A 3D neurovascular microfluidic model consisting of neurons, astrocytes and cerebral endothelial cells as a blood-brain barrier

Organ-on-a-Chip systems for new drugs development

Research on alternatives to the use of animal models and cell cultures has led to the creation of organ-on-a-chip systems, in which organs and their physiological reactions to the presence of external stimuli are simulated. These systems could even replace the use of human beings as subjects for the study of drugs in clinical phases and have an impact on personalized therapies. Organ-on-a-chip technology present higher potential than traditional cell cultures for an appropriate prediction of functional impairments, appearance of adverse effects, the pharmacokinetic and toxicological profile and the efficacy of a drug. This potential is given by the possibility of placing different cell lines in a three-dimensional-arranged polymer piece and simulating and controlling specific conditions. Thus, the normal functioning of an organ, tissue, barrier, or physiological phenomenon can be simulated, as well as the interrelation between different systems. Furthermore, this alternative allows the study of physiological and pathophysiological processes. Its design combines different disciplines such as materials engineering, cell cultures, microfluidics and physiology, among others. This work presents the main considerations of OoC systems, the materials, methods and cell lines used for their design, and the conditions required for their proper functioning. Examples of applications and main challenges for the development of more robust systems are shown. This non-systematic review is intended to be a reference framework that facilitates research focused on the development of new OoC systems, as well as their use as alternatives in pharmacological, pharmacokinetic and toxicological studies.

Development and Application of Endothelial Cells Derived From Pluripotent Stem Cells in Microphysiological Systems Models

The vascular endothelium is present in all organs and blood vessels, facilitates the exchange of nutrients and waste throughout different organ systems in the body, and sets the tone for healthy vessel function. Mechanosensitive in nature, the endothelium responds to the magnitude and temporal waveform of shear stress in the vessels. Endothelial dysfunction can lead to atherosclerosis and other diseases. Modeling endothelial function and dysfunction in organ systems in vitro, such as the blood-brain barrier and tissue-engineered blood vessels, requires sourcing endothelial cells (ECs) for these biomedical engineering applications. It can be difficult to source primary, easily renewable ECs that possess the function or dysfunction in question. In contrast, human pluripotent stem cells (hPSCs) can be sourced from donors of interest and renewed almost indefinitely. In this review, we highlight how knowledge of vascular EC development in vivo is used to differentiate induced pluripotent stem cells (iPSC) into ECs. We then describe how iPSC-derived ECs are being used currently in in vitro models of organ function and disease and in vivo applications.

Advances in microfluidic in vitro systems for neurological disease modeling

Neurological disorders are the leading cause of disability and the second largest cause of death worldwide. Despite significant research efforts, neurology remains one of the most failure-prone areas of drug development. The complexity of the human brain, boundaries to examining the brain directly in vivo, and the significant evolutionary gap between animal models and humans, all serve to hamper translational success. Recent advances in microfluidic in vitro models have provided new opportunities to study human cells with enhanced physiological relevance. The ability to precisely micro-engineer cell-scale architecture, tailoring form and function, has allowed for detailed dissection of cell biology using microphysiological systems (MPS) of varying complexities from single cell systems to "Organ-on-chip" models. Simplified neuronal networks have allowed for unique insights into neuronal transport and neurogenesis, while more complex 3D heterotypic cellular models such as neurovascular unit mimetics and "Organ-on-chip" systems have enabled new understanding of metabolic coupling and blood-brain barrier transport. These systems are now being developed beyond MPS toward disease specific micro-pathophysiological systems, moving from "Organ-on-chip" to "Disease-on-chip." This review gives an outline of current state of the art in microfluidic technologies for neurological disease research, discussing the challenges and limitations while highlighting the benefits and potential of integrating technologies. We provide examples of where such toolsets have enabled novel insights and how these technologies may empower future investigation into neurological diseases.

Selenium at the eural arriers: eview

Selenium (Se) is known to contribute to several vital physiological functions in mammals: antioxidant defense, fertility, thyroid hormone metabolism, and immune response. Growing evidence indicates the crucial role of Se and Se-containing selenoproteins in the brain and brain function. As for the other essential trace elements, dietary Se needs to reach effective concentrations in the central nervous system (CNS) to exert its functions. To do so, Se-species have to cross the blood-brain barrier (BBB) and/or blood-cerebrospinal fluid barrier (BCB) of the choroid plexus. The main interface between the general circulation of the body and the CNS is the BBB. Endothelial cells of brain capillaries forming the so-called tight junctions are the primary anatomic units of the BBB, mainly responsible for barrier function. The current review focuses on Se transport to the brain, primarily including selenoprotein P/low-density lipoprotein receptor-related protein 8 (LRP8, also known as apolipoprotein E receptor-2) dependent pathway, and supplementary transport routes of Se into the brain via low molecular weight Se-species. Additionally, the potential role of Se and selenoproteins in the BBB, BCB, and neurovascular unit (NVU) is discussed. Finally, the perspectives regarding investigating the role of Se and selenoproteins in the gut-brain axis are outlined.

Advances in modelling the human microbiome-gut-brain axis in vitro

The human gut microbiome has emerged as a key player in the bidirectional communication of the gut–brain axis, affecting various aspects of homeostasis and pathophysiology. Until recently, the majority of studies that seek to explore the mechanisms underlying the microbiome–gut–brain axis cross-talk, relied almost exclusively on animal models, and particularly gnotobiotic mice. Despite the great progress made with these models, various limitations, including ethical considerations and interspecies differences that limit the translatability of data to human systems, pushed researchers to seek for alternatives. Over the past decades, the field of in vitro modelling of tissues has experienced tremendous growth, thanks to advances in 3D cell biology, materials, science and bioengineering, pushing further the borders of our ability to more faithfully emulate the in vivo situation. The discovery of stem cells has offered a new source of cells, while their use in generating gastrointestinal and brain organoids, among other tissues, has enabled the development of novel 3D tissues that better mimic the native tissue structure and function, compared with traditional assays. In parallel, organs-on-chips technology and bioengineered tissues have emerged as highly promising alternatives to animal models for a wide range of applications. Here, we discuss how recent advances and trends in this area can be applied in host–microbe and host–pathogen interaction studies. In addition, we highlight paradigm shifts in engineering more robust human microbiome-gut-brain axis models and their potential to expand our understanding of this complex system and hence explore novel, microbiome-based therapeutic approaches.

Thromboinflammation Model-on-A-Chip by Whole Blood Microfluidics on Fixed Human Endothelium

Microfluidic devices have an established role in the study of platelets and coagulation factors in thrombosis, with potential diagnostic applications. However, few microfluidic devices have assessed the contribution of neutrophils to thrombus formation, despite increasing knowledge of neutrophils’ importance in cardiovascular thrombosis. We describe a thromboinflammation model which uses straight channels, lined with fixed human umbilical vein endothelial cells, after treatment with tumour necrosis factor-alpha. Re-calcified whole blood is perfused over the endothelium at venous and arterial shear rate. Neutrophil adhesion, platelet and fibrin thrombus formation, is measured over time by the addition of fluorescent antibodies to a whole blood sample. Fixed endothelium retains surface expression of adhesion molecules ICAM-1 and E-Selectin. Neutrophils adhere preferentially to platelet thrombi on the endothelium. Inhibitors of neutrophil adhesion and anti-inflammatory agents, such as isoquercetin, decrease neutrophil adhesion. Our model offers the advantage of the use of (1) fixed endothelium, (2) whole blood, instead of isolated neutrophils, and (3) a small amount of blood (1 mL). The characteristics of this thromboinflammation model provide the potential for further development for drug screening and point-of-care applications.

Advances in Engineering Human Tissue Models

Research in cell biology greatly relies on cell-based in vitro assays and models that facilitate the investigation and understanding of specific biological events and processes under different conditions. The quality of such experimental models and particularly the level at which they represent cell behavior in the native tissue, is of critical importance for our understanding of cell interactions within tissues and organs. Conventionally, in vitro models are based on experimental manipulation of mammalian cells, grown as monolayers on flat, two-dimensional (2D) substrates. Despite the amazing progress and discoveries achieved with flat biology models, our ability to translate biological insights has been limited, since the 2D environment does not reflect the physiological behavior of cells in real tissues. Advances in 3D cell biology and engineering have led to the development of a new generation of cell culture formats that can better recapitulate the in vivo microenvironment, allowing us to examine cells and their interactions in a more biomimetic context. Modern biomedical research has at its disposal novel technological approaches that promote development of more sophisticated and robust tissue engineering in vitro models, including scaffold- or hydrogel-based formats, organotypic cultures, and organs-on-chips. Even though such systems are necessarily simplified to capture a particular range of physiology, their ability to model specific processes of human biology is greatly valued for their potential to close the gap between conventional animal studies and human (patho-) physiology. Here, we review recent advances in 3D biomimetic cultures, focusing on the technological bricks available to develop more physiologically relevant in vitro models of human tissues. By highlighting applications and examples of several physiological and disease models, we identify the limitations and challenges which the field needs to address in order to more effectively incorporate synthetic biomimetic culture platforms into biomedical research.

Cell type-specific changes in transcriptomic profiles of endothelial cells, iPSC-derived neurons and astrocytes cultured on microfluidic chips

In vitro neuronal models are essential for studying neurological physiology, disease mechanisms and potential treatments. Most in vitro models lack controlled vasculature, despite its necessity in brain physiology and disease. Organ-on-chip models offer microfluidic culture systems with dedicated micro-compartments for neurons and vascular cells. Such multi-cell type organs-on-chips can emulate neurovascular unit (NVU) physiology, however there is a lack of systematic data on how individual cell types are affected by culturing on microfluidic systems versus conventional culture plates. This information can provide perspective on initial findings of studies using organs-on-chip models, and further optimizes these models in terms of cellular maturity and neurovascular physiology. Here, we analysed the transcriptomic profiles of co-cultures of human induced pluripotent stem cell (hiPSC)-derived neurons and rat astrocytes, as well as one-day monocultures of human endothelial cells, cultured on microfluidic chips. For each cell type, large gene expression changes were observed when cultured on microfluidic chips compared to conventional culture plates. Endothelial cells showed decreased cell division, neurons and astrocytes exhibited increased cell adhesion, and neurons showed increased maturity when cultured on a microfluidic chip. Our results demonstrate that culturing NVU cell types on microfluidic chips changes their gene expression profiles, presumably due to distinct surface-to-volume ratios and substrate materials. These findings inform further NVU organ-on-chip model optimization and support their future application in disease studies and drug testing.

Electrophysiology Read-Out Tools for Brain-on-Chip Biotechnology

Brain-on-Chip (BoC) biotechnology is emerging as a promising tool for biomedical and pharmaceutical research applied to the neurosciences. At the convergence between lab-on-chip and cell biology, BoC couples in vitro three-dimensional brain-like systems to an engineered microfluidics platform designed to provide an in vivo-like extrinsic microenvironment with the aim of replicating tissue- or organ-level physiological functions. BoC therefore offers the advantage of an in vitro reproduction of brain structures that is more faithful to the native correlate than what is obtained with conventional cell culture techniques. As brain function ultimately results in the generation of electrical signals, electrophysiology techniques are paramount for studying brain activity in health and disease. However, as BoC is still in its infancy, the availability of combined BoC-electrophysiology platforms is still limited. Here, we summarize the available biological substrates for BoC, starting with a historical perspective. We then describe the available tools enabling BoC electrophysiology studies, detailing their fabrication process and technical features, along with their advantages and limitations. We discuss the current and future applications of BoC electrophysiology, also expanding to complementary approaches. We conclude with an evaluation of the potential translational applications and prospective technology developments.

Abnormal Glu/mGluR2/3/PI3K pathway in the hippocampal neurovascular unit leads to diabetes-related depression

Our previous studies have shown that glutamate and hippocampal neuron apoptosis are key signals and direct factors associated with diabetes-related depression, and structural and functional damage to the hippocampal neurovascular unit has been associated with diabetes-related depression. However, the underlying mechanism remains unclear. We hypothesized that diabetes-related depression might be associated with the glutamate (Glu)/metabotropic glutamate receptor2/3 (mGluR_2/3)/phosphoinositide 3-kinase (PI3K) pathway, activated by glucocorticoid receptors in the hippocampal neurovascular unit. To test this hypothesis, rat hippocampal neurovascular unit models, containing hippocampal neurons, astrocytes, and brain microvascular endothelial cells, were treated with 150 mM glucose and 200 µM corticosterone, to induce diabetes-related depression. Our results showed that under conditions of diabetes complicated by depression, hippocampal neurovascular units were damaged, leading to decreased barrier function; elevated Glu levels; upregulated glucocorticoid receptor, vesicular glutamate transporter 3 (VGLUT-3), and metabotropic glutamate receptor 2/3 (mGluR_2/3) expression; downregulated excitatory amino acid transporter 1 (EAAT-1) expression; and alteration of the balance of key proteins associated with the extracellular signal-regulated kinase (ERK)/glial cell-derived neurotrophic factor (GDNF)/PI3K signaling pathway. Moreover, the viability of neurons was dramatically reduced in the model of diabetes-related depression, and neuronal apoptosis, and caspase-3 and caspase-9 expression levels, were increased. Our results suggest that the Glu/mGluR_2/3/PI3K pathway, induced by glucocorticoid receptor activation in the hippocampal neurovascular unit, may be associated with diabetes-related depression. This study was approved by the Laboratory Animal Ethics Committee of The First Hospital of Hunan University of Chinese Medicine, China (approval No. HN-ZYFY-2019-11-12) on November 12, 2019.

Three-Dimensional in vitro Models of Healthy and Tumor Brain Microvasculature for Drug and Toxicity Screening

Tissue vascularization is essential for its oxygenation and the homogenous diffusion of nutrients. Cutting-edge studies are focusing on the vascularization of three-dimensional (3D) in vitro models of human tissues. The reproduction of the brain vasculature is particularly challenging as numerous cell types are involved. Moreover, the blood-brain barrier, which acts as a selective filter between the vascular system and the brain, is a complex structure to replicate. Nevertheless, tremendous advances have been made in recent years, and several works have proposed promising 3D in vitro models of the brain microvasculature. They incorporate cell co-cultures organized in 3D scaffolds, often consisting of components of the native extracellular matrix (ECM), to obtain a micro-environment similar to the in vivo physiological state. These models are particularly useful for studying adverse effects on the healthy brain vasculature. They provide insights into the molecular and cellular events involved in the pathological evolutions of this vasculature, such as those supporting the appearance of brain cancers. Glioblastoma multiform (GBM) is the most common form of brain cancer and one of the most vascularized solid tumors. It is characterized by a high aggressiveness and therapy resistance. Current conventional therapies are unable to prevent the high risk of recurrence of the disease. Most of the new drug candidates fail to pass clinical trials, despite the promising results shown in vitro. The conventional in vitro models are unable to efficiently reproduce the specific features of GBM tumors. Recent studies have indeed suggested a high heterogeneity of the tumor brain vasculature, with the coexistence of intact and leaky regions resulting from the constant remodeling of the ECM by glioma cells. In this review paper, after summarizing the advances in 3D in vitro brain vasculature models, we focus on the latest achievements in vascularized GBM modeling, and the potential applications for both healthy and pathological models as platforms for drug screening and toxicological assays. Particular attention will be paid to discuss the relevance of these models in terms of cell-cell, cell-ECM interactions, vascularization and permeability properties, which are crucial parameters for improving in vitro testing accuracy.

Toward three-dimensional in vitro models to study neurovascular unit functions in health and disease

The high metabolic demands of the brain require an efficient vascular system to be coupled with neural activity to supply adequate nutrients and oxygen. This supply is coordinated by the action of neurons, glial and vascular cells, known collectively as the neurovascular unit, which temporally and spatially regulate local cerebral blood flow through a process known as neurovascular coupling. In many neurodegenerative diseases, changes in functions of the neurovascular unit not only impair neurovascular coupling but also permeability of the blood-brain barrier, cerebral blood flow and clearance of waste from the brain. In order to study disease mechanisms, we need improved physiologically-relevant human models of the neurovascular unit. Advances towards modeling the cellular complexity of the neurovascular unit in vitro have been made using stem-cell derived organoids and more recently, vascularized organoids, enabling intricate studies of non-cell autonomous processes. Engineering and design innovations in microfluidic devices and tissue engineering are progressing our ability to interrogate the cerebrovasculature. These advanced models are being used to gain a better understanding of neurodegenerative disease processes and potential therapeutics. Continued innovation is required to build more physiologically-relevant models of the neurovascular unit encompassing both the cellular complexity and designed features to interrogate neurovascular unit functionality.

Tumor Targeted Delivery of an Anti-Cancer Therapeutic: An In Vitro and In Vivo Evaluation

The limited effectiveness of current therapeutics against malignant brain gliomas has led to an urgent need for development of new formulations against these tumors. Chelator Dp44mT (di-2-pyridylketone-4,4-dimethyl-3-thiosemicarbazone) presents a promising candidate to defeat gliomas due to its exceptional anti-tumor activity and its unique ability to overcome multidrug resistance. The goal of this study is to develop a targeted nano-carrier for Dp44mT delivery to glioma tumors and to assess its therapeutic efficacy in vitro and in vivo. Dp44mT is loaded into poly(ethylene glycol) (PEG)ylated poly(lactic-co-glycolic acid) (PLGA) nanoparticles (NPs) decorated with glioma-targeting ligand Interlukin 13 (IL13). IL13-conjugation enhanced the NP uptake by glioma cells and also improved their transport across an in vitro blood-brain-barrier (BBB) model. This targeted formulation showed an outstanding toxicity towards glioma cell lines and patient-derived stem cells in vitro, with IC₅₀ values less than 125 nM, and caused no significant death in healthy brain microvascular endothelial cells. In vivo, when tested on a xenograft mouse model, IL13-conjugated Dp44mT-NPs reduced the glioma tumor growth by ≈62% while their untargeted counterparts reduced the tumor growth by only ≈16%. Notably, this formulation does not cause any significant weight loss or kidney/liver toxicity in mice, demonstrating its great therapeutic potential.

3D Self-Organized Human Blood-Brain Barrier in a Microfluidic Chip

A preclinical blood-brain barrier (BBB) model is important for the study of fundamental transport mechanisms and in accessing the delivery of small molecules and antibodies that target brain. Transwell assays for BBB models are easy to create and use but lack the true 3D anatomy of the brain microvasculature and also often the cell-cell and cell-matrix interactions that are important in ensuring a tight BBB. Here we describe the formation of a BBB that expresses neurovascular membrane transporters, tight junction, and extracellular matrix proteins using the coculture of human-induced pluripotent stem cell-derived endothelial cells (iPSC-EC), brain pericytes (PC), and astrocytes (AC) in a microfluidic device. The BBB model recapitulates human brain vascular permeability with values that are lower than conventional in vitro models and are comparable to in vivo measurements in rat brain. This in vitro BBB model can therefore be used to screen for brain-targeting drugs or to study neurovascular functions.

In Situ Surface Modification of Microfluidic Blood-Brain-Barriers for Improved Screening of Small Molecules and Nanoparticles

Here, we have developed and evaluated a microfluidic-based human blood-brain-barrier (μBBB) platform that models and predicts brain tissue uptake of small molecule drugs and nanoparticles (NPs) targeting the central nervous system. By using a photocrosslinkable copolymer that was prepared from monomers containing benzophenone and N-hydroxysuccinimide ester functional groups, we were able to evenly coat and functionalize μBBB chip channels in situ, providing a covalently attached homogenous layer of extracellular matrix proteins. This novel approach allowed the coculture of human endothelial cells, pericytes, and astrocytes and resulted in the formation of a mimic of cerebral endothelium expressing tight junction markers and efflux proteins, resembling the native BBB. The permeability coefficients of a number of compounds, including caffeine, nitrofurantoin, dextran, sucrose, glucose, and alanine, were measured on our μBBB platform and were found to agree with reported values. In addition, we successfully visualized the receptor-mediated uptake and transcytosis of transferrin-functionalized NPs. The BBB-penetrating NPs were able to target glioma cells cultured in 3D in the brain compartment of our μBBB. In conclusion, our μBBB was able to accurately predict the BBB permeability of both small molecule pharmaceuticals and nanovectors and allowed time-resolved visualization of transcytosis. Our versatile chip design accommodates different brain disease models and is expected to be exploited in further BBB studies, aiming at replacing animal experiments.

Real-Time Ratiometric Imaging of Micelles Assembly State in a Microfluidic Cancer-on-a-Chip

The performance of supramolecular nanocarriers as drug delivery systems depends on their stability in the complex and dynamic biological media. After administration, nanocarriers are challenged by physiological barriers such as shear stress and proteins present in blood, endothelial wall, extracellular matrix, and eventually cancer cell membrane. While early disassembly will result in a premature drug release, extreme stability of the nanocarriers can lead to poor drug release and low efficiency. Therefore, comprehensive understanding of the stability and assembly state of supramolecular carriers in each stage of delivery is the key factor for the rational design of these systems. One of the main challenges is that current 2D in vitro models do not provide exhaustive information, as they fail to recapitulate the 3D tumor microenvironment. This deficiency in the 2D model complexity is the main reason for the differences observed in vivo when testing the performance of supramolecular nanocarriers. Herein, we present a real-time monitoring study of self-assembled micelles stability and extravasation, combining spectral confocal microscopy and a microfluidic cancer-on-a-chip. The combination of advanced imaging and a reliable 3D model allows tracking of micelle disassembly by following the spectral properties of the amphiphiles in space and time during the crucial steps of drug delivery. The spectrally active micelles were introduced under flow and their position and conformation continuously followed by spectral imaging during the crossing of barriers, revealing the interplay between carrier structure, micellar stability, and extravasation. Integrating the ability of the micelles to change their fluorescent properties when disassembled, spectral confocal imaging and 3D microfluidic tumor blood vessel-on-a-chip resulted in the establishment of a robust testing platform suitable for real-time imaging and evaluation of supramolecular drug delivery carrier's stability.

Recent progress in translational engineered in vitro models of the central nervous system

The complexity of the human brain poses a substantial challenge for the development of models of the CNS. Current animal models lack many essential human characteristics (in addition to raising operational challenges and ethical concerns), and conventional in vitro models, in turn, are limited in their capacity to provide information regarding many functional and systemic responses. Indeed, these challenges may underlie the notoriously low success rates of CNS drug development efforts. During the past 5 years, there has been a leap in the complexity and functionality of in vitro systems of the CNS, which have the potential to overcome many of the limitations of traditional model systems. The availability of human-derived induced pluripotent stem cell technology has further increased the translational potential of these systems. Yet, the adoption of state-of-the-art in vitro platforms within the CNS research community is limited. This may be attributable to the high costs or the immaturity of the systems. Nevertheless, the costs of fabrication have decreased, and there are tremendous ongoing efforts to improve the quality of cell differentiation. Herein, we aim to raise awareness of the capabilities and accessibility of advanced in vitro CNS technologies. We provide an overview of some of the main recent developments (since 2015) in in vitro CNS models. In particular, we focus on engineered in vitro models based on cell culture systems combined with microfluidic platforms (e.g. 'organ-on-a-chip' systems). We delve into the fundamental principles underlying these systems and review several applications of these platforms for the study of the CNS in health and disease. Our discussion further addresses the challenges that hinder the implementation of advanced in vitro platforms in personalized medicine or in large-scale industrial settings, and outlines the existing differentiation protocols and industrial cell sources. We conclude by providing practical guidelines for laboratories that are considering adopting organ-on-a-chip technologies.

Bioengineered tissue models for the development of dynamic immuno-associated tumor models and high-throughput immunotherapy cytotoxicity assays

Cancer immunotherapy is rapidly developing, with numerous therapies approved over the past decade and more therapies expected to gain approval in the future. However, immunotherapy of solid tumors has been less successful because immunosuppressive barriers limit immune cell trafficking and function against cancer cells. Interactions between suppressive immune cells, cytokines, and inhibitory factors are central to cancer immunotherapy approaches. In this review, we discuss recent advances in utilizing microfluidic platforms for understanding cancer-suppressive immune system interactions. Dendritic cell (DC)-mediated tumor models, infiltrated lymphocyte-mediated tumor models [e.g., natural killer (NK) cells, T cells, chimeric antigen receptor (CAR) T cells, and macrophages], monocyte-mediated tumor models, and immune checkpoint blockade (ICB) tumor models are among the various bioengineered immune cell-cancer cell interactions that we reviewed herein.

Biosynthetic, biomimetic, and self-assembled vascularized Organ-on-a-Chip systems

Organ-on-a-Chip (OOC) devices have seen major advances in the last years with respect to biological complexity, physiological composition and biomedical relevance. In this context, integration of vasculature has proven to be a crucial element for long-term culture of thick tissue samples as well as for realistic pharmacokinetic, toxicity and metabolic modelling. With the emergence of digital production technologies and the reinvention of existing tools, a multitude of design approaches for guided angio- and vasculogenesis is available today. The underlying production methods can be categorized into biosynthetic, biomimetic and self-assembled vasculature formation. The diversity and importance of production approaches, vascularization strategies as well as biomaterials and cell sourcing are illustrated in this work. A comprehensive technological review with a strong focus on the challenge of producing physiologically relevant vascular structures is given. Finally, the remaining obstacles and opportunities in the development of vascularized Organ-on-a-Chip platforms for advancing drug development and predictive disease modelling are noted.

An in vitro bioengineered model of the human arterial neurovascular unit to study neurodegenerative diseases

INTRODUCTION

The neurovascular unit (NVU) - the interaction between the neurons and the cerebrovasculature - is increasingly important to interrogate through human-based experimental models. Although advanced models of cerebral capillaries have been developed in the last decade, there is currently no in vitro 3-dimensional (3D) perfusible model of the human cortical arterial NVU.

METHOD

We used a tissue-engineering technique to develop a scaffold-directed, perfusible, 3D human NVU that is cultured in native-like flow conditions that mimics the anatomy and physiology of cortical penetrating arteries.

RESULTS

This system, composed of primary human vascular cells (endothelial cells, smooth muscle cells and astrocytes) and induced pluripotent stem cell (iPSC) derived neurons, demonstrates a physiological multilayer organization of the involved cell types. It reproduces key characteristics of cortical neurons and astrocytes and enables formation of a selective and functional endothelial barrier. We provide proof-of-principle data showing that this in vitro human arterial NVU may be suitable to study neurovascular components of neurodegenerative diseases such as Alzheimer's disease (AD), as endogenously produced phosphorylated tau and beta-amyloid accumulate in the model over time. Finally, neuronal and glial fluid biomarkers relevant to neurodegenerative diseases are measurable in our arterial NVU model.

CONCLUSION

This model is a suitable research tool to investigate arterial NVU functions in healthy and disease states. Further, the design of the platform allows culture under native-like flow conditions for extended periods of time and yields sufficient tissue and media for downstream immunohistochemistry and biochemistry analyses.

Advances in 3D neural, vascular and neurovascular models for drug testing and regenerative medicine

Clinical trials continue to fall short regarding drugs to effectively treat brain-affecting diseases. Although there are many causes of these shortcomings, the most relevant are the inability of most therapeutic agents to cross the blood-brain barrier (BBB) and the failure to translate effects from animal models to patients. In this review, we analyze the most recent developments in BBB, neural, and neurovascular models, analyzing their impact on the drug development process by considering their quantitative and phenotypical characterization. We offer a perspective of the state-of-the-art of the models that could revolutionize the pharmaceutical industry.

Hemorrhagic Transformation After Tissue Plasminogen Activator Treatment in Acute Ischemic Stroke

Hemorrhagic transformation (HT) is a common complication after thrombolysis with recombinant tissue-type plasminogen activator (rt-PA) in ischemic stroke. In this article, recent research progress of HT in vivo and in vitro studies was reviewed. We have discussed new potential mechanisms and possible experimental models of HT development, as well as possible biomarkers and treatment methods. Meanwhile, we compared and analyzed rodent models, large animal models and in vitro BBB models of HT, and the limitations of these models were discussed. The molecular mechanism of HT was investigated in terms of BBB disruption, rt-PA neurotoxicity and the effect of neuroinflammation, matrix metalloproteinases, reactive oxygen species. The clinical features to predict HT were represented including blood biomarkers and clinical factors. Recent progress in neuroprotective strategies to improve HT after stroke treated with rt-PA is outlined. Further efforts need to be made to reduce the risk of HT after rt-PA therapy and improve the clinical prognosis of patients with ischemic stroke.

Bioprinting Neural Systems to Model Central Nervous System Diseases

To date, pharmaceutical progresses in central nervous system (CNS) diseases are clearly hampered by the lack of suitable disease models. Indeed, animal models do not faithfully represent human neurodegenerative processes and human in vitro 2D cell culture systems cannot recapitulate the in vivo complexity of neural systems. The search for valuable models of neurodegenerative diseases has recently been revived by the addition of 3D culture that allows to re-create the in vivo microenvironment including the interactions among different neural cell types and the surrounding extracellular matrix (ECM) components. In this review, the new challenges in the field of CNS diseases in vitro 3D modeling are discussed, focusing on the implementation of bioprinting approaches enabling positional control on the generation of the 3D microenvironments. The focus is specifically on the choice of the optimal materials to simulate the ECM brain compartment and the biofabrication technologies needed to shape the cellular components within a microenvironment that significantly represents brain biochemical and biophysical parameters.

Innovations in 3-Dimensional Tissue Models of Human Brain Physiology and Diseases

3-dimensional (3D) laboratory tissue cultures have emerged as an alternative to traditional 2-dimensional (2D) culture systems that do not recapitulate native cell behavior. The discrepancy between in vivo and in vitro tissue-cell-molecular responses impedes understanding of human physiology in general and creates roadblocks for the discovery of therapeutic solutions. Two parallel approaches have emerged for the design of 3D culture systems. The first is biomedical engineering methodology, including bioengineered materials, bioprinting, microfluidics and bioreactors, used alone or in combination, to mimic the microenvironments of native tissues. The second approach is organoid technology, in which stem cells are exposed to chemical and/or biological cues to activate differentiation programs that are reminiscent of human (prenatal) development. This review article describes recent technological advances in engineering 3D cultures that more closely resemble the human brain. The contributions of in vitro 3D tissue culture systems to new insights in neurophysiology, neurological diseases and regenerative medicine are highlighted. Perspectives on designing improved tissue models of the human brain are offered, focusing on an integrative approach merging biomedical engineering tools with organoid biology.

Cholic Acid Protects In Vitro Neurovascular Units against Oxygen and Glucose Deprivation-Induced Injury through the BDNF-TrkB Signaling Pathway

Ischemic stroke (IS) can disrupt various types of brain cells in the neurovascular unit (NVU) at both the structural and functional levels. Therefore, NVU is considered to be a more comprehensive target for the treatment of IS. It is necessary to develop drugs which targeted multiple mechanisms and cell types on NVU against IS. As a component of bile acid, cholic acid has been reported to be able to diffuse across phospholipid bilayers and further cross the blood-brain barrier (BBB). However, the effects exerted by cholic acid (CA) on the NVU after stroke remain unclear. Based on our previous research, we established and further supplemented the characteristics of the functional in vitro NVU model and its oxygen-glucose deprivation and reoxygenation (OGD/R) model. Then, we investigated the effect of CA on the maintenance of the in vitro NVU after OGD/R and further discussed the specific molecular targets that CA played a role in. For the first time, we found that CA significantly maintained BBB integrity, downregulated apoptosis, and mitigated oxidative stress and inflammation damage after OGD/R. Meanwhile, CA obviously increased the levels of brain-derived neurotrophic factor (BDNF), which were mainly secreted from astrocytes, in the coculture system after OGD/R. The results demonstrated that CA significantly increased the expression of TrkB, PI3K/Akt, MAPK/Erk, and CREB in neurons. These positive effects on the downstream proteins of BDNF were suppressed by treatment with ANA12 which is an inhibitor of TrkB. In conclusion, the present study demonstrates that CA exerted multiple protective effects on the NVU, mediated by increasing the release of BDNF and further stimulating the BDNF-TrkB-PI3K/Akt and BDNF-TrkB-MAPK/Erk signaling pathways in the context of OGD/R-induced injury. These findings indicate that CA possesses the effect of antagonizing multiple mechanisms of IS and protecting multiple cell types in NVU and may be useful as a treatment for IS.

Advances in nanomedicines for diagnosis of central nervous system disorders

In spite of a great improvement in medical health services and an increase in lifespan, we have witnessed a skyrocket increase in the incidence of central nervous system (CNS) disorders including brain tumors, neurodegenerative diseases (Alzheimer's disease, Parkinson's disease, amyotrophic lateral sclerosis, Huntington's disease), ischemic stroke, and epilepsy, which have seriously undermined the quality of life and substantially increased economic and societal burdens. Development of diagnostic methods for CNS disorders is still in the early stage, and the clinical outcomes suggest these methods are not ready for the challenges associated with diagnosis of CNS disorders, such as early detection, specific binding, sharp contrast, and continuous monitoring of therapeutic interventions. Another challenge is to overcome various barrier structures during delivery of diagnostic agents, especially the blood-brain barrier (BBB). Fortunately, utilization of nanomaterials has been pursued as a potential and promising strategy to address these challenges. This review will discuss anatomical and functional structures of BBB and transport mechanisms of nanomaterials across the BBB, and special emphases will be placed on the state-of-the-art advances in the development of nanomedicines from a variety of nanomaterials for diagnosis of CNS disorders. Meanwhile, current challenges and future perspectives in this field are also highlighted.

An Improved in Vitro Blood-Brain Barrier Model for Applications in Therapeutics' Delivery to Brain

Blood-brain barrier (BBB) imposes a major obstacle for entry of therapeutics to brain. In vitro BBB models that can provide reliable prediction of therapeutics' ability to cross BBB are thus, critical for the advancement of brain therapeutics. Towards the development of an improved BBB model, here we studied the individual and combinatorial effect of few different culture conditions on the quality of the commonly used trans-well BBB model. Specifically, we investigated how the addition of (i) astrocyte co-culture, (ii) astrocyte-conditioned media (ACM), and (iii) astrocyte co-culture along with ACM, affects the characteristics of BBB. The resultant BBB models were characterized for trans-endothelial electrical resistance (TEER), permeability, and expression of a tight-junction protein ZO-1. We found that addition of ACM and astrocytes, individually, had similar impact on BBB's TEER, increasing it by ~2 fold. Interestingly, the presence of both astrocytes and ACM had a significantly greater impact on TEER and increased it by ~3 fold. Addition of ACM, with and without astrocyte co-culture, led to a reduction in permeability of this BBB model. Moreover, addition of ACM and astrocyte co-culture, both individually and in combination, led to a noticeable increase in ZO-1 expression in the BBB endothelial cells. These findings provide a new approach for further improvement of the commonly used trans-well BBB system.

A 3D Biohybrid Real-Scale Model of the Brain Cancer Microenvironment for Advanced In Vitro Testing

The modeling of the pathological microenvironment of the central nervous system (CNS) represents a disrupting approach for drug screening for advanced therapies against tumors and neuronal disorders. The in vitro investigations of the crossing and diffusion of drugs through the blood-brain barrier (BBB) are still not completely reliable, due to technological limits in the replication of 3D microstructures that can faithfully mimic the in vivo scenario. Here, an innovative 1:1 scale 3D-printed realistic biohybrid model of the brain tumor microenvironment, with both luminal and parenchyma compartments, is presented. The dynamically controllable microfluidic device, fabricated through two-photon lithography, enables the triple co-culture of hCMEC/D3 cells, forming the internal biohybrid endothelium of the capillaries, of astrocytes, and of magnetically-driven spheroids of U87 glioblastoma cells. Tumor spheroids are obtained from culturing glioblas-toma cells inside 3D microcages loaded with superparamagnetic iron oxide nanoparticles (SPIONs). The system proves to be capable in hindering dextran diffusion through the bioinspired BBB, while allowing chemotherapy-loaded nanocarriers to cross it. The proper formation of the selective barrier and the good performance of the anti-tumor treatment demonstrate that the proposed device can be successfully exploited as a realistic in vitro model for high-throughput drug screening in CNS diseases.

Study of the neurotoxicity of indoor airborne nanoparticles based on a 3D human blood-brain barrier chip

BACKGROUND

There is growing public awareness regarding the health effects of indoor nanoscale particulate matter (INPM) since people spend the majority of their time indoors. INPM could have a direct entry route into the brain via the axons of the olfactory nerve and migrating across the blood-brain barrier (BBB). Using animals to explore this possibility is not a reliable method to fully demonstrate human physiological responses. We, therefore, set out to develop a human 3D functional blood-brain barrier model to examine the potential effects of INPM on the cerebral nervous system.

METHODS

Human astrocytes were co-cultured and human umbilical vein endothelial cells in 3D within a microfluidic chip to simulate the micro-complex physiological structure of the human BBB. This 3D human organotypic model has then been made to investigate any INPM-induced BBB dysfunction linked to potential cellular responses.

RESULTS

A 3D human functional blood-brain barrier was constructed in this study. We observed the translocation of INPM across the blood-brain barrier. The 3D human organotypic chip initially reflected damage to the nervous system with abnormal astrocyte proliferation and a decline in cell viability. We also looked at the behavior of oxidative stress-related biomarkers (ROS, GSH-Px, and MDA). INPM was implicated in aggravating inflammation via reactive oxygen species (ROS). The Keap1-Nrf2-ARE pathway is a key mechanism in cellular resistance to oxidative stress by mediating and activating a variety of antioxidant and detoxification enzymes. Following ROS accumulation, INPM induced abnormal expression of nuclear transcription factor Nrf2. This behavior disturbed the expression of, γ-glutamate synthase (γ-GCS) and heme oxygenase (HO-1), which further exacerbated the imbalance of the antioxidant system.

CONCLUSIONS

This functional 3D human organotypic chip effectively mimics the physiological response of the human BBB. The chip provides a micro-complex structure to simulate the internal environment of the human blood-brain barrier, and partially simulates the physiological responses of the BBB to INPM exposure. Based on this model, INPM was shown to affect the blood-brain barrier biofunction by disrupting the Keap1-Nrf2-ARE pathways.

A novel tri-culture model for neuroinflammation

Neuroinflammation is believed to play a primary role in the pathogenesis of most neurodegenerative diseases including Alzheimer's disease, Parkinson's disease and schizophrenia. Currently, suitable in vitro neuroinflammation models for studying cellular interactions and inflammatory mechanisms at the neurovascular unit are still scarce. In this study, we established an experimentally flexible tri-culture neuroinflammation model combining murine microglial cells (N11), mouse neuroblastoma Nuro2A cell lines and brain microvascular endothelial MVEC(B3) cells in a transwell co-culture system stimulated with lipopolysaccharides. Neuroinflammation was induced in this tri-culture model as manifested by activated N11 cells via toll-like receptor 4, resulting in increased release of proinflammatory mediators (nitric oxide, interleukin-6 and tumour necrosis factor-α) through the activation of nuclear factor-κB signalling pathway. The released inflammatory cytokines from N11 in turn, damaged the tight junction in microvascular endothelial MVEC(B3) cells, increased permeability of endothelial barrier, and induced tau phosphorylation and up-regulated caspase-3 expression in mouse neuroblastoma Nuro2A cell lines, leading to neuroinflammation injury. In summary, this tri-culture inflammation model mimics the microenvironment, the cellular crosstalk and the molecular events that take place during neuroinflammation. It provides a robust in vitro model for studying neuroinflammation mechanisms and screening for potential therapeutics to treat various neurodegenerative diseases.

Electro-responsive hydrogel-based microfluidic actuator platform for photothermal therapy

Electrical stimuli play an important role in regulating the delivery of plasmonic nanomaterials with cancer targeting peptides. Here, we developed an electro-responsive hydrogel-based microfluidic actuator platform for brain tumor targeting and photothermal therapy (PTT) applications. The electro-responsive hydrogels consisted of highly conductive silver nanowires (AgNWs) and biocompatible collagen I gels. We confirmed that an electrically conductive hydrogel could be used as an effective actuator by applying an electrical signal in the microfluidic platform. Furthermore, we successfully demonstrated PTT efficacy for brain tumor cells using targetable Arg-Gly-Asp (RGD) peptide-conjugated gold nanorods (GNRs). Therefore, our electro-responsive hydrogel-based microfluidic actuator platform could be useful for electro-responsive intelligent nanomaterial delivery and PTT applications.

Integrating Biosensors in Organs-on-Chip Devices: A Perspective on Current Strategies to Monitor Microphysiological Systems

Organs-on-chip (OoC), often referred to as microphysiological systems (MPS), are advanced in vitro tools able to replicate essential functions of human organs. Owing to their unprecedented ability to recapitulate key features of the native cellular environments, they represent promising tools for tissue engineering and drug screening applications. The achievement of proper functionalities within OoC is crucial; to this purpose, several parameters (e.g., chemical, physical) need to be assessed. Currently, most approaches rely on off-chip analysis and imaging techniques. However, the urgent demand for continuous, noninvasive, and real-time monitoring of tissue constructs requires the direct integration of biosensors. In this review, we focus on recent strategies to miniaturize and embed biosensing systems into organs-on-chip platforms. Biosensors for monitoring biological models with metabolic activities, models with tissue barrier functions, as well as models with electromechanical properties will be described and critically evaluated. In addition, multisensor integration within multiorgan platforms will be further reviewed and discussed.

Multiplexed blood-brain barrier organ-on-chip

Organ-on-chip devices are intensively studied in academia and industry due to their high potential in pharmaceutical and biomedical applications. However, most of the existing organ-on-chip models focus on proof of concept of individual functional units without the possibility of testing multiple experimental stimuli in parallel. Here we developed a polydimethylsiloxane (PDMS) multiplexed chip with eight parallel channels branching from a common access port through which all eight channels can be addressed simultaneously without the need for extra pipetting steps thus increasing the reproducibility of the experimental results. At the same time, eight outlets provide individual entry to each channel with the opportunity to create eight different experimental conditions. A multiplexed chip can be assembled as a one-layer device for studying monocultures or as a two-layer device for studying barrier tissue functions. For a two-layer device, a ∼2 μm thick transparent PDMS membrane with 5 μm through-hole pores was fabricated in-house using a soft lithography technique, thereby allowing visual inspection of the cell-culture in real-time. The functionality of the chip was studied by recapitulating the blood-brain barrier. For this, human cerebral microvascular endothelial cells (hCMEC/D3) were cultured in mono- or coculture with human astrocytes. Immunostaining revealed a cellular monolayer with the expression of tight junction ZO-1 and adherence junction VE-cadherin proteins in endothelial cells as well as glial fibrillary acidic protein (GFAP) expression in astrocytes. Furthermore, multiplexed permeability studies of molecule passage through the cellular barrier exhibited expected high permeability coefficients for smaller molecules (4 kDa FITC-dextran) whereas larger molecules (20 kDa) crossed the barrier at a lower rate. With these results, we show that our device can be used as an organ-on-chip model for future multiplexed drug testing.

Three-dimensional co-culture of blood-brain barrier-composing cells in a culture insert with a collagen vitrigel membrane

The blood-brain barrier (BBB) is a structure located in brain capillaries that protects the brain from toxic substances in blood due to its high barrier function. The brain capillaries form a layered structure with pericytes, neurons, glial cells, and extracellular matrix proteins that is called neurovascular unit, and the structure is important to express the high barrier function of BBB. Here, we propose a method to construct a three-dimensional BBB tissue using three human BBB-composing cells, including brain endothelial cells, pericytes, and astrocytes, that mimics the in vivo BBB-like layered structure. Primary human brain endothelial cells were plated on the back side (outside) of the collagen vitrigel membrane of a culture insert, pericytes were plated on the upper side (inside), and astrocytes mixed in Matrigel were plated on the pericyte layer. The layered structure was maintained for at least 2 wk. The BBB tissue-loaded collagen vitrigel membrane can be detached from the insert frame using acetone with the tissue fixed intact and used for vertical cryosectioning to analyze the tissue interior. We also measured transendothelial electrical resistance (TEER) in the three-dimensional BBB co-culture to investigate barrier function of the brain endothelial cells. We believe that our co-culture method is useful to study engineered BBB tissues and develop reliable in vitro human BBB models in the future.

In vitro Models of the Blood-Brain Barrier: Building in physiological complexity

Development of brain therapeutics is significantly hampered by the presence of the blood-brain barrier (BBB). Classical transwell models are able to recapitulate many important aspects of drug transport across the BBB, but are not completely predictive of in vivo brain uptake. Species differences further complicate translation of experimental therapeutics from the benchtop to the clinic. Human BBB models offer some solutions to this problem, and by increasing device complexity both in terms of multicellularity, flow and physical architecture, physiological models of the BBB have been developed that can more faithfully model different aspects of transport and homeostasis BBB. Using these models, it may be possible to improve the predictive capacity in benchmarking candidate therapeutics, and to identify new druggable targets by studying multicellular interactions.

Real time monitoring of oxygen uptake of hepatocytes in a microreactor using optical microsensors

Most in vitro test systems for the assessment of toxicity are based on endpoint measurements and cannot contribute much to the establishment of mechanistic models, which are crucially important for further progress in this field. Hence, in recent years, much effort has been put into the development of methods that generate kinetic data. Real time measurements of the metabolic activity of cells based on the use of oxygen sensitive microsensor beads have been shown to provide access to the mode of action of compounds in hepatocytes. However, for fully exploiting this approach a detailed knowledge of the microenvironment of the cells is required. In this work, we investigate the cellular behaviour of three types of hepatocytes, HepG2 cells, HepG2-3A4 cells and primary mouse hepatocytes, towards their exposure to acetaminophen when the availability of oxygen for the cell is systematically varied. We show that the relative emergence of two modes of action, one NAPQI dependent and the other one transient and NAPQI independent, scale with expression level of CYP3A4. The transient cellular response associated to mitochondrial respiration is used to characterise the influence of the initial oxygen concentration in the wells before exposure to acetaminophen on the cell behaviour. A simple model is presented to describe the behaviour of the cells in this scenario. It demonstrates the level of control over the role of oxygen supply in these experiments. This is crucial for establishing this approach into a reliable and powerful method for the assessment of toxicity.

3D In Vitro Human Organ Mimicry Devices for Drug Discovery, Development, and Assessment

The past few decades have shown significant advancement as complex in vitro humanized systems have substituted animal trials and 2D in vitro studies. 3D humanized platforms mimic the organs of interest with their stimulations (physical, electrical, chemical, and mechanical). Organ-on-chip devices, including in vitro modelling of 3D organoids, 3D microfabrication, and 3D bioprinted platforms, play an essential role in drug discovery, testing, and assessment. In this article, a thorough review is provided of the latest advancements in the area of organ-on-chip devices targeting liver, kidney, lung, gut, heart, skin, and brain mimicry devices for drug discovery, development, and/or assessment. The current strategies, fabrication methods, and the specific application of each device, as well as the advantages and disadvantages, are presented for each reported platform. This comprehensive review also provides some insights on the challenges and future perspectives for the further advancement of each organ-on-chip device.

Biomedical Application of Functional Materials in Organ-on-a-Chip

The organ-on-a-chip (OOC) technology has been utilized in a lot of biomedical fields such as fundamental physiological and pharmacological researches. Various materials have been introduced in OOC and can be broadly classified into inorganic, organic, and hybrid materials. Although PDMS continues to be the preferred material for laboratory research, materials for OOC are constantly evolving and progressing, and have promoted the development of OOC. This mini review provides a summary of the various type of materials for OOC systems, focusing on the progress of materials and related fabrication technologies within the last 5 years. The advantages and drawbacks of these materials in particular applications are discussed. In addition, future perspectives and challenges are also discussed.

Modeling blood-brain barrier pathology in cerebrovascular disease in vitro: current and future paradigms

The complexity of the blood-brain barrier (BBB) and neurovascular unit (NVU) was and still is a challenge to bridge. A highly selective, restrictive and dynamic barrier, formed at the interface of blood and brain, the BBB is a "gatekeeper" and guardian of brain homeostasis and it also acts as a "sensor" of pathological events in blood and brain. The majority of brain and cerebrovascular pathologies are associated with BBB dysfunction, where changes at the BBB can lead to or support disease development. Thus, an ultimate goal of BBB research is to develop competent and highly translational models to understand mechanisms of BBB/NVU pathology and enable discovery and development of therapeutic strategies to improve vascular health and for the efficient delivery of drugs. This review article focuses on the progress being made to model BBB injury in cerebrovascular diseases in vitro.

Three-dimensional induced pluripotent stem-cell models of human brain angiogenesis

During brain development, chemical cues released by developing neurons, cellular signaling with pericytes, and mechanical cues within the brain extracellular matrix (ECM) promote angiogenesis of brain microvascular endothelial cells (BMECs). Angiogenesis is also associated with diseases of the brain due to pathological chemical, cellular, and mechanical signaling. Existing in vitro and in vivo models of brain angiogenesis have key limitations. Here, we develop a high-throughput in vitro blood-brain barrier (BBB) bead assay of brain angiogenesis utilizing 150 μm diameter beads coated with induced pluripotent stem-cell (iPSC)-derived human BMECs (dhBMECs). After embedding the beads within a 3D matrix, we introduce various chemical cues and extracellular matrix components to explore their effects on angiogenic behavior. Based on the results from the bead assay, we generate a multi-scale model of the human cerebrovasculature within perfusable three-dimensional tissue-engineered blood-brain barrier microvessels. A sprouting phenotype is optimized in confluent monolayers of dhBMECs using chemical treatment with vascular endothelial growth factor (VEGF) and wnt ligands, and the inclusion of pro-angiogenic ECM components. As a proof-of-principle that the bead angiogenesis assay can be applied to study pathological angiogenesis, we show that oxidative stress can exert concentration-dependent effects on angiogenesis. Finally, we demonstrate the formation of a hierarchical microvascular model of the human blood-brain barrier displaying key structural hallmarks. We develop two in vitro models of brain angiogenesis: the BBB bead assay and the tissue-engineered BBB microvessel model. These platforms provide a tool kit for studies of physiological and pathological brain angiogenesis, with key advantages over existing two-dimensional models.

Biomaterials and Culture Systems for Development of Organoid and Organ-on-a-Chip Models

The development of novel 3D tissue culture systems has enabled the in vitro study of in vivo processes, thereby overcoming many of the limitations of previous 2D tissue culture systems. Advances in biomaterials, including the discovery of novel synthetic polymers has allowed for the generation of physiologically relevant in vitro 3D culture models. A large number of 3D culture systems, aided by novel organ-on-a-chip and bioreactor technologies have been developed to improve reproducibility and scalability of in vitro organ models. The discovery of induced pluripotent stem cells (iPSCs) and the increasing number of protocols to generate iPSC-derived cell types has allowed for the generation of novel 3D models with minimal ethical limitations. The production of iPSC-derived 3D cultures has revolutionized the field of developmental biology and in particular, the study of fetal brain development. Furthermore, physiologically relevant 3D cultures generated from PSCs or adult stem cells (ASCs) have greatly advanced in vitro disease modelling and drug discovery. This review focuses on advances in 3D culture systems over the past years to model fetal development, disease pathology and support drug discovery in vitro, with a specific focus on the enabling role of biomaterials.

Salidroside as a potential neuroprotective agent for ischemic stroke: a review of sources, pharmacokinetics, mechanism and safety

Salidroside (Sal) is a bioactive extract principally from traditional herbal medicine such as Rhodiola rosea L., which has been commonly used for hundreds of years in Asia countries. The excellent neuroprotective capacity of Sal has been illuminated in recent studies. This work focused on the source, pharmacokinetics, safety and anti-ischemic stroke (IS) effect of Sal, especially emphasizing its mechanism of action and BBB permeability. Extensive databases, including Pubmed, Web of science (WOS), Google Scholar and China National Knowledge Infrastructure (CNKI), were applied to obtain relevant online literatures. Sal exerts powerful therapeutic effects on IS in experimental models either in vitro or in vivo due to its neuroprotection, with significantly diminishing infarct size, preventing cerebral edema and improving neurological function. Also, the findings suggest the underlying mechanisms involve anti-oxidation, anti-inflammation and anti-apoptosis by regulating multiple signaling pathways and key molecules, such as NF-κB, TNF-α and PI3K/Akt pathway. In pharmacokinetics, although showing a rapid absorption and elimination, bioavailability of Sal is elevated under some non-physiological conditions. The component and its metabolite (tyrosol) are capable of distributing to brain tissue and the later keeps a higher level of concentration. Moreover, Sal scarcely has obvious toxicity or side effects in a variety of animal experiments and clinical trials, but combination of drugs and perinatal use of medicine should be taken more attentions. Finally, as an active ingredient, not only is Sal isolated from diverse plants with limited yield, but also large batches of the products can be harvested by biological and chemical synthesis. With higher efficacy and better safety profiles, Sal could sever as a promising neuroprotectant for preventing and treating IS. Nevertheless, further investigations are still required to explore the pharmacodynamic and pharmacokinetic properties of Sal in the treatment of IS.

Recent Developments in Microfluidic Technologies for Central Nervous System Targeted Studies

Neurodegenerative diseases (NDs) bear a lot of weight in public health. By studying the properties of the blood-brain barrier (BBB) and its fundamental interactions with the central nervous system (CNS), it is possible to improve the understanding of the pathological mechanisms behind these disorders and create new and better strategies to improve bioavailability and therapeutic efficiency, such as nanocarriers. Microfluidics is an intersectional field with many applications. Microfluidic systems can be an invaluable tool to accurately simulate the BBB microenvironment, as well as develop, in a reproducible manner, drug delivery systems with well-defined physicochemical characteristics. This review provides an overview of the most recent advances on microfluidic devices for CNS-targeted studies. Firstly, the importance of the BBB will be addressed, and different experimental BBB models will be briefly discussed. Subsequently, microfluidic-integrated BBB models (BBB/brain-on-a-chip) are introduced and the state of the art reviewed, with special emphasis on their use to study NDs. Additionally, the microfluidic preparation of nanocarriers and other compounds for CNS delivery has been covered. The last section focuses on current challenges and future perspectives of microfluidic experimentation.

Lab-on-a-Chip Technologies for the Single Cell Level: Separation, Analysis, and Diagnostics

In the last three decades, microfluidics and its applications have been on an exponential rise, including approaches to isolate rare cells and diagnose diseases on the single-cell level. The techniques mentioned herein have already had significant impacts in our lives, from in-the-field diagnosis of disease and parasitic infections, through home fertility tests, to uncovering the interactions between SARS-CoV-2 and their host cells. This review gives an overview of the field in general and the most notable developments of the last five years, in three parts: 1. What can we detect? 2. Which detection technologies are used in which setting? 3. How do these techniques work? Finally, this review discusses potentials, shortfalls, and an outlook on future developments, especially in respect to the funding landscape and the field-application of these chips.

Evaluation of Cell-Penetrating Peptides Using Microfluidic In Vitro 3D Brain Endothelial Barrier

In drug delivery to the human brain, blood vessels are a significant hurdle because they restrict the entry of most solutes to protect brain. To overcome this hurdle, an in vitro 3D model for brain endothelial barrier is developed using a microfluidic device with hydrogel providing a 3D extracellular matrix scaffold. Using the model, peptides known to utilize receptor-mediated transcytosis are verified, which has been one of the most promising mechanisms for brain-specific penetration. The cytotoxicity and cellular damage to the peptide are investigated and the receptor-mediated transcytosis and brain endothelial specific penetrating abilities of the peptides in a quantitative manner are demonstrated. As a preclinical test, applying the quantification assays conducted in this study are suggested, including the penetrating ability, cytotoxicity, endothelial damage, and receptor specificity. Using this microfluidic device as an in vitro platform for evaluating various brain targeting drugs and drug carrier candidates is also proposed.

Biofabrication Strategies and Engineered In Vitro Systems for Vascular Mechanobiology

The vascular system is integral for maintaining organ-specific functions and homeostasis. Dysregulation in vascular architecture and function can lead to various chronic or acute disorders. Investigation of the role of the vascular system in health and disease has been accelerated through the development of tissue-engineered constructs and microphysiological on-chip platforms. These in vitro systems permit studies of biochemical regulation of vascular networks and parenchymal tissue and provide mechanistic insights into the biophysical and hemodynamic forces acting in organ-specific niches. Detailed understanding of these forces and the mechanotransductory pathways involved is necessary to develop preventative and therapeutic strategies targeting the vascular system. This review describes vascular structure and function, the role of hemodynamic forces in maintaining vascular homeostasis, and measurement approaches for cell and tissue level mechanical properties influencing vascular phenomena. State-of-the-art techniques for fabricating in vitro microvascular systems, with varying degrees of biological and engineering complexity, are summarized. Finally, the role of vascular mechanobiology in organ-specific niches and pathophysiological states, and efforts to recapitulate these events using in vitro microphysiological systems, are explored. It is hoped that this review will help readers appreciate the important, but understudied, role of vascular-parenchymal mechanotransduction in health and disease toward developing mechanotherapeutics for treatment strategies.

Vascularized neural constructs for ex-vivo reconstitution of blood-brain barrier function

Ex-vivo blood-brain barrier (BBB) model is of great value for studying brain function and drug development, but it is still challenging to engineer macroscale three-dimensional (3D) tissue constructs to recapitulate physiological and functional aspects of BBB. Here, we describe a delicate 3D vascularized neural constructs for ex-vivo reconstitution of BBB function. The tissue-engineered tissue construct is based on a multicomponent 3D co-culture of four types of cells, which typically exist in the BBB and were spatially defined and organized to mimic the in vivo BBB structure and function. A porous polycaprolactone/poly (d,l-lactide-co-glycolide) (PCL/PLGA) microfluidic perfusion system works as the vasculature network, which was made by freeze-coating a 3D-printed sacrificial template. Endothelial cells were seeded inside the channels of the network to form 3D interconnected blood vessels; while other types of cells, including pericytes, astrocytes, and neurons, were co-cultured in a collagen matrix wrapping the vasculature network to derive a vascularized neural construct that recapitulates in vivo BBB function with great complexity and delicacy. Using this model, we successfully reconstituted BBB function with parameters that are similar to the in vivo condition, and demonstrated the identification of BBB-penetrating therapeutics by examining the molecular delivery to neuronal cells when relevant biologic molecules were applied to the vasculature circulation system of the neural construct.