Trapping Of Beads Research Articles

Laser-induced heating for the experimental study of critical Casimir forces with optical trapping

Critical Casimir interactions represent a perfect example of bath-induced forces at mesoscales. These forces may have a relevant role in the living systems as well as a role in the design of nanomachines fueled by environmental fluctuations. Since the thermal fluctuations are enhanced in the vicinity of a demixing point of a second-order phase transition, we can modulate the magnitude and range of these Casimir-like forces by slight changes in the temperature. Here, we consider two optical trapped colloidal beads inside a binary mixture. The Casimir interaction is controlled by warming the mixture by laser-induced heating, whose local application ensures high reproducibility. Once this two-particle system is warmed, the critical behavior of different observables allows the system to become its self-thermometer. We use this experimental scheme for analyzing the energetics of a critical colloidal system under a non-equilibrium-driven protocol. We quantify how the injected work can be dissipated to the environment as heat or stored as free energy. Indeed, our system allows us to use the fluctuation theorems framework for analyzing the performance of this critically driven toy model. Our work paves the way for future experimental studies on the non-equilibrium features of bath-induced forces and the design of critically driven nanosystems.

Single-cell trapping and retrieval in open microfluidics

Among various single-cell analysis platforms, hydrodynamic cell trapping systems remain relevant because of their versatility. Among those, deterministic hydrodynamic cell-trapping systems have received significant interest; however, their applications are limited because trapped cells are kept within the closed microchannel, thus prohibiting access to external cell-picking devices. In this study, we develop a hydrodynamic cell-trapping system in an open microfluidics architecture to allow external access to trapped cells. A technique to render only the inside of a polydimethylsiloxane (PDMS) microchannel hydrophilic is developed, which allows the precise confinement of spontaneous capillary flow in the open-type microchannel with a width on the order of several tens of micrometers. Efficient trapping of single beads and single cells is achieved, in which trapped cells can be retrieved via automated robotic pipetting. The present system can facilitate the development of new single-cell analytical systems by bridging between microfluidic devices and macro-scale apparatus used in conventional biology.

A novel visual biosensor for mercury detection based on a cleavable phosphorothioate-RNA probe and microfluidic bead trap

Mercury ion (Hg2+) detection plays a crucial role in safeguarding the environment and protecting human health. In this study, we developed a microfluidic biosensing platform that utilizes a phosphorothioate-RNA (PS-RNA) probe and gold-coated magnetic nanoparticles to quantify Hg2+ visually. The PS-RNA probe was linked to gold-coated magnetic nanoparticles via Au–S interaction and biotin–avidin interaction and then combined with polystyrene microspheres to form a self-assembled structure called a magnetic bead-AuNP-probe-polystyrene (MAPP) bead. We designed and fabricated a microfluidic chip that contained only a magnetic separator and a particle trap. The presence of Hg2+ cut the PS-RNA probe, leading to the separation of magnetic beads and polystyrene microspheres. The MAPP was removed by the magnetic separator, while the cut polystyrene microspheres flowed along the channel and accumulated in the particle trap, forming a long visible strip. The length of the polystyrene microsphere strip was proportional to the Hg2+ concentration. The microfluidic biosensing platform exhibited high sensitivity and specificity for Hg2+ detection, with a detection limit of 21.9 nM. By integrating the microfluidic magnetic manipulation system with a sensitive nucleic acid probe, we developed an automated and visual biosensor that is a valuable reference for the on-site visual detection of heavy metal ions.

Nonlinear effects in optical trapping of titanium dioxide and diamond nanoparticles

Optical trapping in biophysics typically uses micron-scale beads made of materials like polystyrene or glass to probe the target of interest. Using smaller beads made of higher-index materials could increase the time resolution of these measurements. We characterized the trapping of nanoscale beads made of diamond and titanium dioxide (TiO2) in a single-beam gradient trap. Calculating theoretical expectations for the trapping stiffness of these beads, we found good agreement with measured values. Trap stiffness was significantly higher for TiO2 beads, owing to notable enhancement from nonlinear optical effects, not previously observed for continuous-wave trapping. Trap stiffness was over 6-fold higher for TiO2 beads than polystyrene beads of similar size at 70 mW laser power. These results suggest that diamond and TiO2 nanobeads can be used to improve time resolution in optical tweezers measurements.

Recent Development and Applications of Magnetism in Biomedicine

Fluorescence-activated cell sorting (FACS), a cell isolation approach that relies on immunofluorescent signals, is among the most frequently applied approaches. Due to the inherent advantages of microfluidic devices in terms of small size, short detection time and low cost, their application in biotechnology are currently being investigated. Current research by scholars was reviewed by the literature method. Micro-magnet matrices can be used to move multiple cells in cell manipulation, and open tubular with magnetic trapping bead trap beds can be useful in trapping blood cells. The trapping and separation of blood cells have been better progressed, and future studies can concentrate on the concurrent sorting of several targets at high purity, recovery, and throughput levels, label-free continuous separation of cell mixtures, and the sorting of different cells by changing the device environment around the properties of the magnetic device itself.

Geometric-phase metalens to be used for tunable optical tweezers in microfluidics

ABSTRACTGeometric-phase dielectric meta-lenses made of silicon with high numerical aperture and short focal lengths are fabricated and characterised. For circularly polarised light, the same meta-lens can act as a converging or diverging lens, depending on the handedness of the circular polarisation. This effect enables application for optical tweezers that trap or release µm-size polymer beads floating in a microfluidic channel on demand. An electrically addressable polarisation converter based on liquid crystals may be used to switch between the two states of polarisation, at which the light transmitted through the meta-lens is focused (trapping) or defocussed (releasing), respectively.

High-gradient magnetophoretic bead trapping for enhanced electrochemical sensing and particle manipulation.

Magnetic particles are routinely used in many biochemical techniques. As such, the manipulation of these particles is of paramount importance for proper detection and assay preparation. This paper describes a magnetic manipulation and detection paradigm that allows sensing and handling highly sensitive magnetic bead-based assays. The simple manufacturing process presented in this manuscript employs a CNC machining technique and an iron microparticle-doped PDMS (Fe-PDMS) compound to create magnetic microstructures that enhance magnetic forces for magnetic bead confinement. Said confinement, generates increases in local concentrations at the detection site. Higher local concentrations increase the magnitude of the detection signal, leading to higher assay sensitivity and lower limit of detection (LOD). Furthermore, we demonstrate this characteristic signal enhancement in both fluorescence and electrochemical detection techniques. We expect this new technique to allow users to design fully integrated magnetic bead-based microfluidic devices with the goal of preventing sample losses and enhancing signal magnitudes in biological experiments and assays.

Optimal in-depth trapping by tuning the correction collar of a dry objective lens

Optical tweezers are proven and indispensable micro-manipulation tools. It is very common to use an immersion-assisted high NA objective for optical trapping of micrometer-sized beads. However, such objectives suffer from low working depth range. Here we show, both by theory and experiment, that a dry objective can be utilized for optimal trapping of even sub-micrometer objects. For the first time, to the best of our knowledge, we were able to stably trap polystyrene beads with radii of 270 and 175 nm in 3D using an objective with numerical aperture of 0.9.

Dielectrophoretic Traps for Efficient Bead and Cell Trapping and Formation of Aggregates of Controlled Size and Composition.

We present a microfluidic dielectrophoretic-actuated system designed to trap chosen single-cell and form controlled cell aggregates. A novel method is proposed to characterize the efficiency of the dielectrophoretic trapping, considering the flow speed but also the heat generated by the traps as limiting criteria in cell-safe manipulation. Two original designs with different manufacturing processes are experimentally compared. The most efficient design is selected and the cell membrane integrity is monitored by fluorescence imaging to guarantee a safe-cell trapping. Design rules are suggested to adapt the traps to multiple-cells trapping and are experimentally validated as we formed aggregates of controlled size and composition with two different types of cells. We provide hereby a simple manufactured tool allowing the controlled manipulation of particles for the composition of multicellular assemblies.

Heme Detoxification Protein (PfHDP) is essential for the hemoglobin uptake and metabolism in Plasmodium falciparum.

Hemoglobin degradation is crucial for the growth and survival of Plasmodium falciparum in human erythrocytes. Although the process of Hb degradation has been studied in detail, the mechanisms of Hb uptake remain ambiguous to date. Here, we characterized Heme Detoxification Protein (PfHDP); a protein localized in the parasitophorus vacuole, parasite food vacuole, and infected erythrocyte cytosol for its role in Hb uptake. Immunoprecipitation of PfHDP‐GFP fusion protein from a transgenic line using GFP trap beads showed the association of PfHDP with Hb as well as with the members of PTEX translocon complex. Association of PfHDP with Hb or Pfexp‐2, a component of translocon complex was confirmed by protein–protein interaction and immunolocalization tools. Based on these associations, we studied the role of PfHDP in Hb uptake using the PfHDP‐HA‐GlmS transgenic parasites line. PfHDP knockdown significantly reduced the Hb uptake in these transgenic parasites in comparison to the wild‐type parasites. Morphological analysis of PfHDP‐HA‐GlmS transgenic parasites in the presence of GlcN showed food vacuole abnormalities and parasite stress, thereby causing a growth defect in the development of these parasites. Transient knockdown of a member of translocon complex, PfHSP101 in HSP101‐DDDHA parasites also showed a decreased uptake of Hb inside the parasite. Together, these results advocate an interaction between PfHDP and the translocon complex at the parasitophorus vacuole membrane and also suggest a role for PfHDP in the uptake of Hb and parasite development. The study thus reveals new insights into the function of PfHDP, making it an extremely important target for developing new antimalarials.

Deciphering single- and multi-particle trapping dynamics under femtosecond pulsed excitation with simultaneous spatial and temporal resolution

Recent theoretical and experimental studies have shed light on how laser trapping dynamics under femtosecond pulsed excitation are fine-tuned by optical and thermal nonlinearities. Here, we present experimental results of trapping of single and multiple polystyrene beads (of 1 μm diameter). We show how integration and synchronization of bright-field video microscopy with confocal detection of backscatter provide both spatial and temporal resolution required to capture intricate details of nonlinear trapping dynamics. Such spatiotemporal detection is promising to have far-reaching applications in exploring controlled laser trapping and manipulations harnessed by optical and thermal nonlinearities.

Incoherent Optical Tweezers on Black Titanium.

Optical tweezers enable the manipulation of micro- and nanodielectric particles through entrapment using a tightly focused laser. Generally, optical trapping of submicron size particles requires high-intensity light in the order of MW/cm2. Here, we demonstrate a technique of stable optical trapping of submicron polymeric beads on nanostructured titanium surfaces (black-Ti) without the use of lasers. Fluorescent polystyrene beads with a diameter d = 20-500 nm were successfully trapped on black-Ti by low-intensity focused illumination of incoherent light at λ = 370 m from a Hg lamp. Light intensity was 5.5 W/cm2, corresponding to a reduced light intensity of 6 orders of magnitude. Upon switching off illumination, trapped particles were released from the illuminated area, indicating that trapping was optically driven and reversible. Such trapping behavior was not observed on nonstructured Ti surfaces or on nanostructured silicon surfaces. Thus, the Ti nanostructures were demonstrated to play a key role.

Planar hydrodynamic traps and buried channels for bead and cell trapping and releasing.

We present a novel concept for the controlled trapping and releasing of beads and cells in a PDMS microfluidic channel without obstacles present around the particle or in the channel. The trapping principle relies on a two-level microfluidic configuration: a top main PDMS channel interconnected to a buried glass microchannel using round vias. As the fluidic resistances rule the way the liquid flows inside the channels, particles located in the streamlines passing inside the buried level are immobilized by the round via with a smaller diameter, leaving the object motionless in the upper PDMS channel. The particle is maintained by the difference of pressure established across its interface and acts as an infinite fluidic resistance, virtually cancelling the subsequent buried fluidic path. The pressure is controlled at the outlet of the buried path and three modes of operation of a trap are defined: idle, trapping and releasing. The pressure conditions for each mode are defined based on the hydraulic–electrical circuit equivalence. The trapping of polystyrene beads in a compact array of 522 parallel traps controlled by a single pressure was demonstrated with a trapping efficiency of 94%. Pressure conditions necessary to safely trap cells in holes of different diameters were determined and demonstrated in an array of 25 traps, establishing the design and operation rules for the use of planar hydrodynamic traps for biological assays.

Optical Trapping of Polystyrene Nanoparticles on Black Silicon: Implications for Trapping and Studying Bacteria and Viruses

The optical trapping of polymer beads of different diameters d of 100, 200, 500, 1000, and 3000 nm on planar (F-Si) and nanostructured (BSi) crystalline silicon was investigated at laser wavelengths λ = 808 and 1064 nm, respectively. We demonstrate that BSi surfaces can enhance the optical gradient force required to grip nanoparticles (100 nm) in contrast to F-Si surfaces, significantly changing the trapping behavior. Thus, different modes of optical tweezing were characterized by modification of Si surface topography (nanostructuring), wavelength λ, laser irradiation intensity I, and irradiation area. Specifically, we present four separate modes of optical tweezing using planar and nanostructured Si for (i) a single particle trapped by tightly and loosely focused irradiation on F-Si, (ii) a large number of 3D assembled beads trapped by tightly focused irradiation on BSi, (iii) a small number of particles trapped on F-Si, and (iv) a large number of particles trapped on BSi, resulting in a 2D self-ordered assembly. The mechanisms of the optical manipulation of particles in the range 100–1000 nm were discussed with implications for the trapping of bacteria or viruses by using nanostructured semiconductor assisted optical tweezing (NASSCA/OT) and their response to high mechanical shear forces at locally changed temperatures on nanostructured surfaces.

Comparisons between Geotextile Pore Sizes Obtained from Capillary Flow And Dry Sieving Tests

Abstract In the United States of America, two standard methods of measuring the largest pore size of a geotextile have been accepted: Dry Sieving Test (ASTM D4751, Standard Test Methods for Determining Apparent Opening Size of a Geotextile) and Capillary Flow Test (ASTM D6767, Standard Test Method for Pore Size Characteristics of Geotextiles by Capillary Flow Test). Despite several drawbacks of Dry Sieving Tests, such as the amount of time required; tendency of glass beads to become trapped, including trapping of glass beads inside the geotextiles; likelihood of imprecise preparation of glass beads; and potential electrostatic effects, the largest pore size (apparent opening size [AOS], O95) results obtained from Dry Sieving Test still determines many filtration criteria. Although the Capillary Flow Test was approved in 2002, it is still not widely adopted, despite the fact that it provides the largest pore size (Bubble Point, O98) and a complete pore size distribution of a geotextile. Decades of analyses searched for a correlation between the Dry Sieving and Capillary Flow Test results; however, none of them have been widely accepted because of the limited number of geotextiles used and a lack of proper calibration. The current study investigates a possible correlation between the Dry Sieving and Capillary Flow Test results with 51 woven, nonwoven, and composite geotextiles. The Capillary Flow Porometer was calibrated using four thin metal plates with known pore openings and two membranes. This study describes the challenges experienced during the Capillary Flow Test and factors affecting the test results. One finding is that the cleaning of calibration materials and equipment with Methanol plays an important role in the Capillary Flow Test results. The use of different shape factors, contact angles, and wetting liquids in the analyses are also considered. The outliers of Capillary Flow Test results were removed from the analysis using a box plot and whisker diagram, and a good correlation between AOS and Bubble Point (R2 = 0.78) was established.

Reusable Embedded Microcoils for Magnetic Nano-Beads Trapping in Microfluidics: Magnetic Simulation and Experiments.

In this study, a microfluidic chip with integrated coil was designed and fabricated for the aim of effectively trapping magnetic nanobeads (Adembeads®, 300 nm) and measuring the chip’s temperature during the working time. In addition, a reversible technique of bonding Polydimethylsiloxane (PDMS) channels was presented. This bonding process used a coating layer of CYTOP®product as a protection, insulation and low-adhesion layer. The reversible packaging technique allows the bottom substrate to be reused, possibly equipped with sensors, and to use a disposable microchannels network. The FE method was employed to calculate the magnetic field and power consumption by the ANSYS® version 12.1 software. Merit factors were defined in order to synthetically represent the ability of the simulated coil to trap beads for a unit power consumption, i.e. a given heat generation. The simulation results propose a new approach to optimize the design criteria in fabricating planar microcoils. The optimal microcoils were fabricated and then used to realize a magnetic immunoassay in a microfluidic chip. The aim was to integrate these microcoils into a lab-on-chip and obtain a fast and highly sensitive biological element detection.

Thin Film PZT-Based PMUT Arrays for Deterministic Particle Manipulation

Lead zirconate titanate (PZT)-based piezoelectric micromachined ultrasonic transducers (PMUTs) for particle manipulation applications were designed, fabricated, characterized, and tested. The PMUTs had a diaphragm diameter of 60 [Formula: see text], a resonant frequency of ~8 MHz, and an operational bandwidth (BW) of 62.5%. Acoustic pressure output in water was 9.5 kPa at 7.5 mm distance from a PMUT element excited with a unipolar waveform at 5 Vpp . The element consisted of 20 diaphragms connected electrically in parallel. Particle trapping of 4 [Formula: see text] silica beads was shown to be possible with 5 Vpp unipolar excitation. Trapping of multiple beads by a single element and deterministic control of particles via acoustophoresis without the assistance of microfluidic flow were demonstrated. It was found that the particles move toward diaphragm areas of highest pressure, in agreement with literature and simulations. Unique bead patterns were generated at different driving frequencies and were formed at frequencies up to 60 MHz, much higher than the operational BW. Levitation planes were generated above the 30 MHz driving frequency.

An Acoustic Trap for Bead Injection Attenuated Total Reflection Infrared Spectroscopy.

In this work, we introduce a system combining an acoustic trap for bead injection with attenuated total reflection (ATR) infrared (IR) spectroscopy. By mounting an acoustofluidic cell hosting an ultrasound source on top of a custom-built ATR fixture we were able to trap beads labeled with the enzyme alkaline phosphatase without requiring any mechanical retention elements. Sequential injection analysis was employed for reproducible sample handling and bead injection into the acoustic trap. To showcase potential applications of the presented setup for kinetic studies, we monitored the conversion of p-nitrophenylphosphate into p-nitrophenol and phosphate via beads carrying the immobilized enzyme using ATR-IR spectroscopy. Retaining the labeled beads via ultrasound particle manipulation resulted in excellent experimental reproducibility (relative standard deviation, 3.91%). It was demonstrated that trapped beads remained stably restrained with up to eight cell volumes of liquid passing through the acoustofluidic cell. Beads could be discarded in a straightforward manner by switching off the ultrasound, in contrast to systems containing mechanical retention elements, which require backflushing. Multiple experiments were performed by employing different substrate concentrations with the same batch of trapped beads as well as varying the amount of enzyme present in the cell, enabling enzyme kinetic studies and emphasizing the application of the proposed setup in studies where enzymatic reuse is desired. This proves the potential of the acoustic trap combined with ATR-IR spectroscopy to monitor the activity of immobilized enzymes and its ability to perform complex bead-based assays.

A review of polystyrene bead manipulation by dielectrophoresis.

Exploitation of the intrinsic electrical properties of particles has recently emerged as an appealing approach for trapping and separating various scaled particles. Initiative particle manipulation by dielectrophoresis (DEP) showed remarkable advantages including high speed, ease of handling, high precision and being label-free. Herein, we provide a general overview of the manipulation of polystyrene (PS) beads and related particles via DEP; especially, the wide applications of these manipulated PS beads in the quantitative evaluation of device performance for model validation and standardization have been discussed. The motion and polarizability of the PS beads induced by DEP were analyzed and classified into two categories as positive and negative DEP within the time and space domains. The DEP techniques used for bioparticle manipulation were demonstrated, and their applications were conducted in four fields: trapping of single-sized PS beads, separation of multiple-sized PS beads by size, separation of PS beads and non-bioparticles, and separation of PS beads and bioparticles. Finally, future perspectives on DEP-on-a-chip have been proposed to discriminate bio-targets in the network of microfluidic channels.

Optical Trapping and Manipulation of Superparamagnetic Beads Using Annular-Shaped Beams

We propose an optical tweezers setup based on an annular-shaped laser beam that is efficient to trap 2.8 m-diameter superparamagnetic particles. The optical trapping of such particles was fully characterized, and a direct absolute comparison with a geometrical optics model was performed. With this comparison, we were able to show that light absorption by the superparamagnetic particles is negligible for our annular beam tweezers, differing from the case of conventional Gaussian beam tweezers, in which laser absorption by the beads makes stable trapping difficult. In addition, the trap stiffness of the annular beam tweezers increases with the laser power and with the bead distance from the coverslip surface. While this first result is expected and similar to that achieved for conventional Gaussian tweezers, which use ordinary dielectric beads, the second result is quite surprising and different from the ordinary case, suggesting that spherical aberration is much less important in our annular beam geometry. The results obtained here provide new insights into the development of hybrid optomagnetic tweezers, which can apply simultaneously optical and magnetic forces on the same particles.