Oxygen transfer in a diffusion-limited hollow fiber bioartificial liver

Mathematical modelling with Bayesian inference to quantitatively characterize therapeutic cell behaviour in nerve tissue engineering

Cellular engineered neural tissues have significant potential to improve peripheral nerve repair strategies. Traditional approaches depend on quantifying tissue behaviours using experiments in isolation, presenting a challenge for an overarching framework for tissue design. By comparison, mathematical cell-solute models benchmarked against experimental data enable computational experiments to be performed to test the role of biological/biophysical mechanisms, as well as to explore the impact of different design scenarios and thus accelerate the development of new treatment strategies. Such models generally consist of a set of continuous, coupled, partial differential equations relying on a number of parameters and functional forms. They necessitate dedicated in vitro experiments to be informed, which are seldom available and often involve small datasets with limited spatio-temporal resolution, generating uncertainties. We address this issue and propose a pipeline based on Bayesian inference enabling the derivation of experimentally informed cell-solute models describing therapeutic cell behaviour in nerve tissue engineering. We apply our pipeline to three relevant cell types and obtain models that can readily be used to simulate nerve repair scenarios and quantitatively compare therapeutic cells. Beyond parameter estimation, the proposed pipeline enables model selection as well as experiment utility quantification, aimed at improving both model formulation and experimental design.

Modelling-informed cell-seeded nerve repair construct designs for treating peripheral nerve injuries

Millions of people worldwide are affected by peripheral nerve injuries (PNI), involving billions of dollars in healthcare costs. Common outcomes for patients include paralysis and loss of sensation, often leading to lifelong pain and disability. Engineered Neural Tissue (EngNT) is being developed as an alternative to the current treatments for large-gap PNIs that show underwhelming functional recovery in many cases. EngNT repair constructs are composed of a stabilised hydrogel cylinder, surrounded by a sheath of material, to mimic the properties of nerve tissue. The technology also enables the spatial seeding of therapeutic cells in the hydrogel to promote nerve regeneration. The identification of mechanisms leading to maximal nerve regeneration and to functional recovery is a central challenge in the design of EngNT repair constructs. Using in vivo experiments in isolation is costly and time-consuming, offering a limited insight on the mechanisms underlying the performance of a given repair construct. To bridge this gap, we derive a cell-solute model and apply it to the case of EngNT repair constructs seeded with therapeutic cells which produce vascular endothelial growth factor (VEGF) under low oxygen conditions to promote vascularisation in the construct. The model comprises a set of coupled non-linear diffusion-reaction equations describing the evolving cell population along with its interactions with oxygen and VEGF fields during the first 24h after transplant into the nerve injury site. This model allows us to evaluate a wide range of repair construct designs (e.g. cell-seeding strategy, sheath material, culture conditions), the idea being that designs performing well over a short timescale could be shortlisted for in vivo trials. In particular, our results suggest that seeding cells beyond a certain density threshold is detrimental regardless of the situation considered, opening new avenues for future nerve tissue engineering.

Development of a 3D porous chitosan/gelatin liver scaffold for a bioartificial liver device

Functional artificial livers (FALs), with embedded hepatocytes that perform the functions of a normal liver, have been developed during the past decades. It is important to note that the liver scaffold, which is a biologically functional core of bioartificial livers, plays a vital role in the bio-cartridge within a bioartificial liver. In this study, a three-dimensional (3D) liver scaffold for in vitro cultures was fabricated by freeze-drying a chitosan/gelatin (CG) solution. A CG scaffold has advantages such as (i) inexpensive and easy-to-make; (ii) easy to fabricate with varying compressive modulus by changing the concentration of glutaraldehyde; (iii) non-cytotoxicity; and (iv) porous structure is similar to extracellular matrix (ECM), thus facilitating hepatocyte adhesion and proliferation. The results revealed that the compressive modulus and maintainability of a CG scaffold was correlated to the increase in glutaraldehyde. Furthermore, hepatocyte viability and hepatic functions showed the best performances with a 0.61% glutaraldehyde-CG scaffold. This CG scaffold not only had higher hepatocyte biocompatibility and mechanical strength, but also maintained hepatic functions and viability in vitro cultures; especially, the mechanical properties of 0.61% glutaraldehyde-CG scaffold were very similar to those in normal liver. The CG scaffold as a liver scaffold may have high potential for further bioartificial liver design in the near future.

Effects of Bioreactor-Oxygenation During Extracorporeal Granulocytes Treatment in Septic Patients

A granulocyte bioreactor for the extracorporeal treatment was developed to enhance the immune cell function in patients with severe sepsis. The influence of oxygenation on the used cells was tested in a prospective clinical study. Ten patients with severe sepsis were treated twice with the granulocyte bioreactor. The used cells were screened for functionality; values of blood gases, glucose and lactate were obtained from the recirculating bioreactor circuit. Five patients were treated with an oxygenator setup (Oxy group), five without oxygenator (Non-Oxy group). The overall in-hospital mortality was 50%. Significantly lower values of oxygen saturation, partial oxygen pressure, lactate, oxyburst and phagocytosis were seen in the Non-Oxy group compared with the Oxy group in the bioreactor circuit. Further studies with this approach are encouraged and should focus on the influence of oxygenation on production of reactive oxygen species and cytokines of used cells.

Effect of membranes on oxygen transfer rate and consumption within a newly developed three-compartment bioartificial liver device: Advanced experimental and theoretical studies

A mathematical model is developed to predict oxygen transfer in the fiber-in-fiber (FIF) bioartificial liver device. The model parameters are taken from the constructed and tested FIF modules. We extended the Krogh cylinder model by including one more zone for oxygen transfer. Cellular oxygen uptake was based on Michaelis-Menten kinetics. The effect of varying a number of important model parameters is investigated, including (1) oxygen partial pressure at the inlet, (2) the hydraulic permeability of compartment B (cell region), (3) the hydraulic permeability of the inner membrane, and (4) the oxygen diffusivity of the outer membrane. The mathematical model is validated by comparing its output against the experimentally acquired values of an oxygen transfer rate and the hydrostatic pressure drop. Three governing simultaneous linear differential equations are derived to predict and validate the experimental measurements, e.g., the flow rate and the hydrostatic pressure drop. The model output simulated the experimental measurements to a high degree of accuracy. The model predictions show that the cells in the annulus can be oxygenated well even at high cell density or at a low level of gas phase PG if the value of the oxygen diffusion coefficient Dm is 16 × 10(-5) . The mathematical model also shows that the performance of the FIF improves by increasing the permeability of polypropylene membrane (inner fiber). Moreover, the model predicted that 60% of plasma has access to the cells in the annulus within the first 10% of the FIF bioreactor axial length for a specific polypropylene membrane permeability and can reach 95% within the first 30% of its axial length.

Computational fluid model incorporating liver metabolic activities in perfusion bioreactor

The importance of in vitro hepatotoxicity testing during early stages of drug development in the pharmaceutical industry demands effective bioreactor models with optimized conditions. While perfusion bioreactors have been proven to enhance mass transfer and liver specific functions over a long period of culture, the flow-induced shear stress has less desirable effects on the hepatocytes liver-specific functions. In this paper, a two-dimensional human liver hepatocellular carcinoma (HepG2) cell culture flow model, under a specified flow rate of 0.03 mL/min, was investigated. Besides computing the distribution of shear stresses acting on the surface of the cell culture, our numerical model also investigated the cell culture metabolic functions such as the oxygen consumption, glucose consumption, glutamine consumption, and ammonia production to provide a fuller analysis of the interaction among the various metabolites within the cell culture. The computed albumin production of our 2D flow model was verified by the experimental HepG2 culture results obtained over 3 days of culture. The results showed good agreement between our experimental data and numerical predictions with corresponding cumulative albumin production of 2.9 × 10(-5) and 3.0 × 10(-5)  mol/m(3) , respectively. The results are of importance in making rational design choices for development of future bioreactors with more complex geometries.

Microfabricated polymeric vessel mimetics for 3-D cancer cell culture

Modeling tumor growth in vitro is essential for cost-effective testing of hypotheses in preclinical cancer research. 3-D cell culture offers an improvement over monolayer culture for studying cellular processes in cancer biology because of the preservation of cell-cell and cell-ECM interactions. Oxygen transport poses a major barrier to mimicking in vivo environments and is not replicated in conventional cell culture systems. We hypothesized that we can better mimic the tumor microenvironment using a bioreactor system for controlling gas exchange in cancer cell cultures with silicone hydrogel synthetic vessels. Soft-lithography techniques were used to fabricate oxygen-permeable silicone hydrogel membranes containing arrays of micropillars. These membranes were inserted into a bioreactor and surrounded by basement membrane extract (BME) within which fluorescent ovarian cancer (OVCAR8) cells were cultured. Cell clusters oxygenated by synthetic vessels showed a ∼100μm drop-off to anoxia, consistent with in vivo studies of tumor nodules fed by the microvasculature. Oxygen transport in the bioreactor system was characterized by experimental testing with a dissolved oxygen probe and finite element modeling of convective flow. Our study demonstrates differing growth patterns associated with controlling gas distributions to better mimic in vivo conditions.

Nonsteady state oxygen transport in engineered tissue: implications for design

Engineered tissue constructs are limited in size, and thus clinical relevance, when diffusion is the primary mode of oxygen transport. Understanding the extent of oxygen diffusion and cellular consumption is necessary for the design of engineered tissues, particularly those intended for implantation into hypoxic wound sites. This study presents a combined experimental and computation model to predict design constraints for cellularized fibrin tissues subjected to a step change in the oxygen concentration to simulate transplantation. Nonsteady state analysis of oxygen diffusion and consumption was used to estimate the diffusion coefficient of oxygen (mean±SD, 1.7×10(-9)±8.4×10(-11) m(2)/s) in fibrin hydrogels as well as the Michaelis-Menten parameters, Vmax (1.3×10(-17)±9.2×10(-19) mol·cell(-1)·s(-1)), and Km (8.0×10(-3)±3.5×0(-3) mol/m(3)), of normal human lung fibroblasts. Nondimensionalization of the governing diffusion-reaction equation enabled the creation of a single dimensionless parameter, the Thiele modulus (φ), which encompasses the combined effects of oxygen diffusion, consumption, and tissue dimensions. Tissue thickness is the design parameter with the most pronounced influence on the distribution of oxygen within the system. Additionally, tissues designed such that φ<1 achieve a near spatially uniform and adequate oxygen concentration following the step change. Understanding and optimizing the Thiele modulus will improve the design of engineered tissue implants.

Novel in vitro and mathematical models for the prediction of chemical toxicity

The focus of much scientific and medical research is directed towards understanding the disease process and defining therapeutic intervention strategies. The scientific basis of drug safety is very complex and currently remains poorly understood, despite the fact that adverse drug reactions (ADRs) are a major health concern and a serious impediment to development of new medicines. Toxicity issues account for ∼21% drug attrition during drug development and safety testing strategies require considerable animal use. Mechanistic relationships between drug plasma levels and molecular/cellular events that culminate in whole organ toxicity underpins development of novel safety assessment strategies. Current in vitro test systems are poorly predictive of toxicity of chemicals entering the systemic circulation, particularly to the liver. Such systems fall short because of (1) the physiological gap between cells currently used and human hepatocytes existing in their native state, (2) the lack of physiological integration with other cells/systems within organs, required to amplify the initial toxicological lesion into overt toxicity, (3) the inability to assess how low level cell damage induced by chemicals may develop into overt organ toxicity in a minority of patients, (4) lack of consideration of systemic effects. Reproduction of centrilobular and periportal hepatocyte phenotypes in in vitro culture is crucial for sensitive detection of cellular stress. Hepatocyte metabolism/phenotype is dependent on cell position along the liver lobule, with corresponding differences in exposure to substrate, oxygen and hormone gradients. Application of bioartificial liver (BAL) technology can encompass in vitro predictive toxicity testing with enhanced sensitivity and improved mechanistic understanding. Combining this technology with mechanistic mathematical models describing intracellular metabolism, fluid-flow, substrate, hormone and nutrient distribution provides the opportunity to design the BAL specifically to mimic the in vivo scenario. Such mathematical models enable theoretical hypothesis testing, will inform the design of in vitro experiments, and will enable both refinement and reduction of in vivo animal trials. In this way, development of novel mathematical modelling tools will help to focus and direct in vitro and in vivo research, and can be used as a framework for other areas of drug safety science.

Evaluation of a novel hybrid bioartificial liver based on a multi-layer flat-plate bioreactor

AIM: To evaluate the efficacy and safety of a hybrid bioartificial liver (HBAL) system in the treatment of acute liver failure.

METHODS: Canine models with acute liver failure were introduced with intravenous administration of D-galactosamine. The animals were divided into: the HBAL treatment group (n = 8), in which the canines received a 3-h treatment of HBAL; the bioartificial liver (BAL) treatment group (n = 8), in which the canines received a 3-h treatment of BAL; the non-bioartificial liver (NBAL) treatment group (n = 8), in which the canines received a 3-h treatment of NBAL; the control group (n = 8), in which the canines received no additional treatment. Biochemical parameters and survival time were determined. Levels of xenoantibodies, RNA of porcine endogenous retrovirus (PERV) and reverse transcriptase (RT) activity in the plasma were detected.

RESULTS: Biochemical parameters were significantly decreased in all treatment groups. The TBIL level in the HBAL group was lower than that in other groups (2.19 ± 0.55 μmol/L vs 24.2 ± 6.45 μmol/L, 12.47 ± 3.62 μmol/L, 3.77 ± 1.83 μmol/L, P < 0.05). The prothrombin time (PT) in the BAL and HBAL groups was significantly shorter than the NBAL and control groups (18.47 ± 4.41 s, 15.5 ± 1.56 s vs 28.67 ± 5.71 s, 21.71 ± 3.4 s, P < 0.05), and the PT in the HBAL group was shortest of all the groups. The albumin in the BAL and HBAL groups significantly increased and a significantly higher level was observed in the HBAL group compared with the BAL group (27.7 ± 1.7 g/L vs 25.24 ± 1.93 g/L). In the HBAL group, the ammonia levels significantly decreased from 54.37 ± 6.86 to 37.75 ± 6.09 after treatment (P < 0.05); there were significant difference in ammonia levels between other the groups (P < 0.05). The levels of antibodies were similar before and after treatment. The PERV RNA and the RT activity in the canine plasma were all negative.

CONCLUSION: The HBAL showed great efficiency and safety in the treatment of acute liver failure.

Key challenges to the development of extracorporeal bioartificial liver support systems

BACKGROUND

For nearly three decades, extracorporeal bioartificial liver (BAL) support systems have been anticipated as promising tools for the treatment of liver failure. However, these systems are still far from clinical application. This review aimed to analyze the key challenges to the development of BALs.

DATA SOURCE

We carried out a PubMed search of English-language articles relevant to extracorporeal BAL support systems and liver failure.

RESULTS

Extracorporeal BALs face a series of challenges. First, an appropriate cell source for BAL is not readily available. Second, existing bioreactors do not provide in vivo-like oxygenation and bile secretion. Third, emergency needs cannot be met by current BALs. Finally, the effectiveness of BALs, either in animals or in patients, has been difficult to document.

CONCLUSIONS

Extracorporeal BAL support systems are mainly challenged by incompetent cell sources and flawed bioreactors. To advance this technology, future research is needed to provide more insights into interpreting the conditions for hepatocyte differentiation and liver microstructure formation.

A theoretical approach to zonation in a bioartificial liver

Bioartificial livers have yet to gain clinical acceptance. In a previous study, a theoretical model was utilized to create operating region charts that graphically illustrated viable bioartificial liver configurations. On this basis a rationale for the choice of operating and design parameters for the device was created. The concept is extended here to include aspects of liver zonation for further design optimization. In vivo, liver cells display heterogeneity with respect to metabolic activity according to their position in the liver lobule. It is thought that oxygen tension is a primary modulator of this heterogeneity and on this assumption a theoretical model to describe the metabolic zonation within an in vitro bioartificial liver device has been adopted. The distribution of the metabolic zones under varying design and operating parameters is examined. In addition, plasma flow rates are calculated that give rise to an equal distribution of the metabolic zones. The results show that when a clinically relevant number of cells are contained in the BAL (10 billion), it is possible to constrain each of the three metabolic zones to approximately one-third of the cell volume. This is the case for a number of different bioreactor designs. These considerations allow bioartificial liver design to be optimized.

Compartmental hollow fiber capillary membrane-based bioreactor technology for in vitro studies on red blood cell lineage direction of hematopoietic stem cells

Continuous production of red blood cells (RBCs) in an automated closed culture system using hematopoietic stem cell (HSC) progenitor cell populations is of interest for clinical application because of the high demand for blood transfusions. Previously, we introduced a four-compartment bioreactor that consisted of two bundles of hollow fiber microfiltration membranes for transport of culture medium (forming two medium compartments), interwoven with one bundle of hollow fiber membranes for transport of oxygen (O(2)), carbon dioxide (CO(2)), and other gases (forming one gas compartment). Small-scale prototypes were developed of the three-dimensional (3D) perfusion cell culture systems, which enable convection-based mass transfer and integral oxygenation in the cell compartment. CD34(+) HSC were isolated from human cord blood units using a magnetic separation procedure. Cells were inoculated into 2- or 8-mL scaled-down versions of the previously designed 800-mL cell compartment devices and perfused with erythrocyte proliferation and differentiation medium. First, using the small-scale 2-mL analytical scale bioreactor, with an initial seeding density of 800,000 cells/mL, we demonstrated approximately 100-fold cell expansion and differentiation after 7 days of culture. An 8-mL laboratory-scale bioreactor was then used to show pseudocontinuous production by intermediately harvesting cells. Subsequently, we were able to use a model to demonstrate semicontinuous production with up to 14,288-fold expansion using seeding densities of 800,000 cells/mL. The down-scaled culture technology allows for expansion of CD34(+) cells and stimulating these progenitors towards RBC lineage, expressing approximately 40% CD235(+) and enucleation. The 3D perfusion technology provides an innovative tool for studies on RBC production, which is scalable.

Perfusion systems that minimize vascular volume fraction in engineered tissues

This study determines the optimal vascular designs for perfusing engineered tissues. Here, "optimal" describes a geometry that minimizes vascular volume fraction (the fractional volume of a tissue that is occupied by vessels) while maintaining oxygen concentration above a set threshold throughout the tissue. Computational modeling showed that optimal geometries depended on parameters that affected vascular fluid transport and oxygen consumption. Approximate analytical expressions predicted optima that agreed well with the results of modeling. Our results suggest one basis for comparing the effectiveness of designs for microvascular tissue engineering.

Effectiveness factor and diffusion limitations in collagen gel modules containing HepG2 cells

A major obstacle in tissue engineering is overcoming hypoxia in thick, three-dimensional (3D) engineered tissues, which is caused by the diffusional limitations of oxygen and lack of internal vasculature to facilitate mass transfer. Modular tissue engineering is a bio-mimetic strategy that forms scalable, vascularized and uniform 3D constructs by assembling small (sub-mm), cell-containing modules. It was previously assumed that mass transfer resistance within the individual modules was negligible, due to their small size. In the present study, this assumption was tested using theoretical analysis of oxygen transport within the module (effectiveness factor) and experimental studies. Small (400 µm diameter post-contraction) and large (700 µm diameter post-contraction) HepG2-collagen modules were made for a range of seeding densities (2 × 10(6) -1 × 10(7) cells/ml collagen). Cell density, distribution and morphology within the modules showed that the small modules were capable of sustaining high cell densities (8.0 × 10(7) ± 4.4 × 10(7) cells/cm(3) ) with negligible mass transfer inhibition. Conversely, large modules developed a necrotic core and had significantly (p < 0.05) reduced cell densities (1.5 × 10(7) ± 9.2 × 10(6) cells/cm(3) ). It was also observed that the embedded cells responded quickly to the oxygen availability, by proliferating or dying, to reach a sustainable density of approximately 8000 cells/module. Furthermore, a simple effectiveness factor calculation was successful in estimating the maximum cell density per module. The results gathered in this study confirm the previous assumption that the small-diameter modules avoid the internal mass transfer limitations that are often observed in larger constructs.

19F MRI oximetry: simulation of perfluorocarbon distribution impact

In (19)F MRI oximetry, a method used to image tumour hypoxia, perfluorocarbons serve as oxygenation markers. The goal of this study is to evaluate the impact of perfluorocarbon distribution and concentration in (19)F MRI oximetry through a computer simulation. The simulation studies the correspondence between (19)F measured (pO(FNMR)(2)) and actual tissue oxygen tension (pO(2)) for several tissue perfluorocarbon distributions. For this, a Krogh tissue model is implemented which incorporates the presence of perfluorocarbons in blood and tissue. That is, in tissue the perfluorocarbons are distributed homogeneously according to Gaussian diffusion profiles, or the perfluorocarbons are concentrated in the capillary wall. Using these distributions, the oxygen tension in the simulation volume is calculated. The simulated mean oxygen tension is then compared with pO(FNMR)(2), the (19)F MRI-based measure of pO(2) and with pO(0)(2), pO(2) in the absence of perfluorocarbons. The agreement between pO(FNMR)(2) and actual pO(2) is influenced by vascular density and perfluorocarbon distribution. The presence of perfluorocarbons generally gives rise to a pO(2) increase in tissue. This effect is enhanced when perfluorocarbons are also present in blood. Only the homogeneous perfluorocarbon distribution in tissue with no perfluorocarbons in blood guarantees small deviations of pO(FNMR)(2) from pO(2). Hence, perfluorocarbon distribution in tissue and blood has a serious impact on the reliability of (19)F MRI-based measures of oxygen tension. In addition, the presence of perfluorocarbons influences the actual oxygen tension. This finding may be of great importance for further development of (19)F MRI oximetry.

A strategy to determine operating parameters in tissue engineering hollow fiber bioreactors

The development of tissue engineering hollow fiber bioreactors (HFB) requires the optimal design of the geometry and operation parameters of the system. This article provides a strategy for specifying operating conditions for the system based on mathematical models of oxygen delivery to the cell population. Analytical and numerical solutions of these models are developed based on Michaelis–Menten kinetics. Depending on the minimum oxygen concentration required to culture a functional cell population, together with the oxygen uptake kinetics, the strategy dictates the model needed to describe mass transport so that the operating conditions can be defined. If c_min ≫ K_m we capture oxygen uptake using zero-order kinetics and proceed analytically. This enables operating equations to be developed that allow the user to choose the medium flow rate, lumen length, and ECS depth to provide a prescribed value of c_min. When , we use numerical techniques to solve full Michaelis–Menten kinetics and present operating data for the bioreactor. The strategy presented utilizes both analytical and numerical approaches and can be applied to any cell type with known oxygen transport properties and uptake kinetics.

Hemoglobin regulates the metabolic, synthetic, detoxification, and biotransformation functions of hepatoma cells cultured in a hollow fiber bioreactor

Hepatic hollow fiber (HF) bioreactors constitute one type of extracorporeal bioartificial liver assist device (BLAD). Ideally, cultured hepatocytes in a BLAD should closely mimic the in vivo oxygenation environment of the liver sinusoid to yield a device with optimal performance. However, most BLADs, including hepatic HF bioreactors, suffer from O2 limited transport toward cultured hepatocytes, which reduces their performance. We hypothesize that supplementation of hemoglobin-based O2 carriers into the circulating cell culture medium of hepatic HF bioreactors is a feasible and effective strategy to improve bioreactor oxygenation and performance. We examined the effect of bovine hemoglobin (BvHb) supplementation (15g/L) in the circulating cell culture medium of hepatic HF bioreactors on hepatocyte proliferation, metabolism, and varied liver functions, including biosynthesis, detoxification, and biotransformation. It was observed that BvHb supplementation supported the maintenance of a higher cell mass in the extracapillary space, improved hepatocyte metabolic efficiency (i.e., hepatocytes consumed much less glucose), improved hepatocyte capacity for drug metabolism, and conserved both albumin synthesis and ammonia detoxification functions compared to controls (no BvHb supplementation) under the same experimental conditions.

A theoretical method to improve and optimize the design of bioartificial livers

Bioartificial livers (BALs) are a potentially effective countermeasure against liver failure, particularly in cases of acute or fulminant liver failure. It is hoped these devices can sustain a patient's liver function until recovery or transplant. However, no large-scale clinical trial has yet proven that BALs are particularly effective and evidently design issues remain to be addressed. One aspect of BAL design that must be considered is the mass transfer of adequate oxygen to the hepatocytes within the device. We present here a mathematical modeling approach to oxygen mass transport in a BAL. A mathematical model based upon Krogh cylinders is outlined to describe a diffusion-limited hollow fiber bioreactor. In addition, operating constraints are defined on the system--cells should not experience hypoxia and the cell population should be of adequate size. By combining modeling results with these operating constraints and presenting the results graphically, "operating region" charts can be constructed for the hollow fiber BAL (HF-BAL). The effects of varying various operating parameters on the BAL are then established. It is found that smaller radii and short, thin walled fibers are generally advantageous while cell populations in excess of 10 billion could be supported in the BAL with a plasma flow rate of 200 mL/min. For fibers of intermediate length and lumen radius, the minimum number of fibers required to produce a viable design ranges approximately from 7,000-10,000. In theory, this may be enough to support patients with failing livers.

Perfluorocarbon facilitated O(2) transport in a hepatic hollow fiber bioreactor

A mathematical model describing O(2) transport in a hepatic hollow fiber (HF) bioreactor supplemented with perfluorocarbons (PFCs) in the circulating cell culture media was developed to explore the potential of PFCs in properly oxygenating a bioartificial liver assist device (BLAD). The 2-dimensional model is based on the geometry of a commercial HF bioreactor operated under steady-state conditions. The O(2) transport model considers fluid motion of a homogeneous mixture of cell culture media and PFCs, and mass transport of dissolved O(2) in a single HF. Each HF consists of three distinct regions: (1) the lumen (conducts the homogeneous mixture of cell culture media and PFCs), (2) the membrane (physically separates the lumen from the extracapillary space (ECS), and (3) the ECS (hepatic cells reside in this compartment). In a single HF, dissolved O(2) is predominantly transported in the lumen via convection in the axial direction and via diffusion in the radial direction through the membrane and ECS. The resulting transport equations are solved using the finite element method. The calculated O(2) transfer flux showed that supplementation of the cell culture media with PFCs can significantly enhance O(2) transport to the ECS of the HF when compared with a control with no PFC supplementation. Moreover, the O(2) distribution and subsequent analysis of ECS zonation demonstrate that limited in vivo-like O(2) gradients can be recapitulated with proper selection of the operational settings of the HF bioreactor. Taken together, this model can also be used to optimize the operating conditions for future BLAD development that aim to fully recapitulate the liver's varied functions.

Impact of Increased Oxygen Delivery via Bovine Red Blood Cell Supplementation of Culturing Media on Select Metabolic and Synthetic Functions of C3A Hepatocytes Maintained within a Hollow Fiber Bioreactor

Hepatocytes are highly dependent upon appropriate oxygen provision for activity and viability. However, oxygen delivery to hepatocytes cultured within a hollow fiber bioreactor is believed to be problematic because of large diffusion distances, a high hepatocyte oxygen consumption rate and low aqueous media oxygen solubility. Supplementation of bioreactor media with bovine red blood cells (bRBCs) is one means of improving oxygen delivery to hepatocytes as hemoglobin contained within bRBCs binds oxygen. The impact of supplementing hepatocyte culturing media with bRBCs (approximately 5 x 10(8) bRBCs/ml) on hepatocyte activity (albumin and lactate production and glucose consumption) was studied. Decreased hepatocyte lactate production to glucose consumption ratios were found for the case when bRBCs were added to circulating culturing media, which indicated the presence of a more aerobic environment in comparison to the control (no bRBC supplementation). Additionally, albumin synthesis was found to be improved when the circulating media was supplemented with bRBCs. Our results thus support the use of bRBCs to improve oxygen delivery to hepatocytes maintained within a hollow fiber bioreactor.

Low O2 metabolism of HepG2 cells cultured at high density in a 3D microstructured scaffold

Among the features of in vivo liver cells that are rarely mimicked in vitro, especially in microchips, is the very high cell density. In this study, we have cultured HepG2 in a plate-type PDMS scaffold with a three-dimensional ordered microstructure optimally designed to allow cells to attach at a density of 10(8) cells/mL. After the first step of static open culture, the scaffold was sealed to simulate the in vivo oxygen supply, which is supplied only through the perfusion of medium. The oxygen consumption rate at various flow rates was measured. An average maximal cellular oxygen consumption rate of 3.4 x 10(-17) mol/s/cell was found, which is much lower than previously reported values for hepatocytes. Nevertheless, the oxygen concentration in the bulk stream was not the limiting factor. It has been further confirmed by the reported numerical model that the mass transport resistance on the surface of a cell that limits the oxygen supply to the cell. These results further emphasize that access to a sufficient quantity of oxygen, especially through the diffusion-limited layer on the surface of a cell, is very important for the metabolism of hepatocytes at such a high density.

Convection and hemoglobin-based oxygen carrier enhanced oxygen transport in a hepatic hollow fiber bioreactor

Hepatic hollow fiber bioreactors are a promising class of bioartificial liver assist device (BLAD). The development of this type of device is currently hindered by limited oxygen transport to cultured hepatocytes, due to low solubility of oxygen in aqueous media. In order to increase the oxygen spectrum to cultured hepatocytes housed within a hollow fiber bioreactor, several different engineering strategies were explored in this study. These included: supplementing the circulating media stream of the hollow fiber bioreactor with a hemoglobin-based oxygen carrier (bovine red blood cells) with defined oxygen binding and release kinetics and operating the bioreactor with media flow through the hollow fiber membrane into the extracapillary space (ECS). We hypothesize that these two strategies can be used to improve hepatocyte oxygenation and possibly attain an in vivo-like pO(2) spectrum, similar to that observed in vivo in the liver sinusoid. This work is significant, since provision of an in vivo-like pO(2) spectrum should create a fully functional BLAD that could potentially bridge thousands of liver failure patients towards native liver regeneration of damaged tissue or, if necessary, orthotopic liver transplantation.

Targeted Oxygen Delivery within Hepatic Hollow Fiber Bioreactors via Supplementation of Hemoglobin-Based Oxygen Carriers

Hepatic hollow fiber bioreactors are considered a promising class of bioartificial liver assist device (BLAD). Unfortunately, limited oxygen (O(2)) transport to hepatocytes within this device hinders further development. Hepatocytes in vivo (in the liver sinusoid) experience a wide range of oxygen tensions (pO(2) = 25-70 mmHg), which is important for development of proper differentiated function (zonation). Previously, we observed that bovine red blood cell (bRBC) supplementation of the circulating media stream enhanced oxygenation of cultured C3A hepatoma cells compared to a culture with no O(2) carrier (Gordon, J.; Palmer, A. F. Artif. Cells, BloodSubstitutes, Biotechnol. 2006, 33 (3), 297-306). Despite this success, the cells were not exposed to the desired in vivo O(2) spectrum (Sullivan, J.; Gordon, J.; Palmer, A. Biotechnol. Bioeng. 2006, 93 (2) 306-317). We hypothesize that altering the kinetics of O(2) binding/release to/from hemoglobin-based O(2) carriers (HBOCs) could potentially target O(2) delivery to cell cultures. High P(50) (low O(2) affinity) HBOCs preferentially targeted O(2) delivery at high inlet pO(2) values. Conversely, low P(50) (high O(2) affinity) HBOCs targeted O(2) delivery at low inlet pO(2) values. Additionally, inlet pO(2), flow rate, and HBOC concentration were varied to find optimal bioreactor operating conditions. Our results demonstrate that HBOCs can enhance O(2) delivery to cultured hepatocytes, while exposing them to in vivo-like O(2) tensions, which is critical to create a fully functional BLAD.

Optimizing normoxic conditions in liver devices using enhanced gel matrices

For in vitro liver replacement devices, such as packed bed bioreactors, to maintain the essential functions of the liver, they must at least successfully support hepatocytes, the parenchymal cell of the liver. In vivo, the liver is a major consumer of oxygen. Hence it is unsurprising that the limited transport distance of oxygen (O(2)) governs the dimensions of the cellular space of engineered devices. Because cellular space capacity directly affects the device's performance, O(2) transport is a critical issue in the scale up of bioreactor designs. In the current investigation, the microporosity of the extracellular matrix (ECM) has been modified to further improve O(2) transport in packed bed devices beyond that previously reported in the literature. These improvements to the O(2) enhancement technique enabled O(2) transport distances of 481.7 +/- 12.5 microm to be achieved under acellular conditions; and distances of 418.1 +/- 6.0 microm to be attained in the presence of 1 million hepatocytes. Both values are significantly greater than the 170 microm baseline attained when 10(6) hepatocytes are packed within normal non-enhanced ECM gels. The study's results also illustrate that the O(2) enhancement technique has the added benefit of preventing regions of severe hypoxia and hyperoxia from developing within the cellular space. As such, enhanced ECM gels enable packed hepatocytes to maintain better hepatocellular metabolic status than is possible with normal non-enhanced gels.

Enhanced oxygen delivery to primary hepatocytes within a hollow fiber bioreactor facilitated via hemoglobin-based oxygen carriers

The production of a fully functional bioartificial liver assist device (BLAD) would greatly enhance available treatment options for patients suffering from acute liver failure. Currently, inadequate oxygen provision to hepatocytes seeded within hollow fiber bioreactors hampers development of a viable hollow fiber-based BLAD. Experimentally, oxygen provision to primary rat hepatocytes cultured within hollow fiber bioreactors was measured, it was observed that supplementation with an oxygen carrier (bovine red blood cells at approximately 2% human hematocrit) did not significantly improve oxygenation compared to the absence of an oxygen carrier. Therefore, an oxygen transport model of an individual hollow fiber within the bioreactor was developed and simulated (up to approximately 10% human hematocrit) to more fully examine the effect of oxygen carrier supplementation on oxygenation within the bioreactor. The modeling analysis, supported via the experimental results, was utilized to predict optimal bioreactor operating conditions for the delivery of in vivo-like oxygen gradients to cultured hepatocytes in clinically relevant settings.

Radial flow hepatocyte bioreactor using stacked microfabricated grooved substrates

Bioartificial liver (BAL) devices with fully functioning hepatocytes have the potential to provide temporary hepatic support for patients with liver failure. The goal of this study was to optimize the flow environment for the cultured hepatocytes in a stacked substrate, radial flow bioreactor. Photolithographic techniques were used to microfabricate concentric grooves onto the underlying glass substrates. The microgrooves served to protect the seeded hepatocytes from the high shear stresses caused by the volumetric flow rates necessary for adequate convective oxygen delivery. Finite element analysis was used to analyze the shear stresses and oxygen concentrations in the bioreactor. By employing high volumetric flow rates, sufficient oxygen supply to the hepatocytes was possible without an integrated oxygen permeable membrane. To implement this concept, 18 microgrooved glass substrates, seeded with rat hepatocytes cocultured with 3T3-J2 fibroblasts, were stacked in the bioreactor, creating a channel height of 100 microm between each substrate. In this bioreactor configuration, liver-specific functions (i.e., albumin and urea synthesis rates) of the hepatocytes remained stable over 5 days of perfusion, and were significantly increased compared to those in the radial flow bioreactor with stacked substrates without microgrooves. This study suggests that this radial flow bioreactor with stacked microgrooved substrates is scalable and may have potential as a BAL device in the treatment of liver failure.

Preliminary study of the viability of neonatal mini-porcine hepatocytes in extracorporeal circulation

AIM: To observe the viability of neonatal mini-porcine hepatocytes in extracorporeal circulation.

METHODS: A hepatocyte circulation system was constructed with Cello artificial capillary culture equipment and freshly isolated hepatocyte suspensions were circulated at 50 mL/min in vitro. The viability, function and morphological characteristics of hepatocytes were examined within 8 hours.

RESULTS: After 4 hours circulation, the viability and adherence ratio of hepatocytes were 76.1% ± 1.4% and 62.8% ± 1.8%, respectively, and the ratio of amino clearance was about 62.7% ± 14.6% of that in the control group. However, with time, many fragments of hepatocytes were observed in the circulating suspensions, as well as a significant increase in lactate dehydrogenase and aspartate aminotransferase (P < 0.01). The cell viability and adherence ratio, synthesis of urea and albumin, and the clearance rate of ammonia also decreased significantly (P < 0.05).

CONCLUSION: Porcine hepatocyte suspensions can be applied to BAL system in a circulation condition in order to strengthen the mass exchange. However, these cells need to be changed regularly to maintain cellular viability and the supporting effects of BAL.

Mass transfer and metabolic reactions in hepatocyte spheroids cultured in rotating wall gas-permeable membrane system

Isolated hepatocytes in spheroid configuration exhibit a high degree of cell-cell contacts, which are important in the maintenance of viability and liver specific functions. In the absence of a vascular network, the cells in a large spheroid size experience mass transfer limitations of metabolites and oxygen in the core of aggregates. In this paper transport phenomena related to the diffusion and reaction of oxygen, glucose and lactate are mathematically described and experimentally verified for hepatocyte spheroids cultured in a rotating-wall polystyrene system (RWPS) not permeable for gases and in a rotating-wall membrane system (RWMS) with oxygen-permeable membrane. The concentration profiles of glucose, oxygen and lactate in the hepatocyte spheroids were estimated for different diameters of aggregates by solving the mass transfer equations for simultaneous diffusion and reaction, by finite element method. Simulation results evidenced that, for aggregates with size lower than 300 microm cultured in both RWPS and RWMS systems, the concentration profiles of glucose and lactate towards the core of spheroids (effective diffusion coefficients in the order of 10(-11)m(2)/s) are not significantly affected by the metabolic rate (c.a 10(-6)microg/mm(3)/s for glucose and about one order of magnitude less for lactate). On the contrary, the transport of oxygen (diffusion coefficient: 3.4 x 10(-10)m(2)/s, reaction rate: 1.5 x 10(-5)microg/mm(3)/s) is critically affected by the size of the multicellular spheroids and significant gradients in oxygen concentration may develop in spheroids. Aggregates with a size greater than 200 microm suffer severe oxygen limitation in the most part of its size attaining the lowest partial pressure in the centre. The improved viability predicted by the model culturing hepatocyte spheroids in the RWMS, characterized by a higher O(2) permeability with respect to RWPS, was experimentally confirmed. The results demonstrated that the mathematical model used in this study represents a useful support to experimental procedures in order to obtain hepatocyte spheroids with optimal size.

Oxygen uptake rates and liver-specific functions of hepatocyte and 3T3 fibroblast co-cultures

Bioartificial liver (BAL) devices have been developed to treat patients undergoing acute liver failure. One of the most important parameters to consider in designing these devices is the oxygen consumption rate of the seeded hepatocytes which are known to have oxygen consumption rates 10 times higher than most other cell types. Hepatocytes in various culture configurations have been tested in BAL devices including those formats that involve co-culture of hepatocytes with other cell types. In this study, we investigated, for the first time, oxygen uptake rates (OUR)s of hepatocytes co-cultured with 3T3-J2 fibroblasts at various hepatocyte to fibroblast seeding ratios. OURs were determined by measuring the rate of oxygen disappearance using a ruthenium-coated optical probe after closing and sealing the culture dish. Albumin and urea production rates were measured to assess hepatocyte function. Lower hepatocyte density co-cultures demonstrated significantly higher OURs (2 to 3.5-fold) and liver- specific functions (1.6-fold for albumin and 4.5-fold for urea production) on a per cell basis than those seeded at higher densities. Increases in OUR correlated well with increased liver-specific functions. OURs (V(m)) were modeled by fitting Michaelis-Menten kinetics and the model predictions closely correlated with the experimental data. This study provides useful information for predicting BAL design parameters that will avoid oxygen limitations, as well as maximize metabolic functions.

Three-dimensional numerical modeling and computational fluid dynamics simulations to analyze and improve oxygen availability in the AMC bioartificial liver

A numerical model to investigate fluid flow and oxygen (O(2)) transport and consumption in the AMC-Bioartificial Liver (AMC-BAL) was developed and applied to two representative micro models of the AMC-BAL with two different gas capillary patterns, each combined with two proposed hepatocyte distributions. Parameter studies were performed on each configuration to gain insight in fluid flow, shear stress distribution and oxygen availability in the AMC-BAL. We assessed the function of the internal oxygenator, the effect of changes in hepatocyte oxygen consumption parameters in time and the effect of the change from an experimental to a clinical setting. In addition, different methodologies were studied to improve cellular oxygen availability, i.e. external oxygenation of culture medium, culture medium flow rate, culture gas oxygen content (pO(2)) and the number of oxygenation capillaries. Standard operating conditions did not adequately provide all hepatocytes in the AMC-BAL with sufficient oxygen to maintain O(2) consumption at minimally 90% of maximal uptake rate. Cellular oxygen availability was optimized by increasing the number of gas capillaries and pO(2) of the oxygenation gas by a factor two. Pressure drop over the AMC-BAL and maximal shear stresses were low and not considered to be harmful. This information can be used to increase cellular efficiency and may ultimately lead to a more productive AMC-BAL.

Antioxidant Functionality in Hepatocytes Using the Enhanced Collagen Extracellular Matrix Under Different Oxygen Tensions

Improvement of O(2) supply in bioartificial liver devices remains a critical issue in maintaining hepatocyte viability and functions. Therefore, the current study investigates whether enhanced oxygen (O(2)) transport through collagen extracellular matrix (ECM) can produce a more stable antioxidant defense in different O(2) tensions during prolonged incubation times. Total glutathione concentration of cultured hepatocytes in enhanced ECM was significantly higher than in normal ECM under the lowest O(2) tension phase (2.60mm of thickness from O(2) source), and was also significantly increased in 0.52 mm transport distance of hypoxia as compared to normoxic conditions. Catalase and glutathione reductase activities for hepatocytes within enhanced ECM were also significantly preserved relative to their values for the normal collagen ECM. Specifically, the enhanced ECM produced higher activities at a further transport distance (1.56 mm) from the O(2) source at the 24 h time-point, and remained higher up to the 96 h incubation time. In contrast, the glutathione peroxidase activities in both collagen ECM systems were similar. Hepatocyte viability in the enhanced ECM system was also consistently greater than that for normal ECM. These results suggest that the O(2) enhanced collagen ECM preserves the antioxidant defense system as compared to normal collagen ECM, ostensibly via increased micropathways for O(2) transport to the hepatocytes.

Hepatocyte hollow-fibre bioreactors: design, set-up, validation and applications

Hepatocytes carry out many vital biological functions, such as synthetic and catabolic reactions, detoxification and excretion. Due to their ability to restore a tissue-like environment, hollow-fibre bioreactors (HFBs) show great potential among the different systems used to culture hepatocytes. Several designs of HFBs have been proposed in which hepatocytes or hepatocyte-derived cell lines can be cultured in suspensions or on a solid support. Currently the major use of hepatocyte HFBs is as bioartificial livers to sustain patients suffering from acute liver failure, but they can also be used to synthesize cell products and as cellular models for drug metabolism and transport studies. Here, we present an overview of the set-up of hepatocyte HFBs and aim to provide potential users with the basic knowledge necessary to develop their own system. First, general information on HFBs is given, including basic principles, transport phenomena, designs and cell culture conditions. The importance of the tests necessary to assess the performance of the HFBs, i.e. the viability and functionality of hepatocytes, is underlined. Special attention is paid to drug metabolism studies and to adequate analytical methods. Finally, the potential uses of hepatocyte HFBs are described.

Evaluation of a bioartificial liver based on a nonwoven fabric bioreactor with porcine hepatocytes in pigs

BACKGROUND/AIMS

We developed a bioartificial liver (BAL) based on a direct hemoperfusion typed nonwoven fabric bioreactor containing porcine hepatocytes. In this study, the efficacy of our BAL was evaluated with a pig fulminant hepatic failure (FHF) model.

METHODS

FHF was induced with intravenous administration of D-galactosamine (1.3 g/kg) in each pig. Twelve hours post D-galactosamine injection, fifteen pigs were divided into: a BAL group (n = 5), in which pigs received the BAL treatment with 1.0 to 1.3 x 10(9) hepatocytes for 6 h, a sham BAL group (n = 5), in which pigs received the BAL treatment without hepatocytes, and a FHF group (n = 5), in which pigs only received intensive care. Parameters related to liver function and animal survival up to 168 h were determined.

RESULTS

In the BAL group, blood ammonia and plasma lactate levels were lower, and serum glucose levels and Fischer index were higher than those in the other two groups. Survival time of pigs in the BAL group was significantly prolonged as compared with the sham BAL and the FHF group.

CONCLUSIONS

The BAL based on a nonwoven fabric bioreactor containing porcine hepatocytes appears to be effective in the treatment of FHF in pigs.

Simulation of oxygen carrier mediated oxygen transport to C3A hepatoma cells housed within a hollow fiber bioreactor

A priori knowledge of the dissolved oxygen (O2) concentration profile within a hepatic hollow fiber (HF) bioreactor is important in developing an effective bioartificial liver assist device (BLAD). O2 provision is limiting within HF bioreactors and we hypothesize that supplementing a hepatic HF bioreactor's circulating media with bovine red blood cells (bRBCs), which function as an O2 carrier, will improve oxygenation. The dissolved O2 concentration profile within a single HF (lumen, membrane, and representative extra capillary space (ECS)) was modeled with the finite element method, and compared to experimentally measured data obtained on an actual HF bioreactor with the same dimensions housing C3A hepatoma cells. Our results (experimental and modeling) indicate bRBC supplementation of the circulating media leads to an increase in O2 consumed by C3A cells. Under certain experimental conditions (pO2,IN) = 95 mmHg, Q = 8.30 mL/min), the addition of bRBCs at 5% of the average in vivo human red blood cell concentration (% hRBC) results in approximately 50% increase in the O2 consumption rate (OCR). By simply adjusting the operating conditions (pO2,IN) = 25 mmHg, Q = 1.77 mL/min) and increasing bRBC concentration to 25% hRBC the OCR increase is approximately 10-fold. However, the improved O2 concentration profile experienced by the C3A cells could not duplicate the full range of in vivo O2 tensions (25-70 mmHg) typically experienced within the liver sinusoid with this particular HF bioreactor. Nonetheless, we demonstrate that the O2 transport model accurately predicts O2 consumption within a HF bioreactor, thus setting up the modeling framework for improving the design of future hepatic HF bioreactors.

Enhancing Cell Viability with Pulsating Flow in a Hollow Fiber Bioartificial Liver

A pulsating flow of medium was used to alleviate diffusion and transport limitations in a hollow fiber bioreactor containing a human hepatoblastoma cell line. The strategy is easy to implement but effective. The pulsating flow is introduced by a solenoid pinch valve at the outlet of the bioreactor and regulated by a timing circuit. In a permeability test, the system with pulsating flow had much less membrane fouling as compared to the control, a conventional hollow fiber unit. In hepatocyte culture test runs, the pulsating-flow bioreactor demonstrated the ability to maintain a higher cell viability. Histological sections indicated significantly smaller necrotic regions in the pulsating-flow bioreactor as compared to the conventional unit.

Porcine acute liver failure model established by two-phase surgery and treated with hollow fiber bioartificial liver support system

AIM: To establish a highly reproducible animal model of acute liver failure (ALF), for assessing the effect of bioartificial liver support system (BALSS).

METHODS: A two-phase complete liver devascularization procedure was performed in eight loco-hybrid pigs. Blood biochemical index and liver biopsy were studied every 2 h after surgery, and survival time was recorded. The BALSS constructed with high volume recirculating technique was a hollow fiber circulating system consisting of a hepatocyte reactor-hollow fiber module inoculated with microcarrier-adhering hepatocytes, and a double pump, heparinized, thermostabilized, micro-capsulized activated carbon-adsorbing plasmapheresis system. Twelve pigs undergoing two-phase surgery were randomized into: control group (perfused without hepatocytes, n = 6) and treatment group (perfused with hepatocytes, n = 6). Intergroup liver biochemical indexes, survival time, and liver pathological changes were analyzed at regular intervals.

RESULTS: Two-phase surgery was performed in all the experimental pigs, and there was no obvious difference between their biochemical indexes. After 3 h of phase II surgery, ammonia (Amm) increased to (269±37) μmol/L. After 5 h of the surgery, fibrinogen (Fib) decreased to (1.5±0.2) g/L. After 7 h of the surgery, ALT, AST, Tbil and PT were (7.6±1.8) nka/L, (40±5) nka/L, (55±8) μmol/L and (17.5±1.7) nka/L respectively. After 9 h of surgery, ALB and Cr were (27±4) g/L and (87±9) μmol/L. After 13 h of surgery, BUN was (3.5±0.9) μmol/L. All the above values were different from those determined before surgery. Survival time of pigs averaged 13.5±1.4 h. ALF pigs in the other group were treated with BALSS. The comparison analysis between the treated and control animals showed the changes of Tbil, PT, Alb, BUN, Cr, Fib, and Amm (P<0.01), but there was no change of ALT and AST. The survival time was statistically different (P<0.01), and there was no significant difference in histological changes.

CONCLUSION: The porcine ALF model established by two-phase devascularized surgery is valid and reproducible. The hollow fiber BALSS can meet the needs of life support and is effective in treating ALF.