A novel multi-coaxial hollow fiber bioreactor for adherent cell types. Part 1: hydrodynamic studies

Bioreactor design and validation for manufacturing strategies in tissue engineering

The fields of regenerative medicine and tissue engineering offer new therapeutic options to restore, maintain or improve tissue function following disease or injury. To maximize the biological function of a tissue-engineered clinical product, specific conditions must be maintained within a bioreactor to allow the maturation of the product in preparation for implantation. Specifically, the bioreactor should be designed to mimic the mechanical, electrochemical and biochemical environment that the product will be exposed to in vivo. Real-time monitoring of the functional capacity of tissue-engineered products during manufacturing is a critical component of the quality management process. The present review provides a brief overview of bioreactor engineering considerations. In addition, strategies for bioreactor automation, in-line product monitoring and quality assurance are discussed.

Hollow Fiber and Nanofiber Membranes in Bioartificial Liver and Neuronal Tissue Engineering

To date, the creation of biomimetic devices for the regeneration and repair of injured or diseased tissues and organs remains a crucial challenge in tissue engineering. Membrane technology offers advanced approaches to realize multifunctional tools with permissive environments well-controlled at molecular level for the development of functional tissues and organs. Membranes in fiber configuration with precisely controlled, tunable topography, and physical, biochemical, and mechanical cues, can direct and control the function of different kinds of cells toward the recovery from disorders and injuries. At the same time, fiber tools also provide the potential to model diseases in vitro for investigating specific biological phenomena as well as for drug testing. The purpose of this review is to present an overview of the literature concerning the development of hollow fibers and electrospun fiber membranes used in bioartificial organs, tissue engineered constructs, and in vitro bioreactors. With the aim to highlight the main biomedical applications of fiber-based systems, the first part reviews the fibers for bioartificial liver and liver tissue engineering with special attention to their multifunctional role in the long-term maintenance of specific liver functions and in driving hepatocyte differentiation. The second part reports the fiber-based systems used for neuronal tissue applications including advanced approaches for the creation of novel nerve conduits and in vitro models of brain tissue. Besides presenting recent advances and achievements, this work also delineates existing limitations and highlights emerging possibilities and future prospects in this field.

Validation of a novel 3D flow model for the optimization of construct perfusion in radial-flow packed-bed bioreactors (rPBBs) for long-bone tissue engineering

Osteogenic cell culture in three-dimensional (3D) hollow cylindrical porous scaffolds in radial-flow packed-bed bioreactors (rPBBs) may overcome the transport limitations of static and axial perfusion bioreactors in the engineering of long-bone substitutes. Flow models of rPBBs help optimize radial flux distribution of medium and tissue maturation in vitro. Only a 2D model is available for steady flow transport in rPBBs with axisymmetric inlet and outlet accounting for the fluid dynamics of void spaces, assessed against literature information. Here, a novel 3D model is proposed for steady flow transport in the three compartments of rPBBs with a more practical lateral outlet. A 3D model of transient tracer transport was developed based on the flow model to predict bioreactor residence time distribution (RTD). Model-predicted flow patterns were validated in terms of RTD against tracer experiments performed with bioreactor prototypes equipped with commercial scaffolds for bone tissue engineering. Bioreactors were challenged with a step change in entering tracer concentration in an optimized set-up under conditions promoting uniform radial flux distribution and typical shunt flows. Model-predicted RTDs agreed well with those experimentally determined. In conclusion, tracer experiments validate the use of the 3D flow model for optimizing construct perfusion in rPBBs to engineer long-bone substitutes.

Studies of Metabolism Using (13)C MRS of Hyperpolarized Probes

First described in 2003, the dissolution dynamic nuclear polarization (DNP) technique, combined with (13)C magnetic resonance spectroscopy (MRS), has since been used in numerous metabolic studies and has become a valuable metabolic imaging method. DNP dramatically increases the level of polarization of (13)C-labeled compounds resulting in an increase in the signal-to-noise ratio (SNR) of over 50,000 fold for the MRS spectrum of hyperpolarized compounds. The high SNR enables rapid real-time detection of metabolism in cells, tissues, and in vivo. This chapter will present a comprehensive review of the DNP approaches that have been used to monitor metabolism in living systems. First, the list of (13)C DNP probes developed to date will be presented, with a particular focus on the most commonly used probe, namely [1-(13)C] pyruvate. In the next four sections, we will then describe the different factors that need to be considered when designing (13)C DNP probes for metabolic studies, conducting in vitro or in vivo hyperpolarized experiments, as well as acquiring, analyzing, and modeling hyperpolarized (13)C data.

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.

Characterization of a chip-based bioreactor for three-dimensional cell cultivation via Magnetic Resonance Imaging

We describe the characterization of a chip-based platform (3(D)-KITChip) for the three-dimensional cultivation of cells under perfusion conditions via magnetic resonance imaging (MRI). Besides the chip, the microfluidic system is comprised of a bioreactor housing, a medium supply, a pump for generating active flow conditions as well as a gas mixing station. The closed circulation loop is ideally suited for a characterization via MRI since the small bioreactor setup with active perfusion, driven by the pump from outside the coils, not only is completely MRI-compatible but also can be transferred into the magnetic coil of an experimental animal scanner. We have found that the two halves of the chip inside the bioreactor are homogeneously perfused with cell culture medium both with and without cells inside the 3(D)-KITChip. In addition, the homogeneity of perfusion is nearly independent from the flow rates investigated in this study, and furthermore, the setup shows excellent washout characteristics after spiking with Gadolinium-DOTA which makes it an ideal candidate for drug screening purposes. We, therefore, conclude that the 3(D)-KITChip is well suited as a platform for high-density three-dimensional cell cultures, especially those requiring a defined medium flow and/or gas supply in a precisely controllable three dimensional environment, like stem cells.

Construction of a multicoaxial hollow fiber bioreactor

Bioreactors are assembled tools conceived to exploit engineering principles with inbuilt biological -relevance. Such reactors are created as in vitro models to better replicate natural in vivo organs. These biotools are subsets within the interdisciplinary tissue engineering field and are established as inert devices to improve upon biological stimuli while simultaneously allowing tissue functional properties to be nondestructively measured. Design and fabrication efforts are focused on two-dimensional (2D) and three-dimensional (3D) physical constructs while linking environment-cell relations, the microenvironment. Product proficiencies generally involve material scaffolds, nutrient dispersion, compartmentalized units, passive and kinetic flow channels, temperature regulation, pressure management, and cell line or primary cells from assorted organs as tissues. Bioreactor advancements continue with interdisciplinary principles such as energy conservation, cell ecosystems, system-biological approaches, and viable-cell design innovation. Herein, we describe the design and construction of a hollow fiber multicoaxial bioreactor with integral oxygenation (i.e., oxygenation within the bioreactor proper) for use with liver cells, but it could be used with any anchorage-dependent cell type.

New advances in MR-compatible bioartificial liver

MR-compatible bioartificial liver (BAL) studies have been performed for 30 years and are reviewed. There are two types of study: (i) metabolism and drug studies using multinuclear MRS; primarily short-term (< 8 h) studies; (ii) the use of multinuclear MRS and MRI to noninvasively define the features and functions of BAL systems for long-term liver tissue engineering. In the latter, these systems often undergo not only modification of the perfusion system, but also the construction of MR radiofrequency probes around the bioreactor. We present novel MR-compatible BALs and the use of multinuclear MRS ((13)C, (19)F, (31)P) for the noninvasive monitoring of their growth, metabolism and viability, as well as (1)H MRI methods for the determination of flow profiles, diffusion, cell distribution, quality assurance and bioreactor integrity. Finally, a simple flexible coil design and circuit, and life support system, are described that can make almost any BAL MR-compatible.

Stable isotope-resolved metabolomics and applications for drug development

Advances in analytical methodologies, principally nuclear magnetic resonance spectroscopy (NMR) and mass spectrometry (MS), during the last decade have made large-scale analysis of the human metabolome a reality. This is leading to the reawakening of the importance of metabolism in human diseases, particularly cancer. The metabolome is the functional readout of the genome, functional genome, and proteome; it is also an integral partner in molecular regulations for homeostasis. The interrogation of the metabolome, or metabolomics, is now being applied to numerous diseases, largely by metabolite profiling for biomarker discovery, but also in pharmacology and therapeutics. Recent advances in stable isotope tracer-based metabolomic approaches enable unambiguous tracking of individual atoms through compartmentalized metabolic networks directly in human subjects, which promises to decipher the complexity of the human metabolome at an unprecedented pace. This knowledge will revolutionize our understanding of complex human diseases, clinical diagnostics, as well as individualized therapeutics and drug response. In this review, we focus on the use of stable isotope tracers with metabolomics technologies for understanding metabolic network dynamics in both model systems and in clinical applications. Atom-resolved isotope tracing via the two major analytical platforms, NMR and MS, has the power to determine novel metabolic reprogramming in diseases, discover new drug targets, and facilitates ADME studies. We also illustrate new metabolic tracer-based imaging technologies, which enable direct visualization of metabolic processes in vivo. We further outline current practices and future requirements for biochemoinformatics development, which is an integral part of translating stable isotope-resolved metabolomics into clinical reality.

Hyperpolarized (13)C spectroscopy and an NMR-compatible bioreactor system for the investigation of real-time cellular metabolism

The purpose of this study was to combine a three-dimensional NMR-compatible bioreactor with hyperpolarized (13)C NMR spectroscopy in order to probe cellular metabolism in real time. JM1 (immortalized rat hepatoma) cells were cultured in a three-dimensional NMR-compatible fluidized bioreactor. (31)P spectra were acquired before and after each injection of hyperpolarized [1-(13)C] pyruvate and subsequent (13)C spectroscopy at 11.7 T. (1)H and two-dimensional (1)H-(1)H-total correlation spectroscopy spectra were acquired from extracts of cells grown in uniformly labeled (13)C-glucose, on a 16.4 T, to determine (13)C fractional enrichment and distribution of (13)C label. JM1 cells were found to have a high rate of aerobic glycolysis in both two-dimensional culture and in the bioreactor, with 85% of the (13)C label from uniformly labeled (13)C-glucose being present as either lactate or alanine after 23 h. Flux measurements of pyruvate through lactate dehydrogenase and alanine aminotransferase in the bioreactor system were 12.18 +/- 0.49 nmols/sec/10(8) cells and 2.39 +/- 0.30 nmols/sec/10(8) cells, respectively, were reproducible in the same bioreactor, and were not significantly different over the course of 2 days. Although this preliminary study involved immortalized cells, this combination of technologies can be extended to the real-time metabolic exploration of primary benign and cancerous cells and tissues prior to and after therapy.

Bioartificial liver support systems

A variety of bioartificial liver support systems were developed to replace some of the liver's function in case of liver failure. Those systems, in contrast to purely artificial systems, incorporate metabolically active cells to contribute synthetic and regulatory functions as well as detoxification. The selection of the ideal cell source and the design of more sophisticated bioreactors are the main issues in this field of research. Several systems were already introduced into clinical studies to prove their safety. This review briefly introduces a cross-section of experimental and clinically applied systems and tries to give an overview on the problems and limitations of bioartificial liver support.

The effectiveness of a novel cartridge-based bioreactor design in supporting liver cells

There are a number of applications--ranging from temporary strategies for organ failure to pharmaceutical testing--that rely on effective bioreactor designs. The significance of these devices is that they provide an environment for maintaining cells in a way that allows them to perform key cellular and tissue functions. In the current study, a novel cartridge-based bioreactor was developed and evaluated. Its unique features include its capacity for cell support and the adaptable design of its cellular space. Specifically, it is able to accommodate functional and reasonably sized tissue (>2.0 x 10(8) cells), and can be easily modified to support a range of anchorage-dependent cells. To evaluate its efficacy, it was applied to liver support in the current study. This involved evaluating the performance of rat primary hepatocytes within the unique cartridges in culture--sans bioreactor--and after being loaded within the novel bioreactor. Compared to collagen sandwich culture functional controls, hepatocytes within the unique cartridge design demonstrated significantly higher albumin production and urea secretion rates when cultured under dynamic flow conditions--reaching peak values of 170 +/- 22 microg/10(6) cells/day and 195 +/- 18 microg/10(6) cells/day, respectively. The bioreactor's effectiveness in supporting live and functioning primary hepatocytes is also presented. Cell viability at the end of 15 days of culture in the new bioreactor was 84 +/- 18%, suggesting that the new design is effective in maintaining primary hepatocytes for at least 2 weeks in culture. Liver-specific functions of urea secretion, albumin synthesis, and cytochrome P450 activity were also assessed. The results indicate that hepatocytes are able to achieve good functional performance when cultured within the novel bioreactor. This is especially true in the case of cytochrome P450 activity, where by day 15 of culture, hepatocytes within the bioreactor reached values that were 56.6% higher than achieved by the collagen sandwich functional control cultures. The success of the novel cartridge-based bioreactor in supporting hepatocytes with good viability and functional performance suggests that it is an effective design for supporting anchorage-dependent cells.

High-throughput nuclear magnetic resonance metabolomic footprinting for tissue engineering

We report a high-throughput (HTP) nuclear magnetic resonance (NMR) method for analysis of media components and a metabolic schematic to help easily interpret the data. Spin-lattice relaxation values and concentrations were measured for 19 components and 2 internal referencing agents in pure and 2-day conditioned, hormonally defined media from a 3-dimensional (3D) multicoaxial human bioartificial liver (BAL). The (1)H NMR spectral signal-to-noise ratio is 21 for 0.16 mM alanine in medium and is obtained in 12 min using a 400 MHz NMR spectrometer. For comparison, 2D gel cultures and 3D multicoaxial BALs were batch cultured, with medium changed every day for 15 days after inoculation with human liver cells in Matrigel-collagen type 1 gels. Glutamine consumption was higher by day 8 in the BAL than in 2D culture; lactate production was lower through the 15-day culture period. Alanine was the primary amino acid produced and tracked with lactate or urea production. Glucose and pyruvate consumption were similar in the BAL and 2D cultures. NMR analysis permits quality assurance of the bioreactor by identifying contaminants. Ethanol was observed because of a bioreactor membrane "wetting" procedure. A biochemical scheme is presented illustrating bioreactor metabolomic footprint results and demonstrating how this can be translated to modify bioreactor operational parameters or quality assurance issues.

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.

Bioreactors for 3-dimensional high-density culture of human cells

A bioreactor was developed as an instrument to culture human or animal cells that require attachment in a large quantity or at a high density. The purpose for developing such a bioreactor is two-fold: to produce a large quantity of animal or human cells that have been modified by gene recombination technology to accommodate manufacture of physiologically-active substances or human proteins on an industrial scale; and for research to culture animal cells to form a high-density 3-dimensional structure as a morphological or functional tissue or organ entity. In the current report, the circulatory flow bioreactor and radial flow bioreactor (RFB) are introduced, in which the former can be scaled up. As a small bioreactor produced for the latter purpose, a rotary cell culture system and novel multicoaxial hollow-fiber bioreactor are introduced. Finally, a small RFB culture system that was scaled down by the present author and his collaborators for the study of a 3-dimensional high density culture system is described. The RFB can be readily scaled up for manufacturing or scaled down for research purposes. This is a cell culturing system that can induce the functions of human tissues by preparing a high density 3-dimensional organization of cells of human origin.

Bioreactors for tissue mass culture: Design, characterization, and recent advances

This paper reviews reports on three-dimensional mammalian tissue growth in bioreactors and the corresponding mammalian tissue growth requirements. The needs for nutrient and waste removal of several mammalian tissues are reviewed and compared with the environment of many reactors currently in use such as the continuous stirred tank, the hollow fiber, the Couette-Taylor, the airlift, and the rotating-wall reactors developed by NASA. Many studies conclude that oxygen supply appears to be one of the most important factors limiting tissue growth. Various correlations to describe oxygen mass transfer are presented and discussed with the aim to provide some guidance to design, construct, and test reactors for tissue mass culture. To obtain tissue thickness clinically valuable, dimensionless and other types of analysis tend to point out that diffusive transport will have to be matched with an important convection to bring sufficient oxygen molecular flux to the growing cells located within a tissue mass. As learned from solid-state fermentation and hairy root culture, during the growth of large biomass, heterogeneity (i.e., channeling, temperature gradients, non-uniform cell growth, transfer gradients, etc.) can cause some important problems and these should be addressed in tissue engineering as well. Reactors (along with the scaffolds) should be designed to minimize these issues. The role of the uterus, the reactor built by Nature, is examined, and the environment provided to a growing embryo is reported, yielding possible paths for further reactor developments. Finally, the importance of cell seeding methods is also addressed.

Development and validation of a microsomal online cytochrome P450 bioreactor coupled to solid-phase extraction and reversed-phase liquid chromatography

The development and validation of an online cytochrome P450 (CYP)-based bioreactor coupled to automated solid-phase extraction (SPE) and gradient HPLC separation is described. The analytical method was checked on intra- and inter-day repeatability of the ethoxyresorufin-O-demethylation (EROD) reaction with CYP 1Al/1A2 containing beta-NF induced rat liver microsomes as an enzyme source. These experiments showed that CYP activity was linearly decreased with 16% over an 11 h period. Inter-day measurements had a CV of 9.1%. Furthermore, Km and Vmax values of the EROD reaction, measured with the bioreactor, were 2.72 +/- 0.46 microM and 7.9 +/- 0.5 nmol/min/mg protein, respectively. These were in good correspondence with Km and Vmax values, measured with standard batch assay, which amounted 0.66 +/- 0.08 microM and 6.4 +/- 0.2 nmol/min/mg protein respectively. In conclusion the newly developed analytical method can be used effectively and at a microliter scale for online generation, extraction and separation of metabolites.

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.

Effect of flow baffles on the dialysate flow distribution of hollow-fiber hemodialyzers: a nonintrusive experimental study using MRI

We used an innovative, nonintrusive MRI technique called the two-dimensional (2D) Phase-Contrast (2DPC) velocity-imaging technique to investigate the effect of flow baffles on the dialysate-side flow distribution in two different hollow-fiber hemodialyzers (A and B); each with flow rates between 200 and 1000 mL/min (3.33 x 10(-6) and 1.67 x 10(-5) m3/s). Our experimental results show that (1) the dialysate-side flow distribution was nonuniform with channeling flow occurred at the peripheral cross section of these hollow-fiber hemodialyzers, and (2) the existing designs of flow baffles failed to promote uniform dialysate-side flow distribution for all flow rates studies.

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.

Improving the next generation of bioartificial liver devices

Several extracorporeal bioartificial liver (BAL) devices are currently being evaluated as an alternative or adjunct therapy for liver disease. While these hybrid systems show promise, in order to become a clinical reality, BAL devices must clearly demonstrate efficacy in improving patient outcomes. Here, we present aspects of BAL devices that could benefit from fundamental advances in cell and developmental biology. In particular, we examine the development of human hepatocyte cell lines, strategies to stabilize the hepatocyte phenotype in vitro, and emphasize the importance of the cellular microenvironment in bioreactor design. Consideration of these key components of BAL systems will greatly improve next generation devices.

Engineering liver therapies for the future

Treatment of liver disease has been greatly improved by the advent and evolution of liver transplantation. However, as demand for donor organs continues to increase beyond their availability, the need for alternative liver therapies is clear. Several approaches including extracorporeal devices, cell transplantation, and tissue-engineered constructs have been proposed as potential adjuncts or even replacements for transplantation. Simultaneously, experience from the liver biology community have provided valuable insight into tissue morphogenesis and in vitro stabilization of the hepatocyte phenotype. The next generation of cellular therapies must therefore consider incorporating cell sources and cellular microenvironments that provide both a large population of cells and strategies to maintain liver-specific functions over extended time frames. As cell-based therapies evolve, their success will require contribution from many diverse disciplines including regenerative medicine, developmental biology, and transplant medicine.