Large-scale investigation of the olfactory receptor space using a microfluidic microwell array

Biosensors for Odor Detection: A Review

Animals can easily detect hundreds of thousands of odors in the environment with high sensitivity and selectivity. With the progress of biological olfactory research, scientists have extracted multiple biomaterials and integrated them with different transducers thus generating numerous biosensors. Those biosensors inherit the sensing ability of living organisms and present excellent detection performance. In this paper, we mainly introduce odor biosensors based on substances from animal olfactory systems. Several instances of organ/tissue-based, cell-based, and protein-based biosensors are described and compared. Furthermore, we list some other biological materials such as peptide, nanovesicle, enzyme, and aptamer that are also utilized in odor biosensors. In addition, we illustrate the further developments of odor biosensors.

Active Tracking of Temporally Changing Gas-Phase Odor Mixture Using an Array of Cells Expressing Olfactory Receptors

A cell expressing an olfactory receptor (OR) exhibits excellent odorant detection ability and thus is widely applied in odor biosensors. Most of those biosensors, however, could detect only liquid-phase nonchanging single-component odorants. In this paper, we raised up an odor biosensor for the active tracking of temporally changing gas-phase odor mixture by an array of cells expressing ORs. A thin stable liquid film covered the cell, thus allowing gas-phase odorants to penetrate. The online image processing generated individual cell brightness data which were used to compute the biosensor response. Based on the obtained responses, we adjusted the known odor components to be similar with the unknown odor. The function of our biosensor was validated by tracking the variable single-component odorant or the binary odor mixture. The influence from the sensor drift could be overcome by comparing the adjacent unknown and known odor responses. In the odor mixture quantification, adding the OR label to mixed cells and then quantifying separately (named as the pre-label method) was more efficient, while directly using the cell response pattern (named as the label-free method) was still capable even if the OR odor had cross-sensitivity.

Biohybrid sensor for odor detection

Biohybrid odorant sensors that directly integrate a biological olfactory system have been increasingly studied and are suggested to be the next generation of ultrasensitive sensors by taking advantage of the sensitivity and selectivity of living organisms. In this review, we provide a detailed description of the recent developments of biohybrid odorant sensors, especially considering the requisites for their perspective of on-site applications. We introduce the methodologies to effectively capture the biological signals from olfactory systems by readout devices, and describe the essential properties regarding the gaseous detection, stability, quality control, and portability. Moreover, we address the recent progress on multiple odorant recognition using multiple sensors as well as the current screening approaches for pairs of orphan receptors and ligands necessary for the extension of the currently available range of biohybrid sensors. Finally, we discuss our perspectives for the future for the development of practical odorant sensors.

Selective Trapping and Retrieval of Single Cells Using Microwell Array Devices Combined with Dielectrophoresis

We proposed selective manipulation techniques for retrieving and retaining target cells arrayed in microwells based on dielectrophoresis (DEP). The upper substrate with microband electrodes was mounted on the lower substrate with microwells based on the same design of microband electrodes by 90 degree relative to the lower substrate. A repulsive force of negative dielectrophoresis (n-DEP) was employed to retrieve the target cells from the microwell array selectively. Furthermore, the target cells were retained in the microwells after other cells were removed by n-DEP. Thus, the system described in this study could make it possible to retrieve and recover single target cells from a microwell array after determining the function of cells trapped in each microwell.

Market Perspectives and Future Fields of Application of Odor Detection Biosensors within the Biological Transformation-A Systematic Analysis

The technological advantages that biosensors have over conventional technical sensors for odor detection and the role they play in the biological transformation have not yet been comprehensively analyzed. However, this is necessary for assessing their suitability for specific fields of application as well as their improvement and development goals. An overview of biological basics of olfactory systems is given and different odor sensor technologies are described and classified in this paper. Specific market potentials of biosensors for odor detection are identified by applying a tailored methodology that enables the derivation and systematic comparison of both the performance profiles of biosensors as well as the requirement profiles for various application fields. Therefore, the fulfillment of defined requirements is evaluated for biosensors by means of 16 selected technical criteria in order to determine a specific performance profile. Further, a selection of application fields, namely healthcare, food industry, agriculture, cosmetics, safety applications, environmental monitoring for odor detection sensors is derived to compare the importance of the criteria for each of the fields, leading to market-specific requirement profiles. The analysis reveals that the requirement criteria considered to be the most important ones across all application fields are high specificity, high selectivity, high repeat accuracy, high resolution, high accuracy, and high sensitivity. All these criteria, except for the repeat accuracy, can potentially be better met by biosensors than by technical sensors, according to the results obtained. Therefore, biosensor technology in general has a high application potential for all the areas of application under consideration. Health and safety applications especially are considered to have high potential for biosensors due to their correspondence between requirement and performance profiles. Special attention is paid to new areas of application that require multi-sensing capability. Application scenarios for multi-sensing biosensors are therefore derived. Moreover, the role of biosensors within the biological transformation is discussed.

Binary mixture quantification using cell-based odor biosensor system with active sensing

Organisms perceive odorants in the environment through the use of a large number of olfactory receptors. Various odor biosensors have been researched and developed in order to mimic this olfactory mechanism. This study examines the quantification of odorant concentrations through the use of a sensor array comprised of several types of cell-based odor sensors expressing insect olfactory receptors with nonlinear characteristics. The sensor system utilized an active sensing method in order to compare the responses of a target odorant and a prepared odorant in determining the relative concentration of the target odorant. By combining an active sensing method with a real-time reference method in which the target odorant was measured every time the prepared odorant was measured, the relative concentrations were successfully determined even when the response fluctuation was large or odorant sensor cell responses varied as measurement time increased. For proof of concept purposes, the study primarily focused on quantifying odorant concentrations composed of one or two odorant components. It was confirmed that an algorithm to find the optimal relative odorant concentration among a limited number of odorant concentrations is achievable. Though this study is still in the initial stage of the developing odor sensors and has many challenges, it can provide insight into paving the way towards a new type of odor biosensor with active sensing.

Rapid fabrication of sieved microwells and cross-flow microparticle trapping

The use of microwells is popular for a wide range of applications due to its' simplicity. However, the seeding of conventional microwells, which are closed at the bottom, is restricted to gravitational sedimentation for cell or particle deposition and therefore require lengthy settling times to maximize well occupancy. The addition of microfluidics to the capture process has accelerated cell or particle dispersion and improved capture ability but is mostly limited to gravitationally-driven settling for capture into the wells. An alternative approach to conventional closed-microwells, sieved microwells supersedes reliance on gravity by using hydrodynamic forces through the open pores at the bottom of the microwells to draw targets into the wells. We have developed a rapid fabrication method, based on flow lithography techniques, which allows us to easily customize the mesh pore sizes in a simple two-step process. Finally, by combining this microwell design with cross-flow trapping in a microfluidic two-layered channel, we achieve an 88 ± 6% well occupancy in under 10 s.

Enzymatic-based cytometry, a sensitive single-cell cytometric method to assess BCR-ABL1 activity in CML

We developed a simple, rapid and cost-effective enzymatic-based cytometry platform to measure intracellular signaling pathway activity. Our single-cell microwell array platform quantifies protein phosphorylation using enzymatic signal amplification and exploiting Michaelis-Menten kinetics. Our method provides a two-fold increase in resolution compared to conventional flow cytometry.

Membrane protein-based biosensors

This review highlights recent development of biosensors that use the functions of membrane proteins. Membrane proteins are essential components of biological membranes and have a central role in detection of various environmental stimuli such as olfaction and gustation. A number of studies have attempted for development of biosensors using the sensing property of these membrane proteins. Their specificity to target molecules is particularly attractive as it is significantly superior to that of traditional human-made sensors. In this review, we classified the membrane protein-based biosensors into two platforms: the lipid bilayer-based platform and the cell-based platform. On lipid bilayer platforms, the membrane proteins are embedded in a lipid bilayer that bridges between the protein and a sensor device. On cell-based platforms, the membrane proteins are expressed in a cultured cell, which is then integrated in a sensor device. For both platforms we introduce the fundamental information and the recent progress in the development of the biosensors, and remark on the outlook for practical biosensing applications.

Bioinspired Electronics for Artificial Sensory Systems

Humans have a myriad of sensory receptors in different sense organs that form the five traditionally recognized senses of sight, hearing, smell, taste, and touch. These receptors detect diverse stimuli originating from the world and turn them into brain-interpretable electrical impulses for sensory cognitive processing, enabling us to communicate and socialize. Developments in biologically inspired electronics have led to the demonstration of a wide range of electronic sensors in all five traditional categories, with the potential to impact a broad spectrum of applications. Here, recent advances in bioinspired electronics that can function as potential artificial sensory systems, including prosthesis and humanoid robots are reviewed. The mechanisms and demonstrations in mimicking biological sensory systems are individually discussed and the remaining future challenges that must be solved for their versatile use are analyzed. Recent progress in bioinspired electronic sensors shows that the five traditional senses are successfully mimicked using novel electronic components and the performance regarding sensitivity, selectivity, and accuracy have improved to levels that outperform human sensory organs. Finally, neural interfacing techniques for connecting artificial sensors to the brain are discussed.

On-chip immunofluorescence analysis of single cervical cells using an electroactive microwell array with barrier for cervical screening

Several specific tests for cervical screening have been developed recently, including p16/Ki67 dual immunostaining for diagnosing high-risk human papillomavirus positive squamous intraepithelial lesion in the cervix. However, manual screening of cells in an entire glass slide is currently a standard clinical procedure for quantification and interpretation of immunocytochemical features of the cells. Here, we developed a microfluidic device containing an electroactive microwell array with barriers (EMAB) for highly efficient single-cell trapping followed by on-chip immunofluorescence analysis with minimum loss of the sample. EMAB utilizes patterned electrodes at the bottom of cell-sized microwells to trap single cells using dielectrophoresis (DEP) and cell-holding structures behind the microwells to stabilize the position of trapped cells even without DEP. Using the device, we evaluated the performance of p16/Ki67 dual immunostaining of HeLa cells on the chip. The device shows 98% cell-trapping efficiency as well as 92% cell-holding efficiency against the fixed HeLa cells, and we successfully demonstrated high-efficiency on-chip immunofluorescence analysis with minimal loss of sample. p16/Ki67 dual immunostaining using EMAB may be useful for complementary tests for cervical screening in confirming the histopathological diagnosis.

Construction of a Biohybrid Odorant Sensor Using Biological Olfactory Receptors Embedded into Bilayer Lipid Membrane on a Chip

This paper describes an odorant sensor based on mosquito olfactory receptors (ORs) that is sensitive to the volatile organic compound octenol. The ORs and OR coreceptors were reconstructed in the lipid bilayer membrane in a chamber device equipped with electrodes. Using this odorant sensor, we obtained ion current changes caused by specific OR responses to octenol. We installed the odorant sensor into a mobile robot and succeeded in the demonstration of coupling octenol gas detection and robot actuation. We believe that this biohybrid odorant sensing system will be a key technology for future artificial olfaction.

Study on Bombykol Receptor Self-Assembly and Universality of G Protein Cellular Signal Amplification System

The G protein cascade amplification system couples with several receptors to sense/amplify the cellular signal, implying universal application. In order to explore whether GPCRs can trigger G protein signal amplification in tissues/cells from different species, bombykol receptor was isolated and purified from antennas of male Bombyx mori, which subsequently self-assembled on the cell membrane in rat taste buds/rat vomeronasa/catfish tentacles/taste bud tissues of rabbits/pig/cattle in those lacking endogenous bombykol receptor, followed by immobilization between two sheets of nucleopore membranes fixed by sodium alginate-starch gel, forming the sandwich-type sensing membrane, which in turn was immobilized on the glass-carbon electrode. Thus, bombykol receptor sensors were established with different tissues. The response current of bombykol receptor sensor toward bombykol was measured with an electrochemical workstation. Every bombykol receptor sensor could sense bombykol based on enzyme-substrate kinetics. The double reciprocal plot and the activation constant values of bombykol receptor sensors assembled with rat taste buds, rat vomeronasa, catfish tentacles, rabbit taste buds, pig taste buds, and cattle taste buds were calculated. Approximately 2-3 receptors could trigger the G protein cascade amplification system and achieve the maximum signal output. Moreover, the detection lower limit indicated that the bombykol receptor self-assembled on the cell membranes of different tissues that transmitted and amplified the bombykol signal with hypersensitivity. Also, cattle taste bud tissues served as an ideal system for heterogeneous GPCRs self-assembly and signal sensing/amplification. This sensing technique and method had promising potential in studies of biological pest control, sex pheromone detection, and receptor structure and function.

Progress in the development of olfactory-based bioelectronic chemosensors

Artificial chemosensory devices have a wide range of applications in industry, security, and medicine. The development of these devices has been inspired by the speed, sensitivity, and selectivity by which the olfactory system in animals can probe the chemical nature of the environment. In this review, we examine how molecular and cellular components of natural olfactory systems have been incorporated into artificial chemosensors, or bioelectronic sensors. We focus on the biological material that has been combined with signal transduction systems to develop artificial chemosensory devices. The strengths and limitations of different biological chemosensory material at the heart of these devices, as well as the reported overall effectiveness of the different bioelectronic sensor designs, is examined. This review also discusses future directions and challenges for continuing to advance development of bioelectronic sensors.

Target Confinement in Small Reaction Volumes Using Microfluidic Technologies: A Smart Approach for Single-Entity Detection and Analysis

Over the last decades, the study of cells, nucleic acid molecules, and proteins has evolved from ensemble measurements to so-called single-entity studies. The latter offers huge benefits, not only as biological research tools to examine heterogeneities among individual entities within a population, but also as biosensing tools for medical diagnostics, which can reach the ultimate sensitivity by detecting single targets. Whereas various techniques for single-entity detection have been reported, this review focuses on microfluidic systems that physically confine single targets in small reaction volumes. We categorize these techniques as droplet-, microchamber-, and nanostructure-based and provide an overview of their implementation for studying single cells, nucleic acids, and proteins. We furthermore reflect on the advantages and limitations of these techniques and highlight future opportunities in the field.

Ultrasensitive Single-Molecule Enzyme Detection and Analysis Using a Polymer Microarray

This report describes a novel method for isolating and detecting individual enzyme molecules using polymer arrays of picoliter microwells. A fluidic flow-cell device containing an array of microwells is fabricated in cyclic olefin polymer (COP). The use of COP microwell arrays simplifies experiments by eliminating extensive device preparation and surface functionalization that are common in other microwell array formats. Using a simple and robust loading method to introduce the reaction solution, individual enzyme molecules are trapped in picoliter microwells and subsequently isolated and sealed by fluorinated oil. The sealing is stable for hours in the COP device. The picoliter microwell device can measure enzyme concentrations in the low-femtomolar range by counting the number of active wells using a digital read-out. These picoliter microwell arrays can also easily be regenerated and reused.

Applications and Advances in Bioelectronic Noses for Odour Sensing

A bioelectronic nose, an intelligent chemical sensor array system coupled with bio-receptors to identify gases and vapours, resembles mammalian olfaction by which many vertebrates can sniff out volatile organic compounds (VOCs) sensitively and specifically even at very low concentrations. Olfaction is undertaken by the olfactory system, which detects odorants that are inhaled through the nose where they come into contact with the olfactory epithelium containing olfactory receptors (ORs). Because of its ability to mimic biological olfaction, a bio-inspired electronic nose has been used to detect a variety of important compounds in complex environments. Recently, biosensor systems have been introduced that combine nanoelectronic technology and olfactory receptors themselves as a source of capturing elements for biosensing. In this article, we will present the latest advances in bioelectronic nose technology mimicking the olfactory system, including biological recognition elements, emerging detection systems, production and immobilization of sensing elements on sensor surface, and applications of bioelectronic noses. Furthermore, current research trends and future challenges in this field will be discussed.

Massive Parallel Analysis of Single Cells in an Integrated Microfluidic Platform

New tools that facilitate the study of cell-to-cell variability could help uncover novel cellular regulation mechanisms. We present an integrated microfluidic platform to analyze a large number of single cells in parallel. To isolate and analyze thousands of individual cells in multiplexed conditions, our platform incorporates arrays of microwells (7 pL each) in a multilayered microfluidic device. The device allows the simultaneous loading of cells into 16 separate chambers, each containing 4640 microwells, for a total of 74 240 wells per device. We characterized different parameters important for the operation of the microfluidic device including flow rate, solution exchange rate in a microchamber, shear stress, and time to fill up a single microwell with molecules of different molecular weight. In general, after ∼7.5 min of cell loading our device has an 80% microwell occupancy with 1-4 cells, of which 36% of wells contained a single cell. To test the functionality of our device, we carried out a cell viability assay with adherent and nonadherent cells. We also studied the production of neutrophil extracellular traps (NETs) from single neutrophils isolated from peripheral blood, observing the existence of temporal heterogeneity in NETs production, perhaps having implications in the type of the neutrophil response to an infection or inflammation. We foresee our platform will have a variety of applications in drug discovery and cellular biology by facilitating the characterization of phenotypic differences in a monoclonal cell population.

Compartmentalized Platforms for Neuro-Pharmacological Research

Dissociated primary neuronal cell culture remains an indispensable approach for neurobiology research in order to investigate basic mechanisms underlying diverse neuronal functions, drug screening and pharmacological investigation. Compartmentalization, a widely adopted technique since its emergence in 1970s enables spatial segregation of neuronal segments and detailed investigation that is otherwise limited with traditional culture methods. Although these compartmental chambers (e.g. Campenot chamber) have been proven valuable for the investigation of Peripheral Nervous System (PNS) neurons and to some extent within Central Nervous System (CNS) neurons, their utility has remained limited given the arduous manufacturing process, incompatibility with high-resolution optical imaging and limited throughput. The development in the area of microfabrication and microfluidics has enabled creation of next generation compartmentalized devices that are cheap, easy to manufacture, require reduced sample volumes, enable precise control over the cellular microenvironment both spatially as well as temporally, and permit highthroughput testing. In this review we briefly evaluate the various compartmentalization tools used for neurobiological research, and highlight application of the emerging microfluidic platforms towards in vitro single cell neurobiology.

Bioelectronic nose: Current status and perspectives

A characteristic feature of human and animal organs of smell is the ability to identify hundreds of thousands of odours. It is accompanied by particular smell sensations, which are a basic source of information about odour mixture. The main structural elements of biological smell systems are the olfactory receptors. Small differences in a structure of odorous molecules (odorants) can lead to significant change of odour, which is due to the fact that each of the olfactory receptors is coded with different gene and usually corresponds to different type of odour. Discovery and characterisation of the gene family coding the olfactory receptors contributed to the elaboration and development of the electronic smell systems, the so-called bioelectronic noses. The olfactory receptors are employed as a biological element in this type of instruments. An electronic system includes a converter part, which allows measurement and processing of generated signals. A suitable data analysis system is also required to visualise the results. Application potentialities of the bioelectronic noses are focused on the fields of economy and science where highly selective and sensitive analysis of odorous substances is required. The paper presents a review of the latest achievements and critical evaluation of the state of art in the field of bioelectronic noses.

A High-Throughput Automated Microfluidic Platform for Calcium Imaging of Taste Sensing

The human enteroendocrine L cell line NCI-H716, expressing taste receptors and taste signaling elements, constitutes a unique model for the studies of cellular responses to glucose, appetite regulation, gastrointestinal motility, and insulin secretion. Targeting these gut taste receptors may provide novel treatments for diabetes and obesity. However, NCI-H716 cells are cultured in suspension and tend to form multicellular aggregates, preventing high-throughput calcium imaging due to interferences caused by laborious immobilization and stimulus delivery procedures. Here, we have developed an automated microfluidic platform that is capable of trapping more than 500 single cells into microwells with a loading efficiency of 77% within two minutes, delivering multiple chemical stimuli and performing calcium imaging with enhanced spatial and temporal resolutions when compared to bath perfusion systems. Results revealed the presence of heterogeneity in cellular responses to the type, concentration, and order of applied sweet and bitter stimuli. Sucralose and denatonium benzoate elicited robust increases in the intracellular Ca(2+) concentration. However, glucose evoked a rapid elevation of intracellular Ca(2+) followed by reduced responses to subsequent glucose stimulation. Using Gymnema sylvestre as a blocking agent for the sweet taste receptor confirmed that different taste receptors were utilized for sweet and bitter tastes. This automated microfluidic platform is cost-effective, easy to fabricate and operate, and may be generally applicable for high-throughput and high-content single-cell analysis and drug screening.

Deciphering the Receptor Repertoire Encoding Specific Odorants by Time-Lapse Single-Cell Array Cytometry

Mammals can recognize a vast number of odorants by using olfactory receptors (ORs) known as G protein-coupled receptors. The OR gene family is one of the most diverse gene families in mammalian genomes. Because of the vast combinations of ORs and odorants, few ORs have thus far been linked to specific odorants. Here, we established a functional screening method for OR genes by using a microchamber array containing >5,400 single olfactory epithelium-derived cells from mice applied to time-lapse single-cell array cytometry. This method facilitated the prompt isolation of single olfactory sensory neurons (OSNs) responding to the odorant of interest. Subsequent single-cell RT-PCR allowed us to isolate the genes encoding respective ORs. By using volatile molecules recognized as biomarkers for lung cancers, this method could deorphanize ORs and thereby reconstitute the OR-mediated signaling cascade in HEK293T cells. Thus, our system could be applied to identify any receptor by using specific ligands in the fields of physiopathology and pharmacology.

Features of Microsystems for Cultivation and Characterization of Stem Cells with the Aim of Regenerative Therapy

Stem cells have infinite potential for regenerative therapy thanks to their advantageous ability which is differentiable to requisite cell types for recovery and self-renewal. The microsystem has been proved to be more helpful to stem cell studies compared to the traditional methods, relying on its advantageous feature of mimicking in vivo cellular environments as well as other profitable features such as minimum sample consumption for analysis and multiprocedures. A wide variety of microsystems were developed for stem cell studies; however, regenerative therapy-targeted applications of microtechnology should be more emphasized and gain more attractions since the regenerative therapy is one of ultimate goals of biologists and bioengineers. In this review, we introduce stem cell researches harnessing well-known microtechniques (microwell, micropattern, and microfluidic channel) in view point of physical principles and how these systems and principles have been implemented appropriately for characterizing stem cells and finding possible regenerative therapies. Biologists may gain information on the principles of microsystems to apply them to find solutions for their current challenges, and engineers may understand limitations of the conventional microsystems and find new chances for further developing practical microsystems. Through the well combination of engineers and biologists, the regenerative therapy-targeted stem cell researches harnessing microtechnology will find better suitable treatments for human disorders.

Microwell arrays reveal cellular heterogeneity during the clonal expansion of transformed human cells

We developed micromolded microwell arrays to study the proliferation and senescence of single cells. Microwell arrays were designed to be compatible with conventional cell culture protocols to simplify cell loading, cell culture, and imaging. We demonstrated the utility of these arrays by measuring the proliferation and senescence of isogenic cells which expressed or had been depleted of the human Werner syndrome protein. Our results allowed us to reveal cell-to-cell heterogeneity in proliferation in WRN+ and WRN-depleted fibroblasts during clonal growth.

Considerations of Protein Subpockets in Fragment-Based Drug Design

While the fragment-based drug design approach continues to gain importance, gaps in the tools and methods available in the identification and accurate utilization of protein subpockets have limited the scope. The importance of these features of small molecule-protein recognition is highlighted with several examples. A generalized solution for the identification of subpockets and corresponding chemical fragments remains elusive, but there are numerous advancements in methods that can be used in combination to address subpockets. Finally, additional examples of approaches that consider the relative importance of small-molecule co-dependence of protein conformations are highlighted to emphasize an increased significance of subpockets, especially at protein interfaces.

Coupling of olfactory receptor and ion channel for rapid and sensitive visualization of odorant response

In the human smell sensing system, there are about 390 kinds of olfactory receptors (ORs) which bind to various odorants with different affinities and specificities. Characterization and odorant binding pattern analysis of the ORs are essential for understanding of human olfaction and to mimic the olfactory system in various applications. Although various cell-based odorant screening systems have been developed for this purpose, many human ORs (hORs) still remain orphan because of the time-consuming and labor-intensive experimental procedures of the available screening methods. In this study, we constructed an ion channel-coupled hOR for simple odorant detection by rapidly visualizing the odorant response to overcome the limitations of conventional screening systems. The hORs were coupled to the Kir6.2 potassium channel and the fusion proteins were expressed in HEK293 cells. In this system, when an odorant binds to the hORs coupled to the ion channel, a conformational change in the OR occurs, which consequently opens the ion channel to result in ion influx into the cell. This ion influx was then visualized using a membrane potential dye. Cells expressing ion channel-coupled hORs showed high sensitivity and selectivity to their specific odorants, and the odorant-hOR binding pattern was visualized to identify the response of individual hORs to various odorants, as well as the response of various hORs to various odorants. These results indicate that the ion channel-coupled hOR system can be effectively used not only for simple and fast high-throughput odorant screening, but also to visualize the odorant-hOR response pattern.

Versatile Microfluidic Platform for the Assessment of Sialic Acid Expression on Cancer Cells Using Quantum Dots with Phenylboronic Acid Tags

This work describes a versatile microfluidic platform for evaluation of cell-surface glycan expression at the single-cell level using quantum dots (QDs) tagged with phenylboronic acid. The platform was integrated with dual microwell arrays, allowing the introduction of cells in two states using the same cell culture chamber. The simultaneous analysis of cells in the same environment minimized errors resulting from different culture conditions. As proof-of-concept, the expressions of sialic acid (SA) groups on K562 cells, with or without 3'-azido-3'-deoxythymidine (AZT) treatment, were evaluated in the same chamber. 3-Aminophenylboronic acid functionalized CdSeTe@ZnS-SiO2 QDs (APBA-QDs) were prepared as probes to recognize SA groups on K562 cells with only one-step labeling. The results showed that the expression of SA moieties on K562 cells was increased by 18% and 31% after treatment with 20 and 40 μM AZT, respectively. Performing the drug treatment and control experiments simultaneously in the same chamber significantly improved the robustness and effectiveness of the assay. The strategy presented here provides an alternative tool for glycan analysis in a sensitive, high-throughput, and effective manner.

3D-printed microfluidic automation

Microfluidic automation - the automated routing, dispensing, mixing, and/or separation of fluids through microchannels - generally remains a slowly-spreading technology because device fabrication requires sophisticated facilities and the technology's use demands expert operators. Integrating microfluidic automation in devices has involved specialized multi-layering and bonding approaches. Stereolithography is an assembly-free, 3D-printing technique that is emerging as an efficient alternative for rapid prototyping of biomedical devices. Here we describe fluidic valves and pumps that can be stereolithographically printed in optically-clear, biocompatible plastic and integrated within microfluidic devices at low cost. User-friendly fluid automation devices can be printed and used by non-engineers as replacement for costly robotic pipettors or tedious manual pipetting. Engineers can manipulate the designs as digital modules into new devices of expanded functionality. Printing these devices only requires the digital file and electronic access to a printer.

Lab-on-chip device for single cell trapping and analysis

Traditional cell assay gives us an average result of multiple cells and it is assumed that the resultant is the outcome of all cells in population. However, single cell studies have revealed that individual cells of same type may differ dramatically and these differences may have important role to play in cells functionality. Such information can be obscured in only studying cell population experimental approach. To uncover biological principles and ultimately to improve the detection and treatment of disease, new approaches are highly required to single cell analysis. We propose to fabricate a lab on chip device to study high throughput single cell nanotoxicity analysis. The chip incorporates independently addressable active microwell electrodes for cell manipulation and analysis. We employed positive-dielectrophoresis approach to quickly and efficiently capture single cells in each wells with having control over individual microwells. We examined change in impedance properties to verify cell capture in microwell and its health and present a novel model of single cell assay for nanotoxicity, and drug testing.

Quantifying genetically inserted fluorescent protein in single iPS cells to monitor Nanog expression using electroactive microchamber arrays

Interest in the gene expression levels of pluripotent stem cells has increased in order to precisely understand cellular differentiation. Here, we propose a method utilizing a large number of arrayed microchambers to quantitatively measure an intracellular fluorescence protein that is genetically inserted to monitor a pluripotency marker protein, Nanog, in pluripotent stem cells. Individual cells are isolated and lysed by inducing an electric potential on the cell membrane within the tightly enclosed microchambers. The microchambers have a size that is comparable to the target cells, making it possible to trap single cells and restrict the dilution of the cell lysate. The amount of intracellular fluorescence proteins in a single cell is precisely quantified inside the well-defined volume of each microchamber. Our method will be a useful tool for high-throughput and parallelized read-outs of gene expression levels in individual cells in a large population of cells.

Advances in high-throughput single-cell microtechnologies

Micro-scale biological tools that have allowed probing of individual cells--from the genetic, to proteomic, to phenotypic level--have revealed important contributions of single cells to direct normal and diseased body processes. In analyzing single cells, sample heterogeneity between and within specific cell types drives the need for high-throughput and quantitative measurement of cellular parameters. In recent years, high-throughput single-cell analysis platforms have revealed rare genetic subpopulations in growing tumors, begun to uncover the mechanisms of antibiotic resistance in bacteria, and described the cell-to-cell variations in stem cell differentiation and immune cell response to activation by pathogens. This review surveys these recent technologies, presenting their strengths and contributions to the field, and identifies needs still unmet toward the development of high-throughput single-cell analysis tools to benefit life science research and clinical diagnostics.

Wide field-of-view on-chip Talbot fluorescence microscopy for longitudinal cell culture monitoring from within the incubator

Time-lapse or longitudinal fluorescence microscopy is broadly used in cell biology. However, current available time-lapse fluorescence microscopy systems are bulky and costly. The limited field-of-view (FOV) associated with the microscope objective necessitates mechanical scanning if a larger FOV is required. Here we demonstrate a wide FOV time-lapse fluorescence self-imaging Petri dish system, termed the Talbot Fluorescence ePetri, which addresses these issues. This system's imaging is accomplished through the use of the Fluorescence Talbot Microscopy (FTM). By incorporating a microfluidic perfusion subsystem onto the platform, we can image cell cultures directly from within an incubator. Our prototype has a resolution limit of 1.2 μm and an FOV of 13 mm(2). As demonstration, we obtained time-lapse images of HeLa cells expressing H2B-eGFP. We also employed the system to analyze the cells' dynamic response to an anticancer drug, camptothecin (CPT). This method can provide a compact and simple solution for automated fluorescence imaging of cell cultures in incubators.

Over a century of neuron culture: from the hanging drop to microfluidic devices

The brain is the most intricate, energetically active, and plastic organ in the body. These features extend to its cellular elements, the neurons and glia. Understanding neurons, or nerve cells, at the cellular and molecular levels is the cornerstone of modern neuroscience. The complexities of neuron structure and function require unusual methods of culture to determine how aberrations in or between cells give rise to brain dysfunction and disease. Here we review the methods that have emerged over the past century for culturing neurons in vitro, from the landmark finding by Harrison (1910) - that neurons can be cultured outside the body - to studies utilizing culture vessels, micro-islands, Campenot and brain slice chambers, and microfluidic technologies. We conclude with future prospects for neuronal culture and considerations for advancement. We anticipate that continued innovation in culture methods will enhance design capabilities for temporal control of media and reagents (chemotemporal control) within sub-cellular environments of three-dimensional fluidic spaces (microfluidic devices) and materials (e.g., hydrogels). They will enable new insights into the complexities of neuronal development and pathology.

New perspectives on neuronal development via microfluidic environments

Understanding the signals that guide neuronal development and direct formation of axons, dendrites, and synapses during wiring of the brain is a fundamental challenge in developmental neuroscience. Discovery of how local signals shape developing neurons has been impeded by the inability of conventional culture methods to interrogate microenvironments of complex neuronal cytoarchitectures, where different subdomains encounter distinct chemical, physical, and fluidic features. Microfabrication techniques are facilitating the creation of microenvironments tailored to neuronal structures and subdomains with unprecedented access and control. The design, fabrication, and properties of microfluidic devices offer significant advantages for addressing unresolved issues of neuronal development. These high-resolution approaches are poised to contribute new insights into mechanisms for restoring neuronal function and connectivity compromised by injury, stress, and neurodegeneration.

Microfluidic single-cell cultivation chip with controllable immobilization and selective release of yeast cells

We present a microfluidic cell-culture chip that enables trapping, cultivation and release of selected individual cells. The chip is fabricated by a simple hybrid glass-SU-8-PDMS approach, which produces a completely transparent microfluidic system amenable to optical inspection. Single cells are trapped in a microfluidic channel using mild suction at defined cell immobilization orifices, where they are cultivated under controlled environmental conditions. Cells of interest can be individually and independently released for further downstream analysis by applying a negative dielectrophoretic force via the respective electrodes located at each immobilization site. The combination of hydrodynamic cell-trapping and dielectrophoretic methods for cell releasing enables highly versatile single-cell manipulation in an array-based format. Computational fluid dynamics simulations were performed to estimate the properties of the system during cell trapping and releasing. Polystyrene beads and yeast cells have been used to investigate and characterize the different functions and to demonstrate biological compatibility and viability of the platform for single-cell applications in research areas such as systems biology.

Expanding the horizons for single-cell applications on lab-on-a-chip devices

Stochastic events in gene expression, protein synthesis, and metabolite synthesis or degradation lead to cellular heterogeneity essential to life. In a tissue as we see in organs, there is strong heterogeneity among the constituting cells critical to its function. Thus, there exists a strong demand to develop new micro/nanosystems that would enable us to conduct single-cell analysis. This field is rapidly growing, as exemplified below with recent emerging technologies that now reveal sensitive single-cell "omics" analysis. We describe in the review some of the most promising technologies that will certainly transform our view of biology in the near future.

Electroactive microwell arrays for highly efficient single-cell trapping and analysis

We present a novel method, implemented in the form of a microfluidic device, for arraying and analyzing large populations of single cells. The device contains a large array of electroactive microwells where manipulation and analysis of large population of cells are carried out. On the device, single cells can be actively trapped in the microwells by dielectrophoresis (DEP) and then lysed by electroporation (EP) for subsequent analysis of the confined cell lysates. The DEP force in the selected dimensions of the microwells could achieve efficient trapping in nearly all the microwells (95%) in less than three minutes. Moreover, the positions of the cells in the microwells are maintained even when unstable flow of liquid is applied. This makes it possible to exchange the DEP buffer to a solution that will be subsequently used for stimulating or analyzing the trapped cells. After closing the microwells, EP is conducted to lyse the trapped cells by applying short electric pulses. Tight enclosure is critical to prevent dilution, diffusion and cross contamination of the cell lysates. We demonstrated the feasibility of our approach with an enzymatic assay measuring the intracellular-galactosidase activity. The use of this method should greatly help analysis of large populations of cells at the single-cell level. Furthermore, the method offers rapidity in the trapping and analysis of multiple cell types in physiological conditions that will be important to ensure the relevance of single cell analyses.

Biomimetic tissues on a chip for drug discovery

Developing biologically relevant models of human tissues and organs is an important enabling step for disease modeling and drug discovery. Recent advances in tissue engineering, biomaterials and microfluidics have led to the development of microscale functional units of such models also referred to as 'organs on a chip'. In this review, we provide an overview of key enabling technologies and highlight the wealth of recent work regarding on-chip tissue models. In addition, we discuss the current challenges and future directions of organ-on-chip development.

Imaging single-cell signaling dynamics with a deterministic high-density single-cell trap array

Stochasticity in gene expression, protein or metabolite levels contributes to cell-cell variations, the analysis of which could lead to a better understanding of cellular processes and drug responses. Current technologies are limited in their throughput, resolution (in space, time, and tracking individual cells instead of population average) and the ability to control cellular environment. A few microfluidic tools have been developed to trap and image cells; however, in most designs available to date, there is a compromise among loading efficiency, speed, the ability to trap single cells, and density or number of trapped cells. To meet the needs of single-cell imaging studies, we developed a microfluidic platform for high-throughput capture and imaging of thousands of single cells. The optimized trapping mechanism enables 95% of the traps to be occupied with single cells, with a trap density of 860 traps/mm(2). The dense array allows up to 800 cells to be imaged simultaneously with a 4x objective and a typical camera setup. Capture occurs with low shear and 94% viability after 24 h. This platform is compatible with other upstream microfluidic components for complex cell stimulation patterns, and we show here the ability to measure heterogeneity in calcium oscillatory behavior in genetically identical cells and monitor kinetic cellular response to chemical stimuli.