Peristaltic pump vs pressure-based microfluidic flow control systems for Organ on-chip applications
Microfluidic cell culture has significant advantages over macroscopic culture in flasks, Petri dishes, and well-plates. This technology offers new possibilities to accurately reproduce the cellular environment and enables the analysis of biochemical processes that were not accessible before.
To mimic in vivo conditions, the type of perfusion system used is critical. Peristaltic pumps are widely used. They deliver a highly pulsatile flow that oscillates around the set flow rate value, which is not representative of any physiologic condition in the body and can damage cells. Conversely, a pressure-based system can deliver either constant flow or on-demand realistic pulsatile flow patterns simulating aortic flow.
To demonstrate the importance of flow stability in vascular models, endothelial cells seeded in microfluidic chips were perfused either using a peristaltic pump or pressure-based flow controllers.
Figure 1 shows the working principle of the recirculation system using pressure-based flow controllers. Two Flow EZ are connected to two reservoirs. Tubing pass through the L-SWITCH (allowing media recirculation), a flow unit, and the microfluidic device. Software used to control the system are All-in-One (A-i-O), and Microfluidics Automation Tool (MAT).
In the system using the peristaltic pump, the inlet and outlet tubing are both placed in a reservoir containing media which continuously flows within the microfluidic device. In both systems, the flow rate was monitored with a Flow Unit to evaluate fluctuations.
HBEC-i cells are seeded at ~80% confluency in both chips. Before flowing liquid to the microfluidic devices, the two cell cultures are similar in term of viability and confluency.
Media recirculation is performed using the Flow EZ (pressure based flow controllers) or peristaltic pump on each microfluidic device with a set flow rate of 50 µL/min for t = 24h. Figure XXX shows the flow rate as a function of time using the peristaltic pump (in orange) and the Flow EZ (in blue). Using the peristaltic pump the flow rate highly fluctuates, more than 40% flow variation compared to the targeted value.
When using the Flow EZ, we observe a highly stable flow rate with less than 2% flow variation.
After 24h of media recirculation, we observe a decrease in cell density when compared to t=0 with the peristaltic pump, suggesting that the large flow rate fluctuations led to cell detachment. Cells are roundly shaped, as opposed to the well spread cells with trigonal shape observed before perfusion. This suggests that, even though not detached, cells show less adhesion, are less viable, and cell function might have been impacted by the poor flow conditions.
In the cell culture perfused with the Flow EZ, we observe a similar cell confluency of ~80% when compared to t=0. In addition, cells have similar spreading, with trigonal shape. These results confirm that cells perfused with the Flow EZ remained healthy and viable because of ideal flow conditions.
When switching from conventional cell culture in flasks to microfluidics, attention is focused on the chip. We have demonstrated that the perfusion instrument is as important as the chip. Choosing the right instrument to reproduce the flow conditions cells experience in vivo is of major importance as it will impact cell survival, spreading, phenotype and extend their genetic expression.
In the above application, the results are striking. After only 1 day of perfusion, vascular cells grown under erratic pulsatile flow are dying. On the opposite, when submitted to laminar constant shear stress, similar to living conditions, cells survive and are nicely spread in the chip.