Revisiting lab-on-a-chip technology for drug discovery

Accelerating drug discovery via organs-on-chips

Considerable advances have been made in the development of micro-physiological systems that seek to faithfully replicate the complexity and functionality of animal and human physiology in research laboratories. Sometimes referred to as "organs-on-chips", these systems provide key insights into physiological or pathological processes associated with health maintenance and disease control, and serve as powerful platforms for new drug development and toxicity screening. In this Focus article, we review the state-of-the-art designs and examples for developing multiple "organs-on-chips", and discuss the potential of this emerging technology to enhance our understanding of human physiology, and to transform and accelerate the drug discovery and preclinical testing process. This Focus article highlights some of the recent technological advances in this field, along with the challenges that must be addressed for these technologies to fully realize their potential.

Liquid gradient in two-dimensional matrix for high throughput screening

Based on the ingenious combination of two different gradient generation mechanisms, this work reports a novel approach for a high throughput linear liquid gradient in a two-dimensional (2D) matrix. Specifically, a typical Christmas Tree structure with two inlets was designed as the first mixture gradient generator, upon which the second diffusion gradient generator was coupled to produce the desired concentration series on the basis of the distance difference. Rather than a simple 1D line, the integration of the two generators would result in an innovative 2D matrix of reservoirs, which was then characterized both theoretically and experimentally. Theoretically, calculation of fluid field demonstrated the formation of a concentration gradient, which was then confirmed by the dye solution visualization analysis. For high throughput screening application, doxorubicin (Dox) was then selected as model medicine to treat the acute myeloblastic leukemia (HL-60) cells. Cell viability displayed that cell death rate enhanced with the increase of drug concentration, and this result was higher than that on a 96-well plate, and the corresponding mechanism was properly discussed. Subsequently, Dox and quercetin were employed simultaneously to generate an overlapping gradient and its effect on HL-60 cells was investigated. Due to the automatic formation of concentration gradient that could improve the work efficiency, this work provides a promising tool for future high throughput drug screening.

Concise review: microfluidic technology platforms: poised to accelerate development and translation of stem cell-derived therapies

Stem cells are a powerful resource for producing a variety of cell types with utility in clinically associated applications, including preclinical drug screening and development, disease and developmental modeling, and regenerative medicine. Regardless of the type of stem cell, substantial barriers to clinical translation still exist and must be overcome to realize full clinical potential. These barriers span processes including cell isolation, expansion, and differentiation; purification, quality control, and therapeutic efficacy and safety; and the economic viability of bioprocesses for production of functional cell products. Microfluidic systems have been developed for a myriad of biological applications and have the intrinsic capability of controlling and interrogating the cellular microenvironment with unrivalled precision; therefore, they have particular relevance to overcoming such barriers to translation. Development of microfluidic technologies increasingly utilizes stem cells, addresses stem cell-relevant biological phenomena, and aligns capabilities with translational challenges and goals. In this concise review, we describe how microfluidic technologies can contribute to the translation of stem cell research outcomes, and we provide an update on innovative research efforts in this area. This timely convergence of stem cell translational challenges and microfluidic capabilities means that there is now an opportunity for both disciplines to benefit from increased interaction.

A label-free microfluidic biosensor for activity detection of single microalgae cells based on chlorophyll fluorescence

Detection of living microalgae cells is very important for ballast water treatment and analysis. Chlorophyll fluorescence is an indicator of photosynthetic activity and hence the living status of plant cells. In this paper, we developed a novel microfluidic biosensor system that can quickly and accurately detect the viability of single microalgae cells based on chlorophyll fluorescence. The system is composed of a laser diode as an excitation light source, a photodiode detector, a signal analysis circuit, and a microfluidic chip as a microalgae cell transportation platform. To demonstrate the utility of this system, six different living and dead algae samples (Karenia mikimotoi Hansen, Chlorella vulgaris, Nitzschia closterium, Platymonas subcordiformis, Pyramidomonas delicatula and Dunaliella salina) were tested. The developed biosensor can distinguish clearly between the living microalgae cells and the dead microalgae cells. The smallest microalgae cells that can be detected by using this biosensor are 3 μm ones. Even smaller microalgae cells could be detected by increasing the excitation light power. The developed microfluidic biosensor has great potential for in situ ballast water analysis.

Design, fabrication and characterization of drug delivery systems based on lab-on-a-chip technology

Lab-on-a-chip technology is an emerging field evolving from the recent advances of micro- and nanotechnologies. The technology allows the integration of various components into a single microdevice. Microfluidics, the science and engineering of fluid flow in microscale, is the enabling underlying concept for lab-on-a-chip technology. The present paper reviews the design, fabrication and characterization of drug delivery systems based on this amazing technology. The systems are categorized and discussed according to the scales at which the drug is administered. Starting with the fundamentals on scaling laws of mass transfer and basic fabrication techniques, the paper reviews and discusses drug delivery devices for cellular, tissue and organism levels. At the cellular level, a concentration gradient generator integrated with a cell culture platform is the main drug delivery scheme of interest. At the tissue level, the synthesis of smart particles as drug carriers using lab-on-a-chip technology is the main focus of recent developments. At the organism level, microneedles and implantable devices with fluid-handling components are the main drug delivery systems. For drug delivery to a small organism that can fit into a microchip, devices similar to those of cellular level can be used.

Industrial lab-on-a-chip: design, applications and scale-up for drug discovery and delivery

Microfluidics is an emerging and promising interdisciplinary technology which offers powerful platforms for precise production of novel functional materials (e.g., emulsion droplets, microcapsules, and nanoparticles as drug delivery vehicles- and drug molecules) as well as high-throughput analyses (e.g., bioassays, detection, and diagnostics). In particular, multiphase microfluidics is a rapidly growing technology and has beneficial applications in various fields including biomedicals, chemicals, and foods. In this review, we first describe the fundamentals and latest developments in multiphase microfluidics for producing biocompatible materials that are precisely controlled in size, shape, internal morphology and composition. We next describe some microfluidic applications that synthesize drug molecules, handle biological substances and biological units, and imitate biological organs. We also highlight and discuss design, applications and scale up of droplet- and flow-based microfluidic devices used for drug discovery and delivery.

Building functional materials for health care and pharmacy from microfluidic principles and Flow Focusing

In this review, we aim at establishing a relationship between the fundamentals of the microfluidics technologies used in the Pharmacy field, and the achievements accomplished by those technologies. We describe the main methods for manufacturing micrometer drops, bubbles, and capsules, as well as the corresponding underlying physical mechanisms. In this regard, the review is intended to show non-specialist readers the dynamical processes which determine the success of microfluidics techniques. Flow focusing (FF) is a droplet-based method widely used to produce different types of fluid entities on a continuous basis by applying an extensional co-flow. We take this technique as an example to illustrate how microfluidics technologies for drug delivery are progressing from a deep understanding of the physics of fluids involved. Specifically, we describe the limitations of FF, and review novel methods which enhance its stability and robustness. In the last part of this paper, we review some of the accomplishments of microfluidics when it comes to drug manufacturing and delivery. Special attention is paid to the production of the microencapsulated form because this fluidic structure gathers the main functionalities sought for in Pharmacy. We also show how FF has been adapted to satisfy an ample variety of pharmaceutical requirements to date.

3D in vitro tissue models and their potential for drug screening

INTRODUCTION

The development of one standard, simplified in vitro three-dimensional tissue model suitable to biological and pathological investigation and drug-discovery may not yet be feasible, but standardized models for individual tissues or organs are a possibility. Tissue bioengineering, while concerned with finding methods of restoring functionality in disease, is developing technology that can be miniaturized for high throughput screening (HTS) of putative drugs. Through collaboration between biologists, physicists and engineers, cell-based assays are expanding into the realm of tissue analysis. Accordingly, three-dimensional (3D) micro-organoid systems will play an increasing role in drug testing and therapeutics over the next decade. Nevertheless, important hurdles remain before these models are fully developed for HTS.

AREAS COVERED

We highlight advances in the field of tissue bioengineering aimed at enhancing the success of drug candidates through pre-clinical optimization. We discuss models that are most amenable to high throughput screening with emphasis on detection platforms and data modeling.

EXPERT OPINION

Modeling 3D tissues to mimic in-vivo architecture remains a major challenge. As technology advances to provide novel methods of HTS analysis, so do potential pitfalls associated with such models and methods. We remain hopeful that integration of biofabrication with HTS will significantly reduce attrition rates in drug development.

Microfluidic opportunities in the field of nutrition

Nutrition has always been closely related to human health, which is a constant motivational force driving research in a variety of disciplines. Over the years, the rapidly emerging field of microfluidics has been pushing forward the healthcare industry with the development of microfluidic-based, point-of-care (POC) diagnostic devices. Though a great deal of work has been done in developing microfluidic platforms for disease diagnoses, potential microfluidic applications in the field of nutrition remain largely unexplored. In this Focus article, we would like to investigate the potential chances for microfluidics in the field of nutrition. We will first highlight some of the recent advances in microfluidic blood analysis systems that have the capacity to detect biomarkers of nutrition. Then we will examine existing examples of microfluidic devices for the detection of specific biomarkers of nutrition or nutrient content in food. Finally, we will discuss the challenges in this field and provide some insight into the future of applied microfluidics in nutrition.

Scalable alignment of three-dimensional cellular constructs in a microfluidic chip

There have been considerable efforts to engineer three-dimensional (3D) microfluidic environments to enhance cellular function over conventional two-dimensional (2D) cultures in microfluidic chips, but few involve topographical features, such as micro/nano-grooves, which are beneficial for cell types of cardiac, skeletal and neuronal lineages. Here we have developed a cost-effective and scalable method to incorporate micro-topographical cues into microfluidic chips to induce cell alignment. Using commercially available optical media as molds for replica molding, we produced large surface areas of polydimethylsiloxane (PDMS) micro-grooved substrates and plasma-bonded them to multiple microfluidic chips. Besides aligning a 2D monolayer of cells, the micro-grooved substrate can align 3D cellular constructs on chip. C2C12 mouse myoblasts were cultured three-dimensionally in a microfluidic chip with incorporated PDMS micro-grooved substrate remodeled into an aligned 3D cellular construct, where the actin cytoskeleton and nuclei were preferentially oriented along the micro-grooves. Cells within the 3D cellular constructs can align without being in direct contact with the micro-grooves due to synergism between topography and fluid shear stress. Aligned C2C12 3D cellular constructs showed enhanced differentiation into skeletal muscles as compared to randomly aligned ones. This novel method enables the routine inclusion of micro-topographical cues into 2D or 3D microfluidic cultures to generate relevant physiological models for studying tissue morphogenesis and drug screening applications.

An acoustofluidic micromixer based on oscillating sidewall sharp-edges

Rapid and homogeneous mixing inside a microfluidic channel is demonstrated via the acoustic streaming phenomenon induced by the oscillation of sidewall sharp-edges. By optimizing the design of the sharp-edges, excellent mixing performance and fast mixing speed can be achieved in a simple device, making our sharp-edge-based acoustic micromixer a promising candidate for a wide variety of applications.

Innovations in studying in vivo cell behavior and pharmacology in complex tissues--microvascular endothelial cells in the spotlight

Many studies on the molecular control underlying normal cell behavior and cellular responses to disease stimuli and pharmacological intervention are conducted in single-cell culture systems, while the read-out of cellular engagement in disease and responsiveness to drugs in vivo is often based on overall tissue responses. As the majority of drugs under development aim to specifically interact with molecular targets in subsets of cells in complex tissues, this approach poses a major experimental discrepancy that prevents successful development of new therapeutics. In this review, we address the shortcomings of the use of artificial (single) cell systems and of whole tissue analyses in creating a better understanding of cell engagement in disease and of the true effects of drugs. We focus on microvascular endothelial cells that actively engage in a wide range of physiological and pathological processes. We propose a new strategy in which in vivo molecular control of cells is studied directly in the diseased endothelium instead of at a (far) distance from the site where drugs have to act, thereby accounting for tissue-controlled cell responses. The strategy uses laser microdissection-based enrichment of microvascular endothelium which, when combined with transcriptome and (phospho)proteome analyses, provides a factual view on their status in their complex microenvironment. Combining this with miniaturized sample handling using microfluidic devices enables handling the minute sample input that results from this strategy. The multidisciplinary approach proposed will enable compartmentalized analysis of cell behavior and drug effects in complex tissue to become widely implemented in daily biomedical research and drug development practice.

Surface acoustic wave microfluidics

The recent introduction of surface acoustic wave (SAW) technology onto lab-on-a-chip platforms has opened a new frontier in microfluidics. The advantages provided by such SAW microfluidics are numerous: simple fabrication, high biocompatibility, fast fluid actuation, versatility, compact and inexpensive devices and accessories, contact-free particle manipulation, and compatibility with other microfluidic components. We believe that these advantages enable SAW microfluidics to play a significant role in a variety of applications in biology, chemistry, engineering and medicine. In this review article, we discuss the theory underpinning SAWs and their interactions with particles and the contacting fluids in which they are suspended. We then review the SAW-enabled microfluidic devices demonstrated to date, starting with devices that accomplish fluid mixing and transport through the use of travelling SAW; we follow that by reviewing the more recent innovations achieved with standing SAW that enable such actions as particle/cell focusing, sorting and patterning. Finally, we look forward and appraise where the discipline of SAW microfluidics could go next.

The future of the patient-specific Body-on-a-chip

As significant advancements in technology focused on Organ-on-a-chip continue, it is feasible to consider the future of Body-on-a-chip technology. With serious work being done to realize functioning artificial livers, kidneys, hearts, and lungs on chips, the next step is not only to interconnect these organs but also to consider the integration of stem cell technology to create interconnected patient-specific organs. Such a patient-specific Body-on-a-chip requires a sophisticated set of tools for micropattering cell cultures in 3D to create interconnected tissue-like organ structures. This review discusses advanced methods of the past two years in on-Chip organs, the complex 3D patterning of cultures and state-of-the-art scaffolding, and discusses some of the most relevant advancements in human-induced pluripotent stem cell (hiPSC) research applied to these organs and scaffolds for the future of a patient-specific Body-on-a-chip. We anticipate that such a technology would have a wide area of application, primarily benefiting drug development, chemical safety testing, and disease modeling.

Fusion of nonclinical and clinical data to predict human drug safety

Adverse drug reactions continue to be a major cause of morbidity in both patients receiving therapeutics and in drug R&D programs. Predicting and possibly eliminating these adverse events remains a high priority in industry, government agencies and healthcare systems. With small molecule candidates, the fusion of nonclinical and clinical data is essential in establishing an overall system that creates a true translational science approach. Several new advances are taking place that attempt to create a 'patient context' mechanism early in drug research and development and ultimately into the marketplace. This 'life-cycle' approach has as its core the development of human-oriented, nonclinical end points and the incorporation of clinical knowledge at the drug design stage. The next 5 years should witness an explosion of what the author views as druggable and safe chemical space, pharmacosafety molecular targets and the most important aspect, an understanding of unique susceptibilities in patients developing adverse drug reactions. Our current knowledge of clinical safety relies completely on pharmacovigilance data from approved and marketed drugs, with a few exceptions of drugs failing in clinical trials. Massive data repositories now and soon to be available via cloud computing should stimulate a major effort in expanding our view of clinical drug safety and its incorporation into early drug research and development.

Probing cell-cell communication with microfluidic devices

Intercellular communication is a mechanism that regulates critical events during embryogenesis and coordinates signalling within differentiated tissues, such as the nervous and cardiovascular systems. To perform specialized activities, these tissues utilize the rapid exchange of signals among networks that, while are composed of different cell types, are nevertheless functionally coupled. Errors in cellular communication can lead to varied deleterious effects such as degenerative and autoimmune diseases. However, the intercellular communication network is extremely complex in multicellular organisms making isolation of the functional unit and study of basic mechanisms technically challenging. New experimental methods to examine mechanisms of intercellular communication among cultured cells could provide insight into physiological and pathological processes alike. Recent developments in microfluidic technology allow miniaturized and integrated devices to perform intercellular communication experiments on-chip. Microfluidics have many advantages, including the ability to replicate in vitro the chemical, mechanical, and physical cellular microenvironment of tissues with precise spatial and temporal control combined with dynamic characterization, high throughput, scalability and reproducibility. In this Focus article, we highlight some of the recent work and advances in the application of microfluidics to the study of mammalian intercellular communication with particular emphasis on cell contact and soluble factor mediated communication. In addition, we provide some insights into likely direction of the future developments in this field.

Easy monitoring of velocity fields in microfluidic devices using spatiotemporal image correlation spectroscopy

Spatiotemporal image correlation spectroscopy (STICS) is a simple and powerful technique, well established as a tool to probe protein dynamics in cells. Recently, its potential as a tool to map velocity fields in lab-on-a-chip systems was discussed. However, the lack of studies on its performance has prevented its use for microfluidics applications. Here, we systematically and quantitatively explore STICS microvelocimetry in microfluidic devices. We exploit a simple experimental setup, based on a standard bright-field inverted microscope (no fluorescence required) and a high-fps camera, and apply STICS to map liquid flow in polydimethylsiloxane (PDMS) microchannels. Our data demonstrates optimal 2D velocimetry up to 10 mm/s flow and spatial resolution down to 5 μm.

Downscaling the analysis of complex transmembrane signaling cascades to closed attoliter volumes

Cellular signaling is classically investigated by measuring optical or electrical properties of single or populations of living cells. Here we show that ligand binding to cell surface receptors and subsequent activation of signaling cascades can be monitored in single, (sub-)micrometer sized native vesicles with single-molecule sensitivity. The vesicles are derived from live mammalian cells using chemicals or optical tweezers. They comprise parts of a cell's plasma membrane and cytosol and represent the smallest autonomous containers performing cellular signaling reactions thus functioning like minimized cells. Using fluorescence microscopies, we measured in individual vesicles the different steps of G-protein-coupled receptor mediated signaling like ligand binding to receptors, subsequent G-protein activation and finally arrestin translocation indicating receptor deactivation. Observing cellular signaling reactions in individual vesicles opens the door for downscaling bioanalysis of cellular functions to the attoliter range, multiplexing single cell analysis, and investigating receptor mediated signaling in multiarray format.

Pharmacology on microfluidics: multimodal analysis for studying cell-cell interaction

Understanding the mechanisms of cell-cell interaction is a key unanswered question in modern pharmacology, given crosstalk defects are at the basis of many pathologies. Microfluidics represents a valuable tool for analyzing intercellular communication mediated by transmission of soluble signals, as occurring for example between neurons and glial cells in neuroinflammation, or between tumor and surrounding cells in cancer. However, the use of microfluidics for studying cell behavior still encompasses many technical and biological challenges. In this review, a state of the art of successes, potentials and limitations of microfluidics applied to key biological questions in modern pharmacology is analyzed and commented.

A 3D microfluidic platform incorporating methacrylated gelatin hydrogels to study physiological cardiovascular cell-cell interactions

The cardiovascular system is particularly well-suited to modelling with microfluidic technologies, and much progress has been made to create microfluidic devices that mimic the microvasculature. In contrast, microfluidic platforms that model larger blood vessels and heart valves are lacking, despite the clear potential benefits of improved physiological relevance and enhanced throughput over traditional cell culture technologies. To address this need, we developed a bilayer membrane microfluidic device to model the vascular/valvular three-dimensional environment. Key features of the platform include physiologically-relevant spatial arrangement of multiple cell types, fluid flow over an endothelial monolayer, a porous membrane that permits heterotypic cell interactions while maintaining cell compartmentalization, and a photopolymerizable gelatin methacrylate (gel-MA) hydrogel as a physiologically-relevant subendothelial 3D matrix. Processing guidelines were defined for successful in-channel polymerization of gel-MA hydrogels that were mechanically stable, had physiologically-relevant elastic moduli of 2-30 kPa, and supported over 80% primary cell viability for at least four days in culture. The platform was applied to investigate shear stress-regulated paracrine interactions between valvular endothelial cells and valvular interstitial cells. The presence of endothelial cells significantly suppressed interstitial cell pathological differentiation to α-smooth muscle actin-positive myofibroblasts, an effect that was enhanced when the endothelium was exposed to flow-induced shear stress. We expect this versatile organ-on-a-chip platform to have broad utility for mechanistic vascular and valvular biology studies and to be useful for drug screening in physiologically-relevant 3D cardiovascular microenvironments.

The pharmacology of regenerative medicine

Regenerative medicine is a rapidly evolving multidisciplinary, translational research enterprise whose explicit purpose is to advance technologies for the repair and replacement of damaged cells, tissues, and organs. Scientific progress in the field has been steady and expectations for its robust clinical application continue to rise. The major thesis of this review is that the pharmacological sciences will contribute critically to the accelerated translational progress and clinical utility of regenerative medicine technologies. In 2007, we coined the phrase "regenerative pharmacology" to describe the enormous possibilities that could occur at the interface between pharmacology, regenerative medicine, and tissue engineering. The operational definition of regenerative pharmacology is "the application of pharmacological sciences to accelerate, optimize, and characterize (either in vitro or in vivo) the development, maturation, and function of bioengineered and regenerating tissues." As such, regenerative pharmacology seeks to cure disease through restoration of tissue/organ function. This strategy is distinct from standard pharmacotherapy, which is often limited to the amelioration of symptoms. Our goal here is to get pharmacologists more involved in this field of research by exposing them to the tools, opportunities, challenges, and interdisciplinary expertise that will be required to ensure awareness and galvanize involvement. To this end, we illustrate ways in which the pharmacological sciences can drive future innovations in regenerative medicine and tissue engineering and thus help to revolutionize the discovery of curative therapeutics. Hopefully, the broad foundational knowledge provided herein will spark sustained conversations among experts in diverse fields of scientific research to the benefit of all.

Lab-on-a-chip technologies for single-molecule studies

Recent developments on various lab-on-a-chip techniques allow miniaturized and integrated devices to perform on-chip single-molecule studies. Fluidic-based platforms that utilize unique microscale fluidic behavior are capable of conducting single-molecule experiments with high sensitivities and throughputs, while biomolecular systems can be studied on-chip using techniques such as DNA curtains, magnetic tweezers, and solid-state nanopores. The advances of these on-chip single-molecule techniques lead to next-generation lab-on-a-chip devices, such as DNA transistors, and single-molecule real-time (SMRT) technology for rapid and low-cost whole genome DNA sequencing. In this Focus article, we will discuss some recent successes in the development of lab-on-a-chip techniques for single-molecule studies and expound our thoughts on the near future of on-chip single-molecule studies.

Multifunctional porous silicon nanopillar arrays: antireflection, superhydrophobicity, photoluminescence, and surface-enhanced Raman scattering

We have fabricated porous silicon nanopillar arrays over large areas with a rapid, simple, and low-cost technique. The porous silicon nanopillars show unique longitudinal features along their entire length and have porosity with dimensions on the single-nanometer scale. Both Raman spectroscopy and photoluminescence data were used to determine the nanocrystallite size to be <3 nm. The porous silicon nanopillar arrays also maintained excellent ensemble properties, reducing reflection nearly fivefold from planar silicon in the visible range without any optimization, and approaching superhydrophobic behavior with increasing aspect ratio, demonstrating contact angles up to 138°. Finally, the porous silicon nanopillar arrays were made into sensitive surface-enhanced Raman scattering (SERS) substrates by depositing metal onto the pillars. The SERS performance of the substrates was demonstrated using a chemical dye Rhodamine 6G. With their multitude of properties (i.e., antireflection, superhydrophobicity, photoluminescence, and sensitive SERS), the porous silicon nanopillar arrays described here can be valuable in applications such as solar harvesting, electrochemical cells, self-cleaning devices, and dynamic biological monitoring.

An on-chip, multichannel droplet sorter using standing surface acoustic waves

The emerging field of droplet microfluidics requires effective on-chip handling and sorting of droplets. In this work, we demonstrate a microfluidic device that is capable of sorting picoliter water-in-oil droplets into multiple outputs using standing surface acoustic waves (SSAW). This device integrates a single-layer microfluidic channel with interdigital transducers (IDTs) to achieve on-chip droplet generation and sorting. Within the SSAW field, water-in-oil droplets experience an acoustic radiation force and are pushed toward the acoustic pressure node. As a result, by tuning the frequency of the SSAW excitation, the position of the pressure nodes can be changed and droplets can be sorted to different outlets at rates up to 222 droplets s(-1). With its advantages in simplicity, controllability, versatility, noninvasiveness, and capability to be integrated with other on-chip components such as droplet manipulation and optical detection units, the technique presented here could be valuable for the development of droplet-based micro total analysis systems (μTAS).

A simple and versatile microfluidic cell density gradient generator for quantum dot cytotoxicity assay

In this work, a simple and versatile microfluidic cell density gradient generator was successfully developed for cytotoxicity of quantum dots (QDs) assay. The microfluidic cell density gradient generator is composed of eight parallel channels which are respectively surrounded by 1-8 microwells with optimized length and width. The cells fall into microwells by gravity and the cell densities are obviously dependent of microwell number. In a case study, HepG2 and MCF-7 cells were successfully utilized for generating cell density gradients on the microfluidic chip. The microfluidic cell density gradient generator was proved to be easily handled, cell-friendly and could be used to conduct the subsequent cell-based assay. As a proof-of-concept, QD cytotoxicity was evaluated and the results exhibited obvious cell density-dependence. For comparison, QD cytotoxicity was also investigated with a series of cell densities infused by pipette tips. Higher reproducibility was observed on the microfluidic cell density gradient generator and cell density was demonstrated to be a vital factor in cytotoxic study. With higher efficiency, controllability and reproducibility, the microfluidic cell density gradient generator could be integrated into microfluidic analysis systems to promote chip-based biological assay.

A microfluidic system to study the cytotoxic effect of drugs: the combined effect of celecoxib and 5-fluorouracil on normal and cancer cells

We have investigated the response of normal and cancer cells to exposure a combination of celecoxib (Celbx) and 5-fluorouracil (5-FU) using a lab-on-a-chip microfluidic device. Specifically, we have tested the cytotoxic effect of Celbx on normal mouse embryo cells (Balb/c 3T3) and human lung carcinoma cells (A549). The single drugs or their combinations were adjusted to five different concentrations using a concentration gradient generator (CGG) in a single step. The results suggest that Celbx can enhanced the anticancer activity of 5-FU by stronger inhibition of cancer cell growth. We also show that the A549 cancer cells are more sensitive to Celbx than the Balb/c 3T3 normal cells. The results obtained with the microfluidic system were compared to those obtained with a macroscale in vitro cell culture method. In our opinion, the microfluidic system represents a unique approach for an evaluation of cellular response to multidrug exposure that also is more simple than respective microwell plate assays. Figureᅟ

Advances in the development and use of human tissue-based techniques for drug toxicity testing

INTRODUCTION

Unacceptable failure rates in clinical trials are largely responsible for the high costs of bringing successful drugs to market - costs that are passed on to patients, insurers or healthcare providers. Furthermore, failures in clinical trials deny patients much-needed new drugs and potentially expose them to unnecessary risk. With so many medicines reaching their patent expiry date, pressure is on the pharmaceutical industry to not only increase its output of effective medicines but also improve its ability to minimise safety issues.

AREAS COVERED

This review focuses on the availability and use of human tissues and their derivatives to explore potential toxicity problems of new drugs. The growth in the number and quality of human material-based assays and enabling technologies is reviewed, followed by a discussion of the application of such assays to identify specific toxicities, using specific examples.

EXPERT OPINION

Although human tissues are now beginning to be seen as playing an important role in evaluating the potential for toxicity of new drugs in the clinic, their importance deserves to be more widely recognised and their use in the identification of toxicity issues as early as possible in the drug development life cycle should be significantly increased.

Optoacoustic tweezers: a programmable, localized cell concentrator based on opto-thermally generated, acoustically activated, surface bubbles

We present a programmable, biocompatible technique for dynamically concentrating and patterning particles and cells in a microfluidic device. Since our technique utilizes opto-thermally generated, acoustically activated, surface bubbles, we name it "optoacoustic tweezers". The optoacoustic tweezers are capable of concentrating particles/cells at any prescribed locations in a microfluidic chamber without the use of permanent structures, rendering it particularly useful for the formation of flexible, complex cell patterns. Additionally, this technique has demonstrated excellent biocompatibility and can be conveniently integrated with other microfluidic units. In our experiments, micro-bubbles were generated by focusing a 405 nm diode laser onto a gold-coated glass chamber. By properly tuning the laser, we demonstrate precise control over the position and size of the generated bubbles. Acoustic waves were then applied to activate the surface bubbles, causing them to oscillate at an optimized frequency. The resulting acoustic radiation force allowed us to locally trap particles/cells, including 15 μm polystyrene beads and HeLa cells, around each bubble. Cell-adhesion tests were also conducted after cell concentrating to confirm the biocompatibility of this technique.

A fully unsupervised compartment-on-demand platform for precise nanoliter assays of time-dependent steady-state enzyme kinetics and inhibition

The ability to miniaturize biochemical assays in water-in-oil emulsion droplets allows a massive scale-down of reaction volumes, so that high-throughput experimentation can be performed more economically and more efficiently. Generating such droplets in compartment-on-demand (COD) platforms is the basis for rapid, automated screening of chemical and biological libraries with minimal volume consumption. Herein, we describe the implementation of such a COD platform to perform high precision nanoliter assays. The coupling of a COD platform to a droplet absorbance detection set-up results in a fully automated analytical system. Michaelis–Menten parameters of 4-nitrophenyl glucopyranoside hydrolysis by sweet almond β-glucosidase can be generated based on 24 time-courses taken at different substrate concentrations with a total volume consumption of only 1.4 μL. Importantly, kinetic parameters can be derived in a fully unsupervised manner within 20 min: droplet production (5 min), initial reading of the droplet sequence (5 min), and droplet fusion to initiate the reaction and read-out over time (10 min). Similarly, the inhibition of the enzymatic reaction by conduritol B epoxide and 1-deoxynojirimycin was measured, and K_i values were determined. In both cases, the kinetic parameters obtained in droplets were identical within error to values obtained in titer plates, despite a >10⁴-fold volume reduction, from micro- to nanoliters.

Unconventional microfluidics: expanding the discipline

Since its inception, the discipline of microfluidics has been harnessed for innovations in the biomedicine/chemistry fields-and to great effect. This success has had the natural side-effect of stereotyping microfluidics as a platform for medical diagnostics and miniaturized lab processes. But microfluidics has more to offer. And very recently, some researchers have successfully applied microfluidics to fields outside its traditional domains. In this Focus article, we highlight notable examples of such "unconventional" microfluidics applications (e.g., robotics, electronics). It is our hope that these early successes in unconventional microfluidics prompt further creativity, and inspire readers to expand the microfluidics discipline.

Evaluation of a microchip electrophoresis-mass spectrometry platform deploying a pressure-driven make-up flow

Integration of a pressure-driven make-up flow (MUF) into a microchip electrophoresis (MCE) platform in order to facilitate its coupling with electrospray ionization-mass spectrometric detection (ESI-MS) is described. In the glass/PDMS hybrid microchip, a MUF channel was made to intersect with the MCE separation channel at an angle of 45°. The MUF was generated by a syringe pump. Microscopic image results from simulation studies showed that the pressure-driven MUF and the potential-driven electroosmotic flow in the MCE separation channel could be run separately without interfering with each other and mixed well at the joint point by adjusting either the MUF flow rate or the potential applied for MCE separation. The MUF had several desirable functions, including making the start of electrospray easy and cleaning the nanoESI emitter continuously when not spraying. High separation efficiency was achieved with the proposed MCE-nanoESI-MS system in separating an amino acid mixture containing glutamine, serine, threonine, phenylalanine, and glutamic acid. All of them were baseline separated from each other within 3 min. Plate numbers of >10,000 (on a 2.5 cm MCE separation channel) were obtained. The analytical platform also showed a linear response for quantification of DOPA with a detection limit (S/N=3) of 0.10 μM. In addition, on-line derivatization of MCE elutes in order to enhance MS detection sensitivity was easily carried out by adding the tagging reagent into the MUF. These results indicated that the present system might have a good potential in MCE-MS applications.

Effect of wetting on capillary pumping in microchannels

We investigate capillary pumping in microchannels both experimentally and numerically. Putting two droplets of different sizes at the in/outlet of a microchannel, there will in general be a flow from the smaller droplet to the larger one due to the Laplace pressure difference. We show that an unusual flow from a larger droplet into a smaller one is possible by manipulating the wetting properties, notably the contact line pinning. In addition, we propose a way to actively control the flow by electrowetting.

Fluorescence-based high-throughput functional profiling of ligand-gated ion channels at the level of single cells

Ion channels are involved in many physiological processes and are attractive targets for therapeutic intervention. Their functional properties vary according to their subunit composition, which in turn varies in a developmental and tissue-specific manner and as a consequence of pathophysiological events. Understanding this diversity requires functional analysis of ion channel properties in large numbers of individual cells. Functional characterisation of ligand-gated channels involves quantitating agonist and drug dose-response relationships using electrophysiological or fluorescence-based techniques. Electrophysiology is limited by low throughput and high-throughput fluorescence-based functional evaluation generally does not enable the characterization of the functional properties of each individual cell. Here we describe a fluorescence-based assay that characterizes functional channel properties at single cell resolution in high throughput mode. It is based on progressive receptor activation and iterative fluorescence imaging and delivers >100 dose-responses in a single well of a 384-well plate, using α1-3 homomeric and αβ heteromeric glycine receptor (GlyR) chloride channels as a model system. We applied this assay with transiently transfected HEK293 cells co-expressing halide-sensitive yellow fluorescent protein and different GlyR subunit combinations. Glycine EC₅₀ values of different GlyR isoforms were highly correlated with published electrophysiological data and confirm previously reported pharmacological profiles for the GlyR inhibitors, picrotoxin, strychnine and lindane. We show that inter and intra well variability is low and that clustering of functional phenotypes permits identification of drugs with subunit-specific pharmacological profiles. As this method dramatically improves the efficiency with which ion channel populations can be characterized in the context of cellular heterogeneity, it should facilitate systems-level analysis of ion channel properties in health and disease and the discovery of therapeutics to reverse pathological alterations.

Systems approaches in risk assessment

Adverse drug events (ADEs) remain a universal problem in drug development, regulatory review, and clinical practice with a substantial financial burden on the global health-care system. Recent advances in molecular and "omics" technologies, along with online databases and bioinformatics, have enabled a more integrative approach to understanding drug-target (protein) interactions, both desirable and undesirable, within a biological system. This has led to the development of systems approaches to risk assessment in an attempt to complement and improve on contemporary observational and predictive strategies for assessing risk. Although still in an evolutionary phase, systems approaches have the potential to markedly advance our understanding of ADEs and ability to predict them. Systems approaches will also move personalized medicine forward by enabling better identification of individual and subgroup risk factors.

Tunable, pulsatile chemical gradient generation via acoustically driven oscillating bubbles

We present a novel concept of generating both static and pulsatile chemical gradients using acoustically activated bubbles designed in a ladder-like arrangement. Furthermore, by regulating the amplitude of the bubble oscillation, we demonstrate that the chemical gradient profiles can be effectively tuned.

Optofluidic imaging: now and beyond

More than a decade of research work in optofluidics has yielded a large catalogue of optofluidic elements that can manipulate light at the micro-scale (e.g., lenses, prisms). Although these elements have proven useful for many on-chip processes (e.g., miniaturized flow cytometry, interferometry and sample spectroscopy), certain deficiencies have precluded their use in micro-scale imaging. However, recent work in optofluidic imaging has avoided optofluidic elements entirely and focused instead on image capture and composition techniques, demonstrating impressive resolution in both 2D imagery and 3D tomography. In this Focus article, we will discuss some of the recent successes in optofluidic imaging and will expound our expectations for the near future of the optofluidic imaging discipline.

A hybrid microfluidic platform for cell-based assays via diffusive and convective trans-membrane perfusion

We present a novel 3D hybrid assembly of a polymer microfluidic chip with polycarbonate track-etched membrane (PCTEM) enabling membrane-supported cell culture. Two chip designs have been developed to establish either diffusive or convective reagent delivery using the integrated PCTEM. While it is well suited to a range of cell-based assays, we specifically employ this platform for the screening of a common antitumor chemotoxic agent (mitomycin C - MMC) on the HL60 myeloid leukemia cell line. The toxic activity of MMC is based on the generation of severe DNA damage in the cells. Using either mode of operation, the HL60 cells were cultured on-chip before, during, and after exposure to MMC at concentrations ranging from 0 to 50 μM. Cell viability was analysed off-chip by the trypan blue dye exclusion assay. The results of the on-chip viability assay were found to be consistent with those obtained off-chip and indicated ca. 40% cell survival at MMC concentration of 50 μM. The catalogue of capabilities of the here described cell assay platform comprises of (i) the culturing of cells either under shear-free conditions or under induced through-membrane flows, (ii) the tight time control of the reagent exposure, (iii) the straightforward assembly of devices, (iv) the flexibility on the choice of the membrane, and, prospectively, (v) the amenability for large-scale parallelization.

Systematic reviews of animal models: methodology versus epistemology

Systematic reviews are currently favored methods of evaluating research in order to reach conclusions regarding medical practice. The need for such reviews is necessitated by the fact that no research is perfect and experts are prone to bias. By combining many studies that fulfill specific criteria, one hopes that the strengths can be multiplied and thus reliable conclusions attained. Potential flaws in this process include the assumptions that underlie the research under examination. If the assumptions, or axioms, upon which the research studies are based, are untenable either scientifically or logically, then the results must be highly suspect regardless of the otherwise high quality of the studies or the systematic reviews. We outline recent criticisms of animal-based research, namely that animal models are failing to predict human responses. It is this failure that is purportedly being corrected via systematic reviews. We then examine the assumption that animal models can predict human outcomes to perturbations such as disease or drugs, even under the best of circumstances. We examine the use of animal models in light of empirical evidence comparing human outcomes to those from animal models, complexity theory, and evolutionary biology. We conclude that even if legitimate criticisms of animal models were addressed, through standardization of protocols and systematic reviews, the animal model would still fail as a predictive modality for human response to drugs and disease. Therefore, systematic reviews and meta-analyses of animal-based research are poor tools for attempting to reach conclusions regarding human interventions.

A droplet-based, optofluidic device for high-throughput, quantitative bioanalysis

Analysis of chemical or biomolecular contents in a tiny amount of specimen presents a significant challenge in many biochemical studies and diagnostic applications. In this work, we present a single-layer, optofluidic device for real-time, high-throughput, quantitative analysis of droplet contents. Our device integrates an optical fiber-based, on-chip detection unit with a droplet-based microfluidic unit. It can quantitatively analyze the contents of individual droplets in real-time. It also achieves a detection throughput of 2000 droplets per second, a detection limit of 20 nM, and an excellent reproducibility in its detection results. In a proof-of-concept study, we demonstrate that our device can be used to perform detection of DNA and its mutations by monitoring the fluorescent signal changes of the target DNA/molecular beacon complex in single droplets. Our approach can be immediately extended to a real-time, high-throughput detection of other biomolecules (such as proteins and viruses) in droplets. With its advantages in throughput, functionality, cost, size, and reliability, the droplet-based optofluidic device presented here can be a valuable tool for many medical diagnostic applications.