Why do people want to use microfluidics? For sensors that are quite easy, because it has been used fewer reagents, analytes, or volume and those are really thin microfluidics channels in which can have a high sensitivity to their analyte. The focus is that mostly on the fabrication and which kind of microfluidics can make. In the last 20 years, from physics to chemistry to synthesis so part of organic chemistry but also bioengineering or chemical engineering a lot of different fields are moving towards microfluidics. How do make that these devices? Let's check and compare four different methods of making microfluidics with pros and cons.
The first method is glass microfluidics. Some small devices can just buy them, and most of the time, it's using them with something that can close them and can make the connection with a chip. It can be achieved by making the connection with the fluidics with the tubing. The microfluidics of the glass is extremely stable. For example, it can be used in organic solvents, they can be super small and the problem of those is that can rarely do it in the lab. The things that people usually do it's by them and if they want to make the design yourself, then need to send the design to the producer that can produce them. It will take approximately a couple of weeks. And then these are extremely expensive because they have to produce them for custom design. The other problem is probably these chips are fragile so it means that people can break them quite easily. Those are usually made by etching the glass, so make the channel and then attach another piece of glass on top, that's the standard way of making the glass microfluidics. Think about the pros and cons of having glass microfluidics. The field of microfluidics exploded when you gave the possibility to scientists to make their designs. This was mostly due to G. Whitesides who is one of the highest cited scientists in the world. People can see that after he developed this method which is called replica molding the field practically exploded. This is because it's easy that can do it in the lab. Therefore, producers can make their design and then can make your microfluidic devices. Well, how to do this? First of all, the material that he used was PDMS because it's quite similar to glass. The refractive index it's very close, so it's transparent. The chemistry is also very similar, therefore can use for making reactive surfaces. And also another important point, polydimethylsiloxane has a flexible structure. How to make PDMS microfluidic devices? This method that "replica molding". It's part of PDMS and part of the glass, so people want to have their open channel on PDMS and then you seal them with glass. Why it's called replica molding? The need is a master, so a master is something that has been seen with the transistor. Let think about this. Having a surface and something on top of the surface. Then putting methyl silicon on top with curing and removing. After that, there are grooves on the material and attach them to glass. For making the master it's the lithography method and it's the same as the transistor the methodology for making transistors that have been observed. The SU-8 polymer is cured with UV light, using a transparency mask is possible to cure the SU-8 only where the mask is transparent and not where the mask is black. In this case, the master is made called positive. For doing this, naturally, need a cleanroom.
Most of the time, if a scientist wants to go using thin in a microchannel, he/she needs a cleanroom. This is because there is one speck of dust, it's a completely clogging channel. In addition to this, positive pressure inside the room and exist few microns air filters. As mentioned before, these devices making by replica molding. Some steps are the master is fabricated via lithography in a clean room, PDMS is poured over the master and, it is cured. After curing, the dimethyl silicon is peeled off from the master. When you put the polydimethylsiloxane on glass, it will be covalently attached there. This is an important issue, because no desire to delaminate. The covalently attach works with the plasma treatment. Oxidizing both surfaces, the glass, and polydimetylesiloxane are now reactive hydroxyl groups. When putting them together in the oven, then form another sealing either for the bond between them. So now these are covalently attached. If trying to remove them, can not do this. Because they are covalently attached as well.
Why cannot just attach PDMS on PDMS? It means that having a very flexible material, so don't have the glass anymore. It's not fragile anymore and it can have something more interesting because it will be one material flexible and this is extremely important if the scientist is thinking about organ-on-a-chip. Most of the cells don't like to be on glass, and they like to move a little bit to stretch a little bit. Especially if we are talking about lungs or muscle, cells they don't like that much glass or stiff material. They want to have some flexible material. Another interesting thing about having PDMS on PDMS is that again it's stretchable. So, now we can use it for sensors, for example on the skin, because don't have glass anymore. It can stretch, bend and nothing happens to the microfluidics. This is for example one sensor that can be put on skin and it will detect different analytes in sweat. Therefore, it can be explained as a wearable sensors, a device for the capture, storage, and colorimetric sensing of sweat in many sectors. Another super interesting thing is that if have PDMS and PDMS in the field of soft robotics, so if we are using now materials with two different stiffness, but still bendable, can have movement. For example, if we have something on the bottom which is kind of rigid, but still flexible and on top something extremely flexible. It can flex this one and this one will bend a little bit. This application is the field of soft robotics. Of course, this thought was the first actuator ever made and it works with the stiff material on the bottom and soft material on top. And if it applies air pressure on the top, this one will bend. There are no hard parts in this soft robotic and this is why it's called soft robotics. It's practically indestructible, it will not break so we can have actuators that are extremely strong and flexible. But we can have also something like soft robotics which will mimic the heart movement. Lately, this field is moving in 3D printable which can 3D print the full soft robotics. But also inside, we can have chemicals that when they react, will make pressure, because it will evolve carbon dioxide or other gases that can not move. These are some examples of PDMS/PDMS soft robotics.
3D Printed Microfluidics
Naturally, when the 3D printer started to be a little bit better, we had 3D printed microfluidics. It's in our plastic and somehow transparent. It's not transparent, but all the materials for the 3D printer have some problems with transparency. These devices can be made more transparent, but 3D printing is that having the design in three dimensional. If we use 3D printers, then we can have way more space used in three dimensions this is interesting also for having a better mixer because if we want to have two liquids that are mixing having a three-dimensional channel is always better than having a flat channel. Using these devices, it needs to design and 3D print so it's as simple as it sounds. The problem is that we never know which material is used. This is because, we can buy the material, but they rarely tell you which material it is so chemically that have no idea how this will react. The sequence of this type of chips is as follows; CAD design, conversion to STL file, slicing of design, and printing layer by layer. There is another thing that can do with the printer and it's printing the master itself, it can 3d print the master as done with lithography. With the 3D technology, we can make molds for PDMS, so instead of using the lithography and making all the difficult steps with lithography, can just use a 3D printer for making those the master for PDMS on glass or PDMS/PDMS. In this case, it's not printing the final microfluidic device, just printing the master so it means that again put PDMS on top cure the PDMS remove it and attaching and attach it directly on glass or another piece of polydimethylsiloxane. This also simplifies the master fabrication. These are made by masked SLA so both of them are working and have a position that can use for making masters. In this case, check the pros and cons of these devices. We can also use a 3D printer for making either the master of the microfluidic device, but the material is always kind of random which material you can get from different vendors. On the other hand, it's known that about PDMS/PDMS was studied for more than 20 years and extremely well how the PDMS behave. At this point, can we make the 3d Printer work with PDMS? For this, it has developed a method that it's called "escargo" which is a method embedded scaffold removing open technology. With this technology, we print ABS with the normal printer. So with a standard printer also standard material, then putting it inside the liquid PDMS. After that curing process is available, and then it leaves everything in acetone. The acetone will dissolve the ABS but not the PDMS. This will leave you a channel inside the single block of PDMS.
Consequently, there are some methods for the fabrication of microfluidic devices. These are glass, PDMS/glass, PDMS/PDMS, and 3D Printed microfluidic devices. Each method has pros and cros as mentioned. In addition, these production methods may vary according to the application to be worked on.