulse properties. This allows precise streamline manipulation of objects having various physical qualities (e.g., weight and volume). The pulse energy, pulse number, and pulse interval of the femtosecond laser are altered to determine the impulsive force strength. The method is validated in single cell or bead triple-sorting experiments and its capability to perform streamline manipulation in as little as 10 μs is shown. The shift profiles of the beads acting under the impulsive force are studied in order to better understand the sorting mechanism. Additionally, beads and cells with different fluorescence intensities are successfully detected and directed into different microchannels, with maximum success rates of 90% and 64.5%, respectively. To sum up, all results suggest that this method has the potential to sort arbitrary subpopulations by altering the number of femtosecond pulses and that it takes the first step toward developing a single-step multi-selective system.

Multiple virus sorting based on aptamer-modified microspheres in a TSAW device

Due to the overlapping epidemiology and clinical manifestations of flaviviruses, differential diagnosis of these viral diseases is complicated, and the results are unreliable. There is perpetual demand for a simplified, sensitive, rapid and inexpensive assay with less cross-reactivity. The ability to sort distinct virus particles from a mixture of biological samples is crucial for improving the sensitivity of diagnoses. Therefore, we developed a sorting system for the subsequent differential diagnosis of dengue and tick-borne encephalitis in the early stage. We employed aptamer-modified polystyrene (PS) microspheres with different diameters to specifically capture dengue virus (DENV) and tick-borne encephalitis virus (TBEV), and utilized a traveling surface acoustic wave (TSAW) device to accomplish microsphere sorting according to particle size. The captured viruses were then characterized by laser scanning confocal microscopy (LSCM), field emission scanning electron microscopy (FE-SEM) and reverse transcription-polymerase chain reaction (RT‒PCR). The characterization results indicated that the acoustic sorting process was effective and damage-free for subsequent analysis. Furthermore, the strategy can be utilized for sample pretreatment in the differential diagnosis of viral diseases.

Residue-free acoustofluidic manipulation of microparticles via removal of microchannel anechoic corner

Surface acoustic wave (SAW)-based acoustofluidics has shown significant promise to manipulate micro/nanoscale objects for biomedical applications, e.g. cell separation, enrichment, and sorting. A majority of the acoustofluidic devices utilize microchannels with rectangular cross-section where the acoustic waves propagate in the direction perpendicular to the sample flow. A region with weak acoustic wave intensity, termed microchannel anechoic corner (MAC), is formed inside a rectangular microchannel of the acoustofluidic devices where the ultrasonic waves refract into the fluid at the Rayleigh angle with respect to the normal to the substrate. Due to the absence of a strong acoustic field within the MAC, the microparticles flowing adjacent to the microchannel wall remain unaffected by a direct SAW-induced acoustic radiation force (ARF). Moreover, an acoustic streaming flow (ASF) vortex produced within the MAC pulls the particles further into the corner and away from the direct ARF influence. Therefore, a residue of particles continues to flow past the SAWs without intended deflection, causing a decrease in microparticle manipulation efficiency. In this work, we introduce a cross-type acoustofluidic device composed of a half-circular microchannel, fabricated through a thermal reflow of a positive photoresist mold, to overcome the limitations associated with rectangular microchannels, prone to the MAC formation. We investigated the effects of different microchannel cross-sectional shapes with varying contact angles on the microparticle deflection in a continuous flow and found three distinct regimes of particle deflection. By systematically removing the MAC out of the microchannel cross-section, we achieved residue-free acoustofluidic microparticle manipulation via SAW-induced ARF inside a half-circular microchannel. The proposed method was applied to efficient fluorescent coating of the microparticles in a size-selective manner without any residue particles left undeflected in the MAC.

Recent advances in acoustofluidic separation technology in biology

Acoustofluidic separation of cells and particles is an emerging technology that integrates acoustics and microfluidics. In the last decade, this technology has attracted significant attention due to its biocompatible, contactless, and label-free nature. It has been widely validated in the separation of cells and submicron bioparticles and shows great potential in different biological and biomedical applications. This review first introduces the theories and mechanisms of acoustofluidic separation. Then, various applications of this technology in the separation of biological particles such as cells, viruses, biomolecules, and exosomes are summarized. Finally, we discuss the challenges and future prospects of this field.

Surface acoustic wave-assisted microfluidic isolation of aptamers

Aptamers are synthetic single-stranded nucleic acid molecules that bind to biochemical targets with high affinity and specificity. The method of systematic evolution of ligands by exponential enrichment (SELEX) is widely used to isolate aptamers from randomized oligonucleotides. Recently, microfluidic technology has been applied to improve the efficiency and reduce the cost in SELEX processes. In this work, we present an approach that exploits surface acoustic waves to improve the affinity selection process in microfluidic SELEX. Acoustic streaming is used to enhance the interactions of the solution-based oligonucleotide molecules with microbead-immobilized target molecules, allowing the identification of high-affinity aptamer candidates in a more efficient manner. For demonstration, a DNA aptamer is isolated within three rounds of selection in 5 h to specifically bind to immunoglobulin E, a representative target protein, with an equilibrium dissociation constant of approximately 22.6 nM.