Manufacturing a PDMS microfluidic device via a Silicon Wafer Master Eric Lam, Tri Ngo Abstract Mixing the PDMS Degassing the PDMS Pouring the PDMS PDMS Unmolding Post Processing of PDMS device Slide Cleaning Device Bonding Attaching Pipettes or Syringe ReservoirsManufacturing a PDMS microfluidic device via a Silicon Wafer Master Eric Lam, Tri Ngo Abstract This document describes the manufacturing process for a general microfluidic device which uses either pipette tips, O-Ring or non O-Ring syringe reservoirs as a fluid delivery mechanism. The manufacturing process does not involve much labor but requires a lot of waiting. The entire process takes a minimum of 5 hours but optimally the process takes 9 hours. First the PDMS is mixed and degassed for approximate 1 hour. Then the activated PDMS is poured onto the master and baked in an oven. After a minimum of 4 hours of baking, the PDMS is unmolded from the master and processed further. Next a glass slide is cleaned and the PDMS layer containing the microfluidic features are bonded to the glass slide. Finally the device is completed by attaching either pipette tips or pre-cut syringe tubes. Mixing the PDMS In preparation for this stage obtain a some unpowdered disposable gloves, a disposable plastic knife and a disposable plastic cup. PDMS should be mixed in a 1:10 ratio of curing agent and PDMS monomers. The PDMS monomers are much more viscous than the curing agents. For this example implementation, we will be using a mixture which consists of 7g of curing agent and 70g of monomer. Harvard-MIT Division of Health Sciences and TechnologyHST.410J: Projects in Microscale Engineering for the Life Sciences, Spring 2007Course Directors: Prof. Dennis Freeman, Prof. Martha Gray, and Prof. Alexander AranyosiThe PDMS can be mixed in a disposable plastic cup, not unlike the cups found near water coolers. Wider cups may be better because they expose a larger surface of the contained fluid to air, which will aid in degassing the PDMS later. While PDMS is very safe, it is a sticky and messy substance to work with. You should put on some disposable gloves before touching the containers or tools which come into contact with uncured PDMS. Use unpowered gloves to prevent contaminating the PDMS. At this stage, you should not be too concerned with dust or other particles falling into the PDMS, they will be absorbed into the bulk of the device and will usually not affect the channels. However, powdered gloves should not be used the unmolding and bonding stages of fabrication. Turn on the electronic scale and adjust the device so that it reads the correct units, in this case we adjust it to read in grams. Place the cup on the scale, and press the "Tare" button to rezero the scale.Slowly pour 70g of PDMS (base) monomer into the cup. Press the Tare button to rezero the scale. Then pour approximately 1/10 the amount of PDMS curing agent into the cup (7g), for a total mass of 77g for the entire mixture. You will not notice any immediate visible reactions when these two fluids come in contact. In actuality, the order in which the curing agent and PDMS base is added to the cup is not important since they will thoroughly be mixed anyway. Take the cup off the scale and begin to stir the PDMS mixture using the disposable plastic knife. Stir the mixture for approximately 3 minutes. A large stopwatch is useful for keeping time. In general, any stiff disposable rod can be used to stir the mixture and a plastic knife is low cost and works well. After the 3 minutes of stirring the mixture will have many air bubbles. These bubbles must be removed before the PDMS can be used to make a device. Throw away the knife, it is no longer needed.Degassing the PDMS To degas the PDMS, we expose the mixture to a vacuum. The vacuum causes the bubbles in the PDMS mixture to expand and rise to the surface where they pop. Although, the precise amount of time needed to fully degas the mixture will depend on the amount of PDMS being mixed and the width of the cup, 1 hour is a good estimate. However, most of this time is spent just waiting for the process to complete. A desiccator connected to a vacuum line is used to apply a vacuum to the PDMS mixture. First place the plastic cup containing the mixture into a larger, heavier container, such as a Pyrex beaker or an aluminum foil boat. The larger container will catch any PDMS that spills out of the plastic cup during the degassing process and will keep the plastic cup from being knocked over when the vacuum is released. Place both the cup and its container into the desiccator. Multiple cups of PDMS can be degassed simultaneously as long as there is enough room inside the desiccator.Close the lid of the dessicator and adjust the vaccuum valve on the dessicator to connect the vaccuum line. Now turn on a relatively weak vacuum by opening the valve on the vacuum line by only a few turns. A vacuum that is too strong causes the PDMS to actually boil over the side of the cup. Leave the PDMS in the vacuum for 1-2 hours. The PDMS is ready to be taken out of the vacuum when there are nearly no bubbles in the liquid. Release the vacuum valve on the desiccator slowly since the sudden rush of air may knock over the cup. A few small bubbles are acceptable because they will float to the top when making the device. Do not leave the PDMS in the vacuum for too long because it will become more viscous and will prevent bubbles from rising to the surface when the PDMS is poured into the mold. Pouring the PDMS Once the PDMS has been degassed, remove it from the desiccator and place the cup containing it on a lab bench with plenty of space to work with. The pattern for the PDMS device is created by pouring the liquid PDMS onto a surface containing the negative of the pattern and then allowing the PDMS to harden. The solid PDMS now contains the desired pattern of channels and can be bonded to a glass slide or a slab of PDMS to seal the channels. In this project the pattern has been created using SU-8 that has been layered onto a silicon wafer. The SU-8 has been developed by exposing it to a UV light image of the desired pattern. The sections of the SU-8 that were not exposed to UV light are removed leaving a negative mold for the device. The wafer containing the pattern is coated with a thin layer of silane to make it easier to peel the solidified PDMS from the wafer. The completed wafer is known as a "master" for creating the