Numerical Investigations of Hepatic Spheroids Metabolic Reactions in a Perfusion Bioreactor

Computer-aided engineering and additive manufacturing for bioreactors in tissue engineering: State of the art and perspectives

Two main challenges are currently present in the healthcare world, i.e., the limitations given by transplantation and the need to have available 3D in vitro models. In this context, bioreactors are devices that have been introduced in tissue engineering as a support for facing the mentioned challenges by mimicking the cellular native microenvironment through the application of physical stimuli. Bioreactors can be divided into two groups based on their final application: macro- and micro-bioreactors, which address the first and second challenge, respectively. The bioreactor design is a crucial step as it determines the way in which physical stimuli are provided to cells. It strongly depends on the manufacturing techniques chosen for the realization. In particular, in bioreactor prototyping, additive manufacturing techniques are widely used nowadays as they allow the fabrication of customized shapes, guaranteeing more degrees of freedom. To support the bioreactor design, a powerful tool is represented by computational simulations that allow to avoid useless approaches of trial-and-error. In the present review, we aim to discuss the general workflow that must be carried out to develop an optimal macro- and micro-bioreactor. Accordingly, we organize the discussion by addressing the following topics: general and stimulus-specific (i.e., perfusion, mechanical, and electrical) requirements that must be considered during the design phase based on the tissue target; computational models as support in designing bioreactors based on the provided stimulus; manufacturing techniques, with a special focus on additive manufacturing techniques; and finally, current applications and new trends in which bioreactors are involved.

Synergistic Modulation of a Tunable Microenvironment to Fabricate a Liver Fibrosis Chip for Drug Testing

Liver fibrosis is a progressive physiological change that occurs after liver injury and seriously endangers human health. The lack of reliable and physiologically relevant pathological models of liver fibrosis leads to a longer drug development period and sizeable economic investment. The fabrication of a biomimetic liver-on-a-chip is significant for liver disease treatment and drug development. Here, a sandwich chip with a microwell array structure in its bottom layer was fabricated to simulate the Disse space structure of hepatic sinusoids in vitro. By synergistic modulation of the cross-linking degree of gelatin-methacryloyl (GelMA) hydrogels and the induction of transforming growth factor-beta (TGF-β), the early and late stages of liver fibrosis were designed in the chip. Owing to its three-dimensional-mixed-culture strategy, it was possible to construct a liver sinusoid model in vitro to allow for faithful physiological emulation. The model was further subjected to drug treatment, and it presented a significant difference in treatment response in early and late fibrosis progression. Our system provides a unique method for emulating liver function through a vitro liver fibrosis-on-a-chip, potentially paving the way for investigating human liver fibrosis and related drug development.

Mera: A scalable high throughput automated micro-physiological system

There is an urgent need for scalable Microphysiological Systems (MPS's)1 that can better predict drug efficacy and toxicity at the preclinical screening stage. Here we present Mera, an automated, modular and scalable system for culturing and assaying microtissues with interconnected fluidics, inbuilt environmental control and automated image capture. The system presented has multiple possible fluidics modes. Of these the primary mode is designed so that cells may be matured into a desired microtissue type and in the secondary mode the fluid flow can be re-orientated to create a recirculating circuit composed of inter-connected channels to allow drugging or staining. We present data demonstrating the prototype system Mera using an Acetaminophen/HepG2 liver microtissue toxicity assay with Calcein AM and Ethidium Homodimer (EtHD1) viability assays. We demonstrate the functionality of the automated image capture system. The prototype microtissue culture plate wells are laid out in a 3 × 3 or 4 × 10 grid format with viability and toxicity assays demonstrated in both formats. In this paper we set the groundwork for the Mera system as a viable option for scalable microtissue culture and assay development.

Perfusion of MC3T3E1 Preosteoblast Spheroids within Polysaccharide-Based Hydrogel Scaffolds: An Experimental and Numerical Study at the Bioreactor Scale

The traditional 3D culture systems in vitro lack the biological and mechanical spatiotemporal stimuli characteristic to native tissue development. In our study, we combined porous polysaccharide-based hydrogel scaffolds with a bioreactor-type perfusion device that generates favorable mechanical stresses while enhancing nutrient transfers. MC3T3E1 mouse osteoblasts were seeded in the scaffolds and cultivated for 3 weeks under dynamic conditions at a perfusion rate of 10 mL min^-1. The spatial distribution of the cells labeled with superparamagnetic iron oxide nanoparticles was visualized by MRI. Confocal microscopy was used to assess cell numbers, their distribution inside the scaffolds, cell viability, and proliferation. The oxygen diffusion coefficient in the hydrogel was measured experimentally. Numerical simulations of the flow and oxygen transport within the bioreactor were performed using a lattice Boltzmann method with a two-relaxation time scheme. Last, the influence of cell density and spheroid size on cell oxygenation was investigated. The cells spontaneously organized into spheroids with a diameter of 30-100 μm. Cell viability remained unchanged under dynamic conditions but decreased under static culture. The cell proliferation (Ki67 expression) in spheroids was not observed. The flow simulation showed that the local fluid velocity reached 27 mm s^-1 at the height where the cross-sectional area of the flow was the smallest. The shear stress exerted by the fluid on the scaffolds may locally rise to 100 mPa, compared with the average value of 25 mPa. The oxygen diffusion coefficient in the hydrogel was 1.6×10-9 m² s^-1. The simulation of oxygen transport and consumption confirmed that the cells in spheroids did not suffer from hypoxia when the bioreactor was perfused at 10 mL min^-1, and suggested the existence of optimal spheroid size and spacing for appropriate oxygenation. Collectively, these findings enabled us to define the optimal conditions inside the bioreactor for an efficient in vitro cell organization and survival in spheroids, which are paramount to future applications with organoids.

Pre-clinical 2D and 3D toxicity response to a panel of nanomaterials; comparative assessment of NBM-induced liver toxicity

Nanobiomaterials, or NBMs, have been used in medicine and bioimaging for decades, with wide-reaching applications ranging from their uses as carriers of genes and drugs, to acting as sensors and probes. When developing nanomedicine products, it is vitally important to evaluate their safety, ensuring that both biocompatibility and efficacy are achieved so their applications in these areas can be safe and effective. When discussing the safety of nanomedicine in general terms, it is foolish to make generalised statements due to the vast array of different manufactured nanomaterials, formulated from a multitude of different materials, in many shapes and sizes; therefore, NBM pre-clinical screening can be a significant challenge. Outside of their distribution in the various tissues, organs and cells in the body, a key area of interest is the impact of NBMs on the liver. A considerable issue for researchers today is accurately predicting human-specific liver toxicity prior to clinical trials, with hepatotoxicity not only the most cited reasons for withdrawal of approved drugs, but also a primary cause of attrition in pre-launched drug candidates. To date, no simple solution to adequately predict these adverse effects exists prior to entering human experimentation. The limitations of the current pre-clinical toolkit are believed to be one of the main reasons for this, with questions being raised on the relevance of animal models in pre-clinical assessment, and over the ability of conventional, simplified in vitro cell–based assays to adequately assess new drug candidates or NBMs. Common 2D cell cultures are unable to adequately represent the functions of 3D tissues and their complex cell–cell and cell–matrix interactions, as well as differences found in diffusion and transport conditions. Therefore, testing NBM toxicity in conventional 2D models may not be an accurate reflection of the actual toxicity these materials impart on the body. One such method of overcoming these issues is the use of 3D cultures, such as cell spheroids, to more accurately assess NBM-tissue interaction. In this study, we introduce a 3D hepatocellular carcinoma model cultured from HepG2 cells to assess both the cytotoxicity and viability observed following treatment with a variety of NBMs, namely a nanostructured lipid carrier (in the specific technical name = LipImage^™ 815), a gold nanoparticle (AuNP) and a panel of polymeric (in the specific technical name = PACA) NBMs. This model is also in compliance with the 3Rs policy of reduction, refinement and replacement in animal experimentation [1], and meets the critical need for more advanced in vitro models for pre-clinical nanotoxicity assessment.

Natural Hydrogel-Based Bio-Inks for 3D Bioprinting in Tissue Engineering: A Review

Three-dimensional (3D) printing is well acknowledged to constitute an important technology in tissue engineering, largely due to the increasing global demand for organ replacement and tissue regeneration. In 3D bioprinting, which is a step ahead of 3D biomaterial printing, the ink employed is impregnated with cells, without compromising ink printability. This allows for immediate scaffold cellularization and generation of complex structures. The use of cell-laden inks or bio-inks provides the opportunity for enhanced cell differentiation for organ fabrication and regeneration. Recognizing the importance of such bio-inks, the current study comprehensively explores the state of the art of the utilization of bio-inks based on natural polymers (biopolymers), such as cellulose, agarose, alginate, decellularized matrix, in 3D bioprinting. Discussions regarding progress in bioprinting, techniques and approaches employed in the bioprinting of natural polymers, and limitations and prospects concerning future trends in human-scale tissue and organ fabrication are also presented.

Generation of Oxygenating Fluorinated Methacrylamide Chitosan Microparticles to Increase Cell Survival and Function in Large Liver Spheroids

Despite advances in the development of complex culture technologies, the utility, survival, and function of large 3D cell aggregates, or spheroids, are impeded by mass transport limitations. The incorporation of engineered microparticles into these cell aggregates offers a promising approach to increase spheroid integrity through the creation of extracellular spaces to improve mass transport. In this study, we describe the formation of uniform oxygenating fluorinated methacrylamide chitosan (MACF) microparticles via a T-shaped microfluidic device, which when incorporated into spheroids increased extracellular spacing and enhanced oxygen transport via perfluorocarbon substitutions. The addition of MACF microparticles into large liver cell spheroids supported the formation of stable and large spheroids (>500 μm in diameter) made of a heterogeneous population of immortalized human hepatoma (HepG2) and hepatic stellate cells (HSCs) (4 HepG2/1 HSC), especially at a 150:1 ratio of cells to microparticles. Further, as confirmed by the albumin, urea, and CYP3A4 secretion amounts into the culture media, biological functionality was maintained over 10 days due to the incorporation of MACF microparticles as compared to controls without microparticles. Importantly, we demonstrated the utility of fluorinated microparticles in reducing the number of hypoxic cells within the core regions of spheroids, while also promoting the diffusion of other small molecules in and out of these 3D in vitro models.

Development of a Perfusion Reactor for Intervertebral Disk Regeneration

A tissue-engineered biological disk replacement has been proposed as a promising approach for the treatment of degenerative disk disease. A perfusion bioreactor would be a logical consideration to facilitate this scale-up as such reactors have been shown to improve nutrient delivery and provide beneficial mechanical forces that support the cultivation of large three-dimensional constructs. It was hypothesized that perfusion culture of tissue-engineered intervertebral disk (IVD) tissues would be capable of generating outer annulus fibrosus (oAF) and nucleus pulposus (NP) tissues comparable with established spinner reactor or static cultures, respectively, without compromising cellular viability, nutrient delivery, and tissue formation. In this study, the perfusion grown oAF and NP tissues did not show a significant difference in extracellular matrix (ECM) quantity or cellular phenotype when compared with their control conditions. In addition, they maintained cellular viability at the center core of the tissues and received enhanced diffusion of medium throughout the tissue when compared with static conditions. This study lays the groundwork for future studies to grow an entire IVD tissue to a physiologically relevant size.

A miniaturized hydrogel-based in vitro model for dynamic culturing of human cells overexpressing beta-amyloid precursor protein

Recent findings have highlighted an interconnection between intestinal microbiota and the brain, referred to as microbiota–gut–brain axis, and suggested that alterations in microbiota composition might affect brain functioning, also in Alzheimer’s disease. To investigate microbiota–gut–brain axis biochemical pathways, in this work we developed an innovative device to be used as modular unit in an engineered multi-organ-on-a-chip platform recapitulating in vitro the main players of the microbiota–gut–brain axis, and an innovative three-dimensional model of brain cells based on collagen/hyaluronic acid or collagen/poly(ethylene glycol) semi-interpenetrating polymer networks and β-amyloid precursor protein-Swedish mutant-expressing H4 cells, to simulate the pathological scenario of Alzheimer’s disease. We set up the culturing conditions, assessed cell response, scaled down the three-dimensional models to be hosted in the organ-on-a-chip device, and cultured them both in static and in dynamic conditions. The results suggest that the device and three-dimensional models are exploitable for advanced engineered models representing brain features also in Alzheimer’s disease scenario.

A Novel 3D Bioprinter Using Direct-Volumetric Drop-On-Demand Technology for Fabricating Micro-Tissues and Drug-Delivery

Drop-on-demand (DOD) 3D bioprinting technologies currently hold the greatest promise for generating functional tissues for clinical use and for drug development. However, existing DOD 3D bioprinting technologies have three main limitations: (1) droplet volume inconsistency; (2) the ability to print only bioinks with low cell concentrations and low viscosity; and (3) problems with cell viability when dispensed under high pressure. We report our success developing a novel direct-volumetric DOD (DVDOD) 3D bioprinting technology that overcomes each of these limitations. DVDOD can produce droplets of bioink from <10 nL in volume using a direct-volumetric mechanism with <± 5% volumetric percent accuracy in an accurate spatially controlled manner. DVDOD has the capability of dispensing bioinks with high concentrations of cells and/or high viscosity biomaterials in either low- or high-throughput modes. The cells are subjected to a low pressure during the bioprinting process for a very short period of time that does not negatively impact cell viability. We demonstrated the functions of the bioprinter in two distinct manners: (1) by using a high-throughput drug-delivery model; and (2) by bioprinting micro-tissues using a variety of different cell types, including functional micro-tissues of bone, cancer, and induced pluripotent stem cells. Our DVDOD technology demonstrates a promising platform for generating many types of tissues and drug-delivery models.

Liver Bioreactor Design Issues of Fluid Flow and Zonation, Fibrosis, and Mechanics: A Computational Perspective

Tissue engineering, with the goal of repairing or replacing damaged tissue and organs, has continued to make dramatic science-based advances since its origins in the late 1980’s and early 1990’s. Such advances are always multi-disciplinary in nature, from basic biology and chemistry through physics and mathematics to various engineering and computer fields. This review will focus its attention on two topics critical for tissue engineering liver development: (a) fluid flow, zonation, and drug screening, and (b) biomechanics, tissue stiffness, and fibrosis, all within the context of 3D structures. First, a general overview of various bioreactor designs developed to investigate fluid transport and tissue biomechanics is given. This includes a mention of computational fluid dynamic methods used to optimize and validate these designs. Thereafter, the perspective provided by computer simulations of flow, reactive transport, and biomechanics responses at the scale of the liver lobule and liver tissue is outlined, in addition to how bioreactor-measured properties can be utilized in these models. Here, the fundamental issues of tortuosity and upscaling are highlighted, as well as the role of disease and fibrosis in these issues. Some idealized simulations of the effects of fibrosis on lobule drug transport and mechanics responses are provided to further illustrate these concepts. This review concludes with an outline of some practical applications of tissue engineering advances and how efficient computational upscaling techniques, such as dual continuum modeling, might be used to quantify the transition of bioreactor results to the full liver scale.