Air bubbles could form in microfluidic systems for many reasons, such as inappropriate channel design. In some cases these bubbles are intentionally generated, because they have many beneficial features, e.g., flow control, mixing enhancement, pumping improvement. Nevertheless, air bubbles are usually very harmful to microfluidic systems, especially in long-term cell culture experiments, because they cause many problems and are very hard to eliminate. This article discusses some of the complications that can occur as a result of bubble formation and accumulation in microfluidic systems.
Flow instability: Bubbles are dynamic, which means they move in the flow and expand/contract inside the microfluidic channel. As a result, the fluidic resistance increases, and the effective diameter of the microchannels decreases. This could be very harmful, and can excessively increase the pressure in the microchannels, especially when syringe pumps, which use fixed flow rate, are used.
Delayed system response: The microfluidic system response time would be affected by air bubbles accumulation in the microchannels. The expansion and contraction of the bubbles slow down the pressure change within the system, which increase the period needed for the system to reach pressure equilibrium and consequently delay the system response.
Aggregation problems: In addition, bubbles formation can result in aggregation problems in some experiments. The bubble/liquid interface is the region where aggregation of particles and proteins may occur. This phenomenon causes experimental errors and artifacts.
Damage of chemical functionalization of surfaces: Air bubbles can damage chemical grafting on the surfaces of microchannels when they pass through them.
Cell death: In cell culture experiments, bubbles can damage cells, because they increase the interfacial tension. This places stress on the cells, causing the rupture of cell membranes, and eventually leads to cellular death.