Improved Polydimethylsiloxane (PDMS) Double Casting via Silicone Oil Treatment for Densely Packed Microstructure Replication

To address demolding challenges in dense and high aspect ratio microstructures, we modified the PDMS double casting process by adding a silicone oil treatment that applied with a tension thread. Our protocol offers a simple cost-effective solution that avoids the need for expensive equipment or specialized materials, enabling reliable replication of advanced microstructures without damage.

Our laboratory will focus on automating the oil application process and developing custom silicone oils to identify optimal conditions for improved reproducibility and precision.

[Narrator] To begin, mix a solution of hexane and OTS in a 40:1 volume ratio using a clean glass beaker and a stirring rod. Cover the beaker with aluminum foil to minimize evaporation. Sonicate the sealed glass beaker into a room temperature ultrasonic bath filled with water. Then immerse the pre-etched silicon mold with a hole array into the prepared hexane OTS solution. Now, use clean tweezers to gently lift the entire silicon mold vertically out of the solution. Immediately transfer it to a fresh beaker containing neat hexane, then rinse by gently agitating it back and forth five to 10 times. Gently dry the mold using a nitrogen blower held approximately three centimeters away for six to 10 seconds until no visible moisture remains. Next, mix the PDMS base and curing agent in a 10:1 weight ratio in a clean plastic Petri dish for 15 minutes. Place the mixture into a chamber and degas at 160 torr until no visible air bubbles remain. Lightly coat a clean plastic Petri dish with silicone oil of 100 centistokes viscosity using a lint-free wiper pre-soaked in the oil. Then place the OTS-coated silicon mold face up in the center of the coated Petri dish. Slowly pour the degassed PDMS mixture over the mold until it is fully covered. Degas the filled mold again in the vacuum bubble remover until no visible bubbles remain. Now, place the Petri dish on a preheated hot plate set to 90 degrees Celsius to cure the PDMS. Once cured, gently peel the PDMS mold from the silicon mold, ensuring the microstructure remains intact. Place the fabricated PDMS mold on a hot plate set to 150 degrees Celsius. After three days of thermal aging, immerse the mold in silicone oil of 100 centistokes viscosity. Degas the oil-immersed mold under vacuum at 160 torr for 10 to 15 minutes. Then remove the mold using tweezers and gently wipe the surface three to five times with a clean lint-free wiper to leave a uniform thin film of oil. Next, prepare a PDMS mixture at a 10:1 base to curing agent weight ratio using the same mixing and degassing method as before. Gently pour the degassed mixture onto the treated mold, allowing it to spread and reach at least halfway up the micropillars. Secure a 0.5 micrometer nylon thread under gentle tension. Swipe the thread once or twice across the PDMS surface in one direction to reduce excess oil between micropillars. Then degas the mold with the thin PDMS layer in a vacuum chamber until no air bubbles remain. To fabricate a hole array pattern PDMS product, lightly coat a clean plastic Petri dish with silicone oil. Place the PDMS mold into the coated Petri dish and pour the degassed PDMS mixture over it until the desired height is reached. Degas the mold assembly once more to eliminate any trapped air bubbles. Then place the Petri dish on a preheated hot plate set to 110 degrees Celsius and cure until fully solidified. After curing, gently insert clean tweezers between the mold and the replica, and carefully demold the final product from the mold. The silicon mold featured a dense array of circular holes measuring 143 micrometers in diameter with 150 micrometer spacing and a depth of 284 micrometers, yielding a 2:1 aspect ratio. The PDMS mold replicated from the silicon template produced micropillars measuring 142 micrometers in diameter and 283 micrometers in height, closely matching the original dimensions. The PDMS replica thermal aged one day and subsequently treated with 100 centistokes silicone oil showed severe tearing and disintegration of wall structures during demolding. After two days of thermal aging followed by 100 centistoke silicone oil treatment, distinct pillar outlines became visible, but the walls remained heavily damaged due to insufficient release. With three days of thermal aging and no oil layer, cross-linking between cured and uncured PDMS resulted in recombined structures. Using low viscosity MR-100 oil after three days of thermal aging led to poor coverage and tearing as the oil drained off vertical sidewalls. High viscosity oil formed a thick coating, preventing mold filling and causing the final product to appear wavy and malformed. Under optimized conditions of three days thermal aging and 100 centistoke silicone oil coating, the final replica preserved hole patterned microstructures with 148 micrometers diameter and 280 micrometers height, showing minor dimensional variation, but intact structure.