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PhD Defence Martijn Tibbe | IMPROVING ORGAN ON-CHIP PHYSIOLOGY - EXPLORING THE BARRIER IN BLOOD-BRAIN BARRIER ON-CHIP SYSTEMS

Improving organ on-chip physiology - Exploring the barrier in blood-brain barrier on-chip systems

Martijn Tibbe is a PhD student in the research group Biomedical and Environmental Sensorsystems (BIOS). His supervisors are prof.dr.ir. A. van den Berg and prof.dr.ir. L.I. Segerink from the Faculty of Electrical Engineering, Mathematics and Computer Science (EEMCS).

Over the last decades, animal models are considered the gold standard when it comes to disease modelling and drug testing. However, since these models are ethically questionable and they do not provide a proper physiological representation of the human body, a vast body of research has been conducted on in vitro cell culture systems to study specific tissues of the human body. The first systems used were traditional two-dimensional (2D) mono- and co-cultures in cell culture dishes and in Transwell systems. With the development of microfluidic systems, the field of in vitro cell culture took a turn towards so-called organ on-chip (OOC) systems. Cells are cultured inside microchannels under realistic flow conditions, which ensures nutrient supply, waste removal and mechanical forces such as shear stress to the cells. OOC systems are portable, cheap to fabricate, reproducible and due to the small sample volumes, very cost effective and present an interesting alternative to animal testing which explains the growing interest for such systems from various industries. A large class of these OOC barrier systems consist of two channels separated by a porous membrane. One channel represents the luminal side of an organ, whereas the other channel represents the basolateral side. The goal of these systems is to study the interaction between both sides and transport from one side to the other. The used polymer membranes are often relatively thick compared to the thin barrier that separates cells in vivo. In literature, the membrane is visualized schematically as a thin layer in between two cell types, whereas it is in fact a few micrometer thick barrier between the cells.

In this thesis we focus on the BBB, which acts as a gatekeeper for the brain where it prevents harmful substances from entering. The barrier consists of a luminal side; formed by brain endothelial cells, tightly packed together by interconnecting tight junctions; and a basolateral side, which consists of multiple cells such as astrocytes, pericytes and other glial cells. The luminal side is separated from the basolateral side by a basement membrane which is only a few (∼20-200) nanometres thin. As drugs do not pass the barrier easily in vivo, understanding its functionality is crucial. Comparable to other OOC systems, BBB on-chip systems are often two-channel systems which contain a relatively thick (10 µm) polymer membrane for cell separation. However, the cell-cell interaction between brain endothelial cells and astrocytes is thought to be crucial for the development of a tight BBB.

To enable cell-cell contact between the basal and luminal side of the barrier, two different approaches of recreating a BBB on-chip are being described in this thesis. The first being the replacement of the currently used, 10 µm thick, Transwell membranes with a random pore distribution by a thinner membrane with controlled pore dimensions and organization. The second approach is to entirely remove the membrane to create a cell-gel interface. For both approaches, one or more systems were developed to see whether these systems are suitable for the desired application.