The fabrication of PDMS microfluidic devices will be discussed more closely with the given information and specification. There are 2 main parts to focus on the process. Firstly, using soft photolithography to create a negative master pattern on a silicon wafer substrate. Secondly, using this master to create a PDMS microfluidic device.
The first step is to ensure that has a hydrophilic surface on the silicon substrate. For this, it's being used oxygen plasma treatment which raises the surface energy of the substrate. This is essential to promote the adhesion between the silicon wafer and the photoresist on the final master pattern. It must be treating the silicon wafers in an oxygen plasma machine for a sufficient amount of time to improve hydrophilicity. It has been found that a treatment time of 10 minutes at 0.3 millibars gave good results. The hydrophilicity of the silicon wafer is defined by the contact angle of a water droplet. That can do a simple visual test by placing the water droplet on the surface of the wafer. If the droplet is flat, the surface is hydrophilic. Here can be seen the difference between a treated and treated surface notice the dull shape on the untreated surface. Oxygen plasma bonding should be given a clean wafer. However, if it's seen any dirt or dust on the surface of the wafer, can cleaned it using deionized water and isopropanol. It is extremely important to have a clean waste line to avoid defects in the final master pattern. Another step is also to spin called the silicon wafer with a photoresist. There are different photoresists available in the market to suit needs. In that study, SU-8 2100 has been chosen to create channels with 100 micrometers in height. At this point, it's a good idea to download the datasheet for the selected photoresist. And also, useful information is found here. With placing the silicon wafer about its center of rotation in the spin coater, this is important to get an even coating of photoresist. A simple way to manually center the wafer is by rotating it, using a finger on the edge of the wafer. It can be done by observing if the wave is rotating unevenly, and adjusting the position until the wafer is sentenced. It can be set the spin speed which determines the height of the design. It can consult the photoresist datasheet for guidance and the recommended spin speeds that may need to adjust the values from the datasheet depending on the type of equipment. With this work, it has used 10 seconds at 500 RPM and 30 seconds at 2,300 rpm to achieve a coaching height of 100 microns. Having said the spin speed, it can be applied to the photoresist. The best method that has been found to do this is to know the glass syringe of the photoresist and use this to dispense it onto the wafer. At this point, it should not be used plastic syringes as the photoresist can react with the plastic creating unwanted residue. It needs to be sure that there are no air bubbles in the syringe as this will cause defects in the coating. With the applying the photoresist onto the center of the wafer and should be kept the tip of the syringe is in contact with the photoresist to avoid bubbles. But should be also sure that the tip is clean. It's found that four milliliters of photoresist worked well on a four-inch diameter substrate in this study. A good tip here is to clean the lid of spin culture using acetone to avoid any contamination of photoresist. Then should run the spin coater. It is best to do this whole process as quickly as possible to ensure the best quality coat. Step 3 is soft baking the wafer. Soft baking is required to evaporate some solvent from the photoresist. This includes adhesion to the substrate and allows the UV photomask to be applied in the next step without sticking to the photoresist. It can be referred to the photoresist datasheet to get recommended times for the selected product. Here, it's found that five minutes at 65° followed by 20 minutes at 95 degrees worked well. There is another way to cool to room temperature before the next step. The convection within fume hoods can cause too much solvent to evaporate. To avoid this, it can be used a simple aluminum foil to cover the wafer. This also serves to keep the wafer clean and maintain the desired temperature. Step 4 is the UV exposure to cure the photoresist. Researchers should already have a UV photomask with their designs printed on them. Several guides are available online to help create designs on AutoCAD software. It cleans the photomask using distilled water. Any dirt of the channels on the photomask will show up on chosen final design. After this, exist a placing your photomasks onto the photoresist using a mask aligner. There is the correct side for the design. The easiest way to make sure to place the right side down is to add letters or numbers around channels when designing the photomask. A good tip at this stage is to cover as much of the unused areas of photoresist as possible using black pieces of the new photomask. This will prevent unwanted regions of cured photoresist from forming. During the developing stage, these regions can make it difficult to get well-developed channels. As known, prevention is better than cure. The recommended exposure dosage can be found on the datasheet. Based on these, it's calculated that an exposure time of 30 seconds. Step 5 is the post-exposure bake. This evaporates more solvent and improves the adhesion of the photoresist to the wafer. It also reduces stresses by promoting mechanical relaxation of the channel. This shows the referring to the photoresist datasheet for timing guidelines. With this study, that five minutes at 65 degrees followed by 12 minutes at 95 degrees worked well. If the design was sufficiently exposed the pattern should become visible after 2 minutes at 65 degrees. If this is not the case, then the wafer may be underexposed. Then, it can be moved on to developing the challenge.
Developing a high-quality channel is challenging. Final results are dependent on a person's skill and judgment if they are using a manual developing process. The photoresist datasheet has guidelines on timings for the SU8 developer. Timings are dependent on the width and height of channels. Here, it's found that for designs of 50 by 100 microns the timings in the datasheet resulted in significantly overdeveloped channels. It is useful to start with a basic understanding of the substances that are used to develop a designed channel. There have used SU8 developer acetone isopropanol and distilled water for development. Consider a scale from 1 to 10, where 10 is a substance that quickly and effectively removes SU8 and 1 is a substance that does not remove SU8. As expected SU8 developer right to 10. This is the selected primary developer and it will remove issue 8 quickly and effectively. Also, isopropanol ranks at 3. This is significantly less effective than acetone at removing SU8. It is useful for removing small amounts of SU8 without damaging the channels. Finally, distilled water would rank as 1. It does not develop SU8, but it can be used to remove other solvents and residue. It is also useful to dilute the other solvents to reduce the aggressiveness if some subtlety is required in developing. It is just the word of the portion before starting, it is better to underdevelop at this stage with over-deliver. This is can always improve underdeveloped channels, but an overdeveloped channel means that have to start this whole process again. If researchers are unsure, should be conservative with time use. Now, it can be started developing. Start by pouring the small amount of SU8 developer into a glass dish. The amount should be just enough to submerge the silicon wafer. It be should not use a plastic petri dish as the developer will react with the plastic. Plating or wafer into the developer and start the clock. The developing time is largely dependent on designed channel dimensions. In this study, the channel was 50 microns by 100 in height and that is found that three minutes 30 seconds was a good amount of time to leave the channel in the developer. Then it's should be taken out the silicon wafer and give it a quick rinse with the developer. It should be also able to see the photoresist streaking off the wafer. It's recommended that use isopropanol as this gives a good indication of how well developed your tenderness is. Notice the large amounts of white substance on the surface of the wafer. This is a photoresist. If researchers see this, it means that channel is underdeveloped. If the amount of substance is substantial, can put it back into the developer for about 20 seconds. It should be seen the white substance clear away. If the white substance remains that can use acetone to remove it. Remember acetone acts as a developer so be careful not to overdo it. If required that can simultaneously use distilled water and acetone to dilute the solution and prevent over development. It should be used the isopropanol again. The white substance should be significantly reduced. This means the channel is almost developed. Researchers should pay close attention to the channels. They should be darker than the substrate. If they become the same color as a substrate, then they are overdeveloped. Finally, there have uses distilled water to wash off any remaining residue. It should be also tried the silicon wafer using a nitrogen gun. Researchers should be careful not to overdo the nitrogen gun as the channels may fly off. Let the water cool to room temperature. Need to check the channels under a microscope and look for the wool quality and any defects and blockages. It can be seen crack formation in the channel, particularly at junctions. These can be removed by hard baking at 150 Celsius for 5 minutes. Now, there have selected silicon master wafer ready to create a PDMS device.
Part two is creating a PDMS device. It can be moved on to the second phase of the process. The first step is to apply the PDMS layer. It has been used commercially available silicone elastomer and curing agent to create the devices. Also, silicon is used due to its inert properties ease of manufacturing, and excellent optical transparency. It makes ten parts of elastomer to one part curing agent. The device is five millimeters thick approximately 30 milliliters of silicon or sufficient to cover a wafer or four-inch diameter. And placing the wafer in a petri dish to contain the PDMS while it cures. Alternatively, it can be used aluminum foil to contain the PDMS. Pouring the PDMS mixture onto the silicon wafer making sure that the wafer is on a flat surface. The chemical reaction in the PDMS creates bubbles and gets rid of these bubbles using a desiccator and a pump, if one does not have access to a desiccator leave the PDMS at room temperature for approximately one and a half hours. After this place, the silicon wafer in another one at 65 degrees to cure for approximately three to four hours. It should be leaved the PDMS covered to avoid any contamination. Once cured carefully have to cut out the PDMS device into rectangles roughly the size of the cover glass. In here, approval glasses measure 35 by 65 millimeters and need approximately 5 to 10 millimeters on each side of the channel to prevent designed channels from being too close to the edge of the cover glass as this may eventually cause a leak. Also, should be taken care now to keep the PDMS device clean as any contamination may result in blockages from channels. The next step is to punch the holes for the inlets and outlets. It has been used the standard 0.8-millimeter hole punch to create the holes. The holes should be punched from the channel side and should penetrate through the PDMS device. So it must be careful not to make PDMS too thick in the previous stage as hole punch may not be deep enough. It's found that it was preferred to punch the holes on a hard flat surface to prevent tearing of the PDMS on the top side of the device. Step 3. is oxygen plasma bonding to seal the channels with a cover glass. This is placing the cover glass from the PDMS onto the treat for the plasma. It should need to treat both the PDMS and the cover glass to achieve bonding. It is useful to place the device and cover glass on a few microscopes slides to make handling the cover glass easier for the next stage. Remember, there is a need to treat the channel side of the PDMS device so should make sure this is facing up as this is what will be treated. And also, should be sure the channels and the cover glass are clean. It can be cleaned with distilled water. It has been used oxygen plasma treatment for one and a half minutes at a pressure of 0.4 milligrams. Also, can use other gases such as air for treatment, but the parameters may change. The surface treatment effects last for approximately 10 seconds, so must be worked quickly for the next step. It should remove the device and cover glass and place the treated side of the cover glass firmly onto the treated side of the PDMS device. Using fingers to get rid of any air bubbles. Now have some filled channels. The strength of the bond is not at its maximum yet. So should be careful with the device. The next step is to insert the needles and tubes for both inlets and outlets. To seal the needles in the PDMS device that used a two-part epoxy resin. It's found it best to apply a small amount of epoxy resin about three millimeters above the tip of the needle. Again it should be made sure whether not to block the opening. Then must be inserted the needle into the holes made earlier. It should not whether push the needle all the way down to the cover glass as this will block the inlets and outlets. Leaving a small gap so that the particles can flow through the channel. And with applying extra glue if required around the needles on the top surface of the PDMS to completely seal it. A good set at this point is to make sure the natural curve of the tubes points away from the channel. This will make connections in the syringe pump during testing a lot easier. Then will put the whole device in so dry covered at 60 celsius. It's found by leaving the device for at least three days significantly improves the strength of the oxygen plasma bonding and the overall durability of the device. A having completed PDMS microfluidic device ready for testing.
The production steps using PDMS material in microfluidics are as follows. It is quite possible to conclude the production well with some tricks that appear at these stages. You can watch the application video on this subject on our youtube channel.