Mitigating Bubbles in Microfluidics
Have you ever been in the hospital, had a needle placed in your arm for intravenous (IV) therapy, noticed bubbles in the tube lines, and wondered what would happen if these bubbles entered your veins? Are they dangerous? (A little trick of tapping and flicking the line with your fingernail at the level of the bubble will make it disappear. What a relief!)
In microfluidics, bubbles appearing in a tube or enclosed channel are a very common scenario and challenging to tackle because of the small liquid channels and features. Depending on the application, bubbles can have a significant impact on the process performance and the intended use. In the worst case, they will make the microfluidic device nonfunctional or unusable.
In the past couple of years, medical microfluidics applications have expanded rapidly from point-of-care rapid diagnostics tools to more complex high throughput multiplex systems. Microfluidic channel dimensions are at the micro scale, so these bubbles tend to be very stubborn, difficult to remove, and affect the function and performance of the device. The fundamental step in solving any challenge is to determine with certainty the root cause, which in our case, is bubbles. This article presents several potential sources of the bubbles and recommendations on how to mitigate them.
Sources of Microbubbles
In IV lines, it’s easy to see bubbles in the tube; however, in microfluidic channels, bubbles are difficult to detect and see. Whatever the system or microfluidic format, it is crucial to avoid or eliminate these bubbles from the source.
1. Dissolved Gas
The formation of gas bubbles in a liquid is possible for any liquid system. A bubble can be formed when a sudden change in temperature, pressure, or agitation happens. If the amount of a gas in a liquid exceeds the solubility limit of the gas molecules, all the excess gas forms bubbles—provided the amount of liquid is sufficient to contain those bubbles. As a result, there is a steady-state condition in which new portions of molecules are injected into liquid and form bubbles1.
2. Bubbles at the Fluid Introduction Step
In channel microfluidics, the occurrence of bubbles within the fluidic path can be experienced when an air pocket is trapped in a cavity as fluid is introduced. The introduction of fluid may create bubbles inside the fluidic network. The generation and number of bubbles during the reagent introduction depend on a variety of factors such as the material used for the device, flow rates, viscosity, chemical composition, channel features, and the method of fluid introduction. When loading the sample, air bubbles could be trapped in the channel or surrounding features due to the device’s hydrophobicity2.
3. Surface Hydrophobicity and Roughness of the Channel
The surface properties of the microfluidic device are important in the occurrence of air bubbles in the channels. Using hydrophobic materials, air bubbles are entrapped and adhere to the surface of the channel, preventing the smooth flow of the reagents. If the channel surface is rough and imperfect, it changes the flow pattern, which will induce the formation of bubbles3.
4. Parts Acting as Bubble Generators
Microfluidic devices require a network of channels, reservoirs, micro-macro interfaces, and other micro features. Components with fluid flow transitions, such as micro-filters, mixers, chambers, pillars, indents, and ramps, create bubbles as the fluid passes by4.
5. Sudden Expansion Features in the Device (i.e., interfaces and transitions)
Any sudden expansion of fluidic flow in the channel design is considered a bubble generator. When the fluid velocity changes from high to low, bubbles are generated. As an example, an in-line microfilter is a bubble generator. The fluid at a certain velocity is forced to pass a smaller diameter orifice, and with this process, bubbles are generated5.
6. Change of Fluid Properaties and Composition
The viscosity of the fluid flowing through the channel plays a very important role in bubble generation. The fluid’s viscosity provides a direct relation between the velocity and channel features. The material used in the device, the surface properties of the microfluidic channel, and the fluid properties and composition increase the chances of bubble formation6,7.
7. Changes in Fluid Temperature and Pressure
Changes in fluid temperature create bubbles in the channel. This is the combination of the pressure and evaporation of the liquid changing to its gaseous form, creating bubbles. A study demonstrated that bubble diameter decreases and aspect ratio increases with pressure. The effect of temperature is complicated owing to the change of saturated vapor pressure and ratio to system pressure. When the ratio is larger, the bubble diameter increases with temperature due to the vaporization phenomenon. When the ratio is smaller, the bubble diameter decreases with temperature9.
8. Surface-Active Agents
Lastly, the addition of surface-active agents (i.e., surfactant, soap) and foaming agents to lower the surface tension increases bubble formation in the liquid. The purpose of these surface-active agents is to stabilize droplet interfaces, facilitate fluidic flow and biocompatibility, and improve molecular exchanges between droplets10.
Impact of Bubbles in a Microfluidic Device
The identified factors do not work alone but as a combination of each factor. Therefore, during microfluidic device development, it is crucial to consider each contributing factor and design the device as a system. It is challenging to mitigate all the sources of bubbles at once. Minimizing the contribution of the sources in the fluidic system to an acceptable level is good enough if they do not impact the device’s processes and performance. The following are the negative repercussions of bubbles in the microfluidic device:
Bubbles obscure the view of the target particles or objects under investigation. Bubbles usually appear black under a microscope. Furthermore, the presence of bubbles in optical detection provides erroneous results. In most cases, the bubble changes the performance of the detection system by acting as an artifact and making the target particles unrecognizable. Also, it becomes too difficult to see the particles of interest, especially if they are covered by bubbles.
Bubbles change the flow pattern and flow volume within the channel. A big bubble tends to restrict the flow and when accumulating to a certain location, it reduces the flow volume making the assay not workable.
Bubbles increase the fluidic resistance, causing an increase in the governing pressure to actuate the flow. When bubbles restrict the flow, the pneumatic system is required to increase the actuation pressure to allow the amount of flow needed by the system.
Bubbles interact with biological samples. The presence of bubbles interacts with or affects the biological materials under consideration. For cells, bubbles could attach to the wall membrane, affecting the subsequent processes or reactions and putting the assay at risk.
Bubbles tend to clog the microfluidic fluidic network. When clogging occurs, the device can no longer continue the assay execution, making it nonfunctional and failing. Smaller channels (5-75µm) are very susceptible to bubbles. Sometimes, bubbles adhere to the walls of the channel and start to accumulate, preventing the flow. If using an automated sequence actuation, monitoring the pressure and fluidic flow becomes so challenging it results in the device’s failure. When the fluid flow decreases, clogging happens along the fluidic network.
How to Avoid and Mitigate Bubbles in the Microfluidic Network
There are a lot of possible ways to avoid and mitigate the presence and formation of bubbles:
Design the microfluidic device as a system. Bubbles in the fluidic network are the result of not properly thought-out devices. The device should be designed based on a target product profile that covers all requirements from assay to the final product.
Conduct simulations and modeling to understand the fluidic flow characteristics. These will act as a guide in the proper design of microfeatures and understanding processes such as the impact of sudden expansion or the use of off-the-shelf parts and fittings.
Identify and design appropriate fittings and parts. The fewer parts used in the device, the better.
De-gas fluid prior to its introduction to the device to reduce the dissolved gas.
Select the proper materials, parts fabrication techniques, and surface treatment.
Increase the size of the bubbles to separate them from the fluids using a mechanical agitator, or increase and pulse the pressure, and in some cases, slightly increase the temperature.
Introduce an in-line bubble trap or a vent to divert the bubble from the channel.
Use a membrane to remove air bubbles. Choose a membrane wherein gas can pass through, but not liquid. This will reduce the occurrence of bubbles in the network of channels.
Microfluidics bubbles can be macro or microbubbles. Unlike those in IV lines, microfluidic bubbles are stubborn and difficult to remove. Simple tapping or a flick of the fingers to the device will not work.
Microfluidic channels and features are potential sources of bubbles when poorly designed and manufactured. Bubbles are always present. Considering the recommendations outlined in this article as a good engineering and manufacturing practice.
If possible, avoid potential sources of bubbles in the system; however, if they cannot be avoided due to device and assay constraints, minimizing the possible sources and providing mitigation measures will suffice. Acceptance criteria must be set to a level so any bubbles no longer affect the device’s performance. De-gassing the fluids is ideal; however, it will require an additional processing step in the workflow. An in-line bubble trap using a permeable membrane can be used but may not be the optimum alternative when there are constraints in the system, such as footprint, size of the part features, or manufacturability of the components.
A combination of prevention and mitigation options must be considered by balancing the microfluidic device’s overall complexity, cost, and performance. Furthermore, simulation and modeling during the initial design process offer a way to save time and prototyping effort, particularly with features deemed to be potential bubble generators and parts critical to the microfluidic device’s performance.
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Article source: Medical Product Outsourcing