A practical guide to microfluidic perfusion culture of adherent mammalian cells

The culture of A549 cells and its secreted cytokine IL-6 monitoring on the designed multifunctional microfluidic chip

A multifunctional microfluidic chip integrated with perfusion cell culture and in situ SERS detection of cell secretion was designed and developed for the detection of IL-6 secretion from LPS-stimulation of A549 cells in this paper. Researching works were focused on A549 cell activity and secretion in the constructed LPS-stimulated A549 cells model. On the designed microchip, a bubble trap chamber was designed to remove the bubbles in the culture medium which could also be simultaneously preheated by a split hot plate. Then, a long-time perfusion culture process of 549 cells could be realized. Under the optimized conditions the A549 cells could be cultured and kept in good activity for more than 36 h. Subsequently, the model of interaction between LPS and A549 cells was established on the designed microchip. When LPS-stimulated A549 cells, the IL-6 which was one of the secretions formed in this process was detected quantitatively by SERS spectral technique. The silver-coated gold nano-stars were prepared and taken as a sensitive enhancing probe for the SERS detection of IL-6 secreted from LPS-stimulated A549 cells. The immunomagnetic beads, IL-6 antigen, and SERS probes were mixed and incubated in the microchip and form a sandwich structure which was captured by the permanent magnet in the detection zone for SERS detection. The reference material of IL-6 was used to establish the calibration curve, and the linear range and detection limit were 1-10000 pg/mL and 0.75 pg/mL, respectively. Then, the IL-6 secretion from LPS-stimulated A549 cells was detected hourly for 7 h by this established method. The process of LPS stimulation of A594 cells did not lead to a sustained increase in the SERS spectral signature of IL-6. Instead, IL6 secretion initially increased sharply, then decreased and eventually stabilized. It could be due to a potential mechanism that the cells self-regulated to mitigate the inflammatory effects in response to sustained stimulation. The proposed multifunctional microfluidic chip, characterized by high sensitivity and the ability to perform continuous hourly detection, exhibited significant application prospects in the study of external stimulation on cells.

Unveiling the Human Brain on a Chip: An Odyssey to Reconstitute Neuronal Ensembles and Explore Plausible Applications in Neuroscience

The brain is an incredibly complex structure that consists of millions of neural networks. In developmental and cellular neuroscience, probing the highly complex dynamics of the brain remains a challenge. Furthermore, deciphering how several cues can influence neuronal growth and its interactions with different brain cell types (such as astrocytes and microglia) is also a formidable task. Traditional in vitro macroscopic cell culture techniques offer simple and straightforward methods. However, they often fall short of providing insights into the complex phenomena of neuronal network formation and the relevant microenvironments. To circumvent the drawbacks of conventional cell culture methods, recent advancements in the development of microfluidic device-based microplatforms have emerged as promising alternatives. Microfluidic devices enable precise spatiotemporal control over compartmentalized cell cultures. This feature facilitates researchers in reconstituting the intricacies of the neuronal cytoarchitecture within a regulated environment. Therefore, in this review, we focus primarily on modeling neuronal development in a microfluidic device and the various strategies that researchers have adopted to mimic neurogenesis on a chip. Additionally, we have presented an overview of the application of brain-on-chip models for the recapitulation of the blood-brain barrier and neurodegenerative diseases, followed by subsequent high-throughput drug screening. These lab-on-a-chip technologies have tremendous potential to mimic the brain on a chip, providing valuable insights into fundamental brain processes. The brain-on-chip models will also serve as innovative platforms for developing novel neurotherapeutics to address several neurological disorders.

Continuous flow delivery system for the perfusion of scaffold-based 3D cultures

The paper-based culture platform developed by Whitesides readily incorporates tissue-like structures into laboratories with established workflows that rely on monolayer cultures. Cell-laden hydrogels are deposited in these porous scaffolds with micropipettes; these scaffolds support the thin gel slabs, allowing them to be evaluated individually or stacked into thick constructs. The paper-based culture platform has inspired many basic and translational studies, each exploring how readily accessible materials can generate complex structures that mimic aspects of tissues in vivo. Many of these examples have relied on static culture conditions, which result in diffusion-limited environments and cells experiencing pericellular hypoxia. Perfusion-based systems can alleviate pericellular hypoxia and other cell stresses by continually exposing the cells to fresh medium. These perfusion systems are common in microfluidic and organ-on-chip devices supporting cells as monolayer cultures or as 3D constructs. Here, we introduce a continuous flow delivery system, which uses parts readily produced with 3D printing to provide a self-contained culture platform in which cells in paper or other scaffolds are exposed to fresh (flowing) medium. We demonstrate the utility of this device with examples of cells maintained in single cell-laden scaffolds, stacks of cell-laden scaffolds, and scaffolds that contain monolayers of endothelial cells. These demonstrations highlight some possible experimental questions that can be enabled with readily accessible culture materials and a perfusion-based device that can be readily fabricated.

Ultrasound standing wave spatial patterning of human umbilical vein endothelial cells for 3D micro-vascular networks formation

Generating functional and perfusable micro-vascular networks is an important goal for the fabrication of large and three-dimensional tissues. Up to now, the fabrication of micro-vascular networks is a complicated multitask involving several different factors such as time consuming, cells survival, micro-diameter vasculature and strict alignment. Here, we propose a technique combining multi-material extrusion and ultrasound standing wave forces to create a network structure of human umbilical vein endothelial cells within a mixture of calcium alginate and decellularized extracellular matrix. The functionality of the matured microvasculature networks was demonstrated through the enhancement of cell-cell adhesion, angiogenesis process, and perfusion tests with microparticles, FITC-dextran, and whole mouse blood. Moreover, animal experiments exhibited the implantability including that the pre-existing blood vessels of the host sprout towards the preformed vessels of the scaffold over time and the microvessels inside the implanted scaffold matured from empty tubular structures to functional blood-carrying microvessels in two weeks.

A Novel Milli-fluidic Liver Tissue Chip with Continuous Recirculation for Predictive Pharmacokinetics Applications

A crucial step in lead selection during drug development is accurate estimation and optimization of hepatic clearance using in vitro methods. However, current methods are limited by factors such as lack of physiological relevance, short culture/incubation times that are not consistent with drug exposure patterns in patients, use of drug absorbing materials, and evaporation during long-term incubation. To address these technological needs, we developed a novel milli-fluidic human liver tissue chip (LTC) that was designed with continuous media recirculation and optimized for hepatic cultures using human primary hepatocytes. Here, we characterized the LTC using a series of physiologically relevant metrics and test compounds to demonstrate that we could accurately predict the PK of both low- and high-clearance compounds. The non-biological characterization indicated that the cyclic olefin copolymer (COC)-based LTC exhibited negligible evaporation and minimal non-specific binding of drugs of varying ionic states and lipophilicity. Biologically, the LTC exhibited functional and polarized hepatic culture with sustained metabolic CYP activity for at least 15 days. This long-term culture was then used for drug clearance studies for low- and high-clearance compounds for at least 12 days, and clearance was estimated for a range of compounds with high in vitro-in vivo correlation (IVIVC). We also demonstrated that LTC can be induced by rifampicin, and the culture age had insignificant effect on depletion kinetic and predicted clearance value. Thus, we used advances in bioengineering to develop a novel purpose-built platform with high reproducibility and minimal variability to address unmet needs for PK applications.

A biomimetic chip to assess subcutaneous bioavailability of monoclonal antibodies in humans

Subcutaneous (subQ) injection is a common route for delivering biotherapeutics, wherein pharmacokinetics is largely influenced by drug transport in a complex subQ tissue microenvironment. The selection of good drug candidates with beneficial pharmacokinetics for subQ injections is currently limited by a lack of reliable testing models. To address this limitation, we report here a Subcutaneous Co-Culture Tissue-on-a-chip for Injection Simulation (SubCuTIS). SubCuTIS possesses a 3D coculture tissue architecture, and it allows facile quantitative determination of relevant scale independent drug transport rate constants. SubCuTIS captures key in vivo physiological characteristics of the subQ tissues, and it differentiates the transport behavior of various chemically distinct molecules. We supplemented the transport measurements with theoretical modeling, which identified subtle differences in the local absorption rate constants of seven clinically available mAbs. Accounting for first-order proteolytic catabolism, we established a mathematical framework to assess clinical bioavailability using the local absorption rate constants obtained from SubCuTIS. Taken together, the technology described here broadens the applicability of organs-on-chips as a standardized and easy-to-use device for quantitative analysis of subQ drug transport.

Towards a microfluidic H295R steroidogenesis assay-biocompatibility study and steroid detection on a thiol-ene-based chip

The development of cell-based microfluidic assays offers exciting new opportunities in toxicity testing, allowing for integration of new functionalities, automation, and high throughput in comparison to traditional well-plate assays. As endocrine disruption caused by environmental chemicals and pharmaceuticals represents a growing global health burden, the purpose of the current study was to contribute towards the miniaturization of the H295R steroidogenesis assay, from the well-plate to the microfluidic format. Microfluidic chip fabrication with the established well-plate material polystyrene (PS) is expensive and complicated; PDMS and thiol-ene were therefore tested as potential chip materials for microfluidic H295R cell culture, and evaluated in terms of cell attachment, cell viability, and steroid synthesis in the absence and presence of collagen surface modification. Additionally, spike-recovery experiments were performed, to investigate potential steroid adsorption to chip materials. Cell aggregation with poor steroid recoveries was observed for PDMS, while cells formed monolayer cultures on the thiol-ene chip material, with cell viability and steroid synthesis comparable to cells grown on a PS surface. As thiol-ene overall displayed more favorable properties for H295R cell culture, a microfluidic chip design and corresponding cell seeding procedure were successfully developed, achieving repeatable and uniform cell distribution in microfluidic channels. Finally, H295R perfusion culture on thiol-ene chips was investigated at different flow rates (20, 10, and 2.5 µL/min), and 13 steroids were detected in eluting cell medium over 48 h at the lowest flow rate. The presented work and results pave the way for a time-resolved microfluidic H295R steroidogenesis assay.

Single-cell microfluidics in combination with chlorophyll a fluorescence measurements to assess the lifetime of the Chlamydomonas PSBO protein

PSBO is an essential subunit of the oxygen-evolving complex and we recently demonstrated that its lifetime depends on environmental conditions in Chlamydomonas reinhardtii. To assess PSBO lifetime with a high time resolution, we employed (1) a microfluidic platform enabling the trapping of single cells and the parallel measurement of photosynthetic activity, and (2) a nitrate-inducible PSBO amiRNA line. Our microfluidic platform allowed the rapid replacement of the nutrient solution necessary for induction. It also enabled the precise monitoring of the decline in the Fv/Fm value, reflecting PSBO loss. We found that in the dark, at medium and high light intensity, the Fv/Fm value decreased with halftimes of about 25, 12.5, and 5 h, respectively. We also observed that photosynthetic activity was better sustained upon carbon limitation. In the absence of acetate, the halftimes of Fv/Fm diminishment doubled to quadrupled compared with the control, acetate-supplied cultures.

Microfluidic Systems for Neural Cell Studies

Whereas the axons of the peripheral nervous system (PNS) spontaneously regenerate after an injury, the occurring regeneration is rarely successful because axons are usually directed by inappropriate cues. Therefore, finding successful ways to guide neurite outgrowth, in vitro, is essential for neurogenesis. Microfluidic systems reflect more appropriately the in vivo environment of cells in tissues such as the normal fluid flow within the body, consistent nutrient delivery, effective waste removal, and mechanical stimulation due to fluid shear forces. At the same time, it has been well reported that topography affects neuronal outgrowth, orientation, and differentiation. In this review, we demonstrate how topography and microfluidic flow affect neuronal behavior, either separately or in synergy, and highlight the efficacy of microfluidic systems in promoting neuronal outgrowth.

The Spatial-Temporal Effects of Bacterial Growth Substrates on Antibiotic Resistance Gene Spread in the Biofilm

Biofilm is considered as the hotspot of antibiotic resistance gene (ARG) dissemination. Bacterial growth substrates are important factors for biofilm formation, but its spatial-temporal effects on ARG spread in biofilm is still unclear. In this study, microfluidics combined with microscopic observation were used to reveal spatial-temporal effects of bacterial growth substrates on ARG transfer at real time. The initial horizontal gene transfer events were found to be independent of substrate levels. However, subsequent transfer processes varied greatly depending on the availability of growth substrates. The proportion of transconjugants was much higher (~12%) when observed in substrate-rich regions (under the channel) at 24 h, followed by an exponential decline, with the distance far from the channel. Furthermore, three-dimensional observation revealed that vertical gene transfer influenced by the concentrations of bacterial growth substrates was important for ARG spread in biofilm. The transfer frequency was 8.2 times higher in the high substrate concentration (50×) compared to low concentration (0.5×) in simulated sewage, underscoring the substantial impact of bacterial growth substrate variability on ARG dissemination. This study is helpful for in-depth understanding of ARG dissemination through biofilms and indicates that reducing pollutant emission is important for ARG control in the environment.

Recent Advances on Cell Culture Platforms for In Vitro Drug Screening and Cell Therapies: From Conventional to Microfluidic Strategies

The clinical translations of drugs and nanomedicines depend on coherent pharmaceutical research based on biologically accurate screening approaches. Since establishing the 2D in vitro cell culture method, the scientific community has improved cell-based drug screening assays and models. Those advances result in more informative biochemical assays and the development of 3D multicellular models to describe the biological complexity better and enhance the simulation of the in vivo microenvironment. Despite the overall dominance of conventional 2D and 3D cell macroscopic culture methods, they present physicochemical and operational challenges that impair the scale-up of drug screening by not allowing a high parallelization, multidrug combination, and high-throughput screening. Their combination and complementarity with microfluidic platforms enable the development of microfluidics-based cell culture platforms with unequivocal advantages in drug screening and cell therapies. Thus, this review presents an updated and consolidated view of cell culture miniaturization's physical, chemical, and operational considerations in the pharmaceutical research scenario. It clarifies advances in the field using gradient-based microfluidics, droplet-based microfluidics, printed-based microfluidics, digital-based microfluidics, SlipChip, and paper-based microfluidics. Finally, it presents a comparative analysis of the performance of cell-based methods in life research and development to achieve increased precision in the drug screening process.

Mechanical stimulation devices for mechanobiology studies: a market, literature, and patents review

Significant advancements in various research and technological fields have contributed to remarkable findings on the physiological dynamics of the human body. To more closely mimic the complex physiological environment, research has moved from two-dimensional (2D) culture systems to more sophisticated three-dimensional (3D) dynamic cultures. Unlike bioreactors or microfluidic-based culture models, cells are typically seeded on polymeric substrates or incorporated into 3D constructs which are mechanically stimulated to investigate cell response to mechanical stresses, such as tensile or compressive. This review focuses on the working principles of mechanical stimulation devices currently available on the market or custom-built by research groups or protected by patents and highlights the main features still open to improvement. These are the features which could be focused on to perform, in the future, more reliable and accurate mechanobiology studies.

Graphic abstract

Modularized dynamic cell culture platform for efficient production of extracellular vesicles and sequential analysis

Extracellular vesicles (EVs) are nanometer-sized particles naturally secreted by cells for intercellular communication that encapsulate bioactive cargo, such as proteins and RNA, with a lipid bilayer. Tumor cell-derived EVs (tdEVs) are particularly promising biomarkers for cancer research because their contents reflect the cell of origin. In most studies, tdEVs have been obtained from cancer cells cultured under static conditions, thus lacking the ability to recapitulate the microenvironment of cells in vivo. Recent developments in perfusable cell culture systems have allowed oxygen and a nutrient gradient to mimic the physiological and cellular microenvironment. However, as these systems are perfused by circulating the culture medium within the unified structure, independently harvesting cells and EVs at each time point for analysis presents a limitation. In this study, a modularized cell culture system is designed for the perfusion and real-time collection of EVs. The system consists of three detachable chambers, one each for fresh medium, cell culture, and EV collection. The fresh medium flows from the medium chamber to the culture chamber at a flow rate controlled by the hydraulic pressure injected with a syringe pump. When the culture medium containing EVs exceeds a certain volume within the chamber, it overflows into the collection chamber to harvest EVs. The compact and modularized chambers are highly interoperable with conventional cell culture modalities used in the laboratory, thus enabling various EV-based assays.

In vitro interactions of aerosol formulations with human nasal epithelium using real-time monitoring of drug transport in a nasal mucosa-on-a-chip

The current organ-on-chip platforms used for studying respiratory drug delivery are limited to the administration of drug solutions and suspensions, lacking the in vivo aerosol drug administration and aerosol interaction with the respiratory tract barrier. Moreover, they mostly rely on conventional assays that require sample collection and 'off the chip' analyses, which can be labor-intensive and costly. In this study, a human nasal epithelial mucosa (NEM)-on-a-chip is developed that enables the deposition of aerosolized nasal formulations while emulating realistic shear stresses (0.23 and 0.78 Pa), exerted to the inferior and middle turbinate of the human nasal cavity. Under these different dynamic conditions in the donor channel of the NEM-on-a-chip, the deposited dose of aerosols and particle size distributions varied. In addition, the increase in the shear stress to 0.78 Pa adversely affected the cells' viability, reflected by a 36.9 ± 5.4% reduction in the transepithelial electrical resistance. The epithelial transport profiles of aerosolized ibuprofen formulations under 0.23 Pa shear stress were successfully monitored in real-time by an electrochemical sensor embedded in the acceptor channel, where the NEM-on-a-chip was able to monitor the effect of permeation enhancer in the test formulation on the rate of drug transport. The novel NEM-on-a-chip can potentially be a promising physiologically relevant tool for reliable nasal aerosol testing in vitro.

A microfluidic-based approach to investigate the inflammatory response of macrophages to pristine and drug-loaded nanostructured hydroxyapatite

The in vitro biological characterization of biomaterials is largely based on static cell cultures. However, for highly reactive biomaterials such as calcium-deficient hydroxyapatite (CDHA), this static environment has limitations. Drastic alterations in the ionic composition of the cell culture medium can negatively affect cell behavior, which can lead to misleading results or data that is difficult to interpret. This challenge could be addressed by a microfluidics-based approach (i.e. on-chip), which offers the opportunity to provide a continuous flow of cell culture medium and a potentially more physiologically relevant microenvironment. The aim of this work was to explore microfluidic technology for its potential to characterize CDHA, particularly in the context of inflammation. Two different CDHA substrates (chemically identical, but varying in microstructure) were integrated on-chip and subsequently evaluated. We demonstrated that the on-chip environment can avoid drastic ionic alterations and increase protein sorption, which was reflected in cell studies with RAW 264.7 macrophages. The cells grown on-chip showed a high cell viability and enhanced proliferation compared to cells maintained under static conditions. Whereas no clear differences in the secretion of tumor necrosis factor alpha (TNF-α) were found, variations in cell morphology suggested a more anti-inflammatory environment on-chip. In the second part of this study, the CDHA substrates were loaded with the drug Trolox. We showed that it is possible to characterize drug release on-chip and moreover demonstrated that Trolox affects the TNF-α secretion and morphology of RAW 264.7 ​cells. Overall, these results highlight the potential of microfluidics to evaluate (bioactive) biomaterials, both in pristine form and when drug-loaded. This is of particular interest for the latter case, as it allows the biological characterization and assessment of drug release to take place under the same dynamic in vitro environment.

A neurovascular unit-on-a-chip: culture and differentiation of human neural stem cells in a three-dimensional microfluidic environment

Biological studies typically rely on a simple monolayer cell culture, which does not reflect the complex functional characteristics of human tissues and organs, or their real response to external stimuli. Microfluidic technology has advantages of high-throughput screening, accurate control of the fluid velocity, low cell consumption, long-term culture, and high integration. By combining the multipotential differentiation of neural stem cells with high throughput and the integrated characteristics of microfluidic technology, an in vitro model of a functionalized neurovascular unit was established using human neural stem cell-derived neurons, astrocytes, oligodendrocytes, and a functional microvascular barrier. The model comprises a multi-layer vertical neural module and vascular module, both of which were connected with a syringe pump. This provides controllable conditions for cell inoculation and nutrient supply, and simultaneously simulates the process of ischemic/hypoxic injury and the process of inflammatory factors in the circulatory system passing through the blood-brain barrier and then acting on the nerve tissue in the brain. The in vitro functionalized neurovascular unit model will be conducive to central nervous system disease research, drug screening, and new drug development.

The VersaLive platform enables microfluidic mammalian cell culture for versatile applications

Microfluidic-based cell culture allows for precise spatio-temporal regulation of microenvironment, live cell imaging and better recapitulation of physiological conditions, while minimizing reagents’ consumption. Despite their usefulness, most microfluidic systems are designed with one specific application in mind and usually require specialized equipment and expertise for their operation. All these requirements prevent microfluidic-based cell culture to be widely adopted. Here, we designed and implemented a versatile and easy-to-use perfusion cell culture microfluidic platform for multiple applications (VersaLive) requiring only standard pipettes. Here, we showcase the multiple uses of VersaLive (e.g., time-lapse live cell imaging, immunostaining, cell recovery, cell lysis, plasmid transfection) in mammalian cell lines and primary cells. VersaLive could replace standard cell culture formats in several applications, thus decreasing costs and increasing reproducibility across laboratories. The layout, documentation and protocols are open-source and available online at https://versalive.tigem.it/.

VersaLive is a versatile microfluidic platform with flexible input modes and low-volume media reservoirs that can be used for time-lapse live cell imaging, immunostaining, cell recovery, cell lysis and plasmid transfection.

Vascularized pancreas-on-a-chip device produced using a printable simulated extracellular matrix

The extracellular matrix (ECM) influences cellular behavior, function, and fate. The ECM surrounding Langerhans islets has not been investigated in detail to explain its role in the development and maturation of pancreatic β-cells. Herein, a complex combination of the simulated ECM (sECM) has been examined with a comprehensive analysis of cell response and a variety of controls. The most promising results were obtained from group containing fibrin, collagen type I, Matrigel®, hyaluronic acid, methylcellulose, and two compounds of functionalized, ionically crosslinking bacterial cellulose (sECMbc). Even though the cell viability was not significantly impacted, the performance of group of sECMbc showed 2 to 4x higher sprouting number and length, 2 to 4x higher insulin secretion in static conditions, and 2 to 10x higher gene expression of VEGF-A, Endothelin-1, and NOS3 than the control group of fibrin matrix (sECMf). Each material was tested in a hydrogel-based, perfusable, pancreas-on-a-chip device and the best group - sECMbc has been tested with the drug Sunitinib to show the extended possibilities of the device for both diabetes-like screening as well as PDAC chemotherapeutics screening for potential personal medicine approach. It proved its functionality in 7 days dynamic culture and is suitable as a physiological tissue model. Moreover, the device with the pancreatic-like spheroids was 3D bioprintable and perfusable.

Large Area Microfluidic Bioreactor for Production of Recombinant Protein

To produce innovative biopharmaceuticals, highly flexible, adaptable, robust, and affordable bioprocess platforms for bioreactors are essential. In this article, we describe the development of a large-area microfluidic bioreactor (LM bioreactor) for mammalian cell culture that works at laminar flow and perfusion conditions. The 184 cm² 32 cisterns LM bioreactor is the largest polydimethylsiloxane (PDMS) microfluidic device fabricated by photopolymer flexographic master mold methodology, reaching a final volume of 2.8 mL. The LM bioreactor was connected to a syringe pump system for culture media perfusion, and the cells’ culture was monitored by photomicrograph imaging. CHO-ahIFN-α2b adherent cell line expressing the anti-hIFN-a2b recombinant scFv-Fc monoclonal antibody (mAb) for the treatment of systemic lupus erythematosus were cultured on the LM bioreactor. Cell culture and mAb production in the LM bioreactor could be sustained for 18 days. Moreover, the anti-hIFN-a2b produced in the LM bioreactor showed higher affinity and neutralizing antiproliferative activity compared to those mAbs produced in the control condition. We demonstrate for the first-time, a large area microfluidic bioreactor for mammalian cell culture that enables a controlled microenvironment suitable for the development of high-quality biologics with potential for therapeutic use.

MatriGrid® Based Biological Morphologies: Tools for 3D Cell Culturing

Recent trends in 3D cell culturing has placed organotypic tissue models at another level. Now, not only is the microenvironment at the cynosure of this research, but rather, microscopic geometrical parameters are also decisive for mimicking a tissue model. Over the years, technologies such as micromachining, 3D printing, and hydrogels are making the foundation of this field. However, mimicking the topography of a particular tissue-relevant substrate can be achieved relatively simply with so-called template or morphology transfer techniques. Over the last 15 years, in one such research venture, we have been investigating a micro thermoforming technique as a facile tool for generating bioinspired topographies. We call them MatriGrid^®s. In this research account, we summarize our learning outcome from this technique in terms of the influence of 3D micro morphologies on different cell cultures that we have tested in our laboratory. An integral part of this research is the evolution of unavoidable aspects such as possible label-free sensing and fluidic automatization. The development in the research field is also documented in this account.

A Temperature-Based Easy-Separable (TempEasy) 3D Hydrogel Coculture System

Interactions between different cell types are crucial for their behavior in tissues, but are rarely considered in 3D in vitro cell culture experiments. One reason is that such coculture experiments are sometimes difficult to perform in 3D or require specialized equipment or know-how. Here, a new 3D cell coculture system is introduced, TempEasy, which is readily applied in any cell culture lab. The matrix material is based on polyisocyanide hydrogels, which closely resemble the mechanical characteristics of the natural extracellular matrix. Gels with different gelation temperatures, seeded with different cells, are placed on top of each other to form an indirect coculture. Cooling reverses gelation, allowing cell harvesting from each layer separately, which benefits downstream analysis. To demonstrate the potential of TempEasy , human adipose stem cells (hADSCs) with vaginal epithelial fibroblasts are cocultured. The analysis of a 7-day coculture shows that hADSCs promote cell-cell interaction of fibroblasts, while fibroblasts promote proliferation and differentiation of hADSCs. TempEasy provides a straightforward operational platform for indirect cocultures of cells of different lineages in well-defined microenvironments.

Perfused Platforms to Mimic Bone Microenvironment at the Macro/Milli/Microscale: Pros and Cons

As life expectancy increases, the population experiences progressive ageing. Ageing, in turn, is connected to an increase in bone-related diseases (i.e., osteoporosis and increased risk of fractures). Hence, the search for new approaches to study the occurrence of bone-related diseases and to develop new drugs for their prevention and treatment becomes more pressing. However, to date, a reliable in vitro model that can fully recapitulate the characteristics of bone tissue, either in physiological or altered conditions, is not available. Indeed, current methods for modelling normal and pathological bone are poor predictors of treatment outcomes in humans, as they fail to mimic the in vivo cellular microenvironment and tissue complexity. Bone, in fact, is a dynamic network including differently specialized cells and the extracellular matrix, constantly subjected to external and internal stimuli. To this regard, perfused vascularized models are a novel field of investigation that can offer a new technological approach to overcome the limitations of traditional cell culture methods. It allows the combination of perfusion, mechanical and biochemical stimuli, biological cues, biomaterials (mimicking the extracellular matrix of bone), and multiple cell types. This review will discuss macro, milli, and microscale perfused devices designed to model bone structure and microenvironment, focusing on the role of perfusion and encompassing different degrees of complexity. These devices are a very first, though promising, step for the development of 3D in vitro platforms for preclinical screening of novel anabolic or anti-catabolic therapeutic approaches to improve bone health.

Microfluidic Platforms Designed for Morphological and Photosynthetic Investigations of Chlamydomonas reinhardtii on a Single-Cell Level

Chlamydomonas reinhardtii is a model organism of increasing biotechnological importance, yet, the evaluation of its life cycle processes and photosynthesis on a single-cell level is largely unresolved. To facilitate the study of the relationship between morphology and photochemistry, we established microfluidics in combination with chlorophyll a fluorescence induction measurements. We developed two types of microfluidic platforms for single-cell investigations: (i) The traps of the “Tulip” device are suitable for capturing and immobilizing single cells, enabling the assessment of their photosynthesis for several hours without binding to a solid support surface. Using this “Tulip” platform, we performed high-quality non-photochemical quenching measurements and confirmed our earlier results on bulk cultures that non-photochemical quenching is higher in ascorbate-deficient mutants (Crvtc2-1) than in the wild-type. (ii) The traps of the “Pot” device were designed for capturing single cells and allowing the growth of the daughter cells within the traps. Using our most performant “Pot” device, we could demonstrate that the FV/FM parameter, an indicator of photosynthetic efficiency, varies considerably during the cell cycle. Our microfluidic devices, therefore, represent versatile platforms for the simultaneous morphological and photosynthetic investigations of C. reinhardtii on a single-cell level.

Advances in microfluidic 3D cell culture for preclinical drug development

Drug development is often a very long, costly, and risky process due to the lack of reliability in the preclinical studies. Traditional current preclinical models, mostly based on 2D cell culture and animal testing, are not full representatives of the complex in vivo microenvironments and often fail. In order to reduce the enormous costs, both financial and general well-being, a more predictive preclinical model is needed. In this chapter, we review recent advances in microfluidic 3D cell culture showing how its development has allowed the introduction of in vitro microphysiological systems, laying the foundation for organ-on-a-chip technology. These findings provide the basis for numerous preclinical drug discovery assays, which raise the possibility of using micro-engineered systems as emerging alternatives to traditional models, based on 2D cell culture and animals.

A novel design of microfluidic platform for metronomic combinatorial chemotherapy drug screening based on 3D tumor spheroid model

For treating cancer at various stages, chemotherapy drugs administered in combination provide better treatment results with lower side effects compared to single-drug therapy. However, finding the potential drug combinations has been challenging due to the large numbers of possible combinations from approved drugs and the failure of in vitro 2D well plate-based cancer models. 3D spheroid-based high-throughput microfluidic platforms recapitulate some of the important features of native tumor tissue and offer a promising alternative to evaluate the combinatory effects of the drugs. This study develops a novel polydimethylsiloxane (PDMS) based microfluidic design with a dynamic environment and strategically placed U-shaped wells for testing all seven possible combinations (three single-drug treatments, three pairwise combinations, treatment with all three drugs) of three chemotherapy drugs (Paclitaxel, Vinorelbine, and Etoposide) on lung tumor spheroids. The design of U-shaped wells has been validated with computational results. Firstly, we test all combinations of drugs on the conventional well plate in static conditions with 3D tumor spheroids. Based on static drug testing results, we show a proof-of-concept by testing the most effective drug combination on the microfluidic device in a dynamic environment. The concentration of the drugs used in combination falls below the maximum tolerated dose (MTD) of the individual drugs, towards low dose metronomic (LDM) chemotherapy. LDM combinatorial chemotherapy identified in this study can potentially lower toxicity and provide better treatment results in cancer patients. The device can be further used to culture patient-specific tumor spheroids and identify synergistic drug combinations for personalized medicine.

Bridging the academia-to-industry gap: organ-on-a-chip platforms for safety and toxicology assessment

Some organ-on-a-chip (OoC) systems for drug evaluation show better predictive capabilities than planar, static cell cultures and animal models. One of the ongoing initiatives led by OoC developers is to bridge the academia-to-industry gap in the hope of gaining wider adoption by end-users - academic biological researchers and industry. We discuss several recommendations that can help to drive the adoption of OoC systems by the market. We first review some key challenges faced by OoC developers before highlighting current advances in OoC platforms. We then offer recommendations for OoC developers to promote the uptake of OoC systems by the industry.

Piezoelectric Silicon Micropump for Drug Delivery Applications

Subcutaneous injection is crucial for the treatment of many diseases. Especially for regular or continuous injections, automated dosing is beneficial. However, existing devices are large, uncomfortable, visible under clothing, or interfere with physical activity. Thus, the development of small, energy efficient and reliable patch pumps or implantable systems is necessary and research on microelectromechanical system (MEMS) based drug delivery devices has gained increasing interest. However, the requirements of medical applications are challenging and especially the dosing precision and reliability of MEMS pumps are not yet sufficiently evaluated. To enable further miniaturization, we propose a precise 5 × 5 mm2 silicon micropump. Detailed experimental evaluation of ten pumps proves a backpressure capability with air of 12.5 ± 0.8 kPa, which indicates the ability to transport bubbles. The maximal water flow rate is 74 ± 6 µL/min and the pumps’ average blocking pressure is 51 kPa. The evaluation of the dosing precision for bolus deliveries with water and insulin shows a high repeatability of dosed package volumes. The pumps show a mean standard deviation of only 0.02 mg for 0.5 mg packages, and therefore, stay below the generally accepted 5% deviation, even for this extremely small amount. The high precision enables the combination with higher concentrated medication and is the foundation for the development of an extremely miniaturized patch pump.

Microfluidic Devices as Process Development Tools for Cellular Therapy Manufacturing

Cellular therapies are creating a paradigm shift in the biomanufacturing industry. Particularly for autologous therapies, small-scale processing methods are better suited than the large-scale approaches that are traditionally employed in the industry. Current small-scale methods for manufacturing personalized cell therapies, however, are labour-intensive and involve a number of 'open events'. To overcome these challenges, new cell manufacturing platforms following a GMP-in-a-box concept have recently come on the market (GMP: Good Manufacturing Practice). These are closed automated systems with built-in pumps for fluid handling and sensors for in-process monitoring. At a much smaller scale, microfluidic devices exhibit many of the same features as current GMP-in-a-box systems. They are closed systems, fluids can be processed and manipulated, and sensors integrated for real-time detection of process variables. Fabricated from polymers, they can be made disposable, i.e. single-use. Furthermore, microfluidics offers exquisite spatiotemporal control over the cellular microenvironment, promising both reproducibility and control of outcomes. In this chapter, we consider the challenges in cell manufacturing, highlight recent advances of microfluidic devices for each of the main process steps, and summarize our findings on the current state of the art. As microfluidic cell culture devices have been reported for both adherent and suspension cell cultures, we report on devices for the key process steps, or unit operations, of both stem cell therapies and cell-based immunotherapies.

A continuous flow cell culture system for precision cell stimulation and time-resolved profiling of cell secretion

Cells exchange substances with their surroundings during metabolism, signaling, and other functions. These fluxes are dynamic, changing in response to external cues and internal programs. Static cultures are inadequate for measuring these dynamics because the environments of the cells change as substances accumulate or deplete from medium, unintentionally affecting cell behavior. Static cultures offer limited time resolution due to the impracticality of frequent or prolonged manual sampling, and cannot expose cells to smooth, transient changes in stimulus concentrations. In contrast, perfusion cultures constantly maintain cellular environments and continuously sample the effluent stream. Existing perfusion culture systems are either microfluidic, which are difficult to make and use, or macrofluidic devices built from custom parts that neglect solute dispersion. In this study, a multiplexed macrofluidic perfusion culture platform was developed to measure secretion and absorption rates of substances by cells in a temporally controlled environment. The modular platform handles up to 31 streams with automated fraction collection. This paper presents the assembly of this dynamic bioreactor from commercially available parts, and a method for quantitatively handling the effects of dispersion using residence time distributions. The system is then applied to monitor the secretion of a circadian clock gene-driven reporter from engineered cells.

A scalable and sensitive steatosis chip with long-term perfusion of in situ differentiated HepaRG organoids

Nonalcoholic fatty liver disease (NAFLD) is a significant liver disease without approved therapy, lacking human NAFLD models to aid drug development. Existing models are either under-performing or too complex to allow robust drug screening. Here we have developed a 100-well drug testing platform with improved HepaRG organoids formed with uniform size distribution, and differentiated in situ in a perfusion microfluidic device, SteatoChip, to recapitulate major NAFLD features. Compared with the pre-differentiated spheroids, the in situ differentiated HepaRG organoids with perfusion experience well-controlled chemical and mechanical microenvironment, and 3D cellular niche, to exhibit enhanced hepatic differentiation (albumin⁺ cells ratio: 66.2% in situ perfusion vs 46.1% pre-differentiation), enriched and uniform hepatocyte distribution in organoids, higher level of hepatocyte functions (5.2 folds in albumin secretion and 7.6 folds in urea synthesis), enhanced cell polarity and bile canaliculi structures. When induced with free fatty acid (FFA), cells exhibit significantly higher level of lipid accumulation (6.6 folds for in situ perfusion vs 4.4 folds for pre-differentiation), altered glucose regulation and reduced Akt phosphorylation in the organoids. SteatoChip detects reduction of steatosis when cells are incubated with three different anti-steatosis compounds, 78.5% by metformin hydrochloride, 71.3% by pioglitazone hydrochloride and 66.6% by obeticholic acid, versus the control FFA-free media (38% reduction). The precision microenvironment control in SteatoChip enables improved formation, differentiation, and function of HepaRG organoids to serve as a scalable and sensitive drug testing platform, to potentially accelerate the NAFLD drug development.

Combined use of calcium phosphate cement, mesenchymal stem cells and platelet-rich plasma for bone regeneration in critical-size defect of the femoral condyle in mini-pigs

Aim: To investigate the outcome of autologous bone marrow mesenchymal stem cells (BMMSCs) and platelet-rich plasma in combination with calcium phosphate cement (CPC) scaffold to reconstruct femoral critical bone defects in mini-pigs. Materials & methods: Scanning electron microscopy, micro-computed tomography evaluation and quantitative histological assessment were used. Results & conclusion: BMMSCs were attached to the CPC scaffold after 7 days of culture and decreased the residual CPC material in each group at 12 weeks compared with 6 weeks. The newly formed bone area was higher in the CPC+SC+P group than in the CPC group at each time point (all p < 0.05). The strategy of CPC combined with BMMSCs and platelet-rich plasma might be an effective method to repair bone defects.

Bacteriophage uptake by mammalian cell layers represents a potential sink that may impact phage therapy

It is increasingly apparent that bacteriophages, viruses that infect bacteria and more commonly referred to as simply phages, have tropisms outside their bacterial hosts. Using live tissue culture cell imaging, we demonstrate that cell type, phage size, and morphology play a major role in phage internalization. Uptake was validated under physiological conditions using a microfluidic device. Phages adhered to mammalian tissues, with adherent phages being subsequently internalized by macropinocytosis, with functional phages accumulating intracellularly. We incorporated these results into a pharmacokinetic model demonstrating the potential impact of phage accumulation by cell layers, which represents a potential sink for circulating phages in the body. During phage therapy, high doses of phages are directly administered to a patient in order to treat a bacterial infection, thereby facilitating broad interactions between phages and mammalian cells. Understanding these interactions will have important implications on innate immune responses, phage pharmacokinetics, and the efficacy of phage therapy.

Lung carcinoma spheroids embedded in a microfluidic platform

Three-dimensional (3D) spheroid cell cultures are excellent models used in cancer biology research and drug screening. The objective of this study was to develop a lung carcinoma spheroid based microfluidic platform with perfusion function to mimic lung cancer pathology and investigate the effect of a potential drug molecule, panaxatriol. Spheroids were successfully formed on agar microtissue molds at the end of 10 days, reaching an average diameter of about 317.18 ± 4.05 μm and subsequently transferred to 3D dynamic microfluidic system with perfusion function. While the size of the 3D spheroids embedded in the Matrigel matrix in the platform had gradually increased both in the static and dynamic control groups, the size of the spheroids were reduced and fragmented in the drug treated groups. Cell viability results showed that panaxatriol exhibited higher cytotoxic effect on cancer cells than healthy cells and the IC₅₀ value was determined as 61.55 µM. Furthermore, panaxatriol has been more effective on single cells around the spheroid structure, whereas less in 3D spheroid tissues with a compact structure in static conditions compared to dynamic systems, where a flow rate of 2 µL/min leading to a shear stress of 0.002 dyne/cm² was applied. Application of such dynamic systems will contribute to advancing basic research and increasing the predictive accuracy of potential drug molecules, which may accelerate the translation of novel therapeutics to the clinic, possibly decreasing the use of animal models.

Supplementary Information

The online version contains supplementary material available at 10.1007/s10616-021-00470-7.

Microfluidic In Vitro Platform for (Nano)Safety and (Nano)Drug Efficiency Screening

Microfluidic technology is a valuable tool for realizing more in vitro models capturing cellular and organ level responses for rapid and animal-free risk assessment of new chemicals and drugs. Microfluidic cell-based devices allow high-throughput screening and flexible automation while lowering costs and reagent consumption due to their miniaturization. There is a growing need for faster and animal-free approaches for drug development and safety assessment of chemicals (Registration, Evaluation, Authorisation and Restriction of Chemical Substances, REACH). The work presented describes a microfluidic platform for in vivo-like in vitro cell cultivation. It is equipped with a wafer-based silicon chip including integrated electrodes and a microcavity. A proof-of-concept using different relevant cell models shows its suitability for label-free assessment of cytotoxic effects. A miniaturized microscope within each module monitors cell morphology and proliferation. Electrodes integrated in the microfluidic channels allow the noninvasive monitoring of barrier integrity followed by a label-free assessment of cytotoxic effects. Each microfluidic cell cultivation module can be operated individually or be interconnected in a flexible way. The interconnection of the different modules aims at simulation of the whole-body exposure and response and can contribute to the replacement of animal testing in risk assessment studies in compliance with the 3Rs to replace, reduce, and refine animal experiments.

Cultivating Multidisciplinarity: Manufacturing and Sensing Challenges in Cultured Meat Production

Meat cultivation via cellular agriculture holds great promise as a method for future food production. In theory, it is an ideal way of meat production, humane to the animals and sustainable for the environment, while keeping the same taste and nutritional values as traditional meat and having additional benefits such as controlled fat content and absence of antibiotics and hormones used in the traditional meat industry. However, in practice, there is still a number of challenges, such as those associated with the upscale of cultured meat (CM). CM food safety monitoring is a necessary factor when envisioning both the regulatory compliance and consumer acceptance. To achieve this, a multidisciplinary approach is necessary. This includes extensive development of the sensitive and specific analytical devices i.e., sensors to enable reliable food safety monitoring throughout the whole future food supply chain. In addition, advanced monitoring options can help in the further optimization of the meat cultivation which may reduce the currently still high costs of production. This review presents an overview of the sensor monitoring options for the most relevant parameters of importance for meat cultivation. Examples of the various types of sensors that can potentially be used in CM production are provided and the options for their integration into bioreactors, as well as suggestions on further improvements and more advanced integration approaches. In favor of the multidisciplinary approach, we also include an overview of the bioreactor types, scaffolding options as well as imaging techniques relevant for CM research. Furthermore, we briefly present the current status of the CM research and related regulation, societal aspects and challenges to its upscaling and commercialization.

A flexible and cost-effective manual droplet operation platform for miniaturized cell assays and single cell analysis

Herein, we developed a flexible and cost-effective manual droplet operation system (MDOS) for performing miniaturized cell assays as well as single cell analysis. The MDOS consists of a manual x-y-z translation stage for liquid transferring and switching, a high-precision syringe pump for liquid driving and metering, a tapered capillary probe for droplet manipulation, a droplet array chip for droplet loading and reaction, sample/reagent reservoirs for storage, and a microscope for droplet observation, with a total expense of only $4,000. By using the flexible combination of three elementary operations of the x-y-z stage's moving and the pump's aspirating and depositing, the MDOS can manually achieve multiple droplet handling operations in the nanoliter to picoliter range, including droplet generation, assembling, fusion, diluting, and splitting. On this basis, multiple cell-related operations could be performed, such as nanoliter-scale in-droplet cell culture, cell coculture, drug stimulation, cell washing, and cell staining, as well as formation of picoliter single-cell droplets. The feasibility and flexibility of the MDOS was demonstrated in multi-mode miniaturized cell assays, including cell-based drug test, first-pass effect assay, and single-cell enzyme assay. The MDOS with the features of low cost, easy to build and flexible to use, could provide a promising alternative for performing miniaturized assays in routine laboratories, in addition to conventional microfluidic chip-based systems and automated robot systems.

Mechanical loading and the control of stem cell behavior

OBJECTIVE

Mechanical stimulation regulates many cell responses. The present study describes the effects of different in vitro mechanical stimulation approaches on stem cell behavior.

DESIGN

The narrative review approach was performed. The articles published in English language that addressed the effects of mechanical force on stem cells were searched on Pubmed and Scopus database. The effects of extrinsic mechanical force on stem cell response was reviewed and discussed.

RESULTS

Cells sense mechanical stimuli by the function of mechanoreceptors and further transduce force stimulation into intracellular signaling. Cell responses to mechanical stimuli depend on several factors including type, magnitude, and duration. Further, similar mechanical stimuli exhibit distinct cell responses based on numerous factors including cell type and differentiation stage. Various mechanical applications modulate stemness maintenance and cell differentiation toward specific lineages.

CONCLUSIONS

Mechanical force application modulates stemness maintenance and differentiation. Modification of force regimens could be utilized to precisely control appropriate stem cell behavior toward specific applications.

Real-time monitoring of liver fibrosis through embedded sensors in a microphysiological system

Hepatic fibrosis is a foreshadowing of future adverse events like liver cirrhosis, liver failure, and cancer. Hepatic stellate cell activation is the main event of liver fibrosis, which results in excessive extracellular matrix deposition and hepatic parenchyma's disintegration. Several biochemical and molecular assays have been introduced for in vitro study of the hepatic fibrosis progression. However, they do not forecast real-time events happening to the in vitro models. Trans-epithelial electrical resistance (TEER) is used in cell culture science to measure cell monolayer barrier integrity. Herein, we explored TEER measurement's utility for monitoring fibrosis development in a dynamic cell culture microphysiological system. Immortal HepG2 cells and fibroblasts were co-cultured, and transforming growth factor β1 (TGF-β1) was used as a fibrosis stimulus to create a liver fibrosis-on-chip model. A glass chip-based embedded TEER and reactive oxygen species (ROS) sensors were employed to gauge the effect of TGF-β1 within the microphysiological system, which promotes a positive feedback response in fibrosis development. Furthermore, albumin, Urea, CYP450 measurements, and immunofluorescent microscopy were performed to correlate the following data with embedded sensors responses. We found that chip embedded electrochemical sensors could be used as a potential substitute for conventional end-point assays for studying fibrosis in microphysiological systems.

Cilia density and flow velocity affect alignment of motile cilia from brain cells

In many organs, thousands of microscopic ‘motile cilia’ beat in a coordinated fashion generating fluid flow. Physiologically, these flows are important in both development and homeostasis of ciliated tissues. Combining experiments and simulations, we studied how cilia from brain tissue align their beating direction. We subjected cilia to a broad range of shear stresses, similar to the fluid flow that cilia themselves generate, in a microfluidic setup. In contrast to previous studies, we found that cilia from mouse ependyma respond and align to these physiological shear stress at all maturation stages. Cilia align more easily earlier in maturation, and we correlated this property with the increase in multiciliated cell density during maturation. Our numerical simulations show that cilia in densely packed clusters are hydrodynamically screened from the external flow, in agreement with our experimental observation. Cilia carpets create a hydrodynamic screening that reduces the susceptibility of individual cilia to external flows.

Summary: Alignment of motile cilia in mammalian brains is essential for transport of fluids as described in an in vitro model of the developing brain.

The label-free separation and culture of tumor cells in a microfluidic biochip

Circulating tumor cells (CTCs) from liquid biopsy have shown a strong correlation to the clinical outcome of cancer patients. The enumeration and cytological analysis of CTCs have attracted increasing efforts for cancer disease management amid immunotherapy and personalized medicine. However, both enumeration and cytological analysis are challenging due to the rarity of CTCs and the lack of integrated solutions for the minimal risk of cell loss in the course of CTC procurement. We report a simple microfluidic chip permitting a one-stop solution for streamlining the on-chip cell separation, capture, immunofluorescence assay and/or in situ culture of isolated cells devoid of risky manual steps. Our results showed effective trapping of single cells, doublets and cell lumps isolated from blood in the same device. On-chip immunostaining revealed normal cell morphology and the characterization of cell expansion uncovered an altered cell growth curve with a reduced lag phase as compared to the conventional culture despite closely matching cell growth rates. The cells were viable and functional for as long as 11 days inside our chip and cell migration was also readily observed, with lumps showing greater aggressiveness than single cells. With these results, we expect promising applications of our one-stop solution for liquid biopsy via CTCs.

Remodeling of the Microvasculature: May the Blood Flow Be With You

The vasculature ensures optimal delivery of nutrients and oxygen throughout the body, and to achieve this function it must continually adapt to varying tissue demands. Newly formed vascular plexuses during development are immature and require dynamic remodeling to generate well-patterned functional networks. This is achieved by remodeling of the capillaries preserving those which are functional and eliminating other ones. A balanced and dynamically regulated capillary remodeling will therefore ensure optimal distribution of blood and nutrients to the tissues. This is particularly important in pathological contexts in which deficient or excessive vascular remodeling may worsen tissue perfusion and hamper tissue repair. Blood flow is a major determinant of microvascular reshaping since capillaries are pruned when relatively less perfused and they split when exposed to high flow in order to shape the microvascular network for optimal tissue perfusion and oxygenation. The molecular machinery underlying blood flow sensing by endothelial cells is being deciphered, but much less is known about how this translates into endothelial cell responses as alignment, polarization and directed migration to drive capillary remodeling, particularly in vivo. Part of this knowledge is theoretical from computational models since blood flow hemodynamics are not easily recapitulated by in vitro or ex vivo approaches. Moreover, these events are difficult to visualize in vivo due to their infrequency and briefness. Studies had been limited to postnatal mouse retina and vascular beds in zebrafish but new tools as advanced microscopy and image analysis are strengthening our understanding of capillary remodeling. In this review we introduce the concept of remodeling of the microvasculature and its relevance in physiology and pathology. We summarize the current knowledge on the mechanisms contributing to capillary regression and to capillary splitting highlighting the key role of blood flow to orchestrate these processes. Finally, we comment the potential and possibilities that microfluidics offers to this field. Since capillary remodeling mechanisms are often reactivated in prevalent pathologies as cancer and cardiovascular disease, all this knowledge could be eventually used to improve the functionality of capillary networks in diseased tissues and promote their repair.

Microfluidic organ-on-a-chip models of human liver tissue

The liver is the largest internal organ of the body with complex microarchitecture and function that plays critical roles in drug metabolism. Hepatotoxicity and drug-induced liver injury (DILI) caused by various drugs is the main reason for late-stage drug failures. Moreover, liver diseases are among the leading causes of death in the world, with the number of new cases arising each year. Although animal models have been used to understand human drug metabolism and toxicity before clinical trials, tridimensional microphysiological systems, such as liver-on-a-chip (Liver Chip) platforms, could better recapitulate features of human liver physiology and pathophysiology and thus, are often more predictive of human outcome. Liver Chip devices have shown promising results in mimicking in vivo condition by recapitulating the sinusoidal structure of the liver, maintaining high cell viability and cellular phenotypes, and emulating native liver functions. Here, we first review the cellular constituents and physiology of the liver and then critically discuss the state-of-the-art chip-based liver models and their applications in drug screening, disease modeling, and regenerative medicine. We finally address the pending issues of existing platforms and touch upon future directions for developing new, advanced on-chip models.

Voltage-gated ion channels mediate the electrotaxis of glioblastoma cells in a hybrid PMMA/PDMS microdevice

Transformed astrocytes in the most aggressive form cause glioblastoma, the most common cancer in the central nervous system with high mortality. The physiological electric field by neuronal local field potentials and tissue polarity may guide the infiltration of glioblastoma cells through the electrotaxis process. However, microenvironments with multiplex gradients are difficult to create. In this work, we have developed a hybrid microfluidic platform to study glioblastoma electrotaxis in controlled microenvironments with high throughput quantitative analysis by machine learning-powered single cell tracking software. By equalizing the hydrostatic pressure difference between inlets and outlets of the microchannel, uniform single cells can be seeded reliably inside the microdevice. The electrotaxis of two glioblastoma models, T98G and U-251MG, requires an optimal laminin-containing extracellular matrix and exhibits opposite directional and electro-alignment tendencies. Calcium signaling is a key contributor in glioblastoma pathophysiology but its role in glioblastoma electrotaxis is still an open question. Anodal T98G electrotaxis and cathodal U-251MG electrotaxis require the presence of extracellular calcium cations. U-251MG electrotaxis is dependent on the P/Q-type voltage-gated calcium channel (VGCC) and T98G is dependent on the R-type VGCC. U-251MG electrotaxis and T98G electrotaxis are also mediated by A-type (rapidly inactivating) voltage-gated potassium channels and acid-sensing sodium channels. The involvement of multiple ion channels suggests that the glioblastoma electrotaxis is complex and patient-specific ion channel expression can be critical to develop personalized therapeutics to fight against cancer metastasis. The hybrid microfluidic design and machine learning-powered single cell analysis provide a simple and flexible platform for quantitative investigation of complicated biological systems.

Pumpless microfluidic device with open top cell culture under oscillatory shear stress

Here we developed a 96-well plate-based pumpless microfluidic device to mimic bidirectional oscillatory shear stress experienced by osteoblasts at the endosteal niche located at the interface between bone and bone marrow. The culture device was designed to be high-throughput with 32 open top culture chambers for convenient cell seeding and staining. Mathematical modeling was used to simulate the control of oscillatory shear stress with the peak stress in the range of 0.3 to 50 mPa. Osteoblasts, cultured under oscillatory shear stress, were found to be highly viable and significantly aligned along the direction of flow. The modeling and experimental results demonstrate for the first time that cells can be cultured under controllable oscillatory shear stress in the open top culture chamber and pumpless configurations.

Rapid Fabrication of Membrane-Integrated Thermoplastic Elastomer Microfluidic Devices

Leveraging the advantageous material properties of recently developed soft thermoplastic elastomer materials, this work presents the facile and rapid fabrication of composite membrane-integrated microfluidic devices consisting of Flexdym^TM polymer and commercially available porous polycarbonate membranes. The three-layer devices can be fabricated in under 2.5 h, consisting of a 2-min hot embossing cycle, conformal contact between device layers and a low-temperature baking step. The strength of the Flexdym^TM-polycarbonate seal was characterized using a specialized microfluidic delamination device and an automated pressure controller configuration, offering a standardized and high-throughput method of microfluidic burst testing. Given a minimum bonding distance of 200 μm, the materials showed bonding that reliably withstood pressures of 500 mbar and above, which is sufficient for most microfluidic cell culture applications. Bonding was also stable when subjected to long term pressurization (10 h) and repeated use (10,000 pressure cycles). Cell culture trials confirmed good cell adhesion and sustained culture of human dermal fibroblasts on a polycarbonate membrane inside the device channels over the course of one week. In comparison to existing porous membrane-based microfluidic platforms of this configuration, most often made of polydimethylsiloxane (PDMS), these devices offer a streamlined fabrication methodology with materials having favourable properties for cell culture applications and the potential for implementation in barrier model organ-on-chips.

Microfluidic perfusion modulates growth and motor neuron differentiation of stem cell aggregates

Microfluidic technologies provide many advantages for studying differentiation of three-dimensional (3D) stem cell aggregates, including the ability to control the culture microenvironment, isolate individual aggregates for longitudinal tracking, and perform imaging-based assays. However, applying microfluidics to studying mechanisms of stem cell differentiation requires an understanding of how microfluidic culture conditions impact cell phenotypes. Conventional cell culture techniques cannot directly be applied to the microscale, as microscale culture varies from macroscale culture in multiple aspects. Therefore, the objective of this work was to explore key parameters in microfluidic culture of 3D stem cell aggregates and to understand how these parameters influence stem cell behavior and differentiation. These studies were done in the context of differentiation of embryonic stem cells (ESCs) to motor neurons (MNs). We assessed how media exchange frequency modulates the biochemical microenvironment, including availability of exogenous factors (e.g. nutrients, small molecule additives) and cell-secreted molecules, and thereby impacts differentiation. The results of these studies provide guidance on how key characteristics of 3D cell cultures can be considered when designing microfluidic culture parameters. We demonstrate that discontinuous perfusion is effective at supporting stem cell aggregate growth. We find that there is a balance between the frequency of media exchange, which is needed to ensure that cells are not nutrient-limited, and the need to allow accumulation of cell-secreted factors to promote differentiation. Finally, we show how microfluidic device geometries can influence transport of biomolecules and potentially promote asymmetric spatial differentiation. These findings are instructive for future work in designing devices and experiments for culture of cell aggregates.