Laminar flow in organ-on-a-chip

Cell culture media flow in organ-on-a-chip systems often in a “laminar” form, but what does laminar mean? It simply means a very smooth flow, all the layers of the fluid move along each other very smoothly. Imagine cars in a traffic moving next to each other on a highway.

Engineers characterize this smoothness by the so called “Reynolds” number, also known in the short form “Re”. The lower this number, the smoother the flow. You can see the exact definition of laminar flow in the image below. As a reference, Re below 2300 in channels is considered laminar although in organ-on-a-chip devices it is usually well below this number.

A laminar flow is specifically useful when you want to have good control over the flow, for example control over the shear stress on the cells or tissue, or mixing of two culture media. Like the image below, imagine when cars move parallel to each other in traffic, or two cars have to move next to each other in a small street. The boundaries of the street require the cars to move smoothly and that limits the probability of an accident. We call this a smooth movement of the cars; in the case of streams of liquid, (instead of cars), we call this a laminar movement of the streams. Laminar movement, or flow, can be due to lower velocities (v), dimensions (d) or density (ρ). It can also be because of larger viscosity (μ). In all these cases, Re is very small.

Let’s put it this way: flow of water in a small scale, like in organ-on-a-chip, can behave like flow of honey when you squeeze it out of a bottle! You can control better where honey goes on your brie sandwich, right?!

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