Microfluidic platforms for the study of neuronal injury in vitro

Harmine promotes axon regeneration through enhancing glucose metabolism

Axon regeneration requires a substantial mitochondrial energy supply. However, injured mature neurons often fail to regenerate due to their inability to meet these elevated energy demands. Our findings indicate that harmine compensates for the energy deficit following axonal injury by enhancing the coupling between glucose metabolism and mitochondrial homeostasis, thereby promoting axon regeneration. Notably, harmine facilitates mitochondrial biogenesis and enhances mitophagy, ensuring efficient mitochondrial turnover, and energy supply. Thus, harmine plays a crucial role in enhancing glucose metabolism to maintain mitochondrial function, demonstrating significant potential in treating mature neuronal axon injuries and sciatic nerve injuries.

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.

Assessment of corneal nerve regeneration after axotomy in a compartmentalized microfluidic chip model with automated 3D high resolution live-imaging

Introduction

Damage to the corneal nerves can result in discomfort and chronic pain, profoundly impacting the quality of life of patients. Development of novel in vitro method is crucial to better understand corneal nerve regeneration and to find new treatments for the patients. Existing in vitro models often overlook the physiology of primary sensory neurons, for which the soma is separated from the nerve endings.

Methods

To overcome this limitation, our novel model combines a compartmentalized microfluidic culture of trigeminal ganglion neurons from adult mice with live-imaging and automated 3D image analysis offering robust way to assess axonal regrowth after axotomy.

Results

Physical axotomy performed by a two-second aspiration led to a reproducible 70% axonal loss and altered the phenotype of the neurons, increasing the number of substance P-positive neurons 72 h post-axotomy. To validate our new model, we investigated axonal regeneration after exposure to pharmacological compounds. We selected various targets known to enhance or inhibit axonal regrowth and analyzed their basal expression in trigeminal ganglion cells by scRNAseq. NGF/GDNF, insulin, and Dooku-1 (Piezo1 antagonist) enhanced regrowth by 81, 74 and 157%, respectively, while Yoda-1 (Piezo1 agonist) had no effect. Furthermore, SARM1-IN-2 (Sarm1 inhibitor) inhibited axonal regrowth, leading to only 6% regrowth after 72 h of exposure (versus 34% regrowth without any compound).

Discussion

Combining compartmentalized trigeminal neuronal culture with advanced imaging and analysis allowed a thorough evaluation of the extent of the axotomy and subsequent axonal regrowth. This innovative approach holds great promise for advancing our understanding of corneal nerve injuries and regeneration and ultimately improving the quality of life for patients suffering from sensory abnormalities, and related conditions.

Custom-engineered hydrogels for delivery of human iPSC-derived neurons into the injured cervical spinal cord

Cervical damage is the most prevalent type of spinal cord injury clinically, although few preclinical research studies focus on this anatomical region of injury. Here we present a combinatorial therapy composed of a custom-engineered, injectable hydrogel and human induced pluripotent stem cell (iPSC)-derived deep cortical neurons. The biomimetic hydrogel has a modular design that includes a protein-engineered component to allow customization of the cell-adhesive peptide sequence and a synthetic polymer component to allow customization of the gel mechanical properties. In vitro studies with encapsulated iPSC-neurons were used to select a bespoke hydrogel formulation that maintains cell viability and promotes neurite extension. Following injection into the injured cervical spinal cord in a rat contusion model, the hydrogel biodegraded over six weeks without causing any adverse reaction. Compared to cell delivery using saline, the hydrogel significantly improved the reproducibility of cell transplantation and integration into the host tissue. Across three metrics of animal behavior, this combinatorial therapy significantly improved sensorimotor function by six weeks post transplantation. Taken together, these findings demonstrate that design of a combinatorial therapy that includes a gel customized for a specific fate-restricted cell type can induce regeneration in the injured cervical spinal cord.

A Microfluidic High-Capacity Screening Platform for Neurological Disorders

Compartmentalized cell cultures (CCCs) provide the possibility to study mechanisms of neurodegenerative diseases, such as spreading of misfolded proteins in Alzheimer's or Parkinson's disease or functional changes in, e.g., chronic pain, in vitro. However, many CCC devices do not provide the necessary capacity for identifying novel mechanisms, targets, or drugs in a drug discovery context. Here, we present a high-capacity cell culture microtiter microfluidic plate compliant with American National Standard Institute of the Society for Laboratory Automation and Screening (ANSI/SLAS) standards that allows to parallelize up to 96 CCCs/experimental units, where each experimental unit comprises three microchannel-connected compartments. The plate design allows the specific treatment of cells in individual compartments through the application of a fluidic barrier. Moreover, the compatibility of the plate with neuronal cultures was confirmed with rodent primary as well as human-induced pluripotent stem cell-derived neurons of the central or peripheral nervous system for up to 14 days in culture. Using immunocytochemistry, we demonstrated that the plate design restricts neuronal soma to individual compartments, while axons, but not dendrites, can grow through the connecting microchannels to neighboring compartments. In addition, we show that neurons are spontaneously active and, as deemed by the appearance of synchronous depolarizations in neighboring compartments, are synaptically coupled. In summary, the design of the microfluidic plate allows for both morphological and functional studies of neurological in vitro cultures with increased capacity to support identification of novel mechanisms, targets, or drugs.

Microfluidic Aqueous Two-Phase Focusing of Chemical Species for In Situ Subcellular Stimulation

Confinement of chemical species in a controllable micrometer-level (several to a dozen micrometers) space in an aqueous environment is essential for precisely manipulating chemical events in subcellular regions. However, rapid diffusion and hard-to-control micrometer-level fluids make it a tough challenge. Here, a versatile open microfluidic method based on an aqueous two-phase system (ATPS) is developed to restrict species inside an open space with micron-level width. Unequal standard chemical potentials of the chemical species in two phases and space-time correspondence in the microfluidic system prevent outward diffusion across the phase interface, retaining the target species inside its preferred phase flow and creating a sharp boundary with a dramatic concentration change. Then, the chemical flow (the preferred phase with target chemical species) is precisely manipulated by a microfluidic probe, which can be compressed to a micron-level width and aimed at an arbitrary position of the sample. As a demonstration of the feasibility and versatility of the strategy, chemical flow is successfully applied to subcellular regions of various kinds of living single cells. Subcellular regions are successfully labeled (cytomembrane and mitochondria) and damaged. Healing-regeneration behaviors of living single cells are triggered by subcellular damage and analyzed. The method is relatively general regarding the species of chemicals and biosamples, which could promote deeper cell research.

Functional bioengineered tissue models of neurodegenerative diseases

Aging-associated neurodegenerative diseases, such as Alzheimer's and Parkinson's diseases remain poorly understood and no disease-modifying treatments exist despite decades of investigation. Predominant in vitro (e.g., 2D cell culture, organoids) and in vivo (e.g., mouse) models of these diseases are insufficient mimics of human brain tissue structure and function and of human neurodegenerative pathobiology, and have thus contributed to this collective translational failure. This has been a longstanding challenge in the field, and new strategies are required to address both fundamental and translational needs. Bioengineered tissue culture models constitute a class of promising alternatives, as they can overcome the low cell density, poor nutrient exchange, and long term culturability limitations of existing in vitro models. Further, they can reconstruct the structural, mechanical, and biochemical cues of native brain tissue, providing a better mimic of human brain tissues for in vitro pathobiological investigation and drug development. We discuss bioengineering techniques for the generation of these neurodegenerative tissue models, including biomaterials-, organoid-, and microfluidics-based approaches, and design considerations for their construction. To aid the development of the next generation of functional neurodegenerative disease models, we discuss approaches to incorporate greater cellular diversity and simulate aging processes within bioengineered brain tissues.

Models of traumatic brain injury-highlights and drawbacks

Traumatic brain injury (TBI) is the leading cause for high morbidity and mortality rates in young adults, survivors may suffer from long-term physical, cognitive, and/or psychological disorders. Establishing better models of TBI would further our understanding of the pathophysiology of TBI and develop new potential treatments. A multitude of animal TBI models have been used to replicate the various aspects of human TBI. Although numerous experimental neuroprotective strategies were identified to be effective in animal models, a majority of strategies have failed in phase II or phase III clinical trials. This failure in clinical translation highlights the necessity of revisiting the current status of animal models of TBI and therapeutic strategies. In this review, we elucidate approaches for the generation of animal models and cell models of TBI and summarize their strengths and limitations with the aim of exploring clinically meaningful neuroprotective strategies.

Microfluidic 'brain-on chip' systems to supplement neurological practice: development, applications and considerations

Among the greatest general challenges in bioengineering is to mimic human physiology. Advanced efforts in tissue engineering have led to sophisticated 'brain-on-chip' (BoC) microfluidic devices that can mimic structural and functional aspects of brain tissue. BoC may be used to understand the biochemical pathways of neurolgical pathologies and assess promising therapeutic agents for facilitating regenerative medicine. We evaluated the potential of microfluidic BoC devices in various neurological pathologies, such as Alzheimer's, glioblastoma, traumatic brain injury, stroke and epilepsy. We also discuss the principles, limitations and future considerations of BoC technology. Results suggest that BoC models can help understand complex neurological pathologies and augment drug testing efforts for regenerative applications. However, implementing organ-on-chip technology to clinical practice has some practical limitations that warrant greater attention to improve large-scale applicability. Nevertheless, they remain to be versatile and powerful tools that can broaden our understanding of pathophysiological and therapeutic uncertainties to neurological diseases.

Recent Progress and Perspectives on Neural Chip Platforms Integrating PDMS-Based Microfluidic Devices and Microelectrode Arrays

Recent years have witnessed a spurt of progress in the application of the encoding and decoding of neural activities to drug screening, diseases diagnosis, and brain-computer interactions. To overcome the constraints of the complexity of the brain and the ethical considerations of in vivo research, neural chip platforms integrating microfluidic devices and microelectrode arrays have been raised, which can not only customize growth paths for neurons in vitro but also monitor and modulate the specialized neural networks grown on chips. Therefore, this article reviews the developmental history of chip platforms integrating microfluidic devices and microelectrode arrays. First, we review the design and application of advanced microelectrode arrays and microfluidic devices. After, we introduce the fabrication process of neural chip platforms. Finally, we highlight the recent progress on this type of chip platform as a research tool in the field of brain science and neuroscience, focusing on neuropharmacology, neurological diseases, and simplified brain models. This is a detailed and comprehensive review of neural chip platforms. This work aims to fulfill the following three goals: (1) summarize the latest design patterns and fabrication schemes of such platforms, providing a reference for the development of other new platforms; (2) generalize several important applications of chip platforms in the field of neurology, which will attract the attention of scientists in the field; and (3) propose the developmental direction of neural chip platforms integrating microfluidic devices and microelectrode arrays.

Axons-on-a-chip for mimicking non-disruptive diffuse axonal injury underlying traumatic brain injury

Diffuse axonal injury (DAI) is the most severe pathological feature of traumatic brain injury (TBI). However, how primary axonal injury is induced by transient mechanical impacts remains unknown, mainly due to the low temporal and spatial resolution of medical imaging approaches. Here we established an axon-on-a-chip (AoC) model for mimicking DAI and monitoring instant cellular responses. Integrating computational fluid dynamics and microfluidic techniques, DAI was induced by injecting a precisely controlled micro-flux in the transverse direction. The clear correlation between the flow speed of injecting flux and the severity of DAI was elucidated. We next used the AoC to investigate the instant intracellular responses underlying DAI and found that the dynamic formation of focal axonal swellings (FAS) accompanied by Ca²⁺ surge occurs during the flux. Surprisingly, periodic axonal cytoskeleton disruption also occurs rapidly after the flux. These instant injury responses are spatially restricted to the fluxed axon, not affecting the overall viability of the neuron in the acute stage. Compatible with high-resolution live microscopy, the AoC provides a versatile system to identify early mechanisms underlying DAI, offering a platform for screening effective treatments to alleviate TBI.

Microfluidics for Neuronal Cell and Circuit Engineering

The widespread adoption of microfluidic devices among the neuroscience and neurobiology communities has enabled addressing a broad range of questions at the molecular, cellular, circuit, and system levels. Here, we review biomedical engineering approaches that harness the power of microfluidics for bottom-up generation of neuronal cell types and for the assembly and analysis of neural circuits. Microfluidics-based approaches are instrumental to generate the knowledge necessary for the derivation of diverse neuronal cell types from human pluripotent stem cells, as they enable the isolation and subsequent examination of individual neurons of interest. Moreover, microfluidic devices allow to engineer neural circuits with specific orientations and directionality by providing control over neuronal cell polarity and permitting the isolation of axons in individual microchannels. Similarly, the use of microfluidic chips enables the construction not only of 2D but also of 3D brain, retinal, and peripheral nervous system model circuits. Such brain-on-a-chip and organoid-on-a-chip technologies are promising platforms for studying these organs as they closely recapitulate some aspects of in vivo biological processes. Microfluidic 3D neuronal models, together with 2D in vitro systems, are widely used in many applications ranging from drug development and toxicology studies to neurological disease modeling and personalized medicine. Altogether, microfluidics provide researchers with powerful systems that complement and partially replace animal models.

Modeling Neurodegenerative Diseases Using In Vitro Compartmentalized Microfluidic Devices

The human brain is a complex organ composed of many different types of cells interconnected to create an organized system able to efficiently process information. Dysregulation of this delicately balanced system can lead to the development of neurological disorders, such as neurodegenerative diseases (NDD). To investigate the functionality of human brain physiology and pathophysiology, the scientific community has been generated various research models, from genetically modified animals to two- and three-dimensional cell culture for several decades. These models have, however, certain limitations that impede the precise study of pathophysiological features of neurodegeneration, thus hindering therapeutical research and drug development. Compartmentalized microfluidic devices provide in vitro minimalistic environments to accurately reproduce neural circuits allowing the characterization of the human central nervous system. Brain-on-chip (BoC) is allowing our capability to improve neurodegeneration models on the molecular and cellular mechanism aspects behind the progression of these troubles. This review aims to summarize and discuss the latest advancements of microfluidic models for the investigations of common neurodegenerative disorders, such as Alzheimer's disease, Parkinson's disease, and amyotrophic lateral sclerosis.

A Review on Microfluidic Platforms Applied to Nerve Regeneration

In recent decades, microfluidics have significantly advanced nerve regeneration research. Microfluidic devices can provide an accurate simulation of in vivo microenvironment for different research purposes such as analyzing myelin growth inhibitory factors, screening drugs, assessing nerve growth factors, and exploring mechanisms of neural injury and regeneration. The microfluidic platform offers technical supports for nerve regeneration that enable precise spatio-temporal control of cells, such as neuron isolation, single-cell manipulation, neural patterning, and axon guidance. In this paper, we review the development and recent advances of microfluidic platforms for nerve regeneration research.

The Emergence of Model Systems to Investigate the Link Between Traumatic Brain Injury and Alzheimer's Disease

Numerous epidemiological studies have demonstrated that individuals who have sustained a traumatic brain injury (TBI) have an elevated risk for developing Alzheimer's disease and Alzheimer's-related dementias (AD/ADRD). Despite these connections, the underlying mechanisms by which TBI induces AD-related pathology, neuronal dysfunction, and cognitive decline have yet to be elucidated. In this review, we will discuss the various in vivo and in vitro models that are being employed to provide more definite mechanistic relationships between TBI-induced mechanical injury and AD-related phenotypes. In particular, we will highlight the strengths and weaknesses of each of these model systems as it relates to advancing the understanding of the mechanisms that lead to TBI-induced AD onset and progression as well as providing platforms to evaluate potential therapies. Finally, we will discuss how emerging methods including the use of human induced pluripotent stem cell (hiPSC)-derived cultures and genome engineering technologies can be employed to generate better models of TBI-induced AD.

Microfluidic Surgery in Single Cells and Multicellular Systems

Microscale surgery on single cells and small organisms has enabled major advances in fundamental biology and in engineering biological systems. Examples of applications range from wound healing and regeneration studies to the generation of hybridoma to produce monoclonal antibodies. Even today, these surgical operations are often performed manually, but they are labor intensive and lack reproducibility. Microfluidics has emerged as a powerful technology to control and manipulate cells and multicellular systems at the micro- and nanoscale with high precision. Here, we review the physical and chemical mechanisms of microscale surgery and the corresponding design principles, applications, and implementations in microfluidic systems. We consider four types of surgical operations: (1) sectioning, which splits a biological entity into multiple parts, (2) ablation, which destroys part of an entity, (3) biopsy, which extracts materials from within a living cell, and (4) fusion, which joins multiple entities into one. For each type of surgery, we summarize the motivating applications and the microfluidic devices developed. Throughout this review, we highlight existing challenges and opportunities. We hope that this review will inspire scientists and engineers to continue to explore and improve microfluidic surgical methods.

A Brief Review of In Vitro Models for Injury and Regeneration in the Peripheral Nervous System

Nerve axonal injury and associated cellular mechanisms leading to peripheral nerve damage are important topics of research necessary for reducing disability and enhancing quality of life. Model systems that mimic the biological changes that occur during human nerve injury are crucial for the identification of cellular responses, screening of novel therapeutic molecules, and design of neural regeneration strategies. In addition to in vivo and mathematical models, in vitro axonal injury models provide a simple, robust, and reductionist platform to partially understand nerve injury pathogenesis and regeneration. In recent years, there have been several advances related to in vitro techniques that focus on the utilization of custom-fabricated cell culture chambers, microfluidic chamber systems, and injury techniques such as laser ablation and axonal stretching. These developments seem to reflect a gradual and natural progression towards understanding molecular and signaling events at an individual axon and neuronal-soma level. In this review, we attempt to categorize and discuss various in vitro models of injury relevant to the peripheral nervous system and highlight their strengths, weaknesses, and opportunities. Such models will help to recreate the post-injury microenvironment and aid in the development of therapeutic strategies that can accelerate nerve repair.

Bidirectional flow of action potentials in axons drives activity dynamics in neuronal cultures

Objective. Recent technological advances are revealing the complex physiology of the axon and challenging long-standing assumptions. Namely, while most action potential (AP) initiation occurs at the axon initial segment in central nervous system neurons, initiation in distal parts of the axon has been reported to occur in both physiological and pathological conditions. The functional role of these ectopic APs, if exists, is still not clear, nor its impact on network activity dynamics.Approach. Using an electrophysiology platform specifically designed for assessing axonal conduction we show here for the first time regular and effective bidirectional axonal conduction in hippocampal and dorsal root ganglia cultures. We investigate and characterize this bidirectional propagation both in physiological conditions and after distal axotomy.Main results.A significant fraction of APs are not coming from the canonical synapse-dendrite-soma signal flow, but instead from signals originating at the distal axon. Importantly, antidromic APs may carry information and can have a functional impact on the neuron, as they consistently depolarize the soma. Thus, plasticity or gene transduction mechanisms triggered by soma depolarization can also be affected by these antidromic APs. Conduction velocity is asymmetrical, with antidromic conduction being slower than orthodromic.Significance.Altogether these findings have important implications for the study of neuronal functionin vitro, reshaping our understanding on how information flows in neuronal cultures.

Engineered neural circuits for modeling brain physiology and neuropathology

The neural circuits of the central nervous system are the regulatory pathways for feeling, motion control, learning, and memory, and their dysfunction is closely related to various neurodegenerative diseases. Despite the growing demand for the unraveling of the physiology and functional connectivity of the neural circuits, their fundamental investigation is hampered because of the inability to access the components of neural circuits and the complex microenvironment. As an alternative approach, in vitro human neural circuits show principles of in vivo human neuronal circuit function. They allow access to the cellular compartment and permit real-time monitoring of neural circuits. In this review, we summarize recent advances in reconstituted in vitro neural circuits using engineering techniques. To this end, we provide an overview of the fabrication techniques and methods for stimulation and measurement of in vitro neural circuits. Subsequently, representative examples of in vitro neural circuits are reviewed with a particular focus on the recapitulation of structures and functions observed in vivo, and we summarize their application in the study of various brain diseases. We believe that the in vitro neural circuits can help neuroscience and the neuropharmacology. STATEMENT OF SIGNIFICANCE: Despite the growing demand to unravel the physiology and functional connectivity of the neural circuits, the studies on the in vivo neural circuits are frequently limited due to the poor accessibility. Furthermore, single neuron-based analysis has an inherent limitation in that it does not reflect the full spectrum of the neural circuit physiology. As an alternative approach, in vitro engineered neural circuit models have arisen because they can recapitulate the structural and functional characteristics of in vivo neural circuits. These in vitro neural circuits allow the mimicking of dysregulation of the neural circuits, including neurodegenerative diseases and traumatic brain injury. Emerging in vitro engineered neural circuits will provide a better understanding of the (patho-)physiology of neural circuits.

Knowledge transfer across cell lines using hybrid Gaussian process models with entity embedding vectors

To date, a large number of experiments are performed to develop a biochemical process. The generated data is used only once, to take decisions for development. Could we exploit data of already developed processes to make predictions for a novel process, we could significantly reduce the number of experiments needed. Processes for different products exhibit differences in behaviour, typically only a subset behave similar. Therefore, effective learning on multiple product spanning process data requires a sensible representation of the product identity. We propose to represent the product identity (a categorical feature) by embedding vectors that serve as input to a Gaussian process regression model. We demonstrate how the embedding vectors can be learned from process data and show that they capture an interpretable notion of product similarity. The improvement in performance is compared to traditional one-hot encoding on a simulated cross product learning task. All in all, the proposed method could render possible significant reductions in wet-lab experiments.

Tumor-on-a-chip: from bioinspired design to biomedical application

Cancer is one of the leading causes of human death, despite enormous efforts to explore cancer biology and develop anticancer therapies. The main challenges in cancer research are establishing an efficient tumor microenvironment in vitro and exploring efficient means for screening anticancer drugs to reveal the nature of cancer and develop treatments. The tumor microenvironment possesses human-specific biophysical and biochemical factors that are difficult to recapitulate in conventional in vitro planar cell models and in vivo animal models. Therefore, model limitations have hindered the translation of basic research findings to clinical applications. In this review, we introduce the recent progress in tumor-on-a-chip devices for cancer biology research, medicine assessment, and biomedical applications in detail. The emerging tumor-on-a-chip platforms integrating 3D cell culture, microfluidic technology, and tissue engineering have successfully mimicked the pivotal structural and functional characteristics of the in vivo tumor microenvironment. The recent advances in tumor-on-a-chip platforms for cancer biology studies and biomedical applications are detailed and analyzed in this review. This review should be valuable for further understanding the mechanisms of the tumor evolution process, screening anticancer drugs, and developing cancer therapies, and it addresses the challenges and potential opportunities in predicting drug screening and cancer treatment.

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.

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.

Current ex Vivo and in Vitro Approaches to Uncovering Mechanisms of Neurological Dysfunction after Traumatic Brain Injury

Traumatic brain injury often leads to progressive alterations at the molecular to circuit levels resulting in epilepsy and memory impairments. Ex vivo and in vitro models have provided a powerful platform for investigating the multimodal alteration after trauma. Recent ex vivo analyses using voltage sensitive dye imaging, optogenetics, and glutamate uncaging have revealed circuit abnormalities following in vivo brain injury. In vitro injury models have enabled examination of early and progressive changes in activity while development of three-dimensional organoids derived from human induced pluripotent stem cells have opened novel avenues for injury research. Here, we highlight recent advances in ex vivo and in vitro systems, focusing on their potential for advancing mechanistic understandings, possible limitations, and implications for therapeutics.

Current advances in in vitro models of central nervous system trauma

CNS trauma is a prominent cause of mortality and morbidity, and although much effort has focused on developing treatments for CNS trauma-related pathologies, little progress has been made. Pre-clinical models of TBI and SCI suffer from significant drawbacks, which result in substantial failures during clinical translation of promising pre-clinical therapies. Here, we review recent advances made in the development of in vitro models of CNS trauma, the promises and drawbacks of current in vitro CNS injury models, and the attributes necessary for future models to accurately mimic the trauma microenvironment and facilitate CNS trauma drug discovery. The goal is to provide insight for the development of future CNS injury models and to aid researchers in selecting effective models for pre-clinical research of trauma therapeutics.

Refining rodent models of spinal cord injury

This report was produced by an Expert Working Group (EWG) consisting of UK-based researchers, veterinarians and regulators of animal experiments with specialist knowledge of the use of animal models of spinal cord injury (SCI). It aims to facilitate the implementation of the Three Rs (Replacement, Reduction and Refinement), with an emphasis on refinement. Specific animal welfare issues were identified and discussed, and practical measures proposed, with the aim of reducing animal use and suffering, reducing experimental variability, and increasing translatability within this critically important research field.

A Microfluidic Culture Platform to Assess Axon Degeneration

The field of microfluidics allows for the precise spatial manipulation of small amounts of fluids. Within microstructures, laminar flow of fluids can be exploited to control the diffusion of small molecules, creating desired microenvironments for cells. Cellular neuroscience has benefited greatly from devices designed to fluidically isolate cell bodies and axons. Microfluidic devices specialized for neuron compartmentalization are made of polydimethylsiloxane (PDMS) which is gas permeable, is compatible with fluorescence microscopy, and has low cost. These devices are commonly used to study signals initiated exclusively on axons, somatodendritic compartments, or even single synapses. We have also found that microfluidic devices allow for rapid, reproducible interrogation of axon degeneration. Here, we describe the methodology for assessing axonal degeneration in microfluidic devices. We describe several use cases, including enucleation (removal of cell bodies) and trophic deprivation to investigate axon degeneration in pathological and developmental scenarios, respectively.

In Vitro Blood-Brain Barrier-Integrated Neurological Disorder Models Using a Microfluidic Device

The blood-brain barrier (BBB) plays critical role in the human physiological system such as protection of the central nervous system (CNS) from external materials in the blood vessel, including toxicants and drugs for several neurological disorders, a critical type of human disease. Therefore, suitable in vitro BBB models with fluidic flow to mimic the shear stress and supply of nutrients have been developed. Neurological disorder has also been investigated for developing realistic models that allow advance fundamental and translational research and effective therapeutic strategy design. Here, we discuss introduction of the blood-brain barrier in neurological disorder models by leveraging a recently developed microfluidic system and human organ-on-a-chip system. Such models could provide an effective drug screening platform and facilitate personalized therapy of several neurological diseases.

Why Does Brain Trauma Research Fail?

Traumatic brain injury (TBI) represents a major health care problem and a significant social and economic issue worldwide. Considering the generalized failure in introducing effective drugs and clinical protocols, there is an urgent need for efficient treatment modalities, able to improve devastating posttraumatic morbidity and mortality. In this work, the status of brain trauma research is analyzed in all its aspects, including basic and translational science and clinical trials. Implicit and explicit challenges to different lines of research are discussed and clinical trial structures and outcomes are scrutinized, along with possible explanations for systematic therapeutic failures and their implications for future development of drug and clinical trials. Despite significant advances in basic and clinical research in recent years, no specific therapeutic protocols for TBI have been shown to be effective. New potential therapeutic targets have been identified, following a better understanding of pathophysiologic mechanisms underlying TBI, although with disappointing results. Several reasons can be pinpointed at different levels, from inaccurate animal models of disease to faulty preclinical and clinical trials, with poor design and subjective outcome measures. Distinct strategies can be delineated to overcome specific shortcomings of research studies. Identifying and contextualizing the failures that have dominated TBI research is mandatory. This review analyzes current approaches and discusses possible strategies for improving outcomes.

Tissue Models for Neurogenesis and Repair in 3D

Development and maturation of vascular and neuronal tissues occurs simultaneously in utero, and are regulated by significant crosstalk. We report on the development of a 3D tissue system to model neurogenesis and recapitulate developmental signaling conditions. Human umbilical vein endothelial cells (HUVECs) were seeded inside channels within collagen gels to represent nascent vascular networks. Axons extending from chicken dorsal root ganglia (DRGs) grew significantly longer and preferentially towards the HUVEC seeded channels with respect to unloaded channels. To replicate these findings without the vascular component, channels were loaded with brain-derived neurotrophic factor (BDNF), the principle signaling molecule in HUVEC-stimulated axonal growth, and axons likewise were significantly longer and grew preferentially towards the BDNF-loaded channels with respect to controls. This 3D tissue system was then used as an in vitro replicate for peripheral nerve injury, with neural repair observed within 2 weeks. These results demonstrate that our 3D tissue system can model neural network formation, repair after laceration injuries, and can be utilized to further study how these networks form and interact with other tissues, such as skin or skeletal muscle.

Recent Progress in the Development of Microfluidic Vascular Models

The blood vessel is part of the circulatory system, and systemic circulation provides the blood supply to all tissues. Arteries are pathways through which the blood is carried, and the capillaries have a key role in material exchange to maintain the tissue environment. Blood vessels have structures appropriate for their functions, and their sizes and cell types are different. In this review, we introduced recent studies of the microfluidic vascular models. The model structures are classified mainly as poly(dimethylsiloxane) and hydrogel microchannels and self-assembled networks. Basic phenomena and functions were realized in vascular models, including fluid shear stress, cell strain, interstitial flow, endothelial permeation, angiogenesis, and thrombosis. In some models, endothelial cells were co-cultured with smooth muscle cells, pericytes, and fibroblasts in an extracellular matrix. Examples of vascular models involving the brain, lung, liver, kidney, placenta, and cancer were also introduced.

Microfluidic flow cytometry: The role of microfabrication methodologies, performance and functional specification

Flow cytometry is an invaluable tool utilized in modern biomedical research and clinical applications requiring high throughput, high resolution particle analysis for cytometric characterization and/or sorting of cells and particles as well as for analyzing results from immunocytometric assays. In recent years, research has focused on developing microfluidic flow cytometers with the motivation of creating smaller, less expensive, simpler, and more autonomous alternatives to conventional flow cytometers. These devices could ideally be highly portable, easy to operate without extensive user training, and utilized for research purposes and/or point-of-care diagnostics especially in limited resource facilities or locations requiring on-site analyses. However, designing a device that fulfills the criteria of high throughput analysis, automation and portability, while not sacrificing performance is not a trivial matter. This review intends to present the current state of the field and provide considerations for further improvement by focusing on the key design components of microfluidic flow cytometers. The recent innovations in particle focusing and detection strategies are detailed and compared. This review outlines performance matrix parameters of flow cytometers that are interdependent with each other, suggesting trade offs in selection based on the requirements of the applications. The ongoing contribution of microfluidics demonstrates that it is a viable technology to advance the current state of flow cytometry and develop automated, easy to operate and cost-effective flow cytometers.