You could say that valves in microfluidics (or microvalves) are like traffic lights that control flow along microfluidic channels. But I’d say that they’re more like police barricades, stopping anyone they want, wherever they want. The sole purpose of microvalves is to control flow within a microfluidics device, allowing them to become very complex and more automated. Without microvalves, all reactions and mixing must occur in the same space, unless they were premixed elsewhere, which might just eliminate the advantage of microfluidics.
Every microvalve that has ever existed seems to be covered in “A review of microvalves” by Kwang W Oh and Chong H Ahn from the Journal of Micromechanics and Microengineering. I’ll be covering three of the valves mentioned, but feel free to check out all the other microvalves that didn’t make the cut. A companion paper for this post, “Incorporation of prefabricated screw, pneumatic, and solenoid valves into microfluidic devices” by George M Whitesides et al. , featured in Lab on a Chip, describes a way to prefabricate these three valves so that they can easily be added to existing lab-on-a-chips.
You guessed it, solenoid microvalves are based on…solenoids! A solenoid is a coil of wire wound in a helix, like a compressed spring. I won’t go into the physics behind it, but a magnetic field is created by passing current through the solenoid. If you put a metallic object within the coil and vary the current, you could move the object. This is the idea behind solenoid microvalves. A solenoid microvalve is simply a solenoid with an actuator inside of it. The actuator is situated above the channel of a microfluidic device and pushes down on the ceiling of the channel to collapse it and obstruct its flow. In this manner, the solenoid microvalve requires an elastomeric lab-on-a-chip, and is rather bulky.
Whitesides’ prefabrication does not involve a dramatic redesign of solenoid microvalves. Instead, he places the same solenoid in PDMS housing, which can accommodate the solenoid’s large footprint.
This is another one. You guessed it, microvalves are based on…screws! This is an extremely low tech microvalve that requires little more than a screw. The screw is incorporated into the microfluidic device and deflects the membrane of the channel by twisting it. A ball can be placed beneath the screw to prevent damage to the device. This design is great for lab-on-a-chips that are intended to be disposable or for a low-resource setting. It requires no power, has a small profile (although you need access to screw it manually) and is simple to use. Obviously, it’s not completely automated, but I suppose you could incorporate a small electric screwdriver somewhere.
Whitesides’ prefabrication method doesn’t really differ too much from the conventional screw microvalve design. The screw is essentially embedded in a housing which can be joined above an existing microfluidic channel. One of the key advantages argued by Whitesides is that prefabrication regulates the screw microvalves. All the prefabricated screw valves are created in an identical manner, and would require the same degree of rotation to shut off flow. This is important because it may not be possible to see how well the flow is obstructed, resulting in leakage or damage to the channels. Screw valves created by different people at different times may be placed differently. A half-turn of one person’s valve might not properly close the channel, while a half-turn of someone else’s valve might go too far and damage the device.
I know it says pneumatic, but these are commonly referred to as Quake Valves, named after Stephen Quake. Quake valves require additional channels, often perpendicular to the targeted microfluidic channels. The additional channels share a thin, common membrane with the targeted channels. When air at the right pressure is applied through the pneumatic channels, the shared membrane is deflected and obstructs the flow of fluid. This completely changed the field of microfluidics since its arrival in 2000, allowing such feats as 400 simultaneous PCR reactions.
As cool as these microvalves are, they certainly have their drawbacks. First, they require more planning because you not only need to incorporate an additional layer of pneumatic channels, but you also have to route all your channels so that they don’t overlap where you don’t want them to. A small change in the design of your microfluidic device could require a massive redesign. While the solenoid valve certainly has a large footprint over the lab-on-a-chip due to the large size of the solenoid, the Quake valves only require the inclusion of an additional layer, keeping the area directly around the device cleaner. But this setup requires a tank of pressurized gas near the device, so in reality its footprint isn’t so small. This obviously hinders a device from easily leaving the lab. The pneumatic valves can be controlled electronically, allowing a device consisting of multiple independent valves to become more automatic. For instance, Albert Folch of University of Washington created this video of a Microfluidic Ballet by controlling valves according to the frequency of the music by Dimitri Shostakovich.
Whitesides’ prefabrication of Quake valves diverges from the most from the conventional out of all three microvalves shown here. Simply consisting of a chamber attached to an air supply placed over a channel, this method doesn’t require any additional layers of valves. A conventional Quake valve would require an entire redesign of the pneumatic channel layer, while a prefabricated Quake valve can simply be relocated over a new channel. When activated, the chamber presses down on the channel wall and obstructs the flow.
We’ve only examined three different microvalves here, and you can see how radically unique they are. As we develop new microfluidic applications and materials, we’ll continually develop novel ways to control the flow. A high concentration of valves indicates greater device design sophistication, but even so, less can be more, and our best solution might just be the simplest one.
This is one part of my Microfluidics Beginner’s Guide. Check it out the rest of it and keep learning!
Oh, K., & Ahn, C. (2006). A review of microvalves Journal of Micromechanics and Microengineering, 16 (5) DOI: 10.1088/0960-1317/16/5/R01
Elizabeth Hulme, S., Shevkoplyas, S., & Whitesides, G. (2009). Incorporation of prefabricated screw, pneumatic, and solenoid valves into microfluidic devices Lab on a Chip, 9 (1) DOI: 10.1039/b809673b