Blood–brain barrier microfluidic chips and their applications

As a natural dynamic barrier separating blood from brain parenchyma, the blood–brain barrier (BBB) is mainly composed of brain microvascular endothelial cells (BMECs), pericytes, astrocytes, and a variety of neurons. The BBB regulates the highly selective transport of various substances between the brain and blood and maintains the stability of the central nervous system (CNS). Owing to this tight control, the BBB represents a formidable challenge for the delivery of drugs and other exogenous compounds into the CNS, which has bottlenecked the development of many drugs for CNS diseases. Therefore, efficient and precise in vitro models of the BBB are needed to assess the efficacy and toxicity of drugs targeting the CNS to inform drug design and to improve the success rate of agents that enter clinical evaluation. In vitro BBB models have rapidly advanced from the early two-dimensional (2D) static models to the current three-dimensional (3D) dynamic microfluidic chips. Although the commonly used, static, in vitro BBB models are simple to construct and TEER values are convenient to detect, the static models do not provide an ideal (i.e., accurate) BBB environment, since they lack the correct physiological size/scale and hemodynamic shear stress, both of which play substantial roles in promoting and maintaining EC differentiation into a specific BBB phenotype. Compared with traditional static models, 3D microfluidic models thus enable cells to react in a manner more closely resembling in vivo behavior by simulating a microenvironment with more natural signal transduction. As a result, the dynamic 3D BBB model can more accurately recapitulate the structure and function of the human BBB. Here we summarize the recent progress in in vitro microfluidic BBB chips and their research applications as well as discuss the prospects and challenges for where the technology is headed.