The universe works in mysterious ways. For some reason, the inquiries I get are grouped together into similar subjects each week, even though they come from completely unrelated companies in completely unrelated fields. This week is microfluidics week, ladies and gentlemen! So far, and the week has only just begun, I have had no less than five inquiries on that subject, constituting a noticeable portion of my usual inquiries this week.
As I understand them, the basics of microfuidics are this. For a given device application, there is a requirement that tiny amounts of fluid get sucked up or routed through tiny channels made of plastic, usually through capillary action. As you might guess, surface energy of the polymer plays a big part in how easily the fluid moves through them. Coating a chamber with a hydrophilic coating will lower that surface energy, and allow for increased capillary action. Simple, right? Hey, let’s slap a coating on that and crank out some devices tomorrow, shall we?
Hold on a moment. Not all microfluidic applications are equal, and all of them are challenging when it comes to coating. Let’s look at some of the challenges.
1. “My channels are 3 microns wide and 15 cm long.” – Most hydrophilic coatings are applied by directly wetting the surface with the coating solution and then curing and drying it somehow. While it is true that there are some coatings out there that can coat a channel that is 3 microns wide, there is always the possibility with most coatings that the coating liquid will plug up the channel and clog it. Additionally, it is not only about width… it is also about channel length. Longer channels are more difficult to wet with the coating solution. Short channels have a better chance, but still have great risk of clogging with most hydrophilic coatings out there.
2. “My substrate material is XXXX, which will require plasma treatment… and my channels are 3 microns wide and 15 cm long.” – Remember, not all coatings stick to all surfaces. For example, to coat silicone, many hydrophilic coatings will require that you plasma treat the silicone before the coating will adhere. That is easily done if the surface is flat and open to the air, but when the surface is the inner diameter (ID) of a tiny channel, how do you expect to get the plasma inside to treat the surface? If your surface requires plasma treatment to get that coating to stick, you either need to look for a different coating, forget about a coating, or think about changing your substrate material to something that requires no pre-treatment.
3. “My device is exposed to temperatures of 100C under water for 5 years.” – Here’s something about hydrophilic coatings that I wish everyone knew: they degrade and/or delaminate in water at high temperatures. I don’t care what the coating is made from, all hydrophilic coatings have some similar principles in common. They all absorb a lot of water. They all either have a primer coat (that can absorb some water) or they are a single coat that absorbs a lot of water. At high temperatures, especially above the Tg of the polymer, water will infiltrate the primer coat or the single coat and plasticize the polymer further, causing greatly decreased adhesion. If there are no hydrolysis reactions eating away the coating (and that is a BIG if), the coating will eventually just float away. By “eventually”, I mean it will happen anywhere from instantly to not long after that…. certainly not over a period of 5 years, or even 1 day. (I haven’t actually done a degradation study of every hydrophilic coating out there, so I can’t be more specific.) If you ask to coat microfluidic channels, or any device really, the moment you say “water” and “100C”, it’s all over. Find some other way that does not involve a hydrophilic coating.
4. “Great, the coating works! Now how do we manufacture coated devices with it?” – This is a good question. Too often, there is not enough consideration given to whether or not something will be a nightmare to manufacture. Just because the technology works does not mean it is easy to make on any large scale, or even possible to make on a large scale. Start thinking about this early. Microfluidic devices would require highly specialized equipment to coat, and chances are that equipment would need to be designed, built, validated, etc. from scratch. That takes time.
Honestly, if two potential clients come to me and one has a 3 Fr catheter OD and the other has a microfluidic plate with 1-mm channels made out of PVC, I get a lot more excited about the former. The latter is a lot of effort with a very low chance of success.
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