Oxygen transfer in a diffusion-limited hollow fiber bioartificial liver

Microfabricated grooved substrates as platforms for bioartificial liver reactors

An extracorporeal bioartificial liver device has the potential to provide temporary hepatic support for patients with liver failure. Our goal was to optimize the flow environment for the cultured hepatocytes in a flat-plate bioreactor, specifically focusing on oxygen delivery using high medium flow rates while reducing the detrimental effects of the resulting shear stresses. We used photolithographic techniques to fabricate microgrooves onto the underlying glass substrate. The microgrooves, perpendicular to the axial flow direction, protected the hepatocytes from the shear stress induced by the flowing medium. Using finite element analysis, we found that the velocity gradient change near the cell surface (i.e., bottom of the grooves) was smaller than that near the top surface of the flow channel, indicating that the grooves would provide protection to the attached cells from the mechanical effects of the flowing medium. We also determined that the shear stress at the cell surface could be reduced by as much as 30 times (channel height of 100 microm) in the grooved-substrate (0.5 dyn/cm(2)) bioreactor compared to the flat-substrate (15 dyn/cm(2)) bioreactor for a medium flow rate of 4.0 mL/min. Albumin and urea synthesis rates of hepatocytes cocultured with 3T3-J2 fibroblasts remained stable over 5 days of perfusion in the grooved-substrate bioreactor, whereas in the flat-substrate bioreactor they decreased over the same time period. These studies indicate that under "high" flow conditions the microgrooved-substrate in the bioreactor can decrease the detrimental effects of shear stress on the hepatocytes while providing adequate oxygenation, thereby resulting in stable liver-specific function.

Diffusive and convective transport through hollow fiber membranes for liver cell culture

For an efficient membrane bioreactor design, transport phenomena determining the overall mass flux of metabolites, catabolites, cell regulatory factors, and immune-related soluble factors, need to be clarified both experimentally and theoretically. In this work, experiments and calculations aimed at discerning the simultaneous influence of both diffusive and convective mechanisms to the transport of metabolites. In particular, the transmembrane mass flux of glucose, bovine serum albumin (BSA), APO-transferrin, immunoglobulin G, and ammonia was experimentally measured, under pressure and concentration gradients, through high-flux microporous hydrophilic poly-ether-sulphone (PES-HFMs) and poly-sulphone hollow fiber membranes (PS-HFMs). These data were analyzed by means of a model based on the mechanism of capillary pore diffusion, assuming that solute spherical molecules pass through an array of solvent-filled cylindrical pores with a diffusive permeation corrected for friction and steric hindrances. Additionally, resistances to the mass transfer were taken into account. Convective permeation data were discussed in terms of morphological properties of the polymeric membranes, molecular Stokes radius, and solute-membrane interactions according to information given by contact angle measurements. The observed steady-state hydraulic permeance of PS-HFMs was 0.972 L/m2hmbar, about 15.6-fold lower than that measured for PES-HFMs (15.2 L/m2h); in general, PS-HFMs provided a significant hindrance to the transport of target species. Diffusion coefficients of metabolites were found to be similar to the corresponding values in water through PES-HFMs, but significantly reduced through PS-HFMs (D(Glucose)(Membrane)=2.8x10(-6)+/-0.6x10(-6)cm2/s, D(BSA)(Membrane)=6.4 x 10(-7)+/-1 x 10(-7)cm(/s, D(Apotransferrin)(Membrane)=2.3 x 10(-7)+/-0.25 x 10(-7)cm2/s).

Method for determining oxygen consumption rates of static cultures from microplate measurements of pericellular dissolved oxygen concentration

We describe a simple protocol for determining the oxygen consumption of cells in static culture. The protocol is based on a noninvasive oxygen-sensing microplate and a simple mathematical model derived from Fick's Law. The applicability of the model is confirmed by showing the correlation of computed oxygen consumption rate (OCR) values to actual cell densities ascertained by direct cell counting and/or MTT for HL60 and U937 cells cultured in suspension. Correlation between computed OCR and these other indications of cell number was quite good, as long as the cultures were not diffusion-limited for oxygen. The impact of the geometric factors of media depth and well size were confirmed to be consistent with the model. Based on this demonstrated correlation, we also developed a simple, completely noninvasive algorithm for ascertaining the per-cell oxygen utilization rate (OUR), which is the ratio of OCR to cell number, and a fundamental cell characteristic. This is accomplished by correlating the known seed densities to extrapolated determinations of OCR at time zero. Such determinations were performed for numerous cell types, in varying well sizes. Resulting OUR values are consistent with literature values acquired by far more painstaking methods, and ranged from <0.01 fmol.min(-1).cell(-1) for bacteria to 0.1-10 fmol.min(-1).cell(-1) for immortalized mammalian and insect cell lines to >10 fmol.min(-1).cell(-1) for primary hepatocytes. This protocol for determining OCR and OUR is extremely simple and broadly applicable and can afford rapid, informative, and noninvasive insight into the state of the culture.

Convective flow through a hollow fiber bioartificial liver

A study of open shell flow through a hollow fiber bioartificial liver with a cell packed shell region (the extracapillary space between the fibers) was performed. Previous analyses of flow through a hollow fiber bioartificial liver have ignored the porous nature of the shell region. Existing analytic expressions were used to describe the flow through the porous hollow fibers, and the flow through the shell region was described using Darcy's law. The operational parameters were taken from clinical trials of the Hepatix extracorporeal liver assist device (ELAD). From the experimental parameters, the permeability value of the cell packed shell region of the ELAD was determined. The results suggest the possible formation of preferential flow channels and hypoperfusion of many cells leading to an inadequately functioning bioreactor. Some simple design changes may improve the performance of the ELAD.

Oxygen Consumption in a Hollow Fiber Bioartificial Liver-Revisited

Oxygen consumption dynamics in a hollow fiber, hepatocyte-loaded bioartificial liver are investigated both theoretically and experimentally. The theoretical model is based upon the Krogh cylinder, which approximates the bioreactor as a collection of cylindrical elements comprised of an inner fiber lumen for media perfusion, the fiber wall through which oxygen can diffuse, and an annular region of hepatocytes surrounding the fiber. The primary non-dimensional parameters that describe the system are: (i) the Peclet number, Pe, which is the ratio of convective oxygen transport through the lumen to diffusive oxygen transport to the fiber walls; (ii) the hepatocyte saturation parameter, theta, which is the ratio of the inlet oxygen partial pressure to the Michaelis-Menten half-rate oxygen partial pressure; (iii) the Thiele modulus, phi2, which is the ratio of oxygen consumption rate to oxygen diffusion rate in the hepatocyte annulus; (iv) the hepatocyte permeability ratio, beta31, which is the ratio of oxygen permeability in the hepatocyte cell mass to oxygen permeability in the perfusing lumen medium; and (v) the hepatocyte annular thickness, rho3, which is the ratio of the exterior hepatocyte annular radius to the fiber lumen radius. Only Pe and theta are easily manipulated operating variables. phi2, beta31, and rho3 are engineering design parameters that are set when a bioreactor is fabricated. The model results are expressed as the effective hepatocyte utilization ratio, Vratio, which is the ratio of the observed oxygen consumption rate to the intrinsic hepatocyte oxygen consumption rate. Large regions of Vratio > 0.9, which is deemed an acceptable effective hepatocyte utilization are found for parameter values consistent with standard hollow fiber cartridges used in bioartificial liver fabrication. The extent of the Vratio > 0.9 region increases to a plateau with increasing Pe, increases with increasing theta, decreases with increasing phi2, increases with increasing beta31, and decreases with increasing rho3. The theoretical results indicate that Vratio > 0.9 is found whenever the experimentally observed fractional oxygen consumption from the perfusing medium, is less than 0.25. Combination of the theoretical and experimental results indicate that intrinsic, per cell oxygen consumption in the hollow fiber system may decrease as hepatocyte cell density increases and that this decrease may be due to lower intrinsic oxygen requirements in denser suspensions and not due to diffusion limitations in oxygen transport in the hollow fiber system as might be expected from two-dimensional, monolayer culture oxygen consumption measurements.

Effects of Enhanced O2 Transport on Hepatocytes Packed within a Bioartificial Liver Device

The functional performance of an extracorporeal bioartificial liver (BAL) device requires that suitable nutrient pathways exist to support the hepatocytes packed within it. Consequently the limited transport distance of the nutrient oxygen is a limiting factor in the scale-up of many BAL designs. In this study the porosity of a collagen extracellular matrix is increased to evaluate how enhanced O(2) transport alters the viability and functional performance of gel-entrapped hepatocytes packed within a BAL. Our results indicate that the porous collagen increases the number of viable hepatocytes that can be supported by a single O(2) source. Furthermore, the results illustrate that, compared with normal collagen, porous collagen extends the O(2) transport distance such that hepatocytes located at larger distances from the O(2) source of the BAL can be supported. Finally, the function results reveal that hepatocytes within the porous collagen experience significantly improved function over the control cultures. Hence our results demonstrate that enhancing O(2) transport through the extracellular matrix of densely packed BAL designs is one way to significantly improve the functionality of these devices.

Use of Ultrathin Shell Microcapsules of Hepatocytes in Bioartificial Liver-Assist Device

We previously encapsulated hepatocytes in ultrathin shell microcapsules and showed them to have enhanced differentiated functions over cells cultured in monolayer. Here we have used these microencapsulated hepatocytes in a bioartificial liver-assisted device (BLAD) with a rat hepatectomy model. Primary rat hepatocytes were encapsulated in 150- to 200-microm microcapsules, using an electrostatic droplet generator. The microencapsulated hepatocytes exhibited good in vitro urea synthesis activity in plasma from rats with fulminant hepatic failure (FHF). The ex vivo hemoperfusion was conducted in FHF rats by perfusing plasma at a rate of 1-2 mL/min through 1.5-2 x 10(8) encapsulated hepatocytes packed into a packed-bed bioreactor. Hemoperfusion with the bioreactor was initiated 5 h after operative induction of liver failure and continued for 7 h. The BLAD-treated rats showed improvements over the control groups in survival time and metabolic indicators, including ammonia and total bilirubin levels. Furthermore, expanded bed adsorption (EBA) detoxification technology using Streamline-SP resin was explored to complement the bioreactor with microencapsulated hepatocytes. In vitro experiments indicated that serum ammonia could be specifically removed in dose-dependent manner, whereas the total serum proteins were unaffected by the resin. In ex vivo experiments, hemoperfusion with the resin was initiated 3 h after operative induction of liver failure and continued for 7 h. The resin-treated rats showed obvious serum ammonia removal with no observable total blood protein and blood cell adsorption. Therefore, Streamline-SP resin can potentially be integrated into a BLAD for improved efficacy.

Efficacy of an extracorporeal flat-plate bioartificial liver in treating fulminant hepatic failure

BACKGROUND

Fulminant hepatic failure is associated with a high mortality rate. Orthotopic liver transplantation is the only established treatment for patients who do not respond to medical management. A major limitation of this treatment is a shortage of donor organs, resulting in many patients dying while waiting for a transplant. An extracorporeal bioartificial liver (BAL) has the potential to provide temporary support for patients with fulminant hepatic failure (FHF) and for patients awaiting orthotopic liver transplantation. We developed a flat-plate BAL with an internal membrane oxygenator in which porcine hepatocytes were cultured as a monolayer.

MATERIALS AND METHODS

Twenty-four hours after cannulation of the left carotid artery and right jugular vein, FHF was induced in rats by administering 2 intraperitoneal injections of D-galactosamine (GalN) (1.2 g/kg) at a 12-h interval. The rats were connected to a BAL device 24 h after the first GalN injection and underwent extracorporeal perfusion for a duration of 10 h. Liver histology, liver-specific markers, and animal survival up to 168 h (7 days) were examined.

RESULTS

Histologically, liver damage was reduced in the animal group treated with the hepatocyte-based BAL device. Significant reductions occurred in the plasma ammonia levels and prothrombin times in the group treated with the seeded BAL device. Animal survival in the group treated with the seeded BAL device was significantly higher (50.0%) than in the control animal group treated with an unseeded BAL device (11.1%).

CONCLUSIONS

This flat-plate BAL with an internal membrane oxygenator and cultured porcine hepatocytes has yielded encouraging results in the treatment of rats with GalN-induced FHF.

Modeling O2 transport within engineered hepatic devices

Predicting and improving oxygen transport within bioartificial liver (BAL) devices continues to be an important engineering challenge since oxygen is one of the critical nutrients necessary for maintaining hepatocyte viability and function. Such a computational model would not only help predict outcomes but it would also allow system modifications to be analyzed prior to developing experimental protocols. This would help to facilitate future design improvements while reducing both experimental time and capital resource costs, and is the focus of the current study. Specifically, a computational model of O(2) transport through collagen and microporous collagen ECMs is analyzed for hollow fiber (HF), flat plate (FP), and spheroid BAL designs. By modifying the O(2) boundary conditions, hepatocyte O(2) consumption levels, O(2) permeability of the ECM, and ECM void fractions, O(2) transport predictions are determined for each system as a function of time and distance. Accuracy of the predictive model is confirmed by comparing computational vs. experimental results for the HF BAL system. The model's results indicate that O(2) transport within all three BAL designs can be improved significantly by incorporating the enhancement technique. This technique modifies a diffusion-dominant gel ECM into a porous matrix with diffusive and convective flows that mutually transport O(2) through the ECMs. Although tortuous pathways increase the porous ECM's overall effective length of O(2) travel, the decreased transport resistances of these pathways allow O(2) to permeate more effectively into the ECMs. Furthermore, because the HF design employs convective flow on both its inner and outer ECM surfaces, greater control of O(2) transport through its ECM is predicted, as compared with the single O(2) source inputs of the flat plate and spheroid systems. The importance of this control is evaluated by showing how modifying the O(2) concentration and/or transfer coefficients of the convective flows can affect O(2) transport.

Contributions of a finite element model for the geometric optimization of an implantable bioartificial pancreas

The extravascular implantation of islets of Langerhans immunoprotected within a permselective membrane is a promising method to treat diabetes mellitus. However, oxygen limitation due to purely diffusive solute transport was considered to provoke tissue necrosis and graft failure. We built a solute transport model based on a finite element method aiming at optimizing the hollow fiber geometry. With a low islet density, the influence of oxygen axial flux inside the fiber was underlined and a characteristic length for oxygen supply was introduced. This study allowed the conclusion that islet density must be adapted to the fiber diameter chosen for implantation.

A bioartificial liver--state of the art

End-stage liver disease is treated by liver transplantation, but donor organ shortages remain a serious problem. This has prompted the design of bioartificial liver devices to "bridge" patients until they either recover or receive a liver transplant. In these devices, patient plasma is circulated extracorporeally through a bioreactor that houses liver cells (hepatocytes) sandwiched between artificial plates or capillaries.

Advances in bioartificial liver assist devices

Rapid advances in development of bioartificial liver assist devices (BLADs) are exciting clinical interest in the application of BLAD technology for support of patients with acute liver failure. Four devices (Circe Biomedical HepatAssist, Vitagen ELAD, Gerlach BELS, and Excorp Medical BLSS) that rely on hepatocytes cultured in hollow-fiber membrane technology are currently in various stages of clinical evaluation. Several alternative approaches for culture and perfusion of hepatocytes have been evaluated in preclinical, large animal models of liver failure, or at a laboratory scale. Engineering design issues with respect to xenotransplantation, BLAD perfusion, hepatocyte functionality and culture maintenance, and ultimate distribution of a BLAD to a clinical site are delineated.

Oxygen transfer in a convection-enhanced hollow fiber bioartificial liver

A mathematical model was developed to predict oxygen transport in a hollow fiber bioartificial liver device. The model parameters were taken from the HepatAssist 2000 device, a plasma perfused hollow fiber cartridge with primary hepatocytes seeded in the extracapillary space. Cellular oxygen uptake was based on Michaelis-Menten kinetics. Oxygen transport due to the convective flow of plasma into the extracapillary space was considered. The effect of modulating several important parameters was investigated, namely, the Michaelis-Menten constant V(m) (the maximum oxygen consumption per unit volume of the cell mass), the oxygen partial pressure, the flow rate of the plasma at device inlet, and the permeability of the cell mass contained in the extracapillary space. A computer implementation of the model was used to assess whether a given number of cells could be maintained within such a device. The results suggest that a substantial proportion of the hepatocytes are exposed to hypoxic conditions under which metabolism may be impaired.