Connected culture of murine hepatocytes and HUVEC in a multicompartmental bioreactor

gH625-liposomes deliver PACAP through a dynamic in vitro model of the blood–brain barrier

The blood–brain barrier (BBB) selectively protects the central nervous system (CNS) from external insults, but its function can represent a limit for the passage of therapeutic molecules. Numerous in vitro models of the BBB have been realized in order to study the passage of drugs for neurodegenerative diseases, but these in vitro models are not very representative of the physiological conditions because of a limited supply of oxygen and nutrients due to static conditions. To avoid this phenomenon, we used a millifluidic bioreactor model that ensures a circulation of the medium and, therefore, of the nutrients, thanks to the continuous laminar flow. This dynamic model consists of a double-culture chamber separated by a membrane on which brain endothelial cells are cultured in order to evaluate the passage of the drug. Furthermore, in the lower chamber, SH-SY5Y were seeded as 3D spheroids to evaluate the drug passage through these cells. As nanodelivery system, we used liposomes functionalized with viral fusion peptide to evaluate the passage of a neuroprotective agent, pituitary adenylate cyclase-activating polypeptide (PACAP), through the dynamic in vitro model of the BBB. We showed that our nanodelivery system, made of functionalized liposomes and loaded with specific molecules, efficiently crosses the in vitro fluid-dynamic model of the BBB. Our findings represent an important step for further experimental investigations on PACAP administration as a therapeutic agent by an enhanced drug delivery system. Our results can improve the diffusion of good practice in neuroscience laboratories, helping to spread the 3R rules.

Patient-Specific Organoid and Organ-on-a-Chip: 3D Cell-Culture Meets 3D Printing and Numerical Simulation

The last few decades have witnessed diversified in vitro models to recapitulate the architecture and function of living organs or tissues and contribute immensely to advances in life science. Two novel 3D cell culture models: 1) Organoid, promoted mainly by the developments of stem cell biology and 2) Organ-on-a-chip, enhanced primarily due to microfluidic technology, have emerged as two promising approaches to advance the understanding of basic biological principles and clinical treatments. This review describes the comparable distinct differences between these two models and provides more insights into their complementarity and integration to recognize their merits and limitations for applicable fields. The convergence of the two approaches to produce multi-organoid-on-a-chip or human organoid-on-a-chip is emerging as a new approach for building 3D models with higher physiological relevance. Furthermore, rapid advancements in 3D printing and numerical simulations, which facilitate the design, manufacture, and results-translation of 3D cell culture models, can also serve as novel tools to promote the development and propagation of organoid and organ-on-a-chip systems. Current technological challenges and limitations, as well as expert recommendations and future solutions to address the promising combinations by incorporating organoids, organ-on-a-chip, 3D printing, and numerical simulation, are also summarized.

Multi-organ-on-a-chip for pharmacokinetics and toxicokinetic study of drugs

Introduction: Accurate prediction of pharmacokinetic (PK) and toxicokinetics (TK) of drugs is imperative for successful development of new pharmaceutics. Although conventional in vitro methods for predicting the PK and TK of drugs are well established, limitations still exist and more advanced chip-based in vitro platforms combined with mathematical models can help researchers overcome the limitations. Areas covered: We will review recent progress in the development of multi-organ-on-a-chip platforms for predicting PK and TK of drugs, as well as mathematical approaches that can be combined with these platforms for experiment design, data analysis and in vitro-in vivo extrapolation (IVIVE) for application to humans. Expert opinion: Although there remain some challenges to be addressed, the remarkable progress in the area of multi-organ-on-a-chip in recent years indicate that we will see tangible outcomes that can be utilized in the pharmaceutical industry in near future.

Construction of higher-order cellular microstructures by a self-wrapping co-culture strategy using a redox-responsive hydrogel

In this report, a strategy for constructing three-dimensional (3D) cellular architectures comprising viable cells is presented. The strategy uses a redox-responsive hydrogel that degrades under mild reductive conditions, and a confluent monolayer of cells (i.e., cell sheet) cultured on the hydrogel surface peels off and self-folds to wrap other cells. As a proof-of-concept, the self-folding of fibroblast cell sheet was triggered by immersion in aqueous cysteine, and this folding process was controlled by the cysteine concentration. Such folding enabled the wrapping of human hepatocellular carcinoma (HepG2) spheroids, human umbilical vein endothelial cells and collagen beads, and this process improved cell viability, the secretion of metabolites and the proliferation rate of the HepG2 cells when compared with a two-dimensional culture under the same conditions. A key concept of this study is the ability to interact with other neighbouring cells, providing a new, simple and fast method to generate higher-order cellular aggregates wherein different types of cellular components are added. We designated the method of using a cell sheet to wrap another cellular aggregate the 'cellular Furoshiki'. The simple self-wrapping Furoshiki technique provides an alternative approach to co-culture cells by microplate-based systems, especially for constructing heterogeneous 3D cellular microstructures.

Biopolymer Microparticles Prepared by Microfluidics for Biomedical Applications

Biopolymers are macromolecules that are derived from natural sources and have attractive properties for a plethora of biomedical applications due to their biocompatibility, biodegradability, low antigenicity, and high bioactivity. Microfluidics has emerged as a powerful approach for fabricating polymeric microparticles (MPs) with designed structures and compositions through precise manipulation of multiphasic flows at the microscale. The synergistic combination of materials chemistry afforded by biopolymers and precision provided by microfluidic capabilities make it possible to design engineered biopolymer-based MPs with well-defined physicochemical properties that are capable of enabling an efficient delivery of therapeutics, 3D culture of cells, and sensing of biomolecules. Here, an overview of microfluidic approaches is provided for the design and fabrication of functional MPs from three classes of biopolymers including polysaccharides, proteins, and microbial polymers, and their advances for biomedical applications are highlighted. An outlook into the future research on microfluidically-produced biopolymer MPs for biomedical applications is also provided.

Strategies for using mathematical modeling approaches to design and interpret multi-organ microphysiological systems (MPS)

Recent advances in organ-on-a-chip technology have resulted in numerous examples of microscale systems that faithfully mimic the physiology and pathology of human organs and diseases. The next step in this field, which has already been partially demonstrated at a proof-of-concept level, would be integration of organ modules to construct multiorgan microphysiological systems (MPSs). In particular, there is interest in "body-on-a-chip" models, which recapitulate complex and dynamic interactions between different organs. Integration of multiple organ modules, while faithfully reflecting human physiology in a quantitative sense, will require careful consideration of factors such as relative organ sizes, blood flow rates, cell numbers, and ratios of cell types. The use of a mathematical modeling platform will be an essential element in designing multiorgan MPSs and interpretation of experimental results. Also, extrapolation to in vivo will require robust mathematical modeling techniques. So far, several scaling methods and pharmacokinetic and physiologically based pharmacokinetic models have been applied to multiorgan MPSs, with each method being suitable to a subset of different objectives. Here, we summarize current mathematical methodologies used for the design and interpretation of multiorgan MPSs and suggest important considerations and approaches to allow multiorgan MPSs to recapitulate human physiology and disease progression better, as well as help in vitro to in vivo translation of studies on response to drugs or chemicals.

Integrated Microphysiological Systems: Transferable Organ Models and Recirculating Flow

Studying and understanding of tissue and disease mechanisms largely depend on the availability of suitable and representative biological model systems. These model systems should be carefully engineered and faithfully reproduce the biological system of interest to understand physiological effects, pharmacokinetics, and toxicity to better identify new drug compounds. By relying on microfluidics, microphysiological systems (MPSs) enable the precise control of culturing conditions and connections of advanced in vitro 3D organ models that better reproduce in vivo environments. This review focuses on transferable in vitro organ models and integrated MPSs that host these transferable biological units and enable interactions between different tissue types. Interchangeable and transferrable in vitro organ models allow for independent quality control of the biological model before system assembly and building MPS assays on demand. Due to the complexity and different maturation times of individual in vitro tissues, off-chip production and quality control entail improved stability and reproducibility of the systems and results, which is important for large-scale adoption of the technology. Lastly, the technical and biological challenges and open issues for realizing and implementing integrated MPSs with transferable in vitro organ models are discussed.

Systemic and vascular inflammation in an in-vitro model of central obesity

Metabolic disorders due to over-nutrition are a major global health problem, often associated with obesity and related morbidities. Obesity is peculiar to humans, as it is associated with lifestyle and diet, and so difficult to reproduce in animal models. Here we describe a model of human central adiposity based on a 3-tissue system consisting of a series of interconnected fluidic modules. Given the causal link between obesity and systemic inflammation, we focused primarily on pro-inflammatory markers, examining the similarities and differences between the 3-tissue model and evidence from human studies in the literature. When challenged with high levels of adiposity, the in-vitro system manifests cardiovascular stress through expression of E-selectin and von Willebrand factor as well as systemic inflammation (expressing IL-6 and MCP-1) as observed in humans. Interestingly, most of the responses are dependent on the synergic interaction between adiposity and the presence of multiple tissue types. The set-up has the potential to reduce animal experiments in obesity research and may help unravel specific cellular mechanisms which underlie tissue response to nutritional overload.

Multiorgan Microphysiological Systems for Drug Development: Strategies, Advances, and Challenges

Traditional cell culture and animal models utilized for preclinical drug screening have led to high attrition rates of drug candidates in clinical trials due to their low predictive power for human response. Alternative models using human cells to build in vitro biomimetics of the human body with physiologically relevant organ-organ interactions hold great potential to act as "human surrogates" and provide more accurate prediction of drug effects in humans. This review is a comprehensive investigation into the development of tissue-engineered human cell-based microscale multiorgan models, or multiorgan microphysiological systems for drug testing. The evolution from traditional models to macro- and microscale multiorgan systems is discussed in regards to the rationale for recent global efforts in multiorgan microphysiological systems. Current advances in integrating cell culture and on-chip analytical technologies, as well as proof-of-concept applications for these multiorgan microsystems are discussed. Major challenges for the field, such as reproducibility and physiological relevance, are discussed with comparisons of the strengths and weaknesses of various systems to solve these challenges. Conclusions focus on the current development stage of multiorgan microphysiological systems and new trends in the field.

Spontaneous hepatocyte migration towards an endothelial cell tube network

A crucial part of the engineering liver tissue is contribution of nonparenchymal cells and maintenance of a complex three-dimensional (3D) structure in vitro for their normal physiology and function. We generated 3D hepatic tissue using primary isolated rat hepatocytes and an endothelial cell tube network from human endothelial vein epithelial cells (HUVECs). To create the 3D hepatic tissue, coculture of primary hepatocytes and tube-structured HUVECs was performed on a Matrigel®. After the HUVECs formed the tube structures, primary isolated rat hepatocytes were inoculated onto the HUVEC tube-structured layer and cultured for 24 hr. We investigated the cell migration, cellular interaction, and distributions of HUVEC tube structures and hepatocytes using multi cell-imaging incubator, confocal microscopy, and electron microscopy analyses. During the culture time, time-lapse imaging showed spontaneous migration of the hepatocytes in the gel, and after the 24-hr culture period, the vast majority of the hepatocytes had moved and adhered to the surface of the HUVEC tube structures. A confocal microscopy assay confirmed this unique 3D cellular interaction between hepatocytes and HUVEC tube structures. The hepatocytes were able to maintain their spherical shape, as well as HUVECs (tube-like form with tubular cavity). We speculate that coculturing of hepatocytes and endothelial cells replicates part of their normal physiology and may help induce migration in vitro and the growth of complex biological tissue structures.

Building an experimental model of the human body with non-physiological parameters

New advances in engineering and biomedical technology have enabled recent efforts to capture essential aspects of human physiology in microscale, in-vitro systems. The application of these advances to experimentally model complex processes in an integrated platform - commonly called a 'human-on-a-chip (HOC)' - requires that relevant compartments and parameters be sized correctly relative to each other and to the system as a whole. Empirical observation, theoretical treatments of resource distribution systems and natural experiments can all be used to inform rational design of such a system, but technical and fundamental challenges (e.g. small system blood volumes and context-dependent cell metabolism, respectively) pose substantial, unaddressed obstacles. Here, we put forth two fundamental principles for HOC design: inducing in-vivo-like cellular metabolic rates is necessary and may be accomplished in-vitro by limiting O₂ availability and that the effects of increased blood volumes on drug concentration can be mitigated through pharmacokinetics-based treatments of solute distribution. Combining these principles with natural observation and engineering workarounds, we derive a complete set of design criteria for a practically realizable, physiologically faithful, five-organ millionth-scale (× 10^-6) microfluidic model of the human body.

Allometric scaling in-vitro

About two decades ago, West and coworkers established a model which predicts that metabolic rate follows a three quarter power relationship with the mass of an organism, based on the premise that tissues are supplied nutrients through a fractal distribution network. Quarter power scaling is widely considered a universal law of biology and it is generally accepted that were in-vitro cultures to obey allometric metabolic scaling, they would have more predictive potential and could, for instance, provide a viable substitute for animals in research. This paper outlines a theoretical and computational framework for establishing quarter power scaling in three-dimensional spherical constructs in-vitro, starting where fractal distribution ends. Allometric scaling in non-vascular spherical tissue constructs was assessed using models of Michaelis Menten oxygen consumption and diffusion. The models demonstrate that physiological scaling is maintained when about 5 to 60% of the construct is exposed to oxygen concentrations less than the Michaelis Menten constant, with a significant concentration gradient in the sphere. The results have important implications for the design of downscaled in-vitro systems with physiological relevance.

Biomaterials and Culture Technologies for Regenerative Therapy of Liver Tissue

Regenerative approach has emerged to substitute the current extracorporeal technologies for the treatment of diseased and damaged liver tissue. This is based on the use of biomaterials that modulate the responses of hepatic cells through the unique matrix properties tuned to recapitulate regenerative functions. Cells in liver preserve their phenotype or differentiate through the interactions with extracellular matrix molecules. Therefore, the intrinsic properties of the engineered biomaterials, such as stiffness and surface topography, need to be tailored to induce appropriate cellular functions. The matrix physical stimuli can be combined with biochemical cues, such as immobilized functional groups or the delivered actions of signaling molecules. Furthermore, the external modulation of cells, through cocultures with nonparenchymal cells (e.g., endothelial cells) that can signal bioactive molecules, is another promising avenue to regenerate liver tissue. This review disseminates the recent approaches of regenerating liver tissue, with a focus on the development of biomaterials and the related culture technologies.

Biology-inspired microphysiological system approaches to solve the prediction dilemma of substance testing

The recent advent of microphysiological systems - microfluidic biomimetic devices that aspire to emulate the biology of human tissues, organs and circulation in vitro - is envisaged to enable a global paradigm shift in drug development. An extraordinary US governmental initiative and various dedicated research programs in Europe and Asia have led recently to the first cutting-edge achievements of human single-organ and multi-organ engineering based on microphysiological systems. The expectation is that test systems established on this basis would model various disease stages, and predict toxicity, immunogenicity, ADME profiles and treatment efficacy prior to clinical testing. Consequently, this technology could significantly affect the way drug substances are developed in the future. Furthermore, microphysiological system-based assays may revolutionize our current global programs of prioritization of hazard characterization for any new substances to be used, for example, in agriculture, food, ecosystems or cosmetics, thus, replacing laboratory animal models used currently. Thirty-six experts from academia, industry and regulatory bodies present here the results of an intensive workshop (held in June 2015, Berlin, Germany). They review the status quo of microphysiological systems available today against industry needs, and assess the broad variety of approaches with fit-for-purpose potential in the drug development cycle. Feasible technical solutions to reach the next levels of human biology in vitro are proposed. Furthermore, key organ-on-a-chip case studies, as well as various national and international programs are highlighted. Finally, a roadmap into the future is outlined, to allow for more predictive and regulatory-accepted substance testing on a global scale.

Efficient One-Step Production of Microencapsulated Hepatocyte Spheroids with Enhanced Functions

Hepatocyte spheroids microencapsulated in hydrogels can contribute to liver research in various capacities. The conventional approach of microencapsulating spheroids produces a variable number of spheroids per microgel and requires an extra step of spheroid loading into the gel. Here, a microfluidics technology bypassing the step of spheroid loading and controlling the spheroid characteristics is reported. Double-emulsion droplets are used to generate microencapsulated homotypic or heterotypic hepatocyte spheroids (all as single spheroids <200 μm in diameter) with enhanced functions in 4 h. The composition of the microgel is tunable as demonstrated by improved hepatocyte functions during 24 d culture (albumin secretion, urea secretion, and cytochrome P450 activity) when alginate-collagen composite hydrogel is used instead of alginate. Hepatocyte spheroids in alginate-collagen also perform better than hepatocytes cultured in collagen-sandwich configuration. Moreover, hepatocyte functions are significantly enhanced when hepatocytes and endothelial progenitor cells (used as a novel supporting cell source) are co-cultured to form composite spheroids at an optimal ratio of 5:1, which could be further boosted when encapsulated in alginate-collagen. This microencapsulated-spheroid formation technology with high yield, versatility, and uniformity is envisioned to be an enabling technology for liver tissue engineering as well as biomanufacturing.

Multi-Organ toxicity demonstration in a functional human in vitro system composed of four organs

We report on a functional human model to evaluate multi-organ toxicity in a 4-organ system under continuous flow conditions in a serum-free defined medium utilizing a pumpless platform for 14 days. Computer simulations of the platform established flow rates and resultant shear stress within accepted ranges. Viability of the system was demonstrated for 14 days as well as functional activity of cardiac, muscle, neuronal and liver modules. The pharmacological relevance of the integrated modules were evaluated for their response at 7 days to 5 drugs with known side effects after a 48 hour drug treatment regime. The results of all drug treatments were in general agreement with published toxicity results from human and animal data. The presented phenotypic culture model exhibits a multi-organ toxicity response, representing the next generation of in vitro systems, and constitutes a step towards an in vitro "human-on-a-chip" assay for systemic toxicity screening.

Allometric Scaling and Cell Ratios in Multi-Organ in vitro Models of Human Metabolism

Intelligent in vitro models able to recapitulate the physiological interactions between tissues in the body have enormous potential as they enable detailed studies on specific two-way or higher order tissue communication. These models are the first step toward building an integrated picture of systemic metabolism and signaling in physiological or pathological conditions. However, the rational design of in vitro models of cell–cell or cell–tissue interaction is difficult as quite often cell culture experiments are driven by the device used, rather than by design considerations. Indeed, very little research has been carried out on in vitro models of metabolism connecting different cell or tissue types in a physiologically and metabolically relevant manner. Here, we analyze the physiological relationship between cells, cell metabolism, and exchange in the human body using allometric rules, downscaling them to an organ-on-a-plate device. In particular, in order to establish appropriate cell ratios in the system in a rational manner, two different allometric scaling models (cell number scaling model and metabolic and surface scaling model) are proposed and applied to a two compartment model of hepatic-vascular metabolic cross-talk. The theoretical scaling studies illustrate that the design and hence relevance of multi-organ models is principally determined by experimental constraints. Two experimentally feasible model configurations are then implemented in a multi-compartment organ-on-a-plate device. An analysis of the metabolic response of the two configurations demonstrates that their glucose and lipid balance is quite different, with only one of the two models recapitulating physiological-like homeostasis. In conclusion, not only do cross-talk and physical stimuli play an important role in in vitro models, but the numeric relationship between cells is also crucial to recreate in vitro interactions, which can be extrapolated to the in vivo reality.

In silico models for dynamic connected cell cultures mimicking hepatocyte-endothelial cell-adipocyte interaction circle

The biochemistry of a system made up of three kinds of cell is virtually impossible to work out without the use of in silico models. Here, we deal with homeostatic balance phenomena from a metabolic point of view and we present a new computational model merging three single-cell models, already available from our research group: the first model reproduced the metabolic behaviour of a hepatocyte, the second one represented an endothelial cell, and the third one described an adipocyte. Multiple interconnections were created among these three models in order to mimic the main physiological interactions that are known for the examined cell phenotypes. The ultimate aim was to recreate the accomplishment of the homeostatic balance as it was observed for an in vitro connected three-culture system concerning glucose and lipid metabolism in the presence of the medium flow. The whole model was based on a modular approach and on a set of nonlinear differential equations implemented in Simulink, applying Michaelis-Menten kinetic laws and some energy balance considerations to the studied metabolic pathways. Our in silico model was then validated against experimental datasets coming from literature about the cited in vitro model. The agreement between simulated and experimental results was good and the behaviour of the connected culture system was reproduced through an adequate parameter evaluation. The developed model may help other researchers to investigate further about integrated metabolism and the regulation mechanisms underlying the physiological homeostasis.

Reconfigurable microfluidic hanging drop network for multi-tissue interaction and analysis

Integration of multiple three-dimensional microtissues into microfluidic networks enables new insights in how different organs or tissues of an organism interact. Here, we present a platform that extends the hanging-drop technology, used for multi-cellular spheroid formation, to multifunctional complex microfluidic networks. Engineered as completely open, 'hanging' microfluidic system at the bottom of a substrate, the platform features high flexibility in microtissue arrangements and interconnections, while fabrication is simple and operation robust. Multiple spheroids of different cell types are formed in parallel on the same platform; the different tissues are then connected in physiological order for multi-tissue experiments through reconfiguration of the fluidic network. Liquid flow is precisely controlled through the hanging drops, which enable nutrient supply, substance dosage and inter-organ metabolic communication. The possibility to perform parallelized microtissue formation on the same chip that is subsequently used for complex multi-tissue experiments renders the developed platform a promising technology for 'body-on-a-chip'-related research.

Using physiologically-based pharmacokinetic-guided "body-on-a-chip" systems to predict mammalian response to drug and chemical exposure

The continued development of in vitro systems that accurately emulate human response to drugs or chemical agents will impact drug development, our understanding of chemical toxicity, and enhance our ability to respond to threats from chemical or biological agents. A promising technology is to build microscale replicas of humans that capture essential elements of physiology, pharmacology, and/or toxicology (microphysiological systems). Here, we review progress on systems for microscale models of mammalian systems that include two or more integrated cellular components. These systems are described as a "body-on-a-chip", and utilize the concept of physiologically-based pharmacokinetic (PBPK) modeling in the design. These microscale systems can also be used as model systems to predict whole-body responses to drugs as well as study the mechanism of action of drugs using PBPK analysis. In this review, we provide examples of various approaches to construct such systems with a focus on their physiological usefulness and various approaches to measure responses (e.g. chemical, electrical, or mechanical force and cellular viability and morphology). While the goal is to predict human response, other mammalian cell types can be utilized with the same principle to predict animal response. These systems will be evaluated on their potential to be physiologically accurate, to provide effective and efficient platform for analytics with accessibility to a wide range of users, for ease of incorporation of analytics, functional for weeks to months, and the ability to replicate previously observed human responses.

On being the right size: scaling effects in designing a human-on-a-chip

Developing a human-on-a-chip by connecting multiple model organ systems would provide an intermediate screen for therapeutic efficacy and toxic side effects of drugs prior to conducting expensive clinical trials. However, correctly designing individual organs and scaling them relative to each other to make a functional microscale human analog is challenging, and a generalized approach has yet to be identified. In this work, we demonstrate the importance of rational design of both the individual organ and its relationship with other organs, using a simple two-compartment system simulating insulin-dependent glucose uptake in adipose tissues. We demonstrate that inter-organ scaling laws depend on both the number of cells and the spatial arrangement of those cells within the microfabricated construct. We then propose a simple and novel inter-organ 'metabolically supported functional scaling' approach predicated on maintaining in vivo cellular basal metabolic rates by limiting resources available to cells on the chip. This approach leverages findings from allometric scaling models in mammals that limited resources in vivo prompt cells to behave differently than in resource-rich in vitro cultures. Although applying scaling laws directly to tissues can result in systems that would be quite challenging to implement, engineering workarounds may be used to circumvent these scaling issues. Specific workarounds discussed include the limited oxygen carrying capacity of cell culture media when used as a blood substitute and the ability to engineer non-physiological structures to augment organ function, to create the transport-accessible, yet resource-limited environment necessary for cells to mimic in vivo functionality. Furthermore, designing the structure of individual tissues in each organ compartment may be a useful strategy to bypass scaling concerns at the inter-organ level.

How multi-organ microdevices can help foster drug development

Multi-organ microdevices can mimic tissue-tissue interactions that occur as a result of metabolite travel from one tissue to other tissues in vitro. These systems are capable of simulating human metabolism, including the conversion of a pro-drug to its effective metabolite as well as its subsequent therapeutic actions and toxic side effects. Since tissue-tissue interactions in the human body can play a significant role in determining the success of new pharmaceuticals, the development and use of multi-organ microdevices present an opportunity to improve the drug development process. The devices have the potential to predict potential toxic side effects with higher accuracy before a drug enters the expensive phase of clinical trials as well as to estimate efficacy and dose response. Multi-organ microdevices also have the potential to aid in the development of new therapeutic strategies by providing a platform for testing in the context of human metabolism (as opposed to animal models). Further, when operated with human biopsy samples, the devices could be a gateway for the development of individualized medicine. Here we review studies in which multi-organ microdevices have been developed and used in a ways that demonstrate how the devices' capabilities can present unique opportunities for the study of drug action. We will also discuss challenges that are inherent in the development of multi-organ microdevices. Among these are how to design the devices, and how to create devices that mimic the human metabolism with high authenticity. Since single organ devices are testing platforms for tissues that can later be combined with other tissues within multi-organ devices, we will also mention single organ devices where appropriate in the discussion.

Bioreactor technologies to support liver function in vitro

Liver is a central nexus integrating metabolic and immunologic homeostasis in the human body, and the direct or indirect target of most molecular therapeutics. A wide spectrum of therapeutic and technological needs drives efforts to capture liver physiology and pathophysiology in vitro, ranging from prediction of metabolism and toxicity of small molecule drugs, to understanding off-target effects of proteins, nucleic acid therapies, and targeted therapeutics, to serving as disease models for drug development. Here we provide perspective on the evolving landscape of bioreactor-based models to meet old and new challenges in drug discovery and development, emphasizing design challenges in maintaining long-term liver-specific function and how emerging technologies in biomaterials and microdevices are providing new experimental models.

Chip-based liver equivalents for toxicity testing--organotypicalness versus cost-efficient high throughput

Drug-induced liver toxicity dominates the reasons for pharmaceutical product ban, withdrawal or non-approval since the thalidomide disaster in the late-1950s. Hopes to finally solve the liver toxicity test dilemma have recently risen to a historic level based on the latest progress in human microfluidic tissue culture devices. Chip-based human liver equivalents are envisaged to identify liver toxic agents regularly undiscovered by current test procedures at industrial throughput. In this review, we focus on advanced microfluidic microscale liver equivalents, appraising them against the level of architectural and, consequently, functional identity with their human counterpart in vivo. We emphasise the inherent relationship between human liver architecture and its drug-induced injury. Furthermore, we plot the current socio-economic drug development environment against the possible value such systems may add. Finally, we try to sketch a forecast for translational innovations in the field.

Organ-on-a-chip technology and microfluidic whole-body models for pharmacokinetic drug toxicity screening

Microscale cell culture platforms better mimic the in vivo cellular microenvironment than conventional, macroscale systems. Microscale cultures therefore elicit a more authentic response from cultured cells, enabling physiologically realistic in vitro tissue models to be constructed. The fabrication of interconnecting microchambers and microchannels allows drug absorption, distribution, metabolism and elimination to be simulated, and enables precise manipulation of fluid flow to replicate blood circulation. Complex, multi-organ interactions can be investigated using "organ-on-a-chip" toxicology screens. By reproducing the dynamics of multi-organ interaction, the dynamics of various diseases and drug activities can be studied in mechanistic detail. In this review, we summarize the current status of technologies related to pharmacokinetic-based drug toxicity testing, and the use of microtechnology for reproducing the interaction between multiple organs.

Bioreactor engineering of stem cell environments

Stem cells hold promise to revolutionize modern medicine by the development of new therapies, disease models and drug screening systems. Standard cell culture systems have limited biological relevance because they do not recapitulate the complex 3-dimensional interactions and biophysical cues that characterize the in vivo environment. In this review, we discuss the current advances in engineering stem cell environments using novel biomaterials and bioreactor technologies. We also reflect on the challenges the field is currently facing with regard to the translation of stem cell based therapies into the clinic.

On chip porous polymer membranes for integration of gastrointestinal tract epithelium with microfluidic 'body-on-a-chip' devices

We describe a novel fabrication method that creates microporous, polymeric membranes that are either flat or contain controllable 3-dimensional shapes that, when populated with Caco-2 cells, mimic key aspects of the intestinal epithelium such as intestinal villi and tight junctions. The developed membranes can be integrated with microfluidic, multi-organ cell culture systems, providing access to both sides, apical and basolateral, of the 3D epithelial cell culture. Partial exposure of photoresist (SU-8) spun on silicon substrates creates flat membranes with micrometer-sized pores (0.5-4.0 μm) that--supported by posts--span across 50 μm deep microfluidic chambers that are 8 mm wide and 10 long. To create three-dimensional shapes the membranes were air dried over silicon pillars with aspect ratios of up to 4:1. Space that provides access to the underside of the shaped membranes can be created by isotropically etching the sacrificial silicon pillars with xenon difluoride. Depending on the size of the supporting posts and the pore sizes the overall porosity of the membranes ranged from 4.4 % to 25.3 %. The microfabricated membranes can be used for integrating barrier tissues such as the gastrointestinal tract epithelium, the lung epithelium, or other barrier tissues with multi-organ "body-on-a-chip" devices.

The expanding world of tissue engineering: the building blocks and new applications of tissue engineered constructs

The field of tissue engineering has been growing in the recent years as more products have made it to the market and as new uses for the engineered tissues have emerged, motivating many researchers to engage in this multidisciplinary field of research. Engineered tissues are now not only considered as end products for regenerative medicine, but also have emerged as enabling technologies for other fields of research ranging from drug discovery to biorobotics. This widespread use necessitates a variety of methodologies for production of tissue engineered constructs. In this review, these methods together with their non-clinical applications will be described. First, we will focus on novel materials used in tissue engineering scaffolds; such as recombinant proteins and synthetic, self assembling polypeptides. The recent advances in the modular tissue engineering area will be discussed. Then scaffold-free production methods, based on either cell sheets or cell aggregates will be described. Cell sources used in tissue engineering and new methods that provide improved control over cell behavior such as pathway engineering and biomimetic microenvironments for directing cell differentiation will be discussed. Finally, we will summarize the emerging uses of engineered constructs such as model tissues for drug discovery, cancer research and biorobotics applications.

Assuring consumer safety without animals: Applications for tissue engineering

Humans are exposed to a variety of chemicals in their everyday lives through interactions with the environment and through the use of consumer products. It is a basic requirement that these products are tested to assure they are safe under normal and reasonably foreseeable conditions of use. Within the European Union, the majority of tests used for generating toxicological data rely on animals. However recent changes in legislation (e.g., 7(th) amendment of the Cosmetics Directive and REACH) are driving researchers to develop and adopt non-animal alternative methods with which to assure human safety. Great strides have been made to this effect, but what other opportunities/technologies exist that could expedite this? Tissue engineering has increasing scope to contribute to replacing animals with scientifically robust alternatives in basic research and safety testing, but is this application of the technology being fully exploited? This review highlights how the consumer products industry is applying tissue engineering to ensure chemicals are safe for human use without using animals, and identifies areas for future development and application of the technology.

Transplantation of Co-Microencapsulated Hepatocytes and HUVECs for Treatment of Fulminant Hepatic Failure

Purpose:

Microencapsulated hepatocytes might solve immunological rejection, broadening a new perspective for the treatment of fulminant hepatic failure (FHF). However, the transplantation of microcapsulated hepatocytes is limited by low cell viability Nevertheless, the co-microencapsulation of hepatocytes and human umbilical vein endothelial cells (HUVECs) may make the treatment of FHF more promising.

Methods:

We prepared the microcapsules using the high-voltage electrostatic droplet spray method, transplanted the empty microcapsules, isolated hepatocytes, microcapsulated hepatocytes, and co-microencapsulated hepatocytes and HUVEC intraperitoneally into rat models of FHF induced by D-aminogalactose (D-gal). After 1, 3, and 7 days, and 2, 3, and 4 weeks posttransplantation, we calculated the mortality and assessed alanine aminotransferase (ALT), aspartate aminotransferase (AST), and albumin (ALB) levels in the serum of the model; evaluated the integrality and recovery of microcapsules; and stained with hematoxylin and eosin (H&E) the recovered microcapsules as well as the liver of the FHF rats.

Results:

Hepatocyte-specific functions, including the levels of ALT, AST, and ALB in the serum of the co-microencapsulation group, were significantly better than those in the other groups (p<0.05) from 2 to 4 weeks after transplantation. Moreover, cotransplantation of the microencapsulated hepatocytes and HUVECs decreased the mortality rate of the FHF rats. The recovered microcapsules were intact, and recovery was up to 90%. H&E staining showed that the microencapsulated cells were still alive, and the liver tissues had started to recover after 4 weeks posttransplantation.

Conclusion:

The microcapsules have good biocompatibility and immunoprotection to protect the hepatocytes from immunological rejection. Cotransplantation of the microencapsulated hepatocytes and HUVECs could decrease mortality rates and improve liver function in FHF.

The role of body-on-a-chip devices in drug and toxicity studies

High-quality, in vitro screening tools are essential in identifying promising compounds during drug development. Tests with currently used cell-based assays provide an indication of a compound's potential therapeutic benefits to the target tissue, but not to the whole body. Data obtained with animal models often cannot be extrapolated to humans. Multicompartment microfluidic-based devices, particularly those that are physical representations of physiologically based pharmacokinetic (PBPK) models, may contribute to improving the drug development process. These scaled-down devices, termed micro cell culture analogs (μCCAs) or body-on-a-chip devices, can simulate multitissue interactions under near-physiological fluid flow conditions and with realistic tissue-to-tissue size ratios. Because the device can be used with both animal and human cells, it can facilitate cross-species extrapolation. Used in conjunction with PBPK models, the devices permit an estimation of effective concentrations that can be used for studies with animal models or predict the human response. The devices also provide a means for relatively high-throughput screening of drug combinations and, when utilized with a patient's tissue sample, an opportunity for individualized medicine. Here we review efforts made toward the development of microfabricated cell culture systems and give examples that demonstrate their potential use in drug development, such as identifying synergistic drug interactions as well as simulating multiorgan metabolic interactions. In addition to their use in drug development, the devices also can be used to estimate the toxicity of chemicals as occupational hazards and environmental contaminants.

Microtechnology for mimicking in vivo tissue environment

Microtechnology provides a new approach for reproducing the in vivo environment in vitro. Mimicking the microenvironment of the natural tissues allows cultured cells to behave in a more authentic manner, and gives researchers more realistic platforms to study biological systems. In this review article, we discuss the physiochemical aspects of in vivo cellular microenvironment, and relevant technologies that can be used to mimic those aspects. Secondly we identify the core methods used in microtechnology for biomedical applications. Finally we examine the recent application areas of microtechnology, with a focus on reproducing the functions of specific organs, or whole-body response such as homeostasis or metabolism-dependent toxicity of drugs. These new technologies enable researchers to ask and answer questions in a manner that has not been possible with conventional, macroscale technologies.

Engineering Quasi-Vivo in vitro organ models

Cell culture is the workhorse of biologists, toxicologists, tissue engineers and a whole host of research fields in both academia and industry. Having explored individual molecular mechanisms inside cells for decades using traditional cell culture techniques, researchers have only just begun to appreciate that the intricate interconnectivity between cells and cellular networks as well as with the external environment is far more important to cellular orchestration than are single molecular events inside the cell. For example many questions regarding cell, tissue, organ and system response to drugs, environmental toxins, stress and nutrients cannot possibly be answered by concentrating on the minutiae of what goes on in the deepest recesses of single cells. New models are required to investigate cellular cross-talk between different cell types and to construct complex in-vitro models to properly study tissue, organ and system interaction without resorting to animal experiments. This chapter describes how tissue and organ models can be developed using the Quasi-Vivo system and discusses how they may be used in drug toxicity studies.

A flexible bioreactor system for constructing in vitro tissue and organ models

To develop in vitro models of cells, tissues and organs we have designed and realized a series of cell culture chambers. Each chamber is purpose designed to simulate a particular feature of the in vivo environment. The bioreactor system is user friendly, and the chambers are easy to produce, sterilize and assemble. In addition they can be connected together to simulate inter-organ or tissue cross-talk. Here we discuss the design philosophy of the bioreactor system and then describe its construction. Preliminary results of validation tests obtained with hepatocytes and endothelial cells are also reported. The results show that endothelial cells are extremely sensitive to small levels of shear stress and that the presence of heterotypic signals from endothelial cells enhances the endogenous metabolic function of hepatocytes.

Metabolic control through hepatocyte and adipose tissue cross-talk in a multicompartmental modular bioreactor

Physiological processes involve a complex network of signaling molecules that act through paracrinal or endocrinal pathways; however, traditional in vitro models cannot mimic these interactions because of the lack of a dynamic cross-talk between cells belonging to different tissues. The multicompartmental modular bioreactor is a novel cell culture system where hepatocytes and adipose tissue are shown to interact in a more physiological manner. In the multicompartmental modular bioreactor, cells and tissues can be cultured in a common medium, which flows through the system acting as the bloodstream. Primary rat hepatocytes and adipose tissue were cultured separately and together in conventional conditions and in the bioreactor. Urea synthesis, albumin secretion, glycerol, free fatty acid, and glucose concentrations were analyzed and compared. The dynamic connected culture of adipose tissue and hepatocytes led to a significant enhancement of hepatic function in terms of increase of albumin and urea production with respect to conventional cultures. Interestingly, the glycerol gradually released from adipose tissue was buffered in the dynamic connected culture, manifesting a homeostatic-like control. These data show that the dynamic culture not only improves hepatocyte function, but also allows a cross-talk between tissues, leading to enhanced metabolic regulation in vitro.

Integration of in silico and in vitro platforms for pharmacokinetic-pharmacodynamic modeling

IMPORTANCE OF THE FIELD

Pharmacokinetic-pharmacodynamic (PK-PD) modeling enables quantitative prediction of the dose-response relationship. Recent advances in microscale technology enabled researchers to create in vitro systems that mimic biological systems more closely. Combination of mathematical modeling and microscale technology offers the possibility of faster, cheaper and more accurate prediction of the drug's effect with a reduced need for animal or human subjects.

AREAS COVERED IN THIS REVIEW

This article discusses combining in vitro microscale systems and PK-PD models for improved prediction of drug's efficacy and toxicity. First, we describe the concept of PK-PD modeling and its applications. Different classes of PK-PD models are described. Microscale technology offers an opportunity for building physical systems that mimic PK-PD models. Recent progress in this approach during the last decade is summarized.

WHAT THE READER WILL GAIN

This article is intended to review how microscale technology combined with cell cultures, also known as 'cells-on-a-chip', can confer a novel aspect to current PK-PD modeling. Readers will gain a comprehensive knowledge of PK-PD modeling and 'cells-on-a-chip' technology, with the prospect of how they may be combined for synergistic effect.

TAKE HOME MESSAGE

The combination of microscale technology and PK-PD modeling should contribute to the development of a novel in vitro/in silico platform for more physiologically-realistic drug screening.

Flow-regulated glucose and lipid metabolism in adipose tissue, endothelial cell and hepatocyte cultures in a modular bioreactor

Static cell culture has serious limitations in its ability to represent cellular behaviour within a live organism. In vivo, cells are constantly exposed to the flow of bodily fluids and contact with other cell types. Bioreactors provide the opportunity to study cells in an environment that more closely resembles the in vivo setting because cell cultures can be exposed to dynamic flow in contact with or in proximity to other cell types. In this study we compared the metabolic profile of a dynamic cell culture system to that of a static cell culture in three different cellular phenotypes: adipocytes, endothelial cells and hepatocytes. Albumin, glucose, free fatty acids, glycerol, and lactate were measured over 48 h. We show that all three cell types have increased glucose uptake in the presence of flow; lactate release was also significantly affected. We provide robust evidence that the presence of flow significantly modifies cellular metabolism. While flow provides a more uniform nutrient distribution and increases metabolite turnover, our results indicate that different cell types have specific metabolic responses to flow, suggesting cell-specific flow-regulated activation of metabolite signalling pathways.

A low shear stress modular bioreactor for connected cell culture under high flow rates

A generic "system on a plate" modular multicompartmental bioreactor array which enables microwell protocols to be transferred directly to the bioreactor modules, without redesign of cell culture experiments or protocols is described. The modular bioreactors are simple to assemble and use and can be easily compared with standard controls since cell numbers and medium volumes are quite similar. Starting from fluid dynamic and mass transport considerations, a modular bioreactor chamber was first modeled and then fabricated using "milli-molding," a technique adapted from soft lithography. After confirming that the shear stress was extremely low in the system in the range of useful flow rates, the bioreactor chambers were tested using hepatocytes. The results show that the bioreactor chambers can increase or maintain cell viability and function when the flow rates are below 500 microL/min, corresponding to wall shear stresses of 10(-5) Pa or less at the cell culture surface.

A microfluidic device for a pharmacokinetic-pharmacodynamic (PK-PD) model on a chip

Drug discovery is often impeded by the poor predictability of in vitro assays for drug toxicity. One primary reason for this observation is the inability to reproduce the pharmacokinetics (PK) of drugs in vitro. Mathematical models to predict the pharmacokinetics-pharmacodynamics (PK-PD) of drugs are available, but have several limitations, preventing broader application. A microscale cell culture analog (microCCA) is a microfluidic device based on a PK-PD model, where multiple cell culture chambers are connected with fluidic channels to mimic multi-organ interactions and test drug toxicity in a pharmacokinetic-based manner. One critical issue with microfluidics, including the microCCA, is that specialized techniques are required for assembly and operation, limiting its usability to non-experts. Here, we describe a novel design, with enhanced usability while allowing hydrogel-cell cultures of multiple types. Gravity-induced flow enables pumpless operation and prevents bubble formation. Three cell lines representing the liver, tumor and marrow were cultured in the three-chamber microCCA to test the toxicity of an anticancer drug, 5-fluorouracil. The result was analyzed with a PK-PD model of the device, and compared with the result in static conditions. Each cell type exhibited differential responses to 5-FU, and the responses in the microfluidic environment were different from those in static environment. Combination of a mathematical modeling approach (PK-PD modeling) and an in vitro experimental approach (microCCA) provides a novel platform with improved predictability for testing drug toxicity and can help researchers gain a better insight into the drug's mechanism of action.

In vitro microscale systems for systematic drug toxicity study

After administration, drugs go through a complex, dynamic process of absorption, distribution, metabolism and excretion. The resulting time-dependent concentration, termed pharmacokinetics (PK), is critical to the pharmacological response from patients. An in vitro system that can test the dynamics of drug effects in a more systematic way would save time and costs in drug development. Integration of microfabrication and cell culture techniques has resulted in 'cells-on-a-chip' technology, which is showing promise for high-throughput drug screening in physiologically relevant manner. In this review, we summarize current research efforts which ultimately lead to in vitro systems for testing drug's effect in PK-based manner. In particular, we highlight the contribution of microscale systems towards this goal. We envision that the 'cells-on-a-chip' technology will serve as a valuable link between in vitro and in vivo studies, reducing the demand for animal studies, and making clinical trials more effective.