Measurements of scattered light on a microchip flow cytometer with integrated polymer based optical elements

In silico full-angle high-dynamic range scattering of microscopic objects exploiting holotomography

Accurate optical characterization of microscopic objects is crucial in academic research, product development, and clinical diagnosis. We present a method for obtaining full and high-dynamic range, angle-resolved light scattering attributes of microparticles, enabling distinction of variations in both overall morphology and detailed internal structures. This method overcomes previous limitations in observable scattering angles and dynamic range of signals through computationally assisted three-dimensional holotomography. This advancement is significant for particles spanning tens of wavelengths, such as human erythrocytes, which have historically posed measurement challenges due to faint side-scattering signals indicative of their complex interiors. Our technique addresses three key challenges in optical side-scattering analysis: limited observational angular range, reliance on simplified computational models, and low signal-to-noise ratios in both experimental and computational evaluations. We incorporate three-dimensional tomographic complex refractive index data from Fourier-transform light scattering into a tailored finite-difference time-domain simulation space. This approach facilitates precise near-to-far-field transformations. The process yields complete full-angle scattering phase functions, crucial for particles like Plasmodium falciparum-parasitized erythrocytes, predominantly involved in forward scattering. The resultant scattering data exhibit an extreme dynamic range exceeding 100 dB at various incident angles of a He-Ne laser. These findings have the potential to develop point-of-care, cost-effective, and rapid malaria diagnostic tools, inspiring further clinical and research applications in microparticle scattering.

Backscattering-Based Discrimination of Microparticles Using an Optofluidic Multiangle Scattering Chip

In this research, we designed and fabricated an optofluidic chip for the detection and differentiation of single particles via the combination of backscattered (BSC) and forward-scattered (FSC) or side-scattered (SSC) light intensity. The high sensitivity of BSC light to the refractive index of the particles enabled an effective approach for the differentiation of individual particles based on the type of material. By recording BSC as well as FSC and SSC light intensities from single particles, transiting through the illumination zone in a microfluidic channel, the size and type of material could be detected simultaneously. The analysis of model samples of polystyrene (PS), as a primary microplastic particle, and silica microspheres showed substantially higher BSC signal values of PS because of a larger refractive index compared to the silica. The scatter plots correlating contributions of BSC (FSC-BSC and SSC-BSC) allowed a clear differentiation of PS and silica particles. To demonstrate the great potential of this methodology, two "real-life" samples containing different types of particles were tested as application examples. Commercial toothpaste and peeling gel products, as primary sources of microplastics into effluents, were analyzed via the optofluidic chip and compared to results from scanning electron microscopy. The scattering analysis of the complex samples enabled the detection and simultaneous differentiation of particles such as microplastics according to their differences in the refractive index via distinctive areas of high and low BSC signal values. Hence, the contribution of BSC light measurements in multiangle scattering of single particles realized in an optofluidic chip opens the way for the discrimination of single particles in a liquid medium in manifold fields of application ranging from environmental monitoring to cosmetics.

Flow Cytometry as a Rapid Alternative to Quantify Small Microplastics in Environmental Water Samples

The most frequently used method to quantify microplastics (MPs) visually by microscope is time consuming and labour intensive, where the method is also hindered by the size limitation at 10 µm or even higher. A method is proposed to perform pre-concentration of MPs by vacuum filtration, hydrogen peroxide wet digestion, fluorescent staining and flow cytometric determination to rapidly detect and quantify small MPs sized from 1–50 µm. The method performance was evaluated by the spiking of seven different types of polymer, including polystyrene (PS), low-density polyethylene (LDPE), polypropylene (PP), poly(methyl methacrylate) (PMMA), polyvinyl chloride (PVC), polylactic acid (PLA) and acrylonitrile butadiene styrene (ABS) at different levels (400, 4000, 40,000 particles mL−1), with a satisfactory overall % recoveries (101 ± 19.4%) observed, where in general no significant difference between the two methods was observed. Furthermore, a pre-concentration process by vacuum filtration was introduced to reduce the matrix effect. After pre-concentration, satisfactory % recoveries and accuracy in MP counts resulted from both ultrapure water (94.33 ± 11.16%) and sea water (103.17 ± 9.50%) samples. The validated method using flow cytometry can be used to quantify MPs in environmental water samples that can reduce time and human resources.

A microfluidic flow analyzer with integrated lensed optical fibers

Rapid optical interrogation of flowing cells or particles is a powerful tool in the field of biomedical diagnostics. Determination of size and composition of fast-flowing cells, with diameters in the range of 2- 15 μ m , often require complex open-space optics and expensive high-speed cameras. In this work, a method to overcome these challenges by using a hydrodynamic flow-based microfluidic platform coupled with on-chip integrated fiber optics is reported. The lab-scale portable device developed uses a combination of on-chip lensed and non-lensed optical fibers for precision illumination. The narrow light beam produced by the lensed fiber ( f = 150 μ m ) enables precise optical analysis with high sensitivity. A planar arrangement of optical fibers at various angles facilitates multi-parametric analysis from a single point of interrogation. As proof of concept, the laboratory-scale portable bench-top prototype is used to measure fluorescence signals from CD4 immunostained cells and human blood samples. The performance of microfluidic flow analyzer is also compared to the conventional Guava® easyCyte 8HT flow cytometer.

Cell Cytometry: Review and Perspective on Biotechnological Advances

Cell identification and enumeration are essential procedures within clinical and research laboratories. For over 150 years, quantitative investigation of body fluids such as counts of various blood cells has been an important tool for diagnostic analysis. With the current evolution of point-of-care diagnostics and precision medicine, cheap and precise cell counting technologies are in demand. This article reviews the timeline and recent notable advancements in cell counting that have occurred as a result of improvements in sensing including optical and electrical technology, enhancements in image processing capabilities, and contributions of micro and nanotechnologies. Cell enumeration methods have evolved from the use of manual counting using a hemocytometer to automated cell counters capable of providing reliable counts with high precision and throughput. These developments have been enabled by the use of precision engineering, micro and nanotechnology approaches, automation and multivariate data analysis. Commercially available automated cell counters can be broadly classified into three categories based on the principle of detection namely, electrical impedance, optical analysis and image analysis. These technologies have many common scientific uses, such as hematological analysis, urine analysis and bacterial enumeration. In addition to commercially available technologies, future technological trends using lab-on-a-chip devices have been discussed in detail. Lab-on-a-chip platforms utilize the existing three detection technologies with innovative design changes utilizing advanced nano/microfabrication to produce customized devices suited to specific applications.

Standing Surface Acoustic Wave (SSAW)-Based Fluorescence-Activated Cell Sorter

Microfluidic fluorescence-activated cell sorters (μFACS) have attracted considerable interest because of their ability to identify and separate cells in inexpensive and biosafe ways. Here a high-performance μFACS is presented by integrating a standing surface acoustic wave (SSAW)-based, 3D cell-focusing unit, an in-plane fluorescent detection unit, and an SSAW-based cell-deflection unit on a single chip. Without using sheath flow or precise flow rate control, the SSAW-based cell-focusing technique can focus cells into a single file at a designated position. The tight focusing of cells enables an in-plane-integrated optical detection system to accurately distinguish individual cells of interest. In the acoustic-based cell-deflection unit, a focused interdigital transducer design is utilized to deflect cells from the focused stream within a minimized area, resulting in a high-throughput sorting ability. Each unit is experimentally characterized, respectively, and the integrated SSAW-based FACS is used to sort mammalian cells (HeLa) at different throughputs. A sorting purity of greater than 90% is achieved at a throughput of 2500 events s^-1 . The SSAW-based FACS is efficient, fast, biosafe, biocompatible and has a small footprint, making it a competitive alternative to more expensive, bulkier traditional FACS.

Microfluidic flow cytometry: The role of microfabrication methodologies, performance and functional specification

Flow cytometry is an invaluable tool utilized in modern biomedical research and clinical applications requiring high throughput, high resolution particle analysis for cytometric characterization and/or sorting of cells and particles as well as for analyzing results from immunocytometric assays. In recent years, research has focused on developing microfluidic flow cytometers with the motivation of creating smaller, less expensive, simpler, and more autonomous alternatives to conventional flow cytometers. These devices could ideally be highly portable, easy to operate without extensive user training, and utilized for research purposes and/or point-of-care diagnostics especially in limited resource facilities or locations requiring on-site analyses. However, designing a device that fulfills the criteria of high throughput analysis, automation and portability, while not sacrificing performance is not a trivial matter. This review intends to present the current state of the field and provide considerations for further improvement by focusing on the key design components of microfluidic flow cytometers. The recent innovations in particle focusing and detection strategies are detailed and compared. This review outlines performance matrix parameters of flow cytometers that are interdependent with each other, suggesting trade offs in selection based on the requirements of the applications. The ongoing contribution of microfluidics demonstrates that it is a viable technology to advance the current state of flow cytometry and develop automated, easy to operate and cost-effective flow cytometers.

An integrated microfluidic device for the sorting of yeast cells using image processing

The process of detection and separation of yeast cells based on their morphological characteristics is critical to the understanding of cell division cycles, which is of vital importance to the understanding of some diseases such as cancer. The traditional process of manual detection is usually tedious and inconsistent. This paper presents a microfluidic device integrated with microvalves for fluid control for the sorting of yeast cells using image processing algorithms and confirmation based on their fluorescent tag. The proposed device is completely automated, low cost and easy to implement in an academic research setting. Design details of the integrated microfluidic system are highlighted in this paper, along with experimental validation. Real time cell sorting was demonstrated with a cell detection rate of 12 cells per minute.

Micro-optics for microfluidic analytical applications

This critical review summarizes the developments in the integration of micro-optical elements with microfluidic platforms for facilitating detection and automation of bio-analytical applications.

A Perspective on the Rise of Optofluidics and the Future

In the recent past, the field of optofluidics has thrived from the immense efforts of researchers from diverse communities. The concept of optofluidics combines optics and microfluidics to exploit novel properties and functionalities. In the very beginning, the unique properties of liquid, such as mobility, fungibility and deformability, initiated the motivation to develop optical elements or functions using fluid interfaces. Later on, the advancements of microelectromechanical system (MEMS) and microfluidic technologies enabled the realization of optofluidic components through the precise manipulation of fluids at microscale thus making it possible to streamline complex fabrication processes. The optofluidic system aims to fully integrate optical functions on a single chip instead of using external bulky optics, which can consequently lower the cost of system, downsize the system and make it promising for point-of-care diagnosis. This perspective gives an overview of the recent developments in the field of optofluidics. Firstly, the fundamental optofluidic components will be discussed and are categorized according to their basic working mechanisms, followed by the discussions on the functional instrumentations of the optofluidic components, as well as the current commercialization aspects of optofluidics. The paper concludes with the critical challenges that might hamper the transformation of optofluidic technologies from lab-based procedures to practical usages and commercialization.

Microfluidic cytometers with integrated on-chip optical systems for red blood cell and platelet counting

A microfluidic cytometer with integrated on-chip optical systems was designed for red blood cell (RBC) and platelet (PLT) counting. The design, fabrication, and characterization of the microfluidic cytometer with on-chip optical signal detection were described. With process using only a single mask, the device that integrates optical fibers and on-chip microlens with microfluidic channels on a polydimethylsiloxane layer by standard soft photolithography. This compact structure increased the sensitivity of the device and eliminated time-consuming free-space optical alignments. The microfluidic cytometer was used to count red blood cells and platelets. Forward scatter and extinction were collected simultaneously for each cell. Experimental results indicated that the microfluidic cytometer exhibited comparable performance with a conventional cytometer and demonstrated superior capacity to detect on-chip optical signals in a highly compact, simple, truly portable, and low-cost format that is well suitable for point-of-care clinical diagnostics.

Plug and measure - a chip-to-world interface for photonic lab-on-a-chip applications

The integration of detection mechanisms with microfluidics may be one of the most promising routes towards widespread application of Lab-on-a-Chip (LoC) devices. Photonic detection methods like in the so-called Photonic Lab-on-a-Chip (PhLoC) have advantages such as being non-invasive, easy to sterilize and highly sensitive even with short integration times and thus allow in situ monitoring and quantification of biological and chemical processes. The readout of such detection methods usually requires special training of potential users, as in most cases they are confronted with the need of establishing fiber-optics connections to and from the PhLoC and/or rely on the use of complex laboratory equipment. Here, we present a low-cost and robust chip-to-world interface (CWI), fabricated by CO2-laser machining, facilitating the non-expert use of PhLoCs. Fiber-optics with standard SMA-connectors (non-pigtailed) and PhLoCs can be plugged into the CWI without the need for further adjustments. This standardization bestows great versatility on the interface, providing a direct link between PhLoCs and a wide range of light sources and photo-detectors. The ease-of-use of the proposed simple plug mechanism represents a step forward in terms of user-friendliness and may lead PhLoC devices to practical applications.

Scattering pulse of label free fine structure cells to determine the size scale of scattering structures

Scattering pulse is sensitive to the morphology and components of each single label-free cell. The most direct detection result, label free cell's scattering pulse is studied in this paper as a novel trait to recognize large malignant cells from small normal cells. A set of intrinsic scattering pulse calculation method is figured out, which combines both hydraulic focusing theory and small particle's scattering principle. Based on the scattering detection angle ranges of widely used flow cytometry, the scattering pulses formed by cell scattering energy in forward scattering angle 2°-5° and side scattering angle 80°-110° are discussed. Combining the analysis of cell's illuminating light energy, the peak, area, and full width at half maximum (FWHM) of label free cells' scattering pulses for fine structure cells with diameter 1-20 μm are studied to extract the interrelations of scattering pulse's features and cell's morphology. The theoretical and experimental results show that cell's diameter and FWHM of its scattering pulse agree with approximate linear distribution; the peak and area of scattering pulse do not always increase with cell's diameter becoming larger, but when cell's diameter is less than about 16 μm the monotone increasing relation of scattering pulse peak or area with cell's diameter can be obtained. This relationship between the features of scattering pulse and cell's size is potentially a useful but very simple criterion to distinguishing malignant and normal cells by their sizes and morphologies in label free cells clinical examinations.

In situ fabrication of a tunable microlens

We demonstrate an optofluidic variable-focus microlens formed by a solid polydimethylsiloxane (PDMS) meniscus channel wall and a tunable liquid lens body. A novel method for in situ fabrication of the meniscus channel wall is developed by introducing liquid PDMS prepolymer into a microchannel followed by curing. Three-light manipulation techniques including tunable optical focusing, collimating, and diverging are realized by varying the refractive index (RI) of liquid lens body. Also, we present an absorption measurement of methylene blue (MB) with a collimated probing light, achieving a detection limit of 0.25 μM by using a 5-mm-long detection cell.

2D light scattering static cytometry for label-free single cell analysis with submicron resolution

Conventional optical cytometric techniques usually measure fluorescence or scattering signals at fixed angles from flowing cells in a liquid stream. Here we develop a novel cytometer that employs a scanning optical fiber to illuminate single static cells on a glass slide, which requires neither microfluidic fabrication nor flow control. This static cytometric technique measures two dimensional (2D) light scattering patterns via a small numerical aperture (0.25) microscope objective for label-free single cell analysis. Good agreement is obtained between the yeast cell experimental and Mie theory simulated patterns. It is demonstrated that the static cytometer with a microscope objective of a low resolution around 1.30 μm has the potential to perform high resolution analysis on yeast cells with distributed sizes. The capability of the static cytometer for size determination with submicron resolution is validated via measurements on standard microspheres with mean diameters of 3.87 and 4.19 μm. Our 2D light scattering static cytometric technique may provide an easy-to-use, label-free, and flow-free method for single cell diagnostics.

Simulation, fabrication, and characterization of a tunable electrowetting-based lens with a wedge-shaped PDMS dielectric layer

Microlenses with tunable focal length have wide applications in optofluidic devices. This work presents a numerical and experimental investigation on a tunable electrowetting-based concave lens. Optical properties such as focal length of the lens and visibility of images were investigated numerically and experimentally. A finite element analysis and a ZEMAX simulation were used for determination of surface profile and focal length of the lens. The results show that the theoretical surface profile and focal length of the lens are in good agreement with the experimental ones. The lens has a wide tuning focal length equal to 6.5 (cm). Because the polydimethylsiloxane (PDMS) layer is wedge shaped (as both the dielectric and hydrophobic layers), lower applied voltage is needed. A commercial program was used to find the focal length of the lens from maximum visibility value by tuning the applied voltage.

Ship-in-a-bottle femtosecond laser integration of optofluidic microlens arrays with center-pass units enabling coupling-free parallel cell counting with a 100% success rate

Optimal design and fabrication of novel devices for high-performance optofluidic applications is a key issue for the development of advanced lab-on-a-chip systems. Parallel cell counting with a high success rate and simple mode of operation is a challenging goal. Current cell-counting methods, using optical waveguides or flow cytometry, typically require a precise coupling of the probe light and involve complex operations. In the present paper, a novel multifunctional cell counting microdevice is designed. It uses a center-pass optofluidic microlens array (MLA) consisting of seven microlenses and an M-shaped confining wall with 9 μm-diameter apertures. The device can be fabricated in a three-dimensional microchannel by ship-in-a-bottle femtosecond laser integration based on two-photon polymerization with optimized experimental parameters. Each microlens produces approximately the same intensity at the focal positions (within ±5%) under white-light illumination, while the confining wall restricts 6∼8 μm-width cells to passing through the edges of two adjacent microlenses because the aperture opens toward their centers. The device demonstrates coupling-free parallel cell counting with a 100% success rate by monitoring the optical intensity variations at each spot. As a result, this method features both easy operation and high performance. Furthermore, the confining wall can filter deformed cells having 15 μm width.

A multiple path photonic lab on a chip for parallel protein concentration measurements

We propose a PDMS-based photonic system for the accurate measurement of protein concentration with minute amounts of the sample. As opposed to the state of the art approach, in the multiple path photonic lab on a chip (MPHIL), analyte concentration or molar absorptivity is obtained with a single injection step, by performing simultaneous parallel optical measurements varying the optical path length. Also, as opposed to the standard calibration protocol, the MPHIL approach does not require a series of measurements at different concentrations. MPHIL has three main advantages: firstly the possibility of dynamically selecting the path length, always working in the absorbance vs. concentration linear range for each target analyte. Secondly, a dramatic reduction of the total volume of the sample required to obtain statistically reliable results. Thirdly, since only one injection is required, the measurement time is minimized, reducing both contamination and signal drifts. These characteristics are clearly advantageous when compared to commercial micro-spectrophotometers. The MPHIL concept was validated by testing three commercial proteins, lysozyme (HEWL), glucose isomerase (d-xylose-ketol-isomerase, GI) and Aspergillus sp. lipase L (BLL), as well as two proteins expressed and purified for this study, B. cereus formamidase (FASE) and dihydropyrimidinase from S. meliloti CECT41 (DHP). The use of MPHIL is also proposed for any spectrophotometric measurement in the UV-VIS range, as well as for its integration as a concentration measurement platform in more advanced photonic lab on a chip systems.

Micro flow cytometer with self-aligned 3D hydrodynamic focusing

A micro flow cytometer with a single step 3D hydrodynamic flow focusing has been developed. The proposed design is capable to create a single-file particle stream that is self-aligned with an integrated optical fiber-based detection system, regardless of the flow rate ratio between the focusing and core liquids. The design approach provides the ability to adjust the stream size while keeping the position of the focused stream centered with respect to the focusing channel. The device has been fabricated by direct micro milling of PMMA sheets. Experimental validation of the hydrodynamic sheath focusing effect has been presented and sample stream with tuneable size from about 18 to 50 μm was measured. Flow cytometry measurements have been performed by using 10-23 μm fluorescent particles. From the analysis of the signals collected at each transit event we can confirm that the device was capable to align and measure microparticles with a good coefficient of variance.

A sheath-less combined optical and impedance micro-cytometer

We describe a sheath-less micro-cytometer that measures four different parameters, namely fluorescence, large angle side scatter and dual frequency electrical impedance (electrical volume and opacity). The cytometer was benchmarked using both size and fluorescent bead standards and demonstrates excellent size accuracy (CVs ≤ 2.1%), sensitivity and dynamic range (3.5 orders of magnitude) at sample flow rates of 80 μL per minute. The cytometer was evaluated by analysing human blood, and a four part differential leukocyte assay for accurate CD4+ T-cell enumeration was demonstrated. The integration of impedance, fluorescence and side scatter into a single miniature cytometer platform provides the core information content of a classical cytometer in a highly compact, simple, portable and low cost format.

Microbial monitoring of crewed habitats in space-current status and future perspectives

Previous space research conducted during short-term flight experiments and long-term environmental monitoring on board orbiting space stations suggests that the relationship between humans and microbes is altered in the crewed habitat in space. Both human physiology and microbial communities adapt to spaceflight. Microbial monitoring is critical to crew safety in long-duration space habitation and the sustained operation of life support systems on space transit vehicles, space stations, and surface habitats. To address this critical need, space agencies including NASA (National Aeronautics and Space Administration), ESA (European Space Agency), and JAXA (Japan Aerospace Exploration Agency) are working together to develop and implement specific measures to monitor, control, and counteract biological contamination in closed-environment systems. In this review, the current status of microbial monitoring conducted in the International Space Station (ISS) as well as the results of recent microbial spaceflight experiments have been summarized and future perspectives are discussed.

Standing surface acoustic wave (SSAW)-based microfluidic cytometer

The development of microfluidic chip-based cytometers has become an important area due to their advantages of compact size and low cost. Herein, we demonstrate a sheathless microfluidic cytometer which integrates a standing surface acoustic wave (SSAW)-based microdevice capable of 3D particle/cell focusing with a laser-induced fluorescence (LIF) detection system. Using SSAW, our microfluidic cytometer was able to continuously focus microparticles/cells at the pressure node inside a microchannel. Flow cytometry was successfully demonstrated using this system with a coefficient of variation (CV) of less than 10% at a throughput of ~1000 events s(-1) when calibration beads were used. We also demonstrated that fluorescently labeled human promyelocytic leukemia cells (HL-60) could be effectively focused and detected with our SSAW-based system. This SSAW-based microfluidic cytometer did not require any sheath flows or complex structures, and it allowed for simple operation over a wide range of sample flow rates. Moreover, with the gentle, bio-compatible nature of low-power surface acoustic waves, this technique is expected to be able to preserve the integrity of cells and other bioparticles.

Sub-micrometer-precision, three-dimensional (3D) hydrodynamic focusing via "microfluidic drifting"

In this article, we demonstrate single-layered, "microfluidic drifting" based three-dimensional (3D) hydrodynamic focusing devices with particle/cell focal positioning approaching submicron precision along both lateral and vertical directions. By systematically optimizing channel geometries and sample/sheath flow rates, a series of "microfluidic drifting" based 3D hydrodynamic focusing devices with different curvature angles are designed and fabricated. Their performances are then evaluated using confocal microscopy, fast camera imaging, and side-view imaging techniques. Using a device with a curvature angle of 180°, we have achieved a standard deviation of ±0.45 μm in particle focal position and a coefficient of variation (CV) of 2.37% in flow cytometric measurements. To the best of our knowledge, this is the best CV that has been achieved using a microfluidic flow cytometry device. Moreover, the device showed the capability to distinguish 8 peaks when subjected to a stringent 8-peak rainbow calibration test, signifying the ability to perform sensitive, accurate tests similar to commercial flow cytometers. We have further tested and validated our device by detection of HEK-293 cells. With its advantages in simple fabrication (i.e., single-layered device), precise 3D hydrodynamic focusing (i.e., submicrometer precision along both lateral and vertical directions), and high detection resolution (i.e., low CV), our method could serve as an important basis for high-performance, mass-producible microfluidic flow cytometry.

Microfluidic cytometer based on dual photodiode detection for cell size and deformability analysis

Cellular mechanical properties play an important role in disease diagnosis. Distinguishing cells based on their mechanical properties provides a potential method for label-free diagnosis. In this work, a convenient and low-cost microfluidic cytometer was developed to study cell mechanical properties and cell size based on the change of transmission intensity, using a low-cost commercial laser as a light source and two photodiodes as detectors. The cells pass through a narrow microchannel with a width smaller than the cell dimension, integrated in a polydimethylsiloxane chip, below which the laser is focused. The transit time of individual cells is measured by the time difference detected by two photodiodes. This device was used to study the difference in cell mechanical properties between HL60 cells treated with and without Cytochalasin D. Furthermore, it was also applied to distinguish cells with different diameters, HL60 cells and red blood cells, by measuring the transmission intensity.

High-throughput on-chip leukemia diagnosis

Advances in lab-on-a-chip technologies enabled programmable, reconfigurable, and scalable manipulation of a variety of laboratory procedures. Samples, reagents, and fluids can be precisely controlled; buffer temperature, pH, and concentration control systems as well as a variety of detection systems can be integrated on a small chip. These advantages have attracted attention in various fields of clinical application including leukemia diagnosis and research. A lot of research on lab-on-a-chip based diagnosis has been reported and the field is rapidly expanding. This review describes recent developments of lab-on-a-chip technologies as solutions to challenges for high-throughput leukemia diagnosis.

Planar lens integrated capillary action microfluidic immunoassay device for the optical detection of troponin I

Optical based analysis in microfluidic and lab-on-a-chip systems are currently considered the gold standard methodology for the determination of end point reactions for various chemical and biological reaction processes. Typically, assays are performed using bulky ancillary apparatus such as microscopes and complex optical excitation and detection systems. Such instrumentation negates many of the advantages offered by device miniaturisation, particularly with respect to overall portability. In this article, we present a CO2 laser ablation technique for rapidly prototyping on-chip planar lenses, in conjunction with capillary action based autonomous microfluidics, to create a miniaturised and fully integrated optical biosensing platform. The presented self-aligned on-chip optical components offer an efficient means to direct excitation light within microfluidics and to directly couple light from a LED source. The device has been used in conjunction with a miniaturised and bespoke fluorescence detection platform to create a complete, palm sized system (≈60 × 80 × 60 mm) capable of performing fluoro-immunoassays. The system has been applied to the detection of cardiac Troponin I, one of the gold standard biomarkers for the diagnosis of acute myocardial infarction, achieving a lower detection limit of 0.08 ng/ml, which is at the threshold of clinically applicable concentrations. The portable nature of the complete system and the biomarker detection capabilities demonstrate the potential of the devised instrumentation for use as a medical diagnostics device at the point of care.

Integration of optical components on-chip for scattering and fluorescence detection in an optofluidic device

An optofluidic device is demonstrated with photonic components integrated onto the chip for use in fluorescence and scatter detection and counting applications. The device is fabricated by integrating the optical and fluidic components in a single functional layer. Optical excitation on-chip is accomplished via a waveguide integrated with a system of lenses that reforms the geometry of the beam in the microfluidic channel into a specific shape that is more suitable for reliable detection. Separate counting tests by detecting fluorescence and scattered signals from 2.5 and 6.0 μm beads were performed and found to show detection reliability comparable to that of conventional means of excitation and an improvement over other microchip-based designs.

Microfluidic flow cytometer for quantifying photobleaching of fluorescent proteins in cells

Traditional flow cytometers are capable of rapid cellular assays on the basis of fluorescence intensity and light scatter. Microfluidic flow cytometers have largely followed the same path of technological development as their traditional counterparts; however, the significantly smaller transport distance and resulting lower cell speeds in microchannels provides for the opportunity to detect novel spectroscopic signatures based on multiple, nontemporally coincident excitation beams. Here, we characterize the design and operation of a cytometer with a three-beam, probe/bleach/probe geometry, employing HeLa suspension cells expressing fluorescent proteins. The data collection rate exceeds 20 cells/s under a range of beam intensities (5 kW to 179 kW/cm(2)). The measured percent photobleaching (ratio of fluorescence intensities excited by the first and third beams: S(beam3)/S(beam1)) partially resolves a mixture of four red fluorescent proteins in mixed samples. Photokinetic simulations are presented and demonstrate that the percent photobleaching reflects a combination of the reversible and irreversible photobleaching kinetics. By introducing a photobleaching optical signature, which complements traditional fluorescence intensity-based detection, this method adds another dimension to multichannel fluorescence cytometry and provides a means for flow-cytometry-based screening of directed libraries of fluorescent protein photobleaching.

Kinoform microlenses for focusing into microfluidic channels

Optical detection in microflow cytometry requires a tightly focused light beam within a microfluidic channel for effective microparticle analysis. Integrated planar lenses have demonstrated this function, but their design is usually derived from the conventional spherical lens. Compact, efficient, integrated planar kinoform microlenses are proposed for use in microflow cytometry. A detailed design procedure is given and several designs are simulated. A paraxial kinoform lens integrated with a microfluidic channel was then fabricated in a silicate glass material system and characterized for focal position and spotsize, in comparison with light emerging directly from a channel waveguide. Focal spotsizes of 5.6 μm for kinoform lenses have been measured at foci as far as 56 μm into the microfluidic channel.

Simultaneous high speed optical and impedance analysis of single particles with a microfluidic cytometer

We describe a microfluidic cytometer that performs simultaneous optical and electrical characterisation of particles. The microfluidic chip measures side scattered light, signal extinction and fluorescence using integrated optical fibres coupled to photomultiplier tubes. The channel is 80 μm high and 200 μm wide, and made from SU-8 patterned and sandwiched between glass substrates. Particles were focused into the analysis region using 1-D hydrodynamic focusing and typical particle velocities were 0.1 ms(-1). Excitation light is coupled into the detection channel with an optical fibre and focused into the channel using an integrated compound air lens. The electrical impedance of particles is measured at 1 MHz using micro-electrodes fabricated on the channel top and bottom. This data is used to accurately size the particles. The system is characterised using a range of different sized polystyrene beads (fluorescent and non-fluorescent). Single and mixed populations of beads were measured and the data compared with a conventional flow cytometer.

Flow sensing of single cell by graphene transistor in a microfluidic channel

The electronic properties of graphene are strongly influenced by electrostatic forces arising from long-range charge scatterers and by changes in the local dielectric environment. This makes graphene extremely sensitive to the surface charge density of cells interfacing with it. Here, we developed a graphene transistor array integrated with microfluidic flow cytometry for the "flow-catch-release" sensing of malaria-infected red blood cells at the single-cell level. Malaria-infected red blood cells induce highly sensitive capacitively coupled changes in the conductivity of graphene. Together with the characteristic conductance dwell times, specific microscopic information about the disease state can be obtained.

Sealing SU-8 microfluidic channels using PDMS

A simple method of irreversibly sealing SU-8 microfluidic channels using PDMS is reported in this paper. The method is based on inducing a chemical reaction between PDMS and SU-8 by first generating amino groups on PDMS surface using N(2) plasma treatment, then allowing the amino groups to react with the residual epoxy groups on SU-8 surface at an elevated temperature. The N(2) plasma treatment of PDMS can be conducted using an ordinary plasma chamber and high purity N(2), while the residual epoxy groups on SU-8 surface can be preserved by post-exposure baking SU-8 at a temperature no higher than 95 °C. The resultant chemical bonding between PDMS and SU-8 using the method create an interface that can withstand a stress that is greater than the bulk strength of PDMS. The bond is permanent and is long-term resistant to water. The method was applied in fabricating SU-8 microfluidi-photonic integrated devices, and the obtained devices were tested to show desirable performance.

Applying an optical space-time coding method to enhance light scattering signals in microfluidic devices

An "optical space-time coding method" was applied to microfluidic devices to detect the forward and large angle light scattering signals for unlabelled bead and cell detection. Because of the enhanced sensitivity by this method, silicon pin photoreceivers can be used to detect both forward scattering (FS) and large angle (45-60°) scattering (LAS) signals, the latter of which has been traditionally detected by a photomultiplier tube. This method yields significant improvements in coefficients of variation (CV), producing CVs of 3.95% to 10.05% for FS and 7.97% to 26.12% for LAS with 15 μm, 10 μm, and 5 μm beads. These are among the best values ever demonstrated with microfluidic devices. The optical space-time coding method also enables us to measure the speed and position of each particle, producing valuable information for the design and assessment of microfluidic lab-on-a-chip devices such as flow cytometers and complete blood count devices.

Monitoring CD4 in whole blood with an opto-fluidic detector based on spatially modulated fluorescence emission

We describe a new optical detection technique, termed "spatially modulated fluorescence emission," that delivers high signal-to-noise discrimination without precision optics to enable a flow cytometer that can combine high performance, robustness, compactness, low cost, and ease of use. The detection technique is demonstrated with measurements of absolute CD4+ and percentage CD4 counts in human blood. Benchmarking against a commonly used commercial instrument for this test yields excellent agreement for both absolute CD4 and percentage CD4.

Label-free and noninvasive optical detection of the distribution of nanometer-size mitochondria in single cells

A microfluidic flow cytometric technique capable of obtaining information on nanometer-sized organelles in single cells in a label-free, noninvasive optical manner was developed. Experimental two-dimensional (2D) light scattering patterns from malignant lymphoid cells (Jurkat cell line) and normal hematopoietic stem cells (cord blood CD34+ cells) were compared with those obtained from finite-difference time-domain simulations. In the simulations, we assumed that the mitochondria were randomly distributed throughout a Jurkat cell, and aggregated in a CD34+ cell. Comparison of the experimental and simulated light scattering patterns led us to conclude that distinction from these two types of cells may be due to different mitochondrial distributions. This observation was confirmed by conventional confocal fluorescence microscopy. A method for potential cell discrimination was developed based on analysis of the 2D light scattering patterns. Potential clinical applications using mitochondria as intrinsic biological markers in single cells were discussed in terms of normal cells (CD34+ cell and lymphocytes) versus malignant cells (THP-1 and Jurkat cell lines).

A microflow cytometer exploited for the immunological differentiation of leukocytes

In this article, we demonstrate the potential of a microfluidic chip for the differentiation of immunologically stained blood cells. To this end, white blood cells stained with antibodies typically applied for the determination of the immune status were measured in the micro-device. Relative concentrations of lymphocytes and subpopulations of lymphocytes are compared to those obtained with a conventional flow cytometer. The stability of the hydrodynamic focusing and the optical setup was determined by measuring the variation of the signal pulse height of fluorescence calibration beads, being about 2% for the micro-device. This value and the overall performance of the micro-device are similar to conventional flow cytometers. It follows from our results that such microfluidic structures are well suited as modules in a compact, portable read-out instrument. The production process of the microflow cytometers, which we exploited for immunological cell differentiation, is compatible with mass production technology like injection molding and, hence, low cost disposable chips could be available in the future.

Hydrodynamic optical alignment for microflow cytometry

A microfabricated flow cytometer has been developed that is capable of detecting nearly all of the microparticles in an aqueous suspension. Current design allows for integrated coupling between an optical fiber-based detection system and the particle stream via hydrodynamic focusing. By adjusting the relative flow-rates at the auxiliary inputs of the focusing manifold, the particle stream can be steered out-of-plane relative to the illuminating laser, and similarly the particle stream can be squeezed or expanded. The microfabricated device was constructed in polydimethylsiloxane with cross-sectional microfluidic dimensions of 125 µm×125 µm. Using the present device and method, fluorescent microparticles in aqueous solution were counted at an absolute counting efficiency of 91±4%. The coefficient of variation of the fluorescence pulse-heights for far-red fluorescent microparticles was 15%. The device exhibited a linear response to fluorescence intensity calibration microparticles as shown by comparison with a commercial cytometer instrument.

Disposable flow cytometer with high efficiency in particle counting and sizing using an optofluidic lens

Flow cytometers are widely applied to environmental monitoring, industrial testing, and biochemical studies. Integrating a flow cytometer into microfluidic networks helps to miniaturize the system and make it portable for field use. The integration of optical components, such as lenses, further improves the compactness and thus has been intensively studied recently. However, the current designs suffer from severe light scattering due to the roughness of the solid-based lens interface. In this Letter, we propose a flow cytometer using an optofluidic lens to focus the light beam. Benefiting from the smooth liquid-liquid lens interface and the refractive-index matching liquid as cladding streams, a light beam can be well focused without scattering. The variations of the signal peak values are reduced, owing to the small beam width at the beam waist. The device presents an efficient and accurate performance on both the counting and sizing of particles.

Microscope-based label-free microfluidic cytometry

A microscope-based label-free microfluidic cytometer capable of acquiring two dimensional light scatter patterns from single cells, pattern analysis of which determines cellular information such as cell size, orientation and inner nanostructure, was developed. Finite-difference time-domain numerical simulations compared favorably with experimental scatter patterns from micrometer-sized beads and cells. The device was capable of obtaining light scattering patterns from the smallest mature blood cells (platelets) and cord blood hematopoietic stem/progenitor cells 
(CD34 + cells) and myeloid precursor cells. The potential for evaluation of cells using this label-free microfluidic cytometric technique was discussed.

Review Article: Recent advancements in optofluidic flow cytometer

There is an increasing need to develop optofluidic flow cytometers. Optofluidics, where optics and microfluidics work together to create novel functionalities on a small chip, holds great promise for lab-on-a-chip flow cytometry. The development of a low-cost, compact, handheld flow cytometer and microfluorescence-activated cell sorter system could have a significant impact on the field of point-of-care diagnostics, improving health care in, for example, underserved areas of Africa and Asia, that struggle with epidemics such as HIV∕AIDS. In this paper, we review recent advancements in microfluidics, on-chip optics, novel detection architectures, and integrated sorting mechanisms.

Characterization of a microflow cytometer with an integrated three-dimensional optofluidic lens system

Flow cytometry is a standard analytical method in cell biology and clinical diagnostics and is widely distributed for the experimental investigation of microparticle characteristics. In this work, the design, realization, and measurement results of a novel planar optofluidic flow cytometric device with an integrated three-dimensional (3D) adjustable optofluidic lens system for forward-scattering∕extinction-based biochemical analysis fabricated by silicon micromachining are presented. To our knowledge, this is the first planar cytometric system with the ability to focus light three-dimensionally on cells∕particles by the application of fluidic lenses. The single layer microfluidic platform enables versatile 3D hydrodynamic sample focusing to an arbitrary position in the channel and incorporates integrated fiber grooves for the insertion of glass fibers. To confirm the fluid dynamics and raytracing simulations and to characterize the sensor, different cell lines and sets of microparticles were investigated by detecting the extinction (axial light loss) signal, demonstrating the high sensitivity and sample discrimination capability of this analysis system. The unique features of this planar microdevice enable new biotechnological analysis techniques due to the highly increased sensitivity.

Rapid, semiautomated quantification of bacterial cells in freshwater by using a microfluidic device for on-chip staining and counting

A microfluidic device-based system for the rapid and semiautomated counting of bacteria in freshwater was fabricated and examined. Bacteria in groundwater and in potable water, as well as starved Escherichia coli O157:H7 spiked in pond water, were able to be on-chip stained and enumerated within 1 h using this system.