Model-based analysis and design of a microchannel reactor for tissue engineering

Computational model of the cancer necrotic core formation in a tumor-on-a-chip device

The mechanisms underlying the formation of necrotic regions within avascular tumors are complex and poorly understood. In this paper, we investigate the formation of a necrotic core in a 3D tumor cell culture within a microfluidic device, considering oxygen, nutrients, and the microenvironment acidification by means of a computational-mathematical model. Our objective is to simulate cell processes, including proliferation and death inside a microfluidic device, according to the microenvironmental conditions. We employed approximation utilizing finite element models taking into account glucose, oxygen, and hydrogen ions diffusion, consumption and production, as well as cell proliferation, migration and death, addressing how tumor cells evolve under different conditions. The resulting mathematical model was examined under different scenarios, being capable of reproducing cell death and proliferation under different cell concentrations, and the formation of a necrotic core, in good agreement with experimental data reported in the literature. This approach not only advances our fundamental understanding of necrotic core formation but also provides a robust computational platform to study personalized therapeutic strategies, offering an important tool in cancer research and treatment design.

A mechanobiological mathematical model of liver metabolism

The liver plays a complex role in metabolism and detoxification, and better tools are needed to understand its function and to develop liver-targeted therapies. In this study, we establish a mechanobiological model of liver transport and hepatocyte biology to elucidate the metabolism of urea and albumin, the production/detoxification of ammonia, and consumption of oxygen and nutrients. Since hepatocellular shear stress (SS) can influence the enzymatic activities of liver, the effect of SS on the urea and albumin synthesis are empirically modeled through the mechanotransduction mechanisms. The results demonstrate that the rheology and dynamics of the sinusoid flow can significantly affect liver metabolism. We show that perfusate rheology and blood hematocrit can affect urea and albumin production by changing hepatocyte mechanosensitive metabolism. The model can also simulate enzymatic diseases of the liver such as hyperammonemia I, hyperammonemia II, hyperarginemia, citrollinemia, and argininosuccinicaciduria, which disrupt the urea metabolism and ammonia detoxification. The model is also able to predict how aggregate cultures of hepatocytes differ from single cell cultures. We conclude that in vitro perfusable devices for the study of liver metabolism or personalized medicine should be designed with similar morphology and fluid dynamics as patient liver tissue. This robust model can be adapted to any type of hepatocyte culture to determine how hepatocyte viability, functionality, and metabolism are influenced by liver pathologies and environmental conditions.

A Robust Longitudinal Co-culture of Obligate Anaerobic Gut Microbiome With Human Intestinal Epithelium in an Anoxic-Oxic Interface-on-a-Chip

The majority of human gut microbiome is comprised of obligate anaerobic bacteria that exert essential metabolic functions in the human colon. These anaerobic gut bacteria constantly crosstalk with the colonic epithelium in a mucosal anoxic-oxic interface (AOI). However, in vitro recreation of the metabolically mismatched colonic AOI has been technically challenging. Furthermore, stable co-culture of the obligate anaerobic commensal microbiome and epithelial cells in a mechanically dynamic condition is essential for demonstrating the host-gut microbiome crosstalk. Here, we developed an anoxic-oxic interface-on-a-chip (AOI Chip) by leveraging a modified human gut-on-a-chip to demonstrate a controlled oxygen gradient in the lumen-capillary transepithelial interface by flowing anoxic and oxic culture medium at various physiological milieus. Computational simulation and experimental results revealed that the presence of the epithelial cell layer and the flow-dependent conditioning in the lumen microchannel is necessary and sufficient to create the steady-state vertical oxygen gradient in the AOI Chip. We confirmed that the created AOI does not compromise the viability, barrier function, mucin production, and the expression and localization of tight junction proteins in the 3D intestinal epithelial layer. Two obligate anaerobic commensal gut microbiome, Bifidobacterium adolescentis and Eubacterium hallii, that exert metabolic cross-feeding in vivo, were independently co-cultured with epithelial cells in the AOI Chip for up to a week without compromising any cell viability. Our new protocol for creating an AOI in a microfluidic gut-on-a-chip may enable to demonstrate the key physiological interactions of obligate anaerobic gut microbiome with the host cells associated with intestinal metabolism, homeostasis, and immune regulation.

Organ-on-a-Chip Technology for Reproducing Multiorgan Physiology

In the drug development process, the accurate prediction of drug efficacy and toxicity is important in order to reduce the cost, labor, and effort involved. For this purpose, conventional 2D cell culture models are used in the early phase of drug development. However, the differences between the in vitro and the in vivo systems have caused the failure of drugs in the later phase of the drug-development process. Therefore, there is a need for a novel in vitro model system that can provide accurate information for evaluating the drug efficacy and toxicity through a closer recapitulation of the in vivo system. Recently, the idea of using microtechnology for mimicking the microscale tissue environment has become widespread, leading to the development of "organ-on-a-chip." Furthermore, the system is further developed for realizing a multiorgan model for mimicking interactions between multiple organs. These advancements are still ongoing and are aimed at ultimately developing "body-on-a-chip" or "human-on-a-chip" devices for predicting the response of the whole body. This review summarizes recently developed organ-on-a-chip technologies, and their applications for reproducing multiorgan functions.

The histocompatibility research of hair follicle stem cells with bladder acellular matrix

BACKGROUND

Hair follicle stem cells (HFSCs) were reported to have multidirectional differentiation ability and could be differentiated into melanocytes, keratin cells, smooth muscle cells, and neurons. However, the functionality of HFSCs in bladder tissue regeneration is unknown.

METHODS

This study was conducted to build HFSCs vs bladder acellular matrix (BAM) complexes (HFSCs-BAM complexes) in vitro and evaluated whether HFSCs have well biocompatibility with BAM. HFSCs were separated from SD rats. BAM scaffold was prepared from the submucosa of rabbit bladder tissue. Afterwards, HFSCs were inoculated on BAM.

RESULTS

HFSCs-BAM complexes grew rapidly through inverted microscope observation. Cell growth curve showed the proliferation was in stagnate phase at 7th and 8th day. Cytotoxicity assay showed the toxicity grading of BAM was 0 or 1. Scanning electron microscopy, HE staining, and masson staining showed that cells have germinated on the surface of scaffold.

CONCLUSION

The results provide evidence that HFSCs-BAM complexes have well biocompatibility and accumulate important experimental basis for clinical applying of tissue engineering bladder.

Microfluidic device capable of medium recirculation for non-adherent cell culture

We present a microfluidic device designed for maintenance and culture of non-adherent mammalian cells, which enables both recirculation and refreshing of medium, as well as easy harvesting of cells from the device. We demonstrate fabrication of a novel microfluidic device utilizing Braille perfusion for peristaltic fluid flow to enable switching between recirculation and refresh flow modes. Utilizing fluid flow simulations and the human promyelocytic leukemia cell line, HL-60, non-adherent cells, we demonstrate the utility of this RECIR-REFRESH device. With computer simulations, we profiled fluid flow and concentration gradients of autocrine factors and found that the geometry of the cell culture well plays a key role in cell entrapping and retaining autocrine and soluble factors. We subjected HL-60 cells, in the device, to a treatment regimen of 1.25% dimethylsulfoxide, every other day, to provoke differentiation and measured subsequent expression of CD11b on day 2 and day 4 and tumor necrosis factor-alpha (TNF-α) on day 4. Our findings display perfusion sensitive CD11b expression, but not TNF-α build-up, by day 4 of culture, with a 1:1 ratio of recirculation to refresh flow yielding the greatest increase in CD11b levels. RECIR-REFRESH facilitates programmable levels of cell differentiation in a HL-60 non-adherent cell population and can be expanded to other types of non-adherent cells such as hematopoietic stem cells.

Optimal periodic perfusion strategy for robust long-term microfluidic cell culture

Long-term cell culture in microfluidic devices is an essential prerequisite for "on a chip" biological and physiological based studies. We investigated how medium delivery, from continuous to periodic perfusion, affects long-term cell cultures in a microfluidic platform. Computational simulations suggested that different delivery strategies result in different temporal profiles of accumulation and washing out of endogenous (EnF) and exogenous (ExF) factors, respectively. Thus, cultures exposed to the same overall amount of medium with different temporal profiles were analysed in terms of homogeneity, cell morphology and phenotype. Murine and human cell lines (C2C12 and HFF) and mouse embryonic stem cells (mESC) were cultured in microfluidic channels. An ad hoc experimental setup was developed to perform continuous and periodic medium delivery into the chip, tuning the flow rate, the perfusion time, and the interval of perfusion while using the same amount of medium volume. Periodic medium delivery with a short perfusion pulse ensured cell homogeneity compared to standard cell culture. Conversely, a continuous flow resulted in cell heterogeneity, with abnormal morphology and vesiculation. Only dramatic and unfeasible increasing of perfused medium volume in the continuous configuration could rescue normal cell behaviour. Consistent results were obtained for C2C12 and HFF. In order to extend these results to highly sensitive cells, mESC were cultured for 6 days in the microfluidic channels. Our analysis demonstrates that a periodic medium delivery with fast pulses (with a frequency of 4 times per day) resulted in a homogeneous cell culture in terms of cell viability, colony morphology and maintenance of pluripotency markers. According to experimental observations, the computational model provided a rational description of the perfusion strategies and of how they deeply shape the cell microenvironment in microfluidic cell cultures. These results provide new insight to define optimal strategies for homogeneous and robust long-term cell culture in microfluidic systems, an essential prerequisite for lab on chip cell-based applications.

Brain slice on a chip: opportunities and challenges of applying microfluidic technology to intact tissues

Isolated brain tissue, especially brain slices, are valuable experimental tools for studying neuronal function at the network, cellular, synaptic, and single channel levels. Neuroscientists have refined the methods for preserving brain slice viability and function and converged on principles that strongly resemble the approach taken by engineers in developing microfluidic devices. With respect to brain slices, microfluidic technology may 1) overcome the traditional limitations of conventional interface and submerged slice chambers and improve oxygen/nutrient penetration into slices, 2) provide better spatiotemporal control over solution flow/drug delivery to specific slice regions, and 3) permit successful integration with modern optical and electrophysiological techniques. In this review, we highlight the unique advantages of microfluidic devices for in vitro brain slice research, describe recent advances in the integration of microfluidic devices with optical and electrophysiological instrumentation, and discuss clinical applications of microfluidic technology as applied to brain slices and other non-neuronal tissues. We hope that this review will serve as an interdisciplinary guide for both neuroscientists studying brain tissue in vitro and engineers as they further develop microfluidic chamber technology for neuroscience research.

Computational modeling of adherent cell growth in a hollow-fiber membrane bioreactor for large-scale 3-D bone tissue engineering

The use of hollow-fiber membrane bioreactors (HFMBs) has been proposed for three-dimensional bone tissue growth at the clinical scale. However, to achieve an efficient HFMB design, the relationship between cell growth and environmental conditions must be determined. Therefore, in this work, a dynamic double-porous media model was developed to determine nutrient-dependent cell growth for bone tissue formation in a HFMB. The whole hollow-fiber scaffold within the bioreactor was treated as a porous domain in this model. The domain consisted of two interpenetrating porous regions, including a porous lumen region available for fluid flow and a porous extracapillary space filled with a collagen gel that contained adherent cells for promoting long-term growth into tissue-like mass. The governing equations were solved numerically and the model was validated using previously published experimental results. The contributions of several bioreactor design and process parameters to the performance of the bioreactor were studied. The results demonstrated that the process and design parameters of the HFMB significantly affect nutrient transport and thus cell behavior over a long period of culture. The approach presented here can be applied to any cell type and used to develop tissue engineering hollow-fiber scaffolds.

Predictive microfluidic control of regulatory ligand trajectories in individual pluripotent cells

Local (cell-level) signaling environments, regulated by autocrine and paracrine signaling, and modulated by cell organization, are hypothesized to be fundamental stem cell fate control mechanisms used during development. It has, however, been challenging to demonstrate the impact of cell-level organization on stem cell fate control and to relate stem cell fate outcomes to autocrine and paracrine signaling. We address this fundamental problem using a combined in silico and experimental approach in which we directly manipulate, using laminar fluid flow, the local impact of endogenously secreted gp130-activating ligands and their activation of signal transducer and activator of transcription3 (STAT3) signaling in mouse embryonic stem cells (mESC). Our model analysis predicted that flow-dependent changes in autocrine and paracrine ligand binding would impact heterogeneity in cell- and colony-level STAT3 signaling activation and cause a gradient of cell fate determination along the direction of flow. Interestingly, analysis also predicted that local cell density would be inversely proportional to the degree to which endogenous secretion contributed to cell fate determination. Experimental validation using functional activation of STAT3 by secreted factors under microfluidic perfusion culture demonstrated that STAT3 activation and consequently mESC fate were manipulable by flow rate, position in the flow field, and local cell organization. As a unique demonstration of how quantitative control of autocrine and paracrine signaling can be integrated with spatial organization to elicit higher order cell fate effects, this work provides a general template to investigate organizing principles due to secreted factors.

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.

Microfluidic perfusion for regulating diffusible signaling in stem cells

BACKGROUND

Autocrine & paracrine signaling are widespread both in vivo and in vitro, and are particularly important in embryonic stem cell (ESC) pluripotency and lineage commitment. Although autocrine signaling via fibroblast growth factor-4 (FGF4) is known to be required in mouse ESC (mESC) neuroectodermal specification, the question of whether FGF4 autocrine signaling is sufficient, or whether other soluble ligands are also involved in fate specification, is unknown. The spatially confined and closed-loop nature of diffusible signaling makes its experimental control challenging; current experimental approaches typically require prior knowledge of the factor/receptor in order to modulate the loop. A new approach explored in this work is to leverage transport phenomena at cellular resolution to downregulate overall diffusible signaling through the physical removal of cell-secreted ligands.

METHODOLOGY/PRINCIPAL FINDINGS

We develop a multiplex microfluidic platform to continuously remove cell-secreted (autocrine\paracrine) factors to downregulate diffusible signaling. By comparing cell growth and differentiation in side-by-side chambers with or without added cell-secreted factors, we isolate the effects of diffusible signaling from artifacts such as shear, nutrient depletion, and microsystem effects, and find that cell-secreted growth factor(s) are required during neuroectodermal specification. Then we induce FGF4 signaling in minimal chemically defined medium (N2B27) and inhibit FGF signaling in fully supplemented differentiation medium with cell-secreted factors to determine that the non-FGF cell-secreted factors are required to promote growth of differentiating mESCs.

CONCLUSIONS/SIGNIFICANCE

Our results demonstrate for the first time that flow can downregulate autocrine\paracrine signaling and examine sufficiency of extracellular factors. We show that autocrine\paracrine signaling drives neuroectodermal commitment of mESCs through both FGF4-dependent and -independent pathways. Overall, by uncovering autocrine\paracrine processes previously hidden in conventional culture systems, our results establish microfluidic perfusion as a technique to study and manipulate diffusible signaling in cell systems.

A computational and experimental study inside microfluidic systems: the role of shear stress and flow recirculation in cell docking

In this paper, microfluidic devices containing microwells that enabled cell docking were investigated. We theoretically assessed the effect of geometry on recirculation areas and wall shear stress patterns within microwells and studied the relationship between the computational predictions and experimental cell docking. We used microchannels with 150 microm diameter microwells that had either 20 or 80 microm thickness. Flow within 80 microm deep microwells was subject to extensive recirculation areas and low shear stresses (<0.5 mPa) near the well base; whilst these were only presented within a 10 microm peripheral ring in 20 microm thick microwells. We also experimentally demonstrated that cell docking was significantly higher (p < 0.01) in 80 microm thick microwells as compared to 20 microm thick microwells. Finally, a computational tool which correlated physical and geometrical parameters of microwells with their fluid dynamic environment was developed and was also experimentally confirmed.

Model-based analysis and optimization of bioreactor for hematopoietic stem cell cultivation

One of the problems to be solved in attaining the full potentials of hematopoietic stem cell (HSC) applications is the limited availability of the cells. Growing HSCs in a bioreactor offers an alternative solution to this problem. Besides, it also offers the advantages of eliminating labour intensive process as well as the possible contamination involved in the periodic nutrient replenishments in the traditional T-flask stem cell cultivation. In spite of this, the optimization of HSC cultivation in a bioreactor has been barely explored. This manuscript discusses the development of a mathematical model to describe the dynamics in nutrient distribution and cell concentration of an ex vivo HSC cultivation in a microchannel perfusion bioreactor. The model was further used to optimize the cultivation by proposing three alternative feeding strategies in order to prevent the occurrence of nutrient limitation in the bioreactor. The evaluation of these strategies, the periodic step change increase in the inlet oxygen concentration, the periodic step change increase in the media inflow, and the feedback control of media inflow, shows that these strategies can successfully improve the cell yield of the bioreactor. In general, the developed model is useful for the design and optimization of bioreactor operation.

Windows of operation for bioreactor design for the controlled formation of tissue-engineered arteries

The availability of large numbers of units of artificial arteries would offer significant benefits to the clinical management of bypass surgery. Tissue engineering offers the potential of providing vessels that can mimic the morphology, function, and physiological environment of native vessels. Ideally this would involve culturing stem cells in vitro within a biodegradable tubular scaffold so as to construct tissue for implantation. Essential to establishing a robust process for the production of tissue-engineered arteries is the understanding of the impact of changes in the operating conditions and bioreactor design on the construct formation. In this article, models of transport phenomena were developed to predict the critical flow rates and mass transfer requirements of a prototype bioreactor for the formation of tissue-engineered arteries. The impact of the cell concentration, tube geometry, oxygen effective diffusivity in alginate, substrate and metabolite concentration levels, feed rate, and recycle rate on the design of the bioreactor was visualized using windows of operation and contour plots. The result of this analysis determined the best configuration of the bioreactor that meets the cellular transport requirements as well as being reliable in performance while seeking to reduce the amount of nutrients to be used.

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.

Quantitative inference of cellular parameters from microfluidic cell culture systems

Microfluidic cell culture systems offer a convenient way to measure cell biophysical parameters in conditions close to the physiological environment. We demonstrate the application of a mathematical model describing the spatial distribution of nutrient and growth factor concentrations in inferring cellular oxygen uptake rates from experimental measurements. We use experimental measurements of oxygen concentrations in a poly(dimethylsiloxane) (PDMS) microreactor culturing human hepatocellular liver carcinoma cells (HepG2) to infer quantitative information on cellular oxygen uptake rates. We use a novel microchannel design to avoid the parameter correlation problem associated with simultaneous cellular uptake and diffusion of oxygen through the PDMS surface. We find that the cellular uptake of oxygen is dependent on the cell density and can be modeled using a logistic term in the Michaelis-Menten equation. Our results are significant not only for the development of novel assays to quantitatively infer cell response to stimuli, but also for the development, design, and optimization of novel in vitro systems for drug discovery and tissue engineering.

Design of well and groove microchannel bioreactors for cell culture

Microfluidic bioreactors have been shown valuable for various cellular applications. The use of micro-wells/grooves bioreactors, in which micro-topographical features are used to protect sensitive cells from the detrimental effects of fluidic shear stress, is a promising approach to culture sensitive cells in these perfusion microsystems. However, such devices exhibit substantially different fluid dynamics and mass transport characteristics compared to conventional planar microchannel reactors. In order to properly design and optimize these systems, fluid and mass transport issues playing a key role in microscale bioreactors should be adequately addressed. The present work is a parametric study of micro-groove/micro-well microchannel bioreactors. Operation conditions and design parameters were theoretically examined via a numerical model. The complex flow pattern obtained at grooves of various depths was studied and the shear protection factor compared to planar microchannels was evaluated. 3D flow simulations were preformed in order to examine the shear protection factor in micro-wells, which were found to have similar attributes as the grooves. The oxygen mass transport problem, which is coupled to the fluid mechanics problem, was solved for various groove geometries and for several cell types, assuming a defined shear stress limitation. It is shown that by optimizing the groove depth, the groove bioreactor may be used to effectively maximize the number of cells cultured within it or to minimize the oxygen gradient existing in such devices. Moreover, for sensitive cells having a high oxygen demand (e.g., hepatocytes) or low endurance to shear (e.g., human embryonic stem cells), results show that the use of grooves is an enabling technology, since under the same physical conditions the cells cannot be cultured for long periods of time in a planar microchannel. In addition to the theoretical model findings, the culture of human foreskin fibroblasts in groove (30 microm depth) and well bioreactors (35 microm depth) was experimentally examined at various flow rates of medium perfusion and compared to cell culture in regular flat microchannels. It was shown that the wells and the grooves enable a one order of magnitude increase in the maximum perfusion rate compared to planar microchannels. Altogether, the study demonstrates that the proper design and use of microgroove/well bioreactors may be highly beneficial for cell culture assays.

Screening the cellular microenvironment: a role for microfluidics

The cellular microenvironment is an increasingly discussed topic in cell biology as it has been implicated in the progression of cancer and the maintenance of stem cells. The microenvironment of a cell is an organized combination of extracellular matrix (ECM), cells, and interstitial fluid that influence cellular phenotype through physical, mechanical, and biochemical mechanisms. Screening can be used to map combinations of cells and microenvironments to phenotypic outcomes in a way that can help develop more predictive in vitro models and to better understand phenotypic mechanisms from a systems biology perspective. This paper examines microenvironmental screening in terms of outcomes and benefits, key elements of the screening process, challenges for implementation, and a possible role for microfluidics as the screening platform. To assess microfluidics for use in microenvironmental screening, examples and categories of micro-scale and microfluidic technology are highlighted. Microfluidic technology shows promise for simultaneous control of multiple parameters of the microenvironment and can provide a base for scaling advanced cell-based experiments into automated high-throughput formats.

Theoretical analysis of engineered cartilage oxygenation: influence of construct thickness and media flow rate

A novel parallel-plate bioreactor has been shown to modulate the mechanical and biochemical properties of engineered cartilage by the application of fluid-induced shear stress. Flow or perfusion bioreactors may improve tissue development via enhanced transport of nutrients or gases as well as the application of mechanical stimuli, or a combination of these factors. The goal of this study was to complement observed experimental responses to flow by simulating oxygen transport within cartilage constructs of different thicknesses (250 microm or 1 mm). Using numerical computation of convection-diffusion equations, the evaluation of the tissue oxygenation is performed. Four culture conditions are defined based on tissue thickness and flow rates ranging from 0 to approximately 25 mL min(-1). Under these experimental conditions results show a mean oxygen concentration within the tissue varying from 0.01 to 0.19 mol m(-3) as a function of the tissue thickness and the magnitude of the applied shear stress. More generally, the influence of shear stress varying (via flow rate modification) from 10(-3) to 10 dynes cm(-2) on the tissue oxygenation is studied. The influence on the results of important physical parameters such as the maximal oxygen consumption rate of cells is discussed. Lastly, the importance of oxygen concentration in the lower chamber and its relevance to tissue oxygenation are highlighted by the model results.

Three-dimensional modeling of transport of nutrients for multicellular tumor spheroid culture in a microchannel

The growth dynamics of avascular tumors in a microchannel bioreactor is investigated. A three-dimensional flow and nutrient transport model, incorporating the multicellular tumor spheroid (MTS) growth model, has been developed to study the influence of nutrients (oxygen and glucose) supply and distribution on the MTS growth. Numerical simulations based on the EMT6/Ro tumor cells show that the continuous-flow perfusion is more efficient to deliver nutrients to the MTS than the diffusion-only static culture. It is further demonstrated that as long as there is bulk flow, the growth of a single tumor spheroid at the early stage is insensitive to the flow velocity and the channel size. For multiple tumor spheroids in the same microchannel, however, increasing the perfusion velocity can improve the nutrient environment for the disadvantageous downstream tumor spheroid. The flow shear stress exerting on the MTSs in the current microchannel bioreactor is estimated to be far below the critical value to affect the MTS growth, which means that there is still much room for increasing perfusion velocity to satisfy the higher nutrient requirement by the growing tumor spheroids.

Quantitative measurement and control of oxygen levels in microfluidic poly(dimethylsiloxane) bioreactors during cell culture

Microfluidic bioreactors fabricated from highly gas-permeable poly(dimethylsiloxane) (PDMS) materials have been observed, somewhat unexpectedly, to give rise to heterogeneous long term responses along the length of a perfused mammalian cell culture channel, reminiscent of physiologic tissue zonation that arises at least in part due to oxygen gradients. To develop a more quantitative understanding and enable better control of the physical-chemical mechanisms underlying cell biological events in such PDMS reactors, dissolved oxygen concentrations in the channel system were quantified in real time using fluorescence intensity and lifetime imaging of an oxygen sensitive dye, ruthenium tris(2,2'-dipyridyl) dichloride hexahydrate (RTDP). The data indicate that despite oxygen diffusion through PDMS, uptake of oxygen by cells inside the perfused PDMS microchannels induces an axial oxygen concentration gradient, with lower levels recorded in downstream regions. The oxygen concentration gradient generated by a balance of cellular uptake, convective transport by media flow, and permeation through PDMS in our devices ranged from 0.0003 (mg/l)/mm to 0.7 (mg/l)/mm. The existence of such steep gradients induced by cellular uptake can have important biological consequences. Results are consistent with our mathematical model and give insight into the conditions under which flux of oxygen through PDMS into the microchannels will or will not contribute significantly to oxygen delivery to cells and also provide a design tool to manipulate and control oxygen for cell culture and device engineering. The combination of computerized microfluidics, in situ oxygen sensing, and mathematical models opens new windows for microphysiologic studies utilizing oxygen gradients and low oxygen tensions.

A practical guide to microfluidic perfusion culture of adherent mammalian cells

Culturing cells at microscales allows control over microenvironmental cues, such as cell-cell and cell-matrix interactions; the potential to scale experiments; the use of small culture volumes; and the ability to integrate with microsystem technologies for on-chip experimentation. Microfluidic perfusion culture in particular allows controlled delivery and removal of soluble biochemical molecules in the extracellular microenvironment, and controlled application of mechanical forces exerted via fluid flow. There are many challenges to designing and operating a robust microfluidic perfusion culture system for routine culture of adherent mammalian cells. The current literature on microfluidic perfusion culture treats microfluidic design, device fabrication, cell culture, and micro-assays independently. Here we systematically present and discuss important design considerations in the context of the entire microfluidic perfusion culture system. These design considerations include the choice of materials, culture configurations, microfluidic network fabrication and micro-assays. We also present technical issues such as sterilization; seeding cells in both 2D and 3D configurations; and operating the system under optimized mass transport and shear stress conditions, free of air-bubbles. The integrative and systematic treatment of the microfluidic system design and fabrication, cell culture, and micro-assays provides novices with an effective starting point to build and operate a robust microfludic perfusion culture system for various applications.

A parametric study of human fibroblasts culture in a microchannel bioreactor

The culture of cells in a microbioreactor can be highly beneficial for cell biology studies and tissue engineering applications. The present work provides new insights into the relationship between cell growth, cell morphology, perfusion rate, and design parameters in microchannel bioreactors. We demonstrate the long-term culture of mammalian (human foreskin fibroblasts, HFF) cells in a microbioreactor under constant perfusion in a straightforward simple manner. A perfusion system was used to culture human cells for more than two weeks in a plain microchannel (130 microm x 1 mm x 2 cm). At static conditions and at high flow rates (>0.3 ml h(-1)), the cells did not grow in the microchannel for more than a few days. For low flow rates (<0.2 ml h(-1)), the cells grew well and a confluent layer was obtained. We show that the culture of cells in microchannels under perfusion, even at low rates, affects cell growth kinetics as well as cell morphology. The oxygen level in the microchannel was evaluated using a mass transport model and the maximum cell density measured in the microchannel at steady state. The maximum shear stress, which corresponds to the maximum flow rate used for long term culture, was 20 mPa, which is significantly lower than the shear stress cells may endure under physiological conditions. The effect of channel size and cell type on long term cell culture were also examined and were found to be significant. The presented results demonstrate the importance of understanding the relationship between design parameters and cell behavior in microscale culture system, which vary from physiological and traditional culture conditions.

Three dimensional MEMS microfluidic perfusion system for thick brain slice cultures

In vitro tissue culture models are often benchmarked by their ability to replicate in vivo function. One of the limitations of in vitro systems is the difficulty in preserving an orchestrated cell population, especially for generating three-dimensional tissue equivalents. For example, tissue-engineering applications involve large high-density constructs, requiring a perfusing system that is able to apply adequate oxygen and nutrients to the interior region of the tissue. This is particularly true with respect to thick tissue sections harvested for in vitro culture. We have fabricated a microneedle-based perfusion device for high-cell-density in vitro tissue culture from SU-8 photosensitive epoxy and suitable post-processing. The device was tested for its ability to improve viability in slices of harvested brain tissue. This model was chosen due to its acute sensitivity to disruptions in its nutrient supply. Improved viability was visible in the short term as assessed via live-dead discriminating fluorescent staining and confocal microscopy. This perfusion system opens up many possibilities for both neurobiological as well as other culture systems.

Optical imaging in microfluidic bioreactors enables oxygen monitoring for continuous cell culture

For the first time, a fluorescence lifetime calibration method for an oxygen-sensitive dye ruthenium tris(2,2'-dipyridyl) dichloride hexahydrate (RTDP) is applied to image oxygen levels in poly(dimethyl siloxane) (PDMS) bioreactors containing living C2C12 mouse myoblasts. PDMS microsystems are broadly used in bioengineering applications due to their biocompatibility and ease of handling. For these systems, oxygen concentrations are of significance and are likely to play an important role in cell behavior and gene expression. Fluorescence lifetime imaging microscopy (FLIM) bases image contrast on fluorophore excited state lifetimes, which reflect local biochemistry. Unique attributes of the widefield, time-domain FLIM system include tunable excitation (337.1 to 960 nm), large temporal dynamic range (> or =600 ps), high spatial resolution (1.4 microm), calibrated detection (0 to 300+/-8 microM of oxygen), and rapid data acquisition and processing times (10 s). Oxygen levels decrease with increasing cell densities and are consistent with model outcomes obtained by simulating bioreactor oxygen diffusion and cell proliferation. In single bioreactor loops, FLIM detects spatial heterogeneity in oxygen levels with variations as high as 20%. The fluorescence lifetime-based imaging approach we describe avoids intensity-based artifacts (including photobleaching and concentration variations) and provides a technique with high spatial discrimination for oxygen monitoring in continuous cell culture systems.