Electrical and microfluidic technologies for organs-on-chips:Mimicking blood-brain barrier and gut tissues

The goal of the research presented in this thesis is to develop new technologies for organs-on-chips to enable direct measurements of cell layer functions and to move towards high-throughput. In this introduction, a brief description is included of the tissues that were mimicked in the organs-on-chips described in this thesis. Next, conventional in vitro setups for mimicking these tissues are discussed as well as the advantages of organs-on-chips over these conventional in vitro models. Then, the most important tests of tissue function are described. Subsequently, the larger framework for the research described in this thesis is sketched and lastly an outline of the thesis is given

Microfluidic organ-on-chip technology for blood-brain barrier research

Organs-on-chips are a new class of microengineered laboratory models that combine several of the advantages of current in vivo and in vitro models. In this review, we summarize the advances that have been made in the development of organ-on-chip models of the blood-brain barrier (BBBs-on-chips) and the challenges that are still ahead. The BBB is formed by specialized e3ndothelial cells and separates blood from brain tissue. It protects the brain from harmful compounds from the blood and provides homeostasis for optimal neuronal function. Studying BBB function and dysfunction is important for drug development and biomedical research. Microfluidic BBBs-on-chips enable real-time study of (human) cells in an engineered physiological microenvironment, for example incorporating small geometries and fluid flow as well as sensors. Examples of BBBs-on-chips in literature already show the potential of more realistic microenvironments and the study of organ-level functions. A key challenge in the field of BBB-on-chip development is the current lack of standardized quantification of parameters such as barrier permeability and shear stress. This limits the potential for direct comparison of the performance of different BBB-on-chip models to each other and existing models. We give recommendations for further standardization in model characterization and conclude that the rapidly emerging field of BBB-on-chip models holds great promise for further studies in BBB biology and drug development

Studying the blood-brain barrier on a microfluidic chip

A realistic model of the blood-brain barrier (BBB) is valuable to perform drug screening experiments and to improve the understanding of the barrier's physiology at normal and pathological conditions. Although the conventional in vitro systems (e.g. Transwell systems) have been used for this, they lack reproducibility and have a static environment. To overcome these disadvantages so called “organs-on-chips” have been developed, which use microfluidics and (human) cells to mimic organ function.\ud \ud An example of the BBB chip is shown in the work of Griep et al., where human cerebral endothelial cells (hCMEC/D3) were cultured in a microfluidic device made of polydimethyl siloxane (PDMS). Recently we improved this model. Two PDMS parts with microchannels are placed on top of each other, with a porous membrane in between at the intersection serving as scaffold for the cells. hCMEC/D3 cells (kindly provided by INSERM, Paris, France) were cultured in the chip for up to 15 days. With the four integrated electrodes, which did not block view on the intersection, reliable transendothelial electrical resistance measurements were carried out. Additionally, using immunohistochemistry it was shown that the endothelium expressed tight junction proteins, which is an essential characteristic of the BBB.\ud \ud To further improve the physiological relevance of this promising platform, the cells inside the channels will be cultured under fluid flow. As application, this platform will be used to study the transport of nanocarriers with Alzheimer medication through the BBB. In addition, the clearance of Alzheimer-associated proteins (amyloid β) by the BBB can be examined

Simple and stable transendothelial electrical resistance measurement in organs-on-chips

Measuring transendothelial electrical resistance (TEER) is a popular way to monitor cellular barrier tightness in organs-on-chips. However, in these devices integrated electrodes often block sight on the cells and the measured part often includes fluid-filled channels with variable resistance. Therefore, we redesigned our blood-brain barrier on a chip model to be able to measure stable TEER with four electrodes positioned outside the culture area using a simple impedance measurement setu

Direct quantification of transendothelial electrical resistance in organs-on-chips

Measuring transendothelial or transepithelial electrical resistance (TEER) is a widely used method to monitor cellular barrier tightness in organs-on-chips. Unfortunately, integrated electrodes close to the cellular barrier hamper visual inspection of the cells or require specialized cleanroom processes to fabricate see-through electrodes. Out-of-view electrodes inserted into the chip's outlets are influenced by the fluid-filled microchannels with relatively high resistance. In this case, small changes in temperature or medium composition strongly affect the apparent TEER. To solve this, we propose a simple and universally applicable method to directly determine the TEER in microfluidic organs-on-chips without the need for integrated electrodes close to the cellular barrier. Using four electrodes inserted into two channels – two on each side of the porous membrane – and six different measurement configurations we can directly derive the isolated TEER independent of channel properties. We show that this method removes large variation of non-biological origin in chips filled with culture medium. Furthermore, we demonstrate the use of our method by quantifying the TEER of a monolayer of human hCMEC/D3 cerebral endothelial cells, mimicking the blood-brain barrier inside our microfluidic organ-on-chip device. We found stable TEER values of 22 Ω cm2±1.3 Ω cm2 (average ± standard error of the mean of 4 chips), comparable to other TEER values reported for hCMEC/D3 cells in well-established Transwell systems. In conclusion, we demonstrate a simple and robust way to directly determine TEER that is applicable to any organ-on-chip device with two channels separated by a membrane. This enables stable and easily applicable TEER measurements without the need for specialized cleanroom processes and with visibility on the measured cell laye