Acoustic Trapping for Bacteria Identification in Positive Blood Cultures with MALDI-TOF MS

EchoGrid: High-Throughput Acoustic Trapping for Enrichment of Environmental Microplastics

The health hazards of micro- and nanoplastic contaminants in drinking water has recently emerged as an area of concern to policy makers and industry. Plastic contaminants range in size from micro- (5 mm to 1 μm) to nanoplastics (<1 μm). Microfluidics provides many tools for particle manipulation at the microscale, particularly in diagnostics and biomedicine, but has in general a limited capacity to process large volumes. Drinking water and environmental samples with low-level contamination of microplastics require processing of deciliter to liter sample volumes to achieve statistically relevant particle counts. Here, we introduce the EchoGrid, an acoustofluidics device for high throughput continuous flow particle enrichment into a robust array of particle clusters. The EchoGrid takes advantage of highly efficient particle capture through the integration of a micropatterned transducer for surface displacement-based acoustic trapping in a glass and polymer microchannel. Silica seed particles were used as anchor particles to improve capture performance at low particle concentrations and high flow rates. The device was able to maintain the silica grids at a flow rate of 50 mL/min. In terms of enrichment, the device is able to double the final pellet's microplastic concentration every 78 s for 23 μm particles and every 51 s for 10 μm particles at a flow rate of 5 mL/min. In conclusion, we demonstrate the usefulness of the EchoGrid by capturing microplastics in challenging conditions, such as large sample volumes with low microparticle concentrations, without sacrificing the potential of integration with downstream analysis for environmental monitoring.

Interparticle attraction along the direction of the pressure gradient in an acoustic standing wave

Scattering of an acoustic wave by particles gives rise to microstreaming, as well as to acoustic radiation and interaction forces on the particles. We numerically study these steady, nonlinear phenomena for a case of two elastic spheres in a standing wave. We show that if one or both spheres are smaller or comparable to the viscous boundary layer, the microstreaming close to the pressure node can lead to an interparticle attraction along the direction of the pressure gradient of the wave. Similar behavior is observed when, instead of size, density of one of the spheres is sufficiently larger relative to the other sphere. These findings could promote the acoustic manipulation of nanoparticles and bacteria.

Acoustofluidic platforms for particle manipulation

There is an increasing interest in acoustics for microfluidic applications. This field, commonly known as acoustofluidics involves the interaction of ultrasonic standing waves with fluids and dispersed microparticles. The combination of microfluidics and the so-called acoustic standing waves (ASWs) led to the development of integrated systems for contact-less on-chip cell and particle manipulation where it is possible to move and spatially localize these particles based on the different acoustophysical properties. While it was initially suggested that the acoustic forces could be harmful to the cells and could impact cell viability, proliferation, or function via phenotypic or even genotypic changes, further studies disproved such claims. This review is summarizing some interesting applications of acoustofluidics in the manipulations of biomaterials, such as cells or subcellular vesicles, in works published mainly within the last 5 years.

Acoustic tweezing of microparticles in microchannels with sinusoidal cross sections

Acoustic tweezing of bioparticles has distinct advantages over other manipulation methods such as electrophoresis or magnetophoresis in biotechnological applications. This manipulation method guarantees the viability of the bio-particles during and after the process. In this paper, the effects of sinusoidal boundaries of a microchannel on acoustophoretic manipulation of microparticles are studied. Our results show that while top and bottom walls are vertically actuated at the horizontal half-wave resonance frequency, a large mono-vortex appears, which is never achievable in a rectangular geometry with flat walls and one-dimensional oscillations. The drag force caused by such a vortex in combination with the tilted acoustic radiation force leads to trapping and micromixing of microparticles with diameters larger and smaller than the critical size, respectively. Simulation results in this paper show that efficient particle trapping occurs at the intermediate sinusoidal boundary amplitudes. It is also indicated that in a square-sinusoidal geometry there are two strong vortices, instead of one vortex. Sub-micrometer particles tend to be trapped dramatically faster in such a geometry than in the rectangular-sinusoidal ones.

Ultrasonic Particle Manipulation in Glass Capillaries: A Concise Review

Ultrasonic particle manipulation (UPM), a non-contact and label-free method that uses ultrasonic waves to manipulate micro- or nano-scale particles, has recently gained significant attention in the microfluidics community. Moreover, glass is optically transparent and has dimensional stability, distinct acoustic impedance to water and a high acoustic quality factor, making it an excellent material for constructing chambers for ultrasonic resonators. Over the past several decades, glass capillaries are increasingly designed for a variety of UPMs, e.g., patterning, focusing, trapping and transporting of micron or submicron particles. Herein, we review established and emerging glass capillary-transducer devices, describing their underlying mechanisms of operation, with special emphasis on the application of glass capillaries with fluid channels of various cross-sections (i.e., rectangular, square and circular) on UPM. We believe that this review will provide a superior guidance for the design of glass capillary-based UPM devices for acoustic tweezers-based research.

Numerical study of bulk acoustofluidic devices driven by thin-film transducers and whole-system resonance modes

In bulk acoustofluidic devices, acoustic resonance modes for fluid and microparticle handling are traditionally excited by bulk piezoelectric (PZE) transducers. In this work, it is demonstrated by numerical simulations in three dimensions that integrated PZE thin-film transducers, constituting less than 0.1% of the bulk device, work equally well. The simulations are performed using a well-tested and experimentally validated numerical model. A water-filled straight channel embedded in a mm-sized bulk glass chip with a 1- μm-thick thin-film transducer made of Al_0.6Sc_0.4N is presented as a proof-of-concept example. The acoustic energy, radiation force, and microparticle focusing times are computed and shown to be comparable to those of a conventional bulk silicon-glass device actuated by a bulk lead-zirconate-titanate transducer. The ability of thin-film transducers to create the desired acoustofluidic effects in bulk acoustofluidic devices relies on three physical aspects: the in-plane-expansion of the thin-film transducer under the applied orthogonal electric field, the acoustic whole-system resonance of the device, and the high Q-factor of the elastic solid, constituting the bulk part of the device. Consequently, the thin-film device is remarkably insensitive to the Q-factor and resonance properties of the thin-film transducer.

Assessment of bacterial viability by laser desorption ionization mass spectrometry for antimicrobial susceptibility testing

Bacterial infection poses a serious threat to human health worldwide. Rapid antimicrobial susceptibility testing (AST) is essential for the clinical treatment of bacterial infection patients. However, the traditional AST relies on bacteria culture, which is time-consuming and limits the analysis to culturable species. Herein, we present a laser desorption ionization (LDI) mass spectrometry-based method for rapid bacterial viability assessment and AST by tracing the redox of resazurin (RS) by viable bacteria. RS as well as its reduction product, fluorescent resorufin (RF), can be directly detected by LDI-MS in the absence of matrix. The intensity ratio between RF and RS can be used to assess the viability of bacteria in specimens. We have demonstrated the high efficiency of the method using different bacterial species, including K. pneumoniae, S. aureus, E. coli, and P. aeruginosa, and various antibiotic drugs, such as ciprofloxacin, ampicillin, tetracycline, oxytetracycline, ciprofloxacin and levofloxacin. Compared to traditional methods based on optical absorption, the current method is faster and more sensitive. Furthermore, we applied the method to bacterial viability detection and AST using human body fluid samples, i.e. serum and urine, demonstrating that it can screen rapidly appropriate antibiotic drugs for timely clinical treatment of infectious diseases. With the advantages of simplicity in methodology as well as sensitivity and speed in analysis, the current method holds the potential of clinical usages.

A thermoelectrically stabilized aluminium acoustic trap combined with attenuated total reflection infrared spectroscopy for detection of Escherichia coli in water

Acoustic trapping is a non-contact particle manipulation method that holds great potential for performing automated assays. We demonstrate an aluminium acoustic trap in combination with attenuated total reflection Fourier-transform infrared spectroscopy (ATR-FTIR) for detection of E. coli in water. The thermal conductivity of aluminium was exploited to thermo-electrically heat and hold the acoustic trap at the desired assay temperature of 37 °C. Systematic characterisation and optimisation of the acoustic trap allowed high flow rates while maintaining high acoustic trapping performance. The ATR element serves not only as a reflector for ultrasound standing wave generation but also as a sensing interface. The enzyme conversion induced by alkaline phosphatase-labelled bacteria was directly monitored in the acoustic trap using ATR-FTIR spectroscopy. Sequential injection analysis allowed automated liquid handling, including non-contact bacteria retention, washing and enzyme-substrate exchange within the acoustic trap. The presented method was able to detect E. coli concentrations as low as 1.95 × 106 bacteria per mL in 197 min. The demonstrated ultrasound assisted assay paves the way to fully automated bacteria detection devices based on acoustic trapping combined with ATR-FTIR spectroscopy.

Acoustic trapping based on surface displacement of resonance modes

Acoustic trapping is a promising technique for aligning particles in two-dimensional arrays, as well as for dynamic manipulation of particles individually or in groups. The actuating principles used in current systems rely on either cavity modes in enclosures or complex arrangements for phase control. Therefore, available systems either require high power inputs and costly peripheral equipment or sacrifice flexibility. This work presents a different concept for acoustic trapping of particles and cells that enables dynamically defined trapping patterns inside a simple and inexpensive setup. Here, dynamic operation and dexterous trapping are realized through the use of a modified piezoelectric transducer in direct contact with the liquid sample. Physical modeling shows how the transducer induces an acoustic force potential where the conventional trapping in the axial direction is supplemented by surface displacement dependent lateral trapping. The lateral field is a horizontal array of pronounced potential minima with frequency-dependent locations. The resulting system enables dynamic arraying of levitated trapping sites at low power and can be manufactured at ultra-low cost, operated using low-cost electronics, and assembled in less than 5 min. We demonstrate dynamic patterning of particles and biological cells and exemplify potential uses of the technique for cell-based sample preparation and cell culture.

Acoustic dipole and monopole effects in solid particle interaction dynamics during acoustophoresis

A method is presented for measurements of secondary acoustic radiation forces acting on solid particles in a plain ultrasonic standing wave. The method allows for measurements of acoustic interaction forces between particles located in arbitrary positions such as in between a pressure node and a pressure antinode. By utilizing a model that considers both density- and compressibility-dependent effects, the observed particle-particle interaction dynamics can be well understood. Two differently sized polystyrene micro-particles (4.8 and 25 μm, respectively) were used in order to achieve pronounced interaction effects. The particulate was subjected to a 2-MHz ultrasonic standing wave in a microfluidic channel, such as commonly used for acoustophoresis. Observation of deflections in the particle pathways shows that the particle interaction force is not negligible under this circumstance and has to be considered in accurate particle manipulation applications. The effect is primarily pronounced when the distance between two particles is small, the sizes of the particles are different, and the acoustic properties of the particles are different relative to the media. As predicted by theory, the authors also observe that the interaction forces are affected by the angle between the inter-particle centerline and the axis of the standing wave propagation direction.

Fabrication of Micropatterned Dipeptide Hydrogels by Acoustic Trapping of Stimulus-Responsive Coacervate Droplets

Acoustic standing waves offer an excellent opportunity to trap and spatially manipulate colloidal objects. This noncontact technique is used for the in situ formation and patterning in aqueous solution of 1D or 2D arrays of pH-responsive coacervate microdroplets comprising poly(diallyldimethylammonium) chloride and the dipeptide N-fluorenyl-9-methoxy-carbonyl-D-alanine-D-alanine. Decreasing the pH of the preformed droplet arrays results in dipeptide nanofilament self-assembly and subsequent formation of a micropatterned supramolecular hydrogel that can be removed as a self-supporting monolith. Guest molecules such as molecular dyes, proteins, and oligonucleotides are sequestered specifically within the coacervate droplets during acoustic processing to produce micropatterned hydrogels containing spatially organized functional components. Using this strategy, the site-specific isolation of multiple enzymes to drive a catalytic cascade within the micropatterned hydrogel films is exploited.

Acoustic separation in plastic microfluidics for rapid detection of bacteria in blood using engineered bacteriophage

A more effective treatment of bacteremia requires a diagnostic platform that is both sensitive, accurate and rapid. Currently, clinical laboratory techniques require growth of bacteria prior to diagnosis, take days to complete, and leave empiric therapy and broad spectrum antibiotics as the only option at the onset of treatment. In order to bypass this growth requirement, we engineered a system that purifies bacteria from blood to improve performance in a bacteriophage-based luminescence assay. To perform the purification, we used acoustophoresis in plastic microfluidic chips, enabling future development into a low cost point-of-care system. Acoustophoresis achieves differential separation on the basis of size differences between bacteria and blood cells. We show isolation of three known pathogen species, including members of both Gram-negative and positive-bacteria from blood, and show isolation at clinically relevant concentrations. Using the device as a preparation step prior to the bacteriophage-based luminescence assay, we demonstrate a 33-fold improvement in limit of detection, compared with the unpurified sample, achieving a limit of detection of 6 bacteria.

Formation of inverse Chladni patterns in liquids at microscale: roles of acoustic radiation and streaming-induced drag forces

While Chladni patterns in air over vibrating plates at macroscale have been well studied, inverse Chladni patterns in water at microscale have recently been reported. The underlying physics for the focusing of microparticles on the vibrating interface, however, is still unclear. In this paper, we present a quantitative three-dimensional study on the acoustophoretic motion of microparticles on a clamped vibrating circular plate in contact with water with emphasis on the roles of acoustic radiation and streaming-induced drag forces. The numerical simulations show good comparisons with experimental observations and basic theory. While we provide clear demonstrations of three-dimensional particle size-dependent microparticle trajectories in vibrating plate systems, we show that acoustic radiation forces are crucial for the formation of inverse Chladni patterns in liquids on both out-of-plane and in-plane microparticle movements. For out-of-plane microparticle acoustophoresis, out-of-plane acoustic radiation forces are the main driving force in the near-field, which prevent out-of-plane acoustic streaming vortices from dragging particles away from the vibrating interface. For in-plane acoustophoresis on the vibrating interface, acoustic streaming is not the only mechanism that carries microparticles to the vibrating antinodes forming inverse Chladni patterns: In-plane acoustic radiation forces could have a greater contribution. To facilitate the design of lab-on-a-chip devices for a wide range of applications, the effects of many key parameters, including the plate radius R and thickness h and the fluid viscosity μ, on the microparticle acoustophoresis are discussed, which show that the threshold in-plane and out-of-plane particle sizes balanced from the acoustic radiation and streaming-induced drag forces scale linearly with R and μ , but inversely with h .

Fabrication challenges and perspectives on the use of carbon-electrode dielectrophoresis in sample preparation

The focus of this review is to assess the current status of three-dimensional (3D) carbon-electrode dielectrophoresis (carbonDEP) and identify the challenges currently preventing it from its use in high-throughput applications such as sample preparation for diagnostics. The use of 3D electrodes over more traditional planar ones is emphasised here as a way to increase the throughput of DEP devices. Glass-like carbon electrodes are derived through the carbonisation of photoresist structures made using photolithography. These biocompatible carbon electrodes are not ideal electrical conductors but are more electrochemically stable than noble metals such as gold and platinum. They are also significantly less expensive than common electrode materials, both in terms of material cost and fabrication process. CarbonDEP has been demonstrated for the manipulation of microorganisms and biomolecules. This review is divided in three main sections: (i) carbonDEP fabrication process; (ii) applications using 3D carbonDEP; and (iii) challenges and perspectives on the use of carbonDEP for high-throughput applications.

Comparing methods for the modelling of boundary-driven streaming in acoustofluidic devices

Numerical simulations of acoustic streaming flows can be used not only to explain the complex phenomena observed in acoustofluidic manipulation devices, but also to predict and optimise their performances. In this paper, two numerical methods based on perturbation theory are compared in order to demonstrate their viability and applicability for modelling boundary-driven streaming flows in acoustofluidic systems. It was found that the Reynolds stress method, which predicts the streaming fields from their driving terms, can effectively resolve both the inner and outer streaming fields and can be used to demonstrate the driving mechanisms of a broad range of boundary-driven streaming flows. However, computational efficiency typically limits its useful application to two-dimensional models. We highlight the close relationship between the classical boundary-driven streaming vortices and the rotationality of the Reynolds stress force field. The limiting velocity method, which ignores the acoustic boundary layer and solves the outer streaming fields by applying the 'limiting velocities' as boundary conditions, is more computationally efficient and can be used for predicting three-dimensional outer streaming fields and provide insight into their origins, provided that the radius of curvature of the channel surfaces is much greater than the acoustic boundary layer thickness ( δ v ). We also show that for the limiting velocity method to be valid the channel scales must exceed a value of approximately 100 δ v (for an error of ~5% on the streaming velocity magnitudes) for the case presented in this paper. Comparisons of these two numerical methods can provide effective guidance for researchers in the field of acoustofluidics on choosing appropriate methods to predict boundary-driven streaming fields in the design of acoustofluidic particle manipulation devices.

Modeling of Microdevices for SAW-Based Acoustophoresis - A Study of Boundary Conditions

We present a finite-element method modeling of acoustophoretic devices consisting of a single, long, straight, water-filled microchannel surrounded by an elastic wall of either borosilicate glass (pyrex) or the elastomer polydimethylsiloxane (PDMS) and placed on top of a piezoelectric transducer that actuates the device by surface acoustic waves (SAW). We compare the resulting acoustic fields in these full solid-fluid models with those obtained in reduced fluid models comprising of only a water domain with simplified, approximate boundary conditions representing the surrounding solids. The reduced models are found to only approximate the acoustically hard pyrex systems to a limited degree for large wall thicknesses and but not very well for acoustically soft PDMS systems shorter than the PDMS damping length of 3 mm.

Acoustic Force Density Acting on Inhomogeneous Fluids in Acoustic Fields

We present a theory for the acoustic force density acting on inhomogeneous fluids in acoustic fields on time scales that are slow compared to the acoustic oscillation period. The acoustic force density depends on gradients in the density and compressibility of the fluid. For microfluidic systems, the theory predicts a relocation of the inhomogeneities into stable field-dependent configurations, which are qualitatively different from the horizontally layered configurations due to gravity. Experimental validation is obtained by confocal imaging of aqueous solutions in a glass-silicon microchip.

High-throughput malaria parasite separation using a viscoelastic fluid for ultrasensitive PCR detection

A novel microfluidic device for high-throughput particle separation using a viscoelastic fluid, which enables the rapid detection of extremely rare malaria parasites by using PCR analysis, is proposed. Our device consists of two segments: the 1st stage for sheathless pre-alignment and the 2nd stage for separation based on size-dependent viscoelasticity-induced lateral migration. The use of a high-aspect ratio channel and a viscoelastic polymer solution with low viscosity enables high-throughput processing. The device performance was first optimized using synthetic particles. A mixture of 2 and 10 μm particles was focused at the center plane in the 1st stage. The smaller particles, serving as surrogates for malaria parasites, were subsequently separated in the 2nd stage with a recovery rate of ∼96% at 400 μl min(-1). Finally, separation of the malaria parasites from the white blood cells was performed. At 400 μl min(-1), almost all white blood cells were removed and the malaria parasites were separated with a ∼94% recovery rate and ∼99% purity. Although the initial concentration of the malaria parasites was too low to be detected by PCR analysis, WBC depletion and buffer removal increased the parasite concentration sufficiently such that PCR detection was possible.

Enrichment of diluted cell populations from large sample volumes using 3D carbon-electrode dielectrophoresis

Here, we report on an enrichment protocol using carbon electrode dielectrophoresis to isolate and purify a targeted cell population from sample volumes up to 4 ml. We aim at trapping, washing, and recovering an enriched cell fraction that will facilitate downstream analysis. We used an increasingly diluted sample of yeast, 10(6)-10(2) cells/ml, to demonstrate the isolation and enrichment of few cells at increasing flow rates. A maximum average enrichment of 154.2 ± 23.7 times was achieved when the sample flow rate was 10 μl/min and yeast cells were suspended in low electrically conductive media that maximizes dielectrophoresis trapping. A COMSOL Multiphysics model allowed for the comparison between experimental and simulation results. Discussion is conducted on the discrepancies between such results and how the model can be further improved.

Clinical prognostic factors for time to positivity in cancer patients with bloodstream infections

PURPOSE

Time to positivity (TTP) has been used in recent years as a simple and rapid method for the additional characterization of the degree of bacteremia. However, prognostic factors for TTP in cancer patients with bloodstream infections have rarely been studied. The aim of this study was to investigate the clinical factors for TTP involving various isolated organisms in cancer patients.

METHODS

We analyzed 386 episodes of bloodstream infections (BSIs) in patients with or without cancer during a 19 month period. Information on age, gender, tumor type, ICU stay, organisms, multidrug resistance (MDR), TTP and outcome was collected. Multivariate logistic regression analysis was performed.

RESULTS

The mean TTP of Enterobacteriaceae in patients with hepatocellular carcinoma, gastroenterological cancer, and lung cancer was shorter than in non-cancer patients (9.86 ± 3.22, 10.05 ± 3.47, 8.85 ± 2.78 vs 13.11 ± 5.37 h). The mean TTP of nonfermentative bacilli in patients with lung cancer (12.37 ± 5.96 h) and hematologic diseases (8.72 ± 4.21 h) was also shorter than in non-cancer patients (20.74 ± 2.46 h), and the mean TTP of Staphylococcus isolates was significantly different between non-cancer patients (22.06 ± 3.71 h) and hematologic disease patients (11.93 ± 5.44 h). The presence of a benign tumor was a significant prognostic factor for a long TTP only in the Staphylococci group (OR 0.076, 95 % CI 0.014-0.412), according to multivariate analysis. MDR (OR 2.178, 95 % CI 1.196-4.239) was an independent significant predictor in the Enterobacteriaceae group, with a short TTP, and it was also a significant clinical factor for a long TTP in nonfermentative bacilli and the Staphylococci group (OR 5.037, 95 % CI 1.065-23.82; OR 0.167, 95 % CI 0.059-0.474).

CONCLUSION

Time to positivity provides useful diagnostic and prognostic information for the differentiation of frequently isolated organisms. This information may help clinicians to use the correct antibiotics in a timely manner to treat cancer patients with BSIs based on clinical factor analysis.

Highly parallel acoustic assembly of microparticles into well-ordered colloidal crystallites

The precise arrangement of microscopic objects is critical to the development of functional materials and ornately patterned surfaces. Here, we present an acoustics-based method for the rapid arrangement of microscopic particles into organized and programmable architectures, which are periodically spaced within a square assembly chamber. This macroscale device employs two-dimensional bulk acoustic standing waves to propel particles along the base of the chamber toward pressure nodes or antinodes, depending on the acoustic contrast factor of the particle, and is capable of simultaneously creating thousands of size-limited, isotropic and anisotropic assemblies within minutes. We pair experiments with Brownian dynamics simulations to model the migration kinetics and assembly patterns of spherical microparticles. We use these insights to predict and subsequently validate the onset of buckling of the assemblies into three-dimensional clusters by experiments upon increasing the acoustic pressure amplitude and the particle concentration. The simulations are also used to inform our experiments for the assembly of non-spherical particles, which are then recovered via fluid evaporation and directly inspected by electron microscopy. This method for assembly of particles offers several notable advantages over other approaches (e.g., magnetics, electrokinetics and optical tweezing) including simplicity, speed and scalability and can also be used in concert with other such approaches for enhancing the types of assemblies achievable.

On-chip ultrasonic sample preparation for cell based assays

We demonstrate pre-alignment, size-based separation, isolation, trapping, up-concentration and fluorescence monitoring of cells in a sequence by the use of a multi-step, three-transducer acoustophoresis chip designed for cellular sample preparation.