Air bubbles are among the most recurring issues in microfluidics. Because of the micrometric dimensions of the tubes and channels, air bubbles can be very difficult to remove and be very detrimental for your experiment. This review will detail the causes for air bubbles, the issues created, and present a large range of solutions to suppress air bubbles from your microfluidic setup.

Origins of Air Bubbles Inside Microfluidic Device 

Air bubbles inside the microfluidic channels can have several different origins. Identifying what is causing your microfluidic chip to fill with bubbles is the first step to eliminate them.

• Start of the experiment: When the flow controller device is set on, it can take some time before your microfluidic setup is entirely filled with water. During this time, a large amount of air can circulate into your setup. Depending on your setup and chip configuration, some residual bubbles can survive after the setup is fully filled with water.
• Fluid switch: When changing the injected liquid during an experiment, the same phenomenon can appear. If you change the liquid inside the reservoir, you might need some time to eliminate the amount of air introduced into the microfluidic setup.
• Porous material: Porous materials, such as PDMS, can induce air bubbles inside microfluidic chips, especially in long term experiments.
• Leaking issue: Air bubbles can appear during microfluidic experiments if one or several fittings are leaking.
• Dissolved gas: Gas contained in gaseous form in the liquids used during the experiment can cause air bubbles to form. This is the case particularly when the liquids are heated during the experiments.

Issues Created by Air Bubbles Inside Microfluidic Devices 

The issues caused by air bubbles during microfluidic experiments can be classified into two main categories. The first one concerns flow modifications, and the second one covers all the detrimental interactions that bubbles can have with the experiment itself. 

Flow Modifications

• Flow instability: The presence of air bubbles, moving in the fluidic setup, or dilating/contracting can cause an important flow rate instability.

• Compliance increase: When an air bubble is trapped somewhere in your fluidic setup, the time needed to reach pressure equilibration can increase. Indeed, when applying a pressure change, the air bubble will absorb some of the pressure switch by dilating or contracting. This effect can be particularly detrimental when a good fluidic reactivity is needed.
• Resistivity increase: An air bubble trapped inside a microfluidic channel will act as an additional fluidic resistance by reducing the diameter of the channels. This can cause important issues, especially when working with syringe pumps which apply a fixed flow rate. The pressure inside the microfluidic chip will then increase significantly.

Troubleshooting: How to remove bubble in microfluidics 

There is no universal solution that can guarantee a bubble free experiment. However, here are listed some tips than can greatly improve your experiment. They are separated into two categories: preventive and corrective measures.

Preventive Measures 

• Microfluidic chip design: Preventing bubble formation starts with the design of you chip. Avoiding for example acute angles in you chip design will decrease the risk to have air bubbles adhering inside the microfluidic channels.
• Fittings: One of the first measure to avoid air bubbles is to ensure that no fitting is leaking. Using Teflon tape can be really useful in order to obtain a leak-free setup.
• Liquids degasing: When possible, degassing the liquids prior to the experiment can help reducing bubbles formation, especially when the liquids are heated during the experiment.
• Injection loop: Using an injection loop can help you overcome the issue of bubbles entering the system when adding a new liquid. When using an injection loop, a capillary is filled with the sample you want to inject, and some liquid (such as a buffer) pushes the sample. It is possible to use conventional HPLC injection loop  or valve matrices to perform this.

CORRECTIVE MEASURES

• Pressure increase: Increasing the pressure inside your fluidic path can help detaching air bubbles from the tubing and channels walls. This solution is not always appropriate, especially when dealing with cells or fragile microfluidic chips.
• Pressure pulses: Applying pressure pulses is a very good way to detach air bubbles. When using a pressure controller, applying a square shaped pressure signal often works nicely.

 

• Bubbles dissolution: For air bubbles that are very difficult to detach, an alternative solution is to dissolve them. By applying a pressure at each inlet of the micro-fluidic chip for a certain time, the air bubble can be forced to dissolve into the liquid.
• Soft surfactant: In order to help detaching the bubble, a buffer with a soft surfactant (such as SBS) can be flushed through the fluidic path.

 

• Debubbling/Degassing systems: It is possible to use debubbling or degassing devices in-line in the microfluidic setup to get rid of air bubbles coming inside the microfluidic chip. There are also in the scientific literature some examples of bubble trap devices that can be microfabricated and added to the fluidic setup.

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