Discrimination and analysis of phytoplankton using a microfluidic cytometer

Towards Small Scale: Overview and Applications of Microfluidics in Biotechnology

Thanks to recent and continuing technological innovations, modern microfluidic systems are increasingly offering researchers working across all fields of biotechnology exciting new possibilities (especially with respect to facilitating high throughput analysis, portability, and parallelization). The advantages offered by microfluidic devices-namely, the substantially lowered chemical and sample consumption they require, the increased energy and mass transfer they offer, and their comparatively small size-can potentially be leveraged in every sub-field of biotechnology. However, to date, most of the reported devices have been deployed in furtherance of healthcare, pharmaceutical, and/or industrial applications. In this review, we consider examples of microfluidic and miniaturized systems across biotechnology sub-fields. In this context, we point out the advantages of microfluidics for various applications and highlight the common features of devices and the potential for transferability to other application areas. This will provide incentives for increased collaboration between researchers from different disciplines in the field of biotechnology.

Response of Coccomyxa cimbrica sp.nov. to Increasing Doses of Cu(II) as a Function of Time: Comparison between Exposure in a Microfluidic Device or with Standard Protocols

In this study, we explore how the in vitro conditions chosen to cultivate and observe the long-term (up to 72 h) toxic effect of Cu(II) on the freshwater microalga Coccomyxa cimbrica sp.nov. can affect the dose response in time. We test three different cultivation protocols: (i) under static conditions in sealed glass cells, (ii) in a microfluidic device, where the sample is constantly circulated with a peristaltic pump, and (iii) under continuous agitation in plastic falcons on an orbital shaker. The advantage and novelty of this study resides in the fact that each condition can mimic different environmental conditions that alga cells can find in nature. The effect of increasing dose of Cu(II) as a function of time (24, 48, and 72 h) is monitored following chlorophyll a fluorescence intensity from single cells. Fluorescence lifetime imaging experiments are also explored to gain information on the changes induced by Cu(II) in the photosynthetic cycle of this microalga.

Photonic Microfluidic Technologies for Phytoplankton Research

Phytoplankton is a crucial component for the correct functioning of different ecosystems, climate regulation and carbon reduction. Being at least a quarter of the biomass of the world's vegetation, they produce approximately 50% of atmospheric O2 and remove nearly a third of the anthropogenic carbon released into the atmosphere through photosynthesis. In addition, they support directly or indirectly all the animals of the ocean and freshwater ecosystems, being the base of the food web. The importance of their measurement and identification has increased in the last years, becoming an essential consideration for marine management. The gold standard process used to identify and quantify phytoplankton is manual sample collection and microscopy-based identification, which is a tedious and time-consuming task and requires highly trained professionals. Microfluidic Lab-on-a-Chip technology represents a potential technical solution for environmental monitoring, for example, in situ quantifying toxic phytoplankton. Its main advantages are miniaturisation, portability, reduced reagent/sample consumption and cost reduction. In particular, photonic microfluidic chips that rely on optical sensing have emerged as powerful tools that can be used to identify and analyse phytoplankton with high specificity, sensitivity and throughput. In this review, we focus on recent advances in photonic microfluidic technologies for phytoplankton research. Different optical properties of phytoplankton, fabrication and sensing technologies will be reviewed. To conclude, current challenges and possible future directions will be discussed.

Microfluidic Microalgae System: A Review

Microalgae that have recently captivated interest worldwide are a great source of renewable, sustainable and economical biofuels. The extensive potential application in the renewable energy, biopharmaceutical and nutraceutical industries have made them necessary resources for green energy. Microalgae can substitute liquid fossil fuels based on cost, renewability and environmental concern. Microfluidic-based systems outperform their competitors by executing many functions, such as sorting and analysing small volumes of samples (nanolitre to picolitre) with better sensitivities. In this review, we consider the developing uses of microfluidic technology on microalgal processes such as cell sorting, cultivation, harvesting and applications in biofuels and biosensing.

Microfluidic Platforms Designed for Morphological and Photosynthetic Investigations of Chlamydomonas reinhardtii on a Single-Cell Level

Chlamydomonas reinhardtii is a model organism of increasing biotechnological importance, yet, the evaluation of its life cycle processes and photosynthesis on a single-cell level is largely unresolved. To facilitate the study of the relationship between morphology and photochemistry, we established microfluidics in combination with chlorophyll a fluorescence induction measurements. We developed two types of microfluidic platforms for single-cell investigations: (i) The traps of the “Tulip” device are suitable for capturing and immobilizing single cells, enabling the assessment of their photosynthesis for several hours without binding to a solid support surface. Using this “Tulip” platform, we performed high-quality non-photochemical quenching measurements and confirmed our earlier results on bulk cultures that non-photochemical quenching is higher in ascorbate-deficient mutants (Crvtc2-1) than in the wild-type. (ii) The traps of the “Pot” device were designed for capturing single cells and allowing the growth of the daughter cells within the traps. Using our most performant “Pot” device, we could demonstrate that the FV/FM parameter, an indicator of photosynthetic efficiency, varies considerably during the cell cycle. Our microfluidic devices, therefore, represent versatile platforms for the simultaneous morphological and photosynthetic investigations of C. reinhardtii on a single-cell level.

The Fusion of Microfluidics and Optics for On-Chip Detection and Characterization of Microalgae

It has been demonstrated that microalgae play an important role in the food, agriculture and medicine industries. Additionally, the identification and counting of the microalgae are also a critical step in evaluating water quality, and some lipid-rich microalgae species even have the potential to be an alternative to fossil fuels. However, current technologies for the detection and analysis of microalgae are costly, labor-intensive, time-consuming and throughput limited. In the past few years, microfluidic chips integrating optical components have emerged as powerful tools that can be used for the analysis of microalgae with high specificity, sensitivity and throughput. In this paper, we review recent optofluidic lab-on-chip systems and techniques used for microalgal detection and characterization. We introduce three optofluidic technologies that are based on fluorescence, Raman spectroscopy and imaging-based flow cytometry, each of which can achieve the determination of cell viability, lipid content, metabolic heterogeneity and counting. We analyze and summarize the merits and drawbacks of these micro-systems and conclude the direction of the future development of the optofluidic platforms applied in microalgal research.

How does the Internet of Things (IoT) help in microalgae biorefinery?

Microalgae biorefinery is a platform for the conversion of microalgal biomass into a variety of value-added products, such as biofuels, bio-based chemicals, biomaterials, and bioactive substances. Commercialization and industrialization of microalgae biorefinery heavily rely on the capability and efficiency of large-scale cultivation of microalgae. Thus, there is an urgent need for novel technologies that can be used to monitor, automatically control, and precisely predict microalgae production. In light of this, innovative applications of the Internet of things (IoT) technologies in microalgae biorefinery have attracted tremendous research efforts. IoT has potential applications in a microalgae biorefinery for the automatic control of microalgae cultivation, monitoring and manipulation of microalgal cultivation parameters, optimization of microalgae productivity, identification of toxic algae species, screening of target microalgae species, classification of microalgae species, and viability detection of microalgal cells. In this critical review, cutting-edge IoT technologies that could be adopted to microalgae biorefinery in the upstream and downstream processing are described comprehensively. The current advances of the integration of IoT with microalgae biorefinery are presented. What this review discussed includes automation, sensors, lab-on-chip, and machine learning, which are the main constituent elements and advanced technologies of IoT. Specifically, future research directions are discussed with special emphasis on the development of sensors, the application of microfluidic technology, robotized microalgae, high-throughput platforms, deep learning, and other innovative techniques. This review could contribute greatly to the novelty and relevance in the field of IoT-based microalgae biorefinery to develop smarter, safer, cleaner, greener, and economically efficient techniques for exhaustive energy recovery during the biorefinery process.

Microfluidics for microalgal biotechnology

Microalgae have expanded their roles as renewable and sustainable feedstocks for biofuel, smart nutrition, biopharmaceutical, cosmeceutical, biosensing, and space technologies. They accumulate valuable biochemical compounds from protein, carbohydrate, and lipid groups, including pigments and carotenoids. Microalgal biomass, which can be adopted for multivalorization under biorefinery settings, allows not only the production of various biofuels but also other value-added biotechnological products. However, state-of-the-art technologies are required to optimize yield, quality, and the economical aspects of both upstream and downstream processes. As such, the need to use microfluidic-based devices for both fundamental research and industrial applications of microalgae, arises due to their microscale sizes and dilute cultures. Microfluidics-based devices are superior to their competitors through their ability to perform multiple functions such as sorting and analyzing small amounts of samples (nanoliter to picoliter) with higher sensitivities. Here, we review emerging applications of microfluidic technologies on microalgal processes in cell sorting, cultivation, harvesting, and applications in biofuels, biosensing, drug delivery, and nutrition.

Flow-Through Quantification of Microplastics Using Impedance Spectroscopy

Understanding the sources, impacts, and fate of microplastics in the environment is critical for assessing the potential risks of these anthropogenic particles. However, our ability to quantify and identify microplastics in aquatic ecosystems is limited by the lack of rapid techniques that do not require visual sorting or preprocessing. Here, we demonstrate the use of impedance spectroscopy for high-throughput flow-through microplastic quantification, with the goal of rapid measurement of microplastic concentration and size. Impedance spectroscopy characterizes the electrical properties of individual particles directly in the flow of water, allowing for simultaneous sizing and material identification. To demonstrate the technique, spike and recovery experiments were conducted in tap water with 212-1000 μm polyethylene beads in six size ranges and a variety of similarly sized biological materials. Microplastics were reliably detected, sized, and differentiated from biological materials via their electrical properties at an average flow rate of 103 ± 8 mL/min. The recovery rate was ≥90% for microplastics in the 300-1000 μm size range, and the false positive rate for the misidentification of the biological material as plastic was 1%. Impedance spectroscopy allowed for the identification of microplastics directly in water without visual sorting or filtration, demonstrating its use for flow-through sensing.

Biological Living Cell in-Flow Detection Based on Microfluidic Chip and Compact Signal Processing Circuit

Detection and counting of biological living cells in continuous fluidic flows play an essential role in many applications for early diagnosis and treatment of diseases. In this regard, this study highlighted the proposal of a biochip system for detecting and enumerating human lung carcinoma cell flow in the microfluidic channel. The principle of detection was based on the change of impedance between sensing electrodes integrated in the fluidic channel, due to the presence of a biological cell in the sensing region. A compact electronic module was built to sense the unbalanced impedance between the sensing microelectrodes. It consisted of an instrumentation amplifier stage to obtain the difference between the acquired signals, and a lock-in amplifier stage to demodulate the signals at the stimulating frequency as well as to reject noise at other frequencies. The performance of the proposed system was validated through experiments of A549 cells detection as they passed over the microfluidic channel. The experimental results indicated the occurrence of large spikes (up to approximately 180 mV) over the background signal according to the passage of a single A549 cell in the continuous flow. The proposed device is simple-to-operate, inexpensive, portable, and exhibits high sensitivity, which are suitable considerations for developing point-of-care applications.

Coccolithophore calcification studied by single-cell impedance cytometry: Towards single-cell PIC:POC measurements

Since the industrial revolution 30% of the anthropogenic CO2 is absorbed by oceans, resulting in ocean acidification, which is a threat to calcifying algae. As a result, there has been profound interest in the study of calcifying algae, because of their important role in the global carbon cycle. The coccolithophore Emiliania huxleyi is considered to be globally the most dominant calcifying algal species, which creates a unique exoskeleton from inorganic calcium carbonate platelets. The PIC (particulate inorganic carbon): POC (particulate organic carbon) ratio describes the relative amount of inorganic carbon in the algae and is a critical parameter in the ocean carbon cycle. In this research we explore the use of microfluidic single-cell impedance spectroscopy in the field of calcifying algae. Microfluidic impedance spectroscopy enables us to characterize single-cell electrical properties in a non-invasive and label-free way. We use the ratio of the impedance at high frequency vs. low frequency, known as opacity, to discriminate between calcified coccolithophores and coccolithophores with a calcite exoskeleton dissolved by acidification (decalcified). We have demonstrated that using opacity we can discriminate between calcified and decalcified coccolithophores with an accuracy of 94.1%. We have observed a correlation between the measured opacity and the cell height in the channel, which is supported by FEM simulations. The difference in cell density between calcified and decalcified cells can explain the difference in cell height and therefore the measured opacity.

Label-free counting of affinity-enriched circulating tumor cells (CTCs) using a thermoplastic micro-Coulter counter (μCC)

Coulter counters are used for counting particles and biological cells. Most Coulter counters are designed to analyze a sample without the ability to pre-process the sample prior to counting. For the analysis of rare cells, such as circulating tumor cells (CTCs), it is not uncommon to require enrichment before counting due to the modest throughput of μCCs and the high abundance of interfering cells, such as blood cells. We report a microfluidic-based Coulter Counter (μCC) fabricated using simple, low-cost techniques for counting rare cells that can be interfaced to sample pre- and/or post-processing units. In the current work, a microfluidic device for the affinity-based enrichment of CTCs from whole blood into a relatively small volume of ∼10 μL was interfaced to the μCC to allow for exhaustive counting of single CTCs following release of the CTCs from the enrichment chip. When integrated to the CTC affinity enrichment chip, the μCC could count the CTCs without loss and the cells could be collected for downstream molecular profiling or culturing if required. The μCC sensor counting efficiency was >93% and inter-chip variability was ∼1%.

Mechanical stress induced astaxanthin accumulation of H. pluvialis on a chip

Microalgae have been envisioned as a source of food, feed, health nutraceuticals, and cosmetics. Among various microalgae, Haematococcus pluvialis (H. pluvialis) is known to be the richest feedstock of natural astaxanthin. Astaxanthin is a highly effective antioxidation material and is being widely used in aquaculture, nutraceuticals, pharmacology, and feed industries. Here, we present a microfluidic chip consisting of a micropillar array and six sets of culture chambers, which enables sorting of motile flagellated vegetative stage H. pluvialis (15-20 μm) from cyst stage H. pluvialis as well as culture of the selected cells under a mechanically stressed microenvironment. The micropillar array successfully sorted only the motile early vegetative stage cells (avg. size = 19.8 ± 1.6 μm), where these sorted cells were uniformly loaded inside each culture chamber (229 ± 39 cells per chamber). The mechanical stress level applied to the cells was controlled by designing the culture chambers with different heights (5-70 μm). Raman analysis results revealed that the mechanical stress indeed induced the accumulation of astaxanthin in H. pluvialis. Also, the most effective chamber height enhancing the astaxanthin accumulation (i.e., 15 μm) was successfully screened using the developed chip. Approximately 9 times more astaxanthin accumulation was detected after 7 days of culture compared to the no mechanical stress condition. The results clearly demonstrate the capability of the developed chip to investigate bioactive metabolite accumulation of microalgae induced by mechanical stress, where the amount was quantitatively analyzed in a label-free manner. We believe that the developed chip has great potential for studying the effects of mechanical stress on not only H. pluvialis but also various microalgal species in general.

A microfluidic photobioreactor for simultaneous observation and cultivation of single microalgal cells or cell aggregates

Microalgae are an ubiquitous and powerful driver of geochemical cycles which have formed Earth’s biosphere since early in the evolution. Lately, microalgal research has been strongly stimulated by economic potential expected in biofuels, wastewater treatment, and high-value products. Similar to bacteria and other microorganisms, most work so far has been performed on the level of suspensions which typically contain millions of algal cells per millilitre. The thus obtained macroscopic parameters average cells, which may be in various phases of their cell cycle or even, in the case of microbial consortia, cells of different species. This averaging may obscure essential features which may be needed for the correct understanding and interpretation of investigated processes. In contrast to these conventional macroscopic cultivation and measuring tools, microfluidic single-cell cultivation systems represent an excellent alternative to study individual cells or a small number of mutually interacting cells in a well-defined environment. A novel microfluidic photobioreactor was developed and successfully tested by the photoautotrophic cultivation of Chlorella sorokiniana. The reported microbioreactor facilitates automated long-term cultivation of algae with controlled temperature and with an illumination adjustable over a wide range of photon flux densities. Chemical composition of the medium in the microbioreactor can be stabilised or modulated rapidly to study the response of individual cells. Furthermore, the algae are cultivated in one focal plane and separate chambers, enabling single-cell level investigation of over 100 microcolonies in parallel. The developed platform can be used for systematic growth studies, medium screening, species interaction studies, and the thorough investigation of light-dependent growth kinetics.

Microfluidic techniques for enhancing biofuel and biorefinery industry based on microalgae

This review presents a critical assessment of emerging microfluidic technologies for the application on biological productions of biofuels and other chemicals from microalgae. Comparisons of cell culture designs for the screening of microalgae strains and growth conditions are provided with three categories: mechanical traps, droplets, or microchambers. Emerging technologies for the in situ characterization of microalgae features and metabolites are also presented and evaluated. Biomass and secondary metabolite productivities obtained at microscale are compared with the values obtained at bulk scale to assess the feasibility of optimizing large-scale operations using microfluidic platforms. The recent studies in microsystems for microalgae pretreatment, fractionation and extraction of metabolites are also reviewed. Finally, comments toward future developments (high-pressure/-temperature process; solvent-resistant devices; omics analysis, including genome/epigenome, proteome, and metabolome; biofilm reactors) of microfluidic techniques for microalgae applications are provided.

Microfluidic technology for plankton research

Plankton produces numerous chemical compounds used in cosmetics and functional foods. They also play a key role in the carbon budget on the Earth. In a context of global change, it becomes important to understand the physiological response of these microorganisms to changing environmental conditions. Their adaptations and the response to specific environmental conditions are often restricted to a few active cells or individuals in large populations. Using analytical capabilities at the subnanoliter scale, microfluidic technology has also demonstrated a high potential in biological assays. Here, we review recent advances in microfluidic technologies to overcome the current challenges in high content analysis both at population and the single cell level.

A Simple and Robust Event-Detection Algorithm for Single-Cell Impedance Cytometry

Microfluidic impedance cytometry is emerging as a powerful label-free technique for the characterization of single biological cells. In order to increase the sensitivity and the specificity of the technique, suited digital signal processing methods are required to extract meaningful information from measured impedance data. In this study, a simple and robust event-detection algorithm for impedance cytometry is presented. Since a differential measuring scheme is generally adopted, the signal recorded when a cell passes through the sensing region of the device exhibits a typical odd-symmetric pattern. This feature is exploited twice by the proposed algorithm: first, a preliminary segmentation, based on the correlation of the data stream with the simplest odd-symmetric template, is performed; then, the quality of detected events is established by evaluating their E2O index, that is, a measure of the ratio between their even and odd parts. A thorough performance analysis is reported, showing the robustness of the algorithm with respect to parameter choice and noise level. In terms of sensitivity and positive predictive value, an overall performance of 94.9% and 98.5%, respectively, was achieved on two datasets relevant to microfluidic chips with very different characteristics, considering three noise levels. The present algorithm can foster the role of impedance cytometry in single-cell analysis, which is the new frontier in "Omics."

Single Cell Electrical Characterization Techniques

Electrical properties of living cells have been proven to play significant roles in understanding of various biological activities including disease progression both at the cellular and molecular levels. Since two decades ago, many researchers have developed tools to analyze the cell’s electrical states especially in single cell analysis (SCA). In depth analysis and more fully described activities of cell differentiation and cancer can only be accomplished with single cell analysis. This growing interest was supported by the emergence of various microfluidic techniques to fulfill high precisions screening, reduced equipment cost and low analysis time for characterization of the single cell’s electrical properties, as compared to classical bulky technique. This paper presents a historical review of single cell electrical properties analysis development from classical techniques to recent advances in microfluidic techniques. Technical details of the different microfluidic techniques are highlighted, and the advantages and limitations of various microfluidic devices are discussed.

Microfluidic impedance flow cytometry enabling high-throughput single-cell electrical property characterization

This article reviews recent developments in microfluidic impedance flow cytometry for high-throughput electrical property characterization of single cells. Four major perspectives of microfluidic impedance flow cytometry for single-cell characterization are included in this review: (1) early developments of microfluidic impedance flow cytometry for single-cell electrical property characterization; (2) microfluidic impedance flow cytometry with enhanced sensitivity; (3) microfluidic impedance and optical flow cytometry for single-cell analysis and (4) integrated point of care system based on microfluidic impedance flow cytometry. We examine the advantages and limitations of each technique and discuss future research opportunities from the perspectives of both technical innovation and clinical applications.

Microfluidic CD4+ and CD8+ T lymphocyte counters for point-of-care HIV diagnostics using whole blood

Roughly 33 million people worldwide are infected with HIV; disease burden is highest in resource-limited settings. One important diagnostic in HIV disease management is the absolute count of lymphocytes expressing the CD4(+) and CD8(+) receptors. The current diagnostic instruments and procedures require expensive equipment and trained technicians. In response, we have developed microfluidic biochips that count CD4(+) and CD8(+) lymphocytes in whole blood samples, without the need for off-chip sample preparation. The device is based on differential electrical counting and relies on five on-chip modules that, in sequence, chemically lyses erythrocytes, quenches lysis to preserve leukocytes, enumerates cells electrically, depletes the target cells (CD4 or CD8) with antibodies, and enumerates the remaining cells electrically. We demonstrate application of this chip using blood from healthy and HIV-infected subjects. Erythrocyte lysis and quenching durations were optimized to create pure leukocyte populations in less than 1 min. Target cell depletion was accomplished through shear stress-based immunocapture, using antibody-coated microposts to increase the contact surface area and enhance depletion efficiency. With the differential electrical counting method, device-based CD4(+) and CD8(+) T cell counts closely matched control counts obtained from flow cytometry, over a dynamic range of 40 to 1000 cells/μl. By providing accurate cell counts in less than 20 min, from samples obtained from one drop of whole blood, this approach has the potential to be realized as a handheld, battery-powered instrument that would deliver simple HIV diagnostics to patients anywhere in the world, regardless of geography or socioeconomic status.

Assessment of effects of multi drug resistance on dielectric properties of K562 leukemic cells using electrorotation

In this study, dielectric characterization of multidrug resistant (MDR) K562 human leukemia cells was carried out using a MEMS based electrorotation (ER) device with 3D electrodes.

Reconfigurable acquisition system with integrated optics for a portable flow cytometer

Portable and inexpensive scientific instruments that are capable of performing point of care diagnostics are needed for applications such as disease detection and diagnosis in resource-poor settings, for water quality and food supply monitoring, and for biosurveillance activities in autonomous vehicles. In this paper, we describe the development of a compact flow cytometer built from three separate, customizable, and interchangeable modules. The instrument as configured in this work is being developed specifically for the detection of selected Centers for Disease Control (CDC) category B biothreat agents through a bead-based assay: E. coli O157:H7, Salmonella, Listeria, and Shigella. It has two-color excitation, three-color fluorescence and light scattering detection, embedded electronics, and capillary based flow. However, these attributes can be easily modified for other applications such as cluster of differentiation 4 (CD4) counting. Proof of concept is demonstrated through a 6-plex bead assay with the results compared to a commercially available benchtop-sized instrument.

Recent advances in microfluidic techniques for single-cell biophysical characterization

Biophysical (mechanical and electrical) properties of living cells have been proven to play important roles in the regulation of various biological activities at the molecular and cellular level, and can serve as promising label-free markers of cells' physiological states. In the past two decades, a number of research tools have been developed for understanding the association between the biophysical property changes of biological cells and human diseases; however, technical challenges of realizing high-throughput, robust and easy-to-perform measurements on single-cell biophysical properties have yet to be solved. In this paper, we review emerging tools enabled by microfluidic technologies for single-cell biophysical characterization. Different techniques are compared. The technical details, advantages, and limitations of various microfluidic devices are discussed.

Label-free hybridoma cell culture quality control by a chip-based impedance flow cytometer

Impedance flow cytometry (IFC) was evaluated as a possible alternative to fluorescence-based methods for on-line quality monitoring of hybridoma cells. Hybridoma cells were cultured at different cell densities and viability was estimated by means of IFC and fluorescence-based flow cytometry (FCM). Cell death was determined by measuring the impedance phase value at high frequency in low conductivity buffer. IFC data correlate well with reference FCM measurements using AnnexinV and 7-AAD staining. Hybridoma cells growing at different densities in cell culture revealed a density-dependent subpopulation pattern. Living cells of high density cultures show reduced impedance amplitudes, indicating particular cellular changes. Dead cell subpopulations become evident in cultures with increasing cell densities. In addition, a novel intermediate subpopulation, which most probably represents apoptotic cells, was identified. These results emphasize the extraordinary sensitivity of high frequency impedance measurements and their suitability for hybridoma cell culture quality control.

Lab on a chip technologies for algae detection: a review

Over the last few decades, lab on a chip technologies have emerged as powerful tools for high-accuracy diagnosis with minute quantities of liquid and as tools for exploring cell properties in general. In this paper, we present a review of the current status of this technology in the context of algae detection and monitoring. We start with an overview of the detection methods currently used for algae monitoring, followed by a review of lab on a chip devices for algae detection and classification, and then discuss a case study based on our own research activities. We conclude with a discussion on future challenges and motivations for algae-oriented lab on a chip technologies.

Optical classification of algae species with a glass lab-on-a-chip

The identification of submillimetre phytoplankton is important for monitoring environmental and climate changes, as well as evaluating water for health reasons. Current standard methods for phytoplankton species identification require sample collection and ex situ analysis, an expensive procedure which prevents the rapid identification of phytoplankton outbreaks. To address this, we use a glass-based microchip with a microchannel and waveguide included on a monolithic substrate, and demonstrate its use for identifying phytoplankton species. The microchannel and the specimens inside it are illuminated by laser light from the curved waveguide as algae-laden water is passed through the channel. The intensity distribution of the light collected from the biochip is monitored with an external photodetector. Here, we demonstrate that the characteristics of the photodiode signal from this simple and robust system can provide significant and useful information as to the contents of the channel. Specifically, we show first that the signals are correlated to the size of algae cells. Using a pattern-matching neural network, we demonstrate the successful classification of five algae species with an average 78% positive identification rate. Furthermore, as a proof-of-concept for field-operation, we show that the chip can be used to distinguish between detritus in field-collected water and the toxin-producing cyanobacterium Cyanothece.

A new floating electrode structure for generating homogeneous electrical fields in microfluidic channels

In this article a new parallel electrode structure in a microfluidic channel is described that makes use of a floating electrode to get a homogeneous electrical field. Compared to existing parallel electrode structures, the new structure has an easier production process and there is no need for an electrical connection to both sides of the microfluidic chip. With the new chip design, polystyrene beads suspended in background electrolyte have been detected using electrical impedance measurements. The results of electrical impedance changes caused by beads passing the electrodes are compared with results in a similar planar electrode configuration. It is shown that in the new configuration the coefficient of variation of the impedance changes is lower compared to the planar configuration (0.39 versus 0.56) and less dependent on the position of the beads passage in the channel as a result of the homogeneous electrical field. To our knowledge this is the first time that a floating electrode is used for the realization of a parallel electrode structure. The proposed production method for parallel electrodes in microfluidic channels can easily be applied to other applications.

Positional dependence of particles in microfludic impedance cytometry

Single cell impedance cytometry is a label-free electrical analysis method that requires minimal sample preparation and has been used to count and discriminate cells on the basis of their impedance properties. This paper shows experimental and numerically simulated impedance signals for test particles (6 μm diameter polystyrene) flowing through a microfluidic channel. The variation of impedance signal with particle position is mapped using numerical simulation and these results match closely with experimental data. We demonstrate that for a nominal 40 μm × 40 μm channel, the impedance signal is independent of position over the majority of the channel area, but shows large experimentally verifiable variation at extreme positions. The parabolic flow profile in the channel ensures that most of the sample flows through the area of uniform signal. At high flow rates inertial focusing is observed; the particles flow in equal numbers through two equilibrium positions reducing the coefficient of variance (CV) in the impedance signals to negligible values.

Microfluidic impedance-based flow cytometry

Microfabricated flow cytometers can detect, count, and analyze cells or particles using microfluidics and electronics to give impedance-based characterization. Such systems are being developed to provide simple, low-cost, label-free, and portable solutions for cell analysis. Recent work using microfabricated systems has demonstrated the capability to analyze micro-organisms, erythrocytes, leukocytes, and animal and human cell lines. Multifrequency impedance measurements can give multiparametric, high-content data that can be used to distinguish cell types. New combinations of microfluidic sample handling design and microscale flow phenomena have been used to focus and position cells within the channel for improved sensitivity. Robust designs will enable focusing at high flowrates while reducing requirements for control over multiple sample and sheath flows. Although microfluidic impedance-based flow cytometers have not yet or may never reach the extremely high throughput of conventional flow cytometers, the advantages of portability, simplicity, and ability to analyze single cells in small populations are, nevertheless, where chip-based cytometry can make a large impact.

A Microfluidic Passive Pumping Coulter Counter

A microfluidic device using on-chip passive pumping was characterized for use as a particle counter. Flow occurred due to a Young-Laplace pressure gradient between two 1.2 mm diameter inlets and a 4 mm diameter reservoir when 0.5μ L fluid droplets were applied to the inlets using a micropipette. Polystyrene particles (10μm diameter) were enumerated using the resistive pulse technique. Particle counts using passive pumping were within 13% of counts from a device using syringe pumping. All pumping methods produced particle counts that were within 16% of those obtained with a hemocytometer. The effect of intermediate wash steps on particle counts within the passive pumping device was determined. Zero, one, or two wash droplets were loaded after the first of two sample droplets. No statistical difference was detected in the mean particle counts among the loading patterns (p > 0.05). Hydrodynamic focusing using passive pumping was also demonstrated.

On-chip determination of spermatozoa concentration using electrical impedance measurements

In this article we describe the development of a microfluidic chip to determine the concentration of spermatozoa in semen, which is a main quality parameter for the fertility of a man. A microfluidic glass-glass chip is used, consisting of a microchannel with a planar electrode pair that allows the detection of spermatozoa passing the electrodes using electrical impedance measurements. Cells other than spermatozoa in semen also cause a change in impedance when passing the electrodes, interfering with the spermatozoa count. We demonstrate that the change in electrical impedance is related to the size of cells passing the electrodes, allowing to distinguish between spermatozoa and HL-60 cells suspended in washing medium. In the same way we are able to distinguish between polystyrene beads and spermatozoa. Thus, by adding a known concentration of polystyrene beads to a boar semen sample, the spermatozoa concentrations of seven mixtures are measured and show a good correlation with the actual concentration (R(2)-value = 0.97). To our knowledge this is the first time that the concentration of spermatozoa has been determined on chip using electrical impedance measurements without a need to know the actual flow speed. The proposed method to determine the concentration can be easily applied to other cells. The described on-chip determination of the spermatozoa concentration is a first step towards a microfluidic system for a complete quality analysis of semen.

The application of microfluidics in biology

Recent advances in the bio- and nanotechnologies have led to the development of novel microsystems for bio-particle separation and analysis. Microsystems are already revolutionising the way we do science and have led to the development of a number of ultrasensitive bioanalytical devices capable of analysing complex biological samples. These devices have application in a number of diverse areas such as pollution monitoring, clinical diagnostics, drug discovery and biohazard detection. In this chapter we give an overview of the physical principles governing the behaviour of fluids and particles at the micron scale, which are relevant to the operation of microfluidic devices. We briefly discuss some of the fabrication technologies used in the production of microfluidic systems and then present a number of examples of devices and applications relevant to the biological and life sciences.