LAB Chip Lid Molding LAB PROCEDURE The purpose of this lab is to expose the student to a lab-on-a-chip fabrication technique. The student will utilize polydimethylsiloxane (PDMS) poured in four Petri dishes to produce the lids for the acrylic chip bases To make the lids, first the mixed PDMS resin and curing agent will be poured into the petridishes. The student will then remove air content from the PDMS by utilizing a desiccator and vacuum system. The student will be introduced to the issues concerning material safety data sheets (MSDS) and safe or appropriate handling of materials based on the information provided in an MSDS. CHIP MOLDING LATEX OR VINYL GLOVES SHOULD BE WORN DURING THIS PROCEDURE Read the MSDS for both components contained at the end of this procedure. Cleanliness in this process is mandatory for successful results. 1. PDMS is an inexpensive elastomeric polymer very similar to the silicone sealant you can purchase in a hardware store. It is comprised of a resin (prepolymer) and a curing agent. These should be mixed at approximately a 10 to 1 ratio and poured into the mold, as detailed in the following steps. 2. Take the plastic cup to the table where resin, hardener, and 20ml and 5ml syringes are located. 3. Use the syringes to extract the following amounts of resin and hardener. Insure no large bubbles are in the syringe. If there are large bubbles, turn the syringe upside down and as the bubbles move to top, expel them. To save time, if you are making four molds (four Petri dishes), mix all the PDMS resin and hardener in one batch. Use the 20 ml syringe to obtain a total of 72.4 ml of resin. Use the 5ml syringe to obtain 7.6 ml of hardener. Expel the contents into the cup. This will be enough for four molds (four Petri dishes), two lids and two wafer molds. 4. Be careful not to contaminate the supply of resin or hardener. Take the cup with the total of the contents of resin and hardener back to your table. 5. Use your glass stirring rod and STIR the contents VIGOROUSLY for 5 minutes. The mixture should turn cloudy from many small bubbles forming. 6. Remove the lids from your four Petri dishes. 7. Carefully EQUALLY DISTRIBUTE THE MIXTURE from the cup into each of the Petri dishes. You may line up the Petri dishes beside each other to gauge the relative level of each Petri dish. 8. Use the stirring rod to scrape out any remaining resin. It is OK if the Petri dishes are not exactly even, but they should be close. 9. DEGASSING INSTRUCTIONS
a. On the lab tables in the front half of the room, there is one plastic desiccator for each group. They are attached, with 3-way stopcocks and vacuum hoses, to a vacuum pump. You will use a vacuum desiccator to remove air bubbles since low pressure will expand the bubbles (mechanically releasing them from the chip) and lower the solubility of air in the PDMS (reducing the potential for more bubbles showing up later). This process is known as degassing and is very important as bubbles in the PDMS, particularly in the channels, can ruin your device. b. As illustrated in Figure 1, stack each topless Petri dish containing a PDMS lid into the desiccator separated by the wire grates between the Petri dishes. c. Place the desiccator lid back on. The desiccator lid simply sits on the body and is sealed against a rubber gasket under vacuum. Figure 1. contents. There is no mechanical locking mechanism for the lid and the weight of the vacuum hose can sometimes topple the desiccator so always make sure someone is holding the lid until a seal is made with vacuum. The outlet of the desiccator, located on the lid, has a 3-way stopcock with attached hose. Look carefully at the arrows molded onto the stopcock control. The arrows depict which passageways are open. The only acceptable settings are those connecting the desiccator to vacuum and the desiccator to the. DO NOT set the stopcock to a position connecting the vacuum to. d. With the desiccator lid in place and the Petri dishes and their contents sealed inside the desiccator, turn the stopcock control so the desiccator is connected to the vacuum pump. If the vacuum pump is running, the pressure inside the desiccator should begin to drop. You can check this by checking if your lid is now sealed. to to pump Acceptable Stopcock Positions, desiccator to Unacceptable Stopcock Positions
e. Continue to watch the system while the pressure drops for several minutes. Eventually, bubbles will start to rise to the top of the PDMS. f. If the PDMS starts to FOAM, carefully turn the stopcock so that the desiccator is connected to the. This will release the vacuum, equalize the pressure between the desiccator and, and allow the foam to recede. g. Once the foam has dispersed, RETURN the stopcock control to the vacuuming position, with the desiccator and vacuum hose connected. h. Periodically release the vacuum on your desiccator to help pop bubbles, then let the vacuum return for several minutes. Continue this until no more bubbles are visible in the Petri dishes. i. Alternately, the glass stirring rod may be used to pop or move stubborn bubbles away from the central area (2 diameter) after at least 20 minutes of vacuum. 10. Once you have NO BUBBLES near the central area for all Petri dishes, release the vacuum to the desiccator, and carefully remove all Petri dishes. If, after 30 minutes, bubbles remain, have an instructor determine if you should stop. Place the wire mesh back in the desiccator. Keep the Petri dishes level!!! 11. Place LIDS on all of the Petri dishes. 12. PLACE your covered lids carefully in the location designated by your instructor where they will cure until next lab session. 13. Throw the plastic mixing cup away and CLEAN stirring rod with paper towels. Throw away the paper towels. Put the stirring rod away with your supplies. 14. WIPE up any spilled PDMS or other mess on your TABLE with a paper towel. END Attachments MSDS for Sylgard 184 Silicone Elastomer Base MSDS for Sylgard 184 Silicone Elastomer Curing Agent to
LAB 6 Item Description Quantity per table Quantity per room Location PDMS and curing agent 2 Front rubber cement 2 Front 20 cc syringe (dispose after use) 1 Table 5 cc syringe (dispose after use) 1 Table compressed air 1 Table cups for mixing (standard drinking size) 1 Table (s) attached to vacuum pumps 1 Table Petri dishes and lids 4 Table wire grating (included in desiccator boxes) 3 Table
Requirements of Lab Chip Lid Molding Memo Note This is a LAB Chip Lid Molding TEAM MEMO. Content Lab Chip Lid Molding Memo Guidelines: Header Information Point Value 5 pts (for header and memo format) Points Received Introductory Paragraph 5 pts 1. Objectives/Goals of the Labs 3 2. Brief Description of Contents of the memo 2 Discussion 75 pts 1. Briefly discuss the general techniques and results from this lab. 10 2. Why is it important to remove the bubbles from the PDMS before it cures? 7 3. Specifically state why reducing the pressure in the desiccator causes the release of air from the PDMS. 10 4. NTM 5, #1. 8 5. NTM 5, #3. 8 6. NTM 5, #5. 8 7. NTM 6, #1. 8 8. NTM 6, #3. 8 9. NTM 6, #5. 8 Conclusion 10 pts 1. Briefly state the goals achieved by undertaking this lab activity. 5 2. Describe potential problems that would arise if the PDMS does not cure leveled. 5 Lab Participation Agreement 5 pts