Advances in Microfluidic Blood–Brain Barrier (BBB) Models

Microfluidic-based blood–brain barrier-on-chip (μBBB) technology is a powerful approach to study the physiological function of the BBB in vitro and to facilitate drug discovery targeting brain disorders. Mimicry of the complexity of multiple cell crosstalk and thin extracellular matrix as basal membrane are essential but challenging. Different biomaterials and chip designs have been explored in the fabrication of μBBBs. Other key features such as shear stress, cell type/origin, and cell co-culture spatial configuration must be carefully controlled and selected. Appropriate BBB permeability assays and parameters (e.g., TEER measurement, small molecule drugs, and fluorescent probes) should be standardized and compared with in vivo data. μBBBs hold great potential in disease modeling, drug discovery, neurotoxicity screening, and personalized medicine applications. Therapeutic options for neurological disorders currently remain limited. The intrinsic complexity of the brain architecture prevents potential therapeutics from reaching their cerebral target, thus limiting their efficacy. Recent advances in microfluidic technology and organ-on-chip systems have enabled the development of a new generation of in vitro platforms that can recapitulate complex in vivo microenvironments and physiological responses. In this context, microfluidic-based in vitro models of the blood–brain barrier (BBB) are of particular interest as they provide an innovative approach for conducting research related to the brain, including modeling of neurodegenerative diseases and high-throughput drug screening. Here, we present the most recent advances in BBB-on-chip devices and examine validation steps that will strengthen their future applications. Therapeutic options for neurological disorders currently remain limited. The intrinsic complexity of the brain architecture prevents potential therapeutics from reaching their cerebral target, thus limiting their efficacy. Recent advances in microfluidic technology and organ-on-chip systems have enabled the development of a new generation of in vitro platforms that can recapitulate complex in vivo microenvironments and physiological responses. In this context, microfluidic-based in vitro models of the blood–brain barrier (BBB) are of particular interest as they provide an innovative approach for conducting research related to the brain, including modeling of neurodegenerative diseases and high-throughput drug screening. Here, we present the most recent advances in BBB-on-chip devices and examine validation steps that will strengthen their future applications. enzyme responsible for the degradation of acetylcholine (ACh). Its inhibition leads to accumulation of ACh in synapses and neuromuscular junctions and consequent hyper-cholinergic activity, resulting in seizures and brain damage. class of proteins with important roles in vascular development and angiogenesis. star-shaped glial cells in the central nervous system. The end-feet of astrocytes encircle ECs and crosstalk with neurons in the neurovascular units. inner layer of the basement membrane, constituted by the extracellular matrix secreted by the epithelial cells, on which the epithelium sits. also known as basement membrane, a highly specialized extracellular matrix composed of two layers: basal lamina and underlying layer of reticular connective tissue. It is characterized by sheet-like networks, which physically separate the vasculature and the pial surface of the brain. The basal membrane consists of structural and functional proteins, such as collagen IV, fibronectin, laminin, and proteoglycans. major component of the brain interstitial system, the complex region defined by the membranes of the assembled cells, and acts as the conduit for metabolic substrates to and from the brain. It contains the extracellular matrix and interstitial fluid, which is rich in ions, gaseous molecules, and organic molecules. branched, high molecular weight glycan (3–2000 kDa) that does not pass through the BBB actively. main type of cells that line the interior surface of blood vessels, lymphatic vessels, and the heart. They determine changes in the vessel intraluminal diameters through vasodilatation and vasoconstriction, maintaining homeostasis. 3D network of extracellular macromolecules which support surrounding cells structurally and biochemically. The common components of ECM are collagen, fibronectin, laminin, proteoglycans, hyaluronic acid, etc. Specifically, laminins are glycoproteins having 18 different isoforms, some of which are only expressed in BMECs and astrocytes. As a main structural protein, collagen IV’s function is to stabilize BMs by retaining laminin, nidogen, and perlecan. Knockdown of either collagen IV or laminin in mice results in embryonic lethality. subpopulation of cells within a tumor mass, responsible for tumorigenesis and resistance to conventional therapies. maintenance of a steady state condition in spite of external changes. Cerebral homeostasis ensures a continuous and consistent supply of oxygen, ions, and nutrients via blood supply to the brain and is a finely regulated mechanism. a type of pluripotent stem cell that is generated directly from adult somatic cells by directly transfecting or chemically inducing the expression of specific sets of pluripotency-associated genes to target cells. class of signaling molecules that are excreted from immune cells like helper T cells and macrophages, and certain other cell types that promote inflammation. For instance, IL-1 and TNF are proinflammatory cytokines, and when they are administered to humans they produce fever, inflammation, tissue destruction, and, in some cases, shock and death. malignant type of brain tumor that begins as cancer elsewhere in the body. Most common metastatic brain tumors start in the lung, breast, colon, or skin. primary innate immune cells in the central nervous system. They contribute to many physiological functions, including synaptic plasticity, immune cell recruitment, and neurogenesis. However, in many diseases microglia lose their homeostatic function and become chronically inflammatory. 3D multicellular in vitro tissue construct that mimics its corresponding in vivo organ, such that it can be used to study aspects of that organ in the tissue culture dish. class of chemicals often used in agriculture as insecticides. They were first developed as chemical warfare agents, as potent nerve agents. Their neurotoxicity is related to the inhibition of acetylcholinesterase in nerve cells. non-cell in vitro model based on the use of a specially designed artificial lipid-impregnated membrane established on a solid filter support. It models the passive diffusion of molecules penetrating BBB but does not model active transport nor paracellular permeation in the endothelium. smooth muscle-derived cells that play a key role in maintaining the structural stability of the vessel walls and regulate vasodynamic mechanisms. These cells wrap around endothelial cells to form microvessels, including capillaries, venules, and arterioles. BBB permeability of an analyte in a BBB device is calculated as apparent permeability coefficient independent of flow rate and analyte size according to the following equation:Papp=ValCalAClt,whent≪ValA∙Papp;1Papp=1PBBB+1P0[1] where Val (ml) and Cal (mol/ml) are the volume and concentration of target analyte in the ‘brain’ channel, respectively; A (cm2) is the contact area between abluminal and luminal channels; Cl (mol/ml) is the analyte concentration in the ‘blood’ channel; and t is the perfusion period. P0 (cm/s) represents the permeability from a blank device with ECM coating, and PBBB represents the BBB permeability of the target analyte (cm/s). include embryonic stem cells and induced pluripotent stem cells. They are self-renewable and able to differentiate into any cell or tissue of the body. This ‘master’ property is called pluripotency. optically clear and inert polymer with broad biomedical applications. Since being introduced in the late 1990s as a microfluidic device building block, PDMS has now become the most common microfluidics fabrication material among the research community. tumor that originates in the brain. Primary brain tumors may be grouped into ‘benign’ and ‘malignant’ tumors. Glioblastoma multiforme (GBM), astrocytoma, medulloblastoma, and ependymoma are examples of primary brain tumors. enzyme responsible for the phosphorylation of a wide variety of protein targets and is involved in cellular signaling pathways, including growth, apoptosis, and differentiation. It also plays a major role in human diseases, including cancer. areas within glioblastoma, characterized by dense cell populations and high cell apoptosis. The release of procoagulant factors secreted by glioblastoma tumor cells might induce thrombosis. As a consequence, the supply of oxygen and nutrients is compromised, forcing tumor cells to migrate towards nutrients and oxygen-enriched regions. This mechanism may explain the aggressiveness of glioblastoma, which alternate cycles of migration with cycles of high proliferation. they include astrocytes, microglia, and ependymal cells, and are the most abundant cells in the central nervous system. Glial cells are a non-neuronal cell type in the central nervous system and do not conduct electrical impulses. They surround neurons and provide support and insulation between them. Dysfunction of glial cells is associated with many neurodegenerative diseases. type of cell–cell junctions that prevents the majority of soluble molecules from being transported between the two sides of the epithelium. They are mainly composed by claudin and occludin proteins. measurement of electrical resistance across a cellular monolayer. Some of the biological barrier models that have been widely characterized utilizing TEER include BBB, gastrointestinal tract, and pulmonary models. Since paracellular and transcellular transport across the brain endothelium are highly regulated, this results in higher electrical resistance, measured as TEER. Thus, increased transport of charged species, such as ions and amino acids, is associated with leaky TJs. a membrane insert that separates well plates into an upper compartment and a lower compartment. Transwell inserts are widely used in permeability and cell migration assays, with different sizes, membrane types, pore sizes, and densities being commercially available. any foreign substance or exogenous chemical not normally present in the body, such as drugs and pollutants.