A microfabricated array bioreactor for perfused 3D liver culture

384 hanging drop arrays give excellent Z-factors and allow versatile formation of co-culture spheroids

We previously reported the development of a simple, user-friendly, and versatile 384 hanging drop array plate for 3D spheroid culture and the importance of utilizing 3D cellular models in anti-cancer drug sensitivity testing. The 384 hanging drop array plate allows for high-throughput capabilities and offers significant improvements over existing 3D spheroid culture methods. To allow for practical 3D cell-based high-throughput screening and enable broader use of the plate, we characterize the robustness of the 384 hanging drop array plate in terms of assay performance and demonstrate the versatility of the plate. We find that the 384 hanging drop array plate performance is robust in fluorescence- and colorimetric-based assays through Z-factor calculations. Finally, we demonstrate different plate capabilities and applications, including: spheroid transfer and retrieval for Janus spheroid formation, sequential addition of cells for concentric layer patterning of different cell types, and culture of a wide variety of cell types.

COMPUTATIONAL FLUID DYNAMICS FOR TISSUE ENGINEERING APPLICATIONS

Hydrodynamic cellular environment plays an important role in translating engineered tissue constructs into clinically useful grafts. However, the cellular fluid dynamic environment inside bioreactor systems is highly complex and it is normally impractical to experimentally characterize the local flow patterns at the cellular scale. Computational fluid dynamics (CFD) has been recognized as an invaluable and reliable alternative to investigate the complex relationship between hydrodynamic environments and the regeneration of engineered tissues at both the macroscopic and microscopic scales. This review describes the applications of CFD simulations to probe the hydrodynamic environment parameters (e.g., flow rate, shear stress, etc.) and the corresponding experimental validations. We highlight the use of CFD to optimize bioreactor design and scaffold architectures for improved ex-vivo hydrodynamic environments. It is envisioned that CFD could be used to customize specific hydrodynamic cellular environments to meet the unique requirements of different cell types in combination with advanced manufacturing techniques and finally facilitate the maturation of tissue-engineered constructs.

LIVER TISSUE MODEL FOR DRUG TOXICITY SCREENING

Understanding the mechanisms involved in the biotransformation of new drugs and their toxicological implications is important for drug development. In this regard, a lot of effort has been put into research to recreate the liver tissue in the laboratory for the purpose of drug screening. This has also helped to minimize the use of laboratory animal and reduce incidence of post-market withdrawal of drugs. Despite the progress made so far, cell source remains a major limitation since primary human hepatocytes are scarce and the various cell alternatives do not express all the genes found in the normal liver. In terms of tissue construct, there is a current shift to 3D models since the cell–cell interactions found in the 3D configuration enhance the morphology and function of hepatocytes. Furthermore, the engineered tissue's performance can be optimized by cocultures, perfusion-based systems, and the use of scaffolds. Nanotechnology seems promising in the field of tissue engineering, as it has been proven that cell–matrix interactions at the nano level can influence greatly on the outcome of the tissue. The review explores the various cell sources, the 3D model, flow-based systems, cocultures, and nanoscaffolds use in hepatocytes in vitro drug testing

TISSUE ENGINEERING APPROACH TO OSTEOCHONDRAL REPAIR AND REGENERATION

Repair of osteochondral lesions remains difficult in current clinical medicine. This is due to the lack of self-reparatory capacity in adult cartilage to respond to injuries. Furthermore, current surgical based treatment is unable to achieve long-term satisfactory results. Cell therapies combined with scaffolds has become a promising tissue engineering approach for osteochondral regeneration. This article briefly outlines the approaches and limitations in osteochondral tissue engineering from three key aspects, namely: (1) Cells and Cell Source; (2) Biomaterials and Scaffold design and fabrication; and (3) Mechanical and Biochemical Stimulus. Current optimal candidate cells for tissue engineering include bone marrow and adipose tissue derived mesenchymal stem cells. As for scaffolds, the structural design and biomaterials used should support cell growth and the organization of new functional tissue formation. Using Fused Deposition Modeling (FDM) technique, the authors developed a novel polycaprolactone osteochondral scaffold which was shown to have the ability to recruit mesenchymal stem cells and the potential for repairing defects in vivo. The article also discussed mechanical and biological stimulus for enhancing in vitro growth of tissue-engineered constructs. The final challenge is the integration of the tissue-engineered tissues into a living system as a functional device.

Microfluidic platforms for hepatocyte cell culture: new technologies and applications

In this article, we summarize the key elements of microfluidic platforms for mimicking in vivo hepatocyte cell culture and the major recent advances in this area. Specifically, we will give brief background and rationale for key design requirements for mimicking in vivo hepatocyte cell culture, and then summarize findings, applications, and limitations from microfluidic platforms that addressed these design requirements. Although no ideal microfluidic platform has so far been developed for fully mimicking in vivo hepatocyte cell culture, some approaches and designs have demonstrated great potential in this area.

From 3D cell culture to organs-on-chips

3D cell-culture models have recently garnered great attention because they often promote levels of cell differentiation and tissue organization not possible in conventional 2D culture systems. We review new advances in 3D culture that leverage microfabrication technologies from the microchip industry and microfluidics approaches to create cell-culture microenvironments that both support tissue differentiation and recapitulate the tissue-tissue interfaces, spatiotemporal chemical gradients, and mechanical microenvironments of living organs. These 'organs-on-chips' permit the study of human physiology in an organ-specific context, enable development of novel in vitro disease models, and could potentially serve as replacements for animals used in drug development and toxin testing.

Microfluidic cell culture models for tissue engineering

Microfluidic systems have emerged as revolutionary new platform technologies for a range of applications, from consumer products such as inkjet printer cartridges to lab-on-a-chip diagnostic systems. Recent developments have opened the door to a new set of opportunities for microfluidic systems, in the field of tissue and organ engineering. Advances in the design of physiologically relevant structures and networks, fabrication processes for biomaterials suitable for in vivo use, and techniques for scaling towards large, three-dimensional constructs, are converging towards therapeutic applications of microfluidic technologies in engineering complex tissues and organs. These advances herald a new generation of microfluidics-based approaches designed for specific tissue and organ applications, incorporating microvascular networks, structures for transport and filtration, and a three-dimensional microenvironment suitable for supporting phenotypic cell behavior, tissue function, and implantation and host integration.

Exploitation of physical and chemical constraints for three-dimensional microtissue construction in microfluidics

There are a plethora of approaches to construct microtissues as building blocks for the repair and regeneration of larger and complex tissues. Here we focus on various physical and chemical trapping methods for engineering three-dimensional microtissue constructs in microfluidic systems that recapitulate the in vivo tissue microstructures and functions. Advances in these in vitro tissue models have enabled various applications, including drug screening, disease or injury models, and cell-based biosensors. The future would see strides toward the mesoscale control of even finer tissue microstructures and the scaling of various designs for high throughput applications. These tools and knowledge will establish the foundation for precision engineering of complex tissues of the internal organs for biomedical applications.

3D culture of adult mouse neural stem cells within functionalized self-assembling peptide scaffolds

Three-dimensional (3D) in vitro models of cell culture aim to fill the gap between the standard two-dimensional cell studies and the in vivo environment. Especially for neural tissue regeneration approaches where there is little regenerative capacity, these models are important for mimicking the extracellular matrix in providing support, allowing the natural flow of oxygen, nutrients, and growth factors, and possibly favoring neural cell regrowth. We have previously demonstrated that a new self-assembling nanostructured biomaterial, based on matrigel, was able to support adult neural stem cell (NSC) culture. In this study, we developed a new 3D cell culture system that takes advantage of the nano- and microfiber assembling process, under physiologic conditions, of these biomaterials. The assembled scaffold forms an intricate and biologically active matrix that displays specifically designed functional motifs: RGD (Arg-Gly-Asp), BMHP1 (bone marrow homing peptide 1), and BMHP2, for the culture of adult NSCs. These scaffolds were prepared at different concentrations, and microscopic examination of the cell-embedded scaffolds showed that NSCs are viable and they proliferate and differentiate within the nanostructured environment of the scaffold. Such a model has the potential to be tailored to develop ad hoc designed peptides for specific cell lines.

Cell therapeutic options in liver diseases: cell types, medical devices and regulatory issues

Although significant progress has been made in the field of orthotopic liver transplantation, cell-based therapies seem to be a promising alternative to whole-organ transplantation. The reasons are manifold but organ shortage is the main cause for this approach. However, many problems such as the question which cell type should be used or which application site is best for transplantation have been raised. In addition, some clinicians have had success by cultivating liver cells in bioreactors for temporary life support. Besides answering the question which cell type, which injection site or even which culture form should be used for liver support recent international harmonization of legal requirements is needed to be addressed by clinicians, scientists and companies dealing with cellular therapies. We here briefly summarize the possible cell types used to partially or temporarily correct liver diseases, the most recent development of bioreactor technology and important regulatory issues.

A microfluidic membrane device to mimic critical components of the vascular microenvironment

Vascular function, homeostasis, and pathological development are regulated by the endothelial cells that line blood vessels. Endothelial function is influenced by the integrated effects of multiple factors, including hemodynamic conditions, soluble and insoluble biochemical signals, and interactions with other cell types. Here, we present a membrane microfluidic device that recapitulates key components of the vascular microenvironment, including hemodynamic shear stress, circulating cytokines, extracellular matrix proteins, and multiple interacting cells. The utility of the device was demonstrated by measuring monocyte adhesion to and transmigration through a porcine aortic endothelial cell monolayer. Endothelial cells grown in the membrane microchannels and subjected to 20 dynes∕cm(2) shear stress remained viable, attached, and confluent for several days. Consistent with the data from macroscale systems, 25 ng∕ml tumor necrosis factor (TNF)-α significantly increased RAW264.7 monocyte adhesion. Preconditioning endothelial cells for 24 h under static or 20 dynes∕cm(2) shear stress conditions did not influence TNF-α-induced monocyte attachment. In contrast, simultaneous application of TNF-α and 20 dynes∕cm(2) shear stress caused increased monocyte adhesion compared with endothelial cells treated with TNF-α under static conditions. THP-1 monocytic cells migrated across an activated endothelium, with increased diapedesis in response to monocyte chemoattractant protein (MCP)-1 in the lower channel of the device. This microfluidic platform can be used to study complex cell-matrix and cell-cell interactions in environments that mimic those in native and tissue engineered blood vessels, and offers the potential for parallelization and increased throughput over conventional macroscale systems.

A microfluidic traps system supporting prolonged culture of human embryonic stem cells aggregates

The unlimited proliferative and differentiative capacities of embryonic stem cells (ESCs) are tightly regulated by their microenvironment. Local concentrations of soluble factors, cell-cell interactions and extracellular matrix signaling are just a few variables that influence ESC fate. A common method employed to induce ESC differentiation involves the formation of cell aggregates called embryoid bodies (EBs), which recapitulate early stages of embryonic development. EBs are normally formed in suspension cultures, producing heterogeneously shaped and sized aggregates. The present study demonstrates the usage of a microfluidic traps system which supports prolonged EB culturing. The traps are uniquely designed to facilitate cell capture and aggregation while offering efficient gas/nutrients exchange. A finite element simulation is presented with emphasis on several aspects critical to appropriate design of such bioreactors for ESC culture. Finally, human ESC, mouse Nestin-GFP ESC and OCT4-EGFP ESCs were cultured using this technique and demonstrated extended viability for more than 5 days. In addition, EBs developed and maintained a polarized differentiation pattern, possibly as a result of the nutrient gradients imposed by the traps bioreactor. The novel microbioreactor presented here can enhance future embryogenesis research by offering tight control of culturing conditions.

Microarrays for the scalable production of metabolically relevant tumour spheroids: a tool for modulating chemosensitivity traits

We report the use of thin film poly(dimethylsiloxane) (PDMS) prints for the arrayed mass production of highly uniform 3-D human HT29 colon carcinoma spheroids. The spheroids have an organotypic density and, as determined by 3-axis imaging, were genuinely spherical. Critically, the array density impacts growth kinetics and can be tuned to produce spheroids ranging in diameter from 200 to 550 µm. The diffusive limit of competition for media occurred with a pitch of ≥1250 µm and was used for the optimal array-based culture of large, viable spheroids. During sustained culture mass transfer gradients surrounding and within the spheroids are established, and lead to growth cessation, altered expression patterns and the formation of a central secondary necrosis. These features reflect the microenvironment of avascularised tumours, making the array format well suited for the production of model tumours with defined sizes and thus defined spatio-temporal pathophysiological gradients. Experimental windows, before and after the onset of hypoxia, were identified and used with an enzyme activity-based viability assay to measure the chemosensitivity towards irinotecan. Compared to monolayer cultures, a marked reduction in the drug efficacy towards the different spheroid culture states was observed and attributed to cell cycle arrest, the 3-D character, scale and/or hypoxia factors. In summary, spheroid culture using the array format has great potential to support drug discovery and development, as well as tumour biology research.

Autocrine-controlled formation and function of tissue-like aggregates by primary hepatocytes in micropatterned hydrogel arrays

The liver carries out a variety of essential functions regulated in part by autocrine signaling, including hepatocyte-produced growth factors and extracellular matrix (ECM). The local concentrations of autocrine factors are governed by a balance between receptor-mediated binding at the cell surface and diffusion into the local matrix and are thus expected to be influenced by the dimensionality of the cell culture environment. To investigate the role of growth factor and ECM-modulated autocrine signaling in maintaining appropriate primary hepatocyte survival, metabolic functions, and polarity, we created three-dimensional cultures of defined geometry using micropatterned semisynthetic polyethylene glycol-fibrinogen hydrogels to provide a mechanically compliant, nonadhesive material platform that could be modified by cell-secreted factors. We found that in the absence of exogenous peptide growth factors or ECM, hepatocytes retain the epidermal growth factor (EGF) receptor ligands (EGF and transforming growth factor-α) and the proto-oncogenic mesenchymal epithelial transition factor (c-MET) ligand hepatocyte growth factor (HGF), along with fibronectin. Further, hepatocytes cultured in this three-dimensional microenvironment maintained high levels of liver-specific functions over the 10-day culture period. Function-blocking inhibitors of α5β1 or EGF receptor dramatically reduced cell viability and function, suggesting that signaling by both these receptors is needed for in vitro survival and function of hepatocytes in the absence of other exogenous signals.

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.

A microfluidic bioreactor with integrated transepithelial electrical resistance (TEER) measurement electrodes for evaluation of renal epithelial cells

We have developed a bilayer microfluidic system with integrated transepithelial electrical resistance (TEER) measurement electrodes to evaluate kidney epithelial cells under physiologically relevant fluid flow conditions. The bioreactor consists of apical and basolateral fluidic chambers connected via a transparent microporous membrane. The top chamber contains microfluidic channels to perfuse the apical surface of the cells. The bottom chamber acts as a reservoir for transport across the cell layer and provides support for the membrane. TEER electrodes were integrated into the device to monitor cell growth and evaluate cell-cell tight junction integrity. Immunofluorescence staining was performed within the microchannels for ZO-1 tight junction protein and acetylated α-tubulin (primary cilia) using human renal epithelial cells (HREC) and MDCK cells. HREC were stained for cytoskeletal F-actin and exhibited disassembly of cytosolic F-actin stress fibers when exposed to shear stress. TEER was monitored over time under normal culture conditions and after disruption of the tight junctions using low Ca(2+) medium. The transport rate of a fluorescently labeled tracer molecule (FITC-inulin) was measured before and after Ca(2+) switch and a decrease in TEER corresponded with a large increase in paracellular inulin transport. This bioreactor design provides an instrumented platform with physiologically meaningful flow conditions to study various epithelial cell transport processes.

An Optically Controlled 3D Cell Culturing System

A novel 3D cell culture system was developed and tested. The cell culture device consists of a microfluidic chamber on an optically absorbing substrate. Cells are suspended in a thermoresponsive hydrogel solution, and optical patterns are utilized to heat the solution, producing localized hydrogel formation around cells of interest. The hydrogel traps only the desired cells in place while also serving as a biocompatible scaffold for supporting the cultivation of cells in 3D. This is demonstrated with the trapping of MDCK II and HeLa cells. The light intensity from the optically induced hydrogel formation does not significantly affect cell viability.

Microprinting of liver micro-organ for drug metabolism study

In their normal in vivo matrix milieu, tissues assume complex well-organized 3D architectures. Therefore, a primary aim in the tissue engineering design process is to fabricate an optimal analog of the in vivo scenario, in which the precise configuration and composition of cells and bioactive matrix components can establish the well-defined biomimetic microenvironments that promote cell-cell and cell-matrix interactions. With the advent and refinements in microfabricated systems which can present physical and chemical cues to cells in a controllable and reproducible fashion unrealizable with conventional tissue culture, high-fidelity, high-throughput in vitro models are achieved. The convergence of solid freeform fabrication (SFF) technologies, namely microprinting, along with microfabrication techniques, a 3D microprinted micro-organ, can serve as an in vitro platform for cell culture, drug screening, or to elicit further biological insights. This chapter firstly details the principles, methods, and applications that undergird the fabrication process development and adaptation of microfluidic devices for the creation of a drug screening model. This model involves the combinatorial setup of an automated syringe-based, layered direct cell writing microprinting process with soft lithographic micropatterning techniques to fabricate a microscale in vitro device housing a chamber of microprinted 3D micro-organ that biomimics the cell's natural microenvironment for enhanced performance and functionality. In order to assess the structural formability and biological feasibility of such a micro-organ, 3D cell-encapsulated hydrogel-based tissue constructs are microprinted reproducibly in defined design patterns and biologically characterized for both viability and cell-specific function. Another key facet of the in vivo microenvironment that is recapitulated with the in vitro system is the necessary dynamic perfusion of the 3D microscale liver analog with cells probed for their collective drug metabolic function and suitability as a drug metabolism model.

Perfusion-based microfluidic device for three-dimensional dynamic primary human hepatocyte cell culture in the absence of biological or synthetic matrices or coagulants

We describe a perfusion-based microfluidic device for three-dimensional (3D) dynamic primary human hepatocyte cell culture. The microfluidic device was used to promote and maintain 3D tissue-like cellular morphology and cell-specific functionality of primary human hepatocytes by restoring membrane polarity and hepatocyte transport function in vitro without the addition of biological or synthetic matrices or coagulants. A unique feature of our dynamic cell culture device is the creation of a microenvironment, without the addition of biological or synthetic matrices or coagulants, that promotes the 3D organization of hepatocytes into cord-like structures that exhibit functional membrane polarity as evidenced by the expression of gap junctions and the formation of an extended, functionally active, bile canalicular network.

Microplate-reader compatible perfusion microbioreactor array for modular tissue culture and cytotoxicity assays

One important application of tissue engineering is to provide novel in vitro models for cell-based assays. Perfusion microbioreactor array provides a useful tool for microscale tissue culture in parallel. However, high-throughput data generation has been a challenge. In this study, a 4 x 4 array of perfusion microbioreactors was developed for plate-reader compatible, time-series quantification of cell proliferation, and cytotoxicity assays. The device was built through multilayer soft lithography. Low-cost nonwoven polyethylene terephthalate fibrous matrices were integrated as modular tissue culture scaffolds. Human colon cancer HT-29 cells with stable expression of enhanced green fluorescent protein were cultured in the device with continuous perfusion and reached a cell density over 5 x 10(7) cells/mL. The microbioreactor array was used to test a chemotherapeutic drug 5-FU for its effect on HT-29 cells in continuous perfusion 3D culture. Compared with conventional 2D cytotoxicity assay, significant drug resistance was observed in the 3D perfusion culture.

Biofabrication of a three-dimensional liver micro-organ as an in vitro drug metabolism model

In their normal in vivo matrix milieu, tissues assume complex well-organized three-dimensional architectures. Therefore, the primary aim in the tissue engineering design process is to fabricate an optimal analog of the in vivo scenario. This challenge can be addressed by applying emerging layered biofabrication approaches in which the precise configuration and composition of cells and bioactive matrix components can recapitulate the well-defined three-dimensional biomimetic microenvironments that promote cell-cell and cell-matrix interactions. Furthermore, the advent of and refinements in microfabricated systems can present physical and chemical cues to cells in a controllable and reproducible fashion unmatched with conventional cultures, resulting in the precise construction of engineered biomimetic microenvironments on the cellular length scale in geometries that are readily parallelized for high throughput in vitro models. As such, the convergence of layered solid freeform fabrication (SFF) technologies along with microfabrication techniques enables the creation of a three-dimensional micro-organ device to serve as an in vitro platform for cell culture, drug screening or to elicit further biological insights, particularly for NASA's interest in a flight-suitable high-fidelity microscale platform to study drug metabolism in space and planetary environments. The proposed model in this paper involves the combinatorial setup of an automated syringe-based, layered direct cell writing bioprinting process with micro-patterning techniques to fabricate a microscale in vitro device housing a chamber of bioprinted three-dimensional liver cell-encapsulated hydrogel-based tissue constructs in defined design patterns that biomimic the cell's natural microenvironment for enhanced biological functionality. In order to assess the structural formability and biological feasibility of such a micro-organ, reproducibly fabricated tissue constructs were biologically characterized for liver cell-specific function. Another key facet of the in vivo microenvironment that was recapitulated with the in vitro system included the necessary dynamic perfusion of the three-dimensional microscale liver analog with cells probed for their collective drug metabolic function and suitability as a drug metabolism model. This paper details the principles and methods that undergird the direct cell writing biofabrication process development and adaptation of microfluidic devices for the creation of a drug screening model, thereby establishing a novel drug metabolism study platform for NASA's interest to adopt a microfluidic microanalytical device with an embedded three-dimensional microscale liver tissue analog to assess drug pharmacokinetic profiles in planetary environments.

A method for patterned in situ biofunctionalization in injection-molded microfluidic devices

We developed a method to modify the surface in injection molded polymer microdevices prior to bonding and to pattern biomolecules in the completed microsystem in situ by a sequence of simple perfusion steps directly before utilization of the device. This method is compatible with production technology such as injection molding and bonding processes currently employed in the fabrication of polymer microsystems. It solves the problem of the inherent incompatibility of biomolecules with microfabrication technology as it allows for the biofunctionalization step to be performed after completion of the microsystem. Injection molded cyclic olefin copolymer (COC) microfluidic chips were modified by irradiating the surface with UV-light at lambda = 185 nm. This results in the formation of stable acidic groups which were further modified by binding of the extracellular matrix protein collagen type I. Non-irradiated surfaces were modified by binding of Pluronic® F-127 to become non-adhesive. Density of acid groups decreases to 50% within 45 days and to 25% within 19 weeks after irradiation. However, even then the remaining density of functional groups was shown to be sufficient to bind proteins and promote cell adhesion. Selective adhesion of primary hepatocytes on surfaces patterned by UV-irradiation and a biofunctional coating with collagen type I were demonstrated in injection molded microsystems.

Directing the flow of medium in controlled cocultures of HeLa cells and human umbilical vein endothelial cells with a microfluidic device

A microfluidic device was integrated with a controlled coculture system of HeLa cells and human umbilical vein endothelial cells (HUVECs). This integrated assembly allowed control of the direction of flow of medium (along with signaling factors secreted from cells) across the cultured cells. We grew HeLa cells and HUVECs to confluency on separate substrates and then joined the two substrates. A microfluidic device was then assembled onto the substrates and a cell coculture was initiated with controlled perfusion of the medium. When the medium flow was directed from the HeLa side to the HUVEC side, the HUVECs retreated and the HeLa cells migrated into the newly vacated areas. By contrast, when the medium flow was in the opposite direction, there was essentially no net movement of either cell type. Our results suggest that the migration of HeLa cells and HUVECs in coculture was likely mediated by soluble factors produced by HeLa cells.

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.

Characterization of collagen fibrils films formed on polydimethylsiloxane surfaces for microfluidic applications

Type I collagen fibrillar thin films have been prepared on hydrophobic recovered poly(dimethylsiloxane) (PDMS) surfaces and inside of irreversibly sealed PDMS microfluidic devices. Fibrillar films prepared on PDMS surfaces have been characterized with optical microscopy and atomic force microscopy and compared with films prepared using more traditional bulk methods on thiol-coated gold substrates. Collagen fibril films formed after 18 h of incubation on PDMS surfaces were observed to have similar underlying film thicknesses (15 nm), fibril size (67 nm), fibril coverage (45%), and physiologically supermolecular structure when compared to films on gold substrates. Collagen fibrils formed within devices were also determined to be usable across physiologically relevant cell perfusion rates. To validate the utility of these collagen fibril thin films for cell culture applications, vascular smooth muscle cells are shown to attach to collagen fibrils and exhibit cell spread areas equivalent to those seen on collagen fibrils created via bulk cell culture methods on thiol-coated gold substrates. These results extend the use and benefits of collagen fibril thin films into microfluidic-based cellular studies.

Extracellular matrix provides an optimal niche for the maintenance and propagation of mesenchymal stem cells

Relatively little is known about the cellular and molecular mechanisms underlying the control of mesenchymal stem cell (MSC) proliferation, differentiation, and survival. This presents difficulties in following and characterizing cells along the lineage because of our inability to isolate and obtain a sufficient number of homogeneous MSCs using current culture systems for in vitro expansion. Adjusting the cellular machinery to allow greater proliferation can lead to other unwanted outcomes, such as unmanageable precancerous changes, or differentiation down an undesired pathway. Recently, it has become increasingly evident that the extracellular matrix (ECM) is an important component of the cellular niche in a tissue, supplying critical biochemical and physical signals to initiate and sustain cellular functions. Indeed, it is very doubtful that the intricate and highly ordered nature of the ECM could be reproduced with synthetic or purified components. This review cites evidence that supports an alternative approach for maintenance of MSCs by simulating in vitro the bone marrow ECM, where MSCs reside in vivo, and discusses the potential mechanisms whereby the ECM regulates the exposure of cells to growth factors that subsequently control MSC replication and differentiation, and also how the ECM provides unique cues that govern the lineage specification and differentiation of MSCs.

Liver Cell Culture Devices

In the last 15 years many different liver cell culture devices, consisting of functional liver cells and artificial materials, have been developed. They have been devised for numerous different applications, such as temporary organ replacement (a bridge to liver transplantation or native liver regeneration) and as in vitro screening systems in the early stages of the drug development process, like assessing hepatotoxicity, hepatic drug metabolism, and induction/inhibition studies. Relevant literature is summarized about artificial human liver cell culture systems by scrutinizing PubMed from 2003 to 2009. Existing devices are divided in 2D configurations (e.g., static monolayer, sandwich, perfused cells, and flat plate) and 3D configurations (e.g., liver slices, spheroids, and different types of bioreactors). The essential features of an ideal liver cell culture system are discussed: different types of scaffolds, oxygenation systems, extracellular matrixes (natural and artificial), cocultures with nonparenchymal cells, and the role of shear stress problems. Finally, miniaturization and high-throughput systems are discussed. All these factors contribute in their own way to the viability and functionality of liver cells in culture. Depending on the aim for which they are designed, several good systems are available for predicting hepatotoxicity and hepatic metabolism within the general population. To predict hepatotoxicity in individual cases genomic analysis might be essential as well.

Microfluidic platforms for studies of angiogenesis, cell migration, and cell-cell interactions. Sixth International Bio-Fluid Mechanics Symposium and Workshop March 28-30, 2008 Pasadena, California

Recent advances in microfluidic technologies have opened the door for creating more realistic in vitro cell culture methods that replicate many aspects of the true in vivo microenvironment. These new designs (i) provide enormous flexibility in controlling the critical biochemical and biomechanical factors that influence cell behavior, (ii) allow for the introduction of multiple cell types in a single system, (iii) provide for the establishment of biochemical gradients in two- or three-dimensional geometries, and (iv) allow for high quality, time-lapse imaging. Here, some of the recent developments are reviewed, with a focus on studies from our own laboratory in three separate areas: angiogenesis, cell migration in the context of tumor cell-endothelial interactions, and liver tissue engineering.

Reconstituting organ-level lung functions on a chip

Here, we describe a biomimetic microsystem that reconstitutes the critical functional alveolar-capillary interface of the human lung. This bioinspired microdevice reproduces complex integrated organ-level responses to bacteria and inflammatory cytokines introduced into the alveolar space. In nanotoxicology studies, this lung mimic revealed that cyclic mechanical strain accentuates toxic and inflammatory responses of the lung to silica nanoparticles. Mechanical strain also enhances epithelial and endothelial uptake of nanoparticulates and stimulates their transport into the underlying microvascular channel. Similar effects of physiological breathing on nanoparticle absorption are observed in whole mouse lung. Mechanically active "organ-on-a-chip" microdevices that reconstitute tissue-tissue interfaces critical to organ function may therefore expand the capabilities of cell culture models and provide low-cost alternatives to animal and clinical studies for drug screening and toxicology applications.

Microbioreactors for Raman microscopy of stromal cell differentiation

We present the development of microbioreactors with a sensitive and accurate optical coupling to a confocal Raman microspectrometer. We show that such devices enable in situ and in vitro investigation of cell cultures for tissue engineering by chemically sensitive Raman spectroscopic imaging techniques. The optical resolution of the Raman microspectrometer allows recognition and chemical analysis of subcellular features. Human bone marrow stromal cells (hBMSCs) have been followed after seeding through a phase of early proliferation until typically 21 days later, well after the cells have differentiated to osteoblasts. Long-term perfusion of cells in the dynamic culture conditions was shown to be compatible with experimental optical demands and off-line optical analysis. We show that Raman optical analysis of cells and cellular differentiation in microbioreactors is feasible down to the level of subcellular organelles during development. We conclude that microbioreactors combined with Raman microspectroscopy are a valuable tool to study hBMSC proliferation, differentiation, and development into tissues under in situ and in vitro conditions.

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.

Heparin-based hydrogel as a matrix for encapsulation and cultivation of primary hepatocytes

Primary hepatocytes are commonly used as liver surrogates in toxicology and tissue engineering fields, therefore, maintenance of functional hepatocytes in vitro is an important topic of investigation. This paper sought to characterize heparin-based hydrogel as a three-dimensional scaffold for hepatocyte culture. The primary rat hepatocytes were mixed with a prepolymer solution comprised of thiolated heparin and acrylated poly(ethylene glycol) (PEG). Raising the temperature from 25 degrees to 37 degrees C initiated Michael addition reaction between the thiol and acrylated moieties and resulted in formation of hydrogel with entrapped cells. Analysis of liver-specific products, albumin and urea, revealed that the heparin hydrogel was non-cytotoxic to cells and, in fact, promoted hepatic function. Hepatocytes entrapped in the heparin-based hydrogel maintained high levels of albumin and urea synthesis after three weeks in culture. Because heparin is known to bind growth factors, we incorporated hepatocyte growth factor (HGF)-an important liver signaling molecule - into the hydrogel. HGF release from heparin hydrogel matrix was analyzed using enzyme linked immunoassay (ELISA) and was shown to occur in a controlled manner with only 40% of GF molecules released after 30 days in culture. Importantly, hepatocytes cultured within HGF-containing hydrogels exhibited significantly higher levels of albumin and urea synthesis compared to cells cultured in the hydrogel alone. Overall, heparin-based hydrogel showed to be a promising matrix for encapsulation and maintenance of difficult-to-culture primary hepatocytes. In the future, we envision employing heparin-based hyrogels as matrices for in vitro differentiation of hepatocytes or stem cells and as vehicles for transplantation of these cells.

Microfluidic biochip for the perifusion of precision-cut rat liver slices for metabolism and toxicology studies

Early detection of kinetic, metabolic, and toxicity (ADME-Tox) profiles for new drug candidates is of crucial importance during drug development. This article describes a novel in vitro system for the incubation of precision-cut liver slices (PCLS) under flow conditions, based on a poly(dimethylsiloxane) (PDMS) device containing 25-microL microchambers for integration of the slices. The microdevice is coupled to a perifusion system, which enables a constant delivery of nutrients and oxygen and a continuous removal of waste products. Both a highly controlled incubation environment and high metabolite detection sensitivity could be achieved using microfluidics. Liver slices were viable for at least 24 h in the microdevice. The compound, 7-ethoxycoumarin (7-EC), was chosen to test metabolism, since its metabolism includes both phase I and phase II metabolism and when tested in the conventional well plate system, correlates well with the in vivo situation (De Kanter et al. 2004. Xenobiotica 34(3): 229-241.). The metabolic rate of 7-EC was found to be 214 +/- 5 pmol/min/mg protein in the microdevice, comparable to well plates, and was constant over time for at least 3 h. This perifusion system better mimics the in vivo situation, and has the potential to significantly contribute to drug metabolism and toxicology studies of novel chemical entities.

Multicellular tumor spheroids: an underestimated tool is catching up again

The present article highlights the rationale, potential and flexibility of tumor spheroid mono- and cocultures for implementation into state of the art anti-cancer therapy test platforms. Unlike classical monolayer-based models, spheroids strikingly mirror the 3D cellular context and therapeutically relevant pathophysiological gradients of in vivo tumors. Some concepts for standardization and automation of spheroid culturing, monitoring and analysis are discussed, and the challenges to define the most convenient analytical endpoints for therapy testing are outlined. The potential of spheroids to contribute to either the elimination of poor drug candidates at the pre-animal and pre-clinical state or the identification of promising drugs that would fail in classical 2D cell assays is emphasised. Microtechnologies, in the form of micropatterning and microfluidics, are also discussed and offer the exciting prospect of standardized spheroid mass production to tackle high-throughput screening applications within the context of traditional laboratory settings. The extension towards more sophisticated spheroid coculture models which more closely reflect heterologous tumor tissues composed of tumor and various stromal cell types is also covered. Examples are given with particular emphasis on tumor-immune cell cocultures and their usefulness for testing novel immunotherapeutic treatment strategies. Finally, tumor cell heterogeneity and the extraordinary possibilities of putative cancer stem/tumor-initiating cell populations that can be maintained and expanded in sphere-forming assays are introduced. The relevance of the cancer stem cell hypothesis for cancer cure is highlighted, with the respective sphere cultures being envisioned as an integral tool for next generation drug development offensives.

Perfused multiwell plate for 3D liver tissue engineering

In vitro models that capture the complexity of in vivo tissue and organ behaviors in a scalable and easy-to-use format are desirable for drug discovery. To address this, we have developed a bioreactor that fosters maintenance of 3D tissue cultures under constant perfusion and we have integrated multiple bioreactors into an array in a multiwell plate format. All bioreactors are fluidically isolated from each other. Each bioreactor in the array contains a scaffold that supports formation of hundreds of 3D microscale tissue units. The tissue units are perfused with cell culture medium circulated within the bioreactor by integrated pneumatic diaphragm micropumps. Electronic controls for the pumps are kept outside the incubator and connected to the perfused multiwell by pneumatic lines. The docking design and open-well bioreactor layout make handling perfused multiwell plates similar to using standard multiwell tissue culture plates. A model of oxygen consumption and transport in the circulating culture medium was used to predict appropriate operating parameters for primary liver cultures. Oxygen concentrations at key locations in the system were then measured as a function of flow rate and time after initiation of culture to determine oxygen consumption rates. After seven days of culture, tissue formed from cells seeded in the perfused multiwell reactor remained functionally viable as assessed by immunostaining for hepatocyte and liver sinusoidal endothelial cell (LSEC) phenotypic markers.