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

Summary

This protocol presents an improved demolding procedure for the Polydimethylsiloxane (PDMS) double casting technique by introducing silicone oil as a non-adhesive barrier between PDMS layers. Unlike plasma or chemical surface modification methods, this approach requires no specialized or costly equipment, making it a more accessible and cost-effective alternative.

Abstract

Polydimethylsiloxane (PDMS) double casting is widely used for replicating microscale structures, including microfluidic channels and biomedical microdevices such as Bio-MEMS. Despite its versatility, strong interfacial adhesion between PDMS layers often results in incomplete replication or structural damage, particularly in densely packed microfeatures. Conventional approaches to reduce adhesion, such as plasma or chemical surface modification, require specialized equipment and can be time-consuming and costly. To address these limitations, we present an improved demolding strategy that combines thermal aging with the application of a thin silicone oil layer as a non-adhesive release barrier. A critical step in the protocol involves sweeping a tensioned thread across the surface to distribute the oil and promote even PDMS penetration between micropillars. This enhances mold release and preserves structural integrity without compromising delicate microstructures. The method is demonstrated using densely packed hole-array patterns, achieving clean replication with minimal dimensional deviation. This simple, reproducible, and cost-effective approach is well-suited for fabricating PDMS-based microsystems, including multilayer devices and geometrically complex features in soft lithography applications.

Introduction

Soft-lithographic casting underpins microfluidics^1,2, biosensing^3,4, and analytical diagnostics⁵ by enabling rapid, high-fidelity pattern transfer. Various polymeric materials have been explored for this purpose, including thermoplastics^6,7, UV-curable polymers^8,and hydrogels⁹. However, these materials often involve trade-offs: thermoplastic materials are generally rigid and require high-temperature processing, which limits their ability to conform to soft or flexible features; UV-curable polymers require specialized equipment and may suffer from limited flexibility; and hydrogels are mechanically fragile and unstable under dry conditions. Among these, polydimethylsiloxane (PDMS) has become the most widely adopted casting material due to its mechanical flexibility, ease of molding, biocompatibility, and gas permeability. In particular, PDMS double casting is frequently used to replicate delicate or expensive silicon and SU-8 molds, or to duplicate non-inverted geometries such as biomimetic topographies¹⁰. Nevertheless, a persistent challenge in this method is the strong adhesion between the cured and uncured PDMS layers during the secondary casting step, driven by interfacial chain entanglement. This often results in incomplete demolding or structural failure, thereby limiting its broader applicability¹¹.

To address this issue, various surface treatment strategies have been developed to reduce adhesion between PDMS layers. These include surface coatings^12,13, varying curing conditions¹⁴, and plasma treatment^15,16. However, these approaches typically require expensive equipment and involve delicate surface modification processes. As a simpler alternative, thermal aging has been proposed^11,17. This technique involves heat-treating the PDMS mold to reduce the presence of uncrosslinked or low-molecular-weight chains on the surface. Studies by Kwapiszewska et al.¹⁷ and Li et al.¹¹ have shown that thermally aged PDMS molds significantly improve replication fidelity without requiring additional chemical modification. However, even with thermal aging, problems such as adhesion-induced tearing and feature distortion persist, particularly in structures with high density and complex geometries.

This study presents an improved PDMS double casting method that combines thermal aging with an additional surface treatment using silicone oil. After the PDMS mold undergoes thermal aging, a thin layer of silicone oil is coated on the mold surface, forming a non-adhesive barrier that reduces interfacial adhesion. This simple modification significantly improves mold release performance, even for densely packed and high-aspect-ratio microfeatures, and eliminates the need for plasma treatment or specialized equipment. The protocol is designed to be reproducible and easily adoptable by users with limited access to surface modification infrastructure. Figure 1 and Figure 2 provide a detailed, step-by-step description of the fabrication and surface treatment procedure. In contrast, Figure 6 demonstrates the successful replication of high-density hole-array structures, confirming the method's effectiveness under challenging geometric conditions.

Protocol

1. Fabrication of the PDMS mold (M2) from a silicon mold (M1) (Figure 1)

NOTE: All hexane procedures should be conducted in a fume hood or glove box to prevent inhalation of volatile organic compounds and ensure operator safety.

1. Silicon mold coating (M1)
   1. Mix a solution of hexane and octadecyltrichlorosilane (OTS) in a 40:1 volume ratio in a clean glass beaker with a stirring rod.
   2. Cover the glass beaker with aluminum foil to minimize evaporation.
   3. Place the sealed glass beaker into a room-temperature ultrasonic bath filled with water. Sonicate at 40 kHz and 135 W for 5 min to ensure complete mixing.
   4. Immerse the silicon mold (previously fabricated via deep reactive ion etching to feature a hole array) into the prepared hexane-OTS mixture for 30 min at room temperature (RT) (Figure 1A).
      NOTE: In this study, a silicon mold with dimensions of 5 cm × 0.5 cm × 725 µm was used.
   5. Using clean tweezers, gently lift the entire silicon mold vertically out of the hexane-OTS solution. Immediately transfer the mold into a beaker containing neat hexane. Rinse the mold by gently agitating it back and forth 5-10 times in hexane.
   6. Gently dry the mold using an N₂ blower from a distance of ~3 cm for 6-10 s. Ensure that no visible moisture remains on the surface.
2. PDMS mold fabrication (M2)
   1. Mix the PDMS base and curing agent at a 10:1 ratio (w/w) in a clean plastic Petri dish for 15 min.
   2. Place the mixed PDMS in a chamber and degas at 160 torr for approximately 30 min, or longer if needed, until no visible air bubbles remain on the surface.
   3. Lightly coat a clean plastic Petri dish (e.g., 90 mm in diameter) with SO-100cSt (silicone oil with a viscosity of 100 cSt) using a lint-free wiper pre-soaked in SO-100cSt.
   4. 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 (Figure 1B).
   5. Degas the mold again in the vacuum bubble remover until no visible bubbles remain.
   6. Place the Petri dish on a preheated hot plate set to 90 °C and cure the PDMS for 50 min. After curing, allow the mold to cool to RT before demolding (Figure 1C).
   7. Once cured, gently peel the PDMS mold from the silicon mold, taking care to preserve the microstructure (Figure 1D).

2. Fabrication of the final PDMS product using the PDMS mold (M2) (Figure 2)

1. PDMS mold (M2) surface treatment
   1. Place the fabricated PDMS mold on a hot plate set to 150 °C and thermally age it for 3 days in ambient air (Figure 2A).
   2. After the thermal aging, immerse the mold in SO-100cSt.
   3. Degas the mold under vacuum 160 torr for 10-15 min, or until no visible bubbles remain.
   4. Remove the mold using tweezers and gently wipe the surface 3-5 times with a clean lint-free wiper to remove excess oil while leaving a uniform thin film (Figure 2B).
2. Minimizing the thickness of the oil layer between PDMS pillars
   NOTE: This step is crucial, as an excessively thick oil layer can lead to inaccurate replication. The diameter of the thread should be determined depending on the geometry of the microstructure.
   1. Prepare a PDMS mixture (10:1 w/w base to curing agent), following the same mixing and degassing steps described in step 1.2.
   2. Gently pour the degassed PDMS mixture onto the PDMS mold treated with SO-100cSt, allowing it to spread across the surface (Figure 2C). Apply a sufficient volume so that the poured PDMS reaches at least halfway up the height of the micropillars.
   3. Secure a 0.5 µm nylon thread under gentle tension. Swipe it once or twice across the PDMS surface in a single direction to thin out excess oil between micropillars and ensure uniform coating (Figure 2D).
   4. Degas the PDMS mold with the thin PDMS layer in a vacuum chamber until no visible air bubbles remain.
3. Hole-array pattern PDMS fabrication
   1. Lightly coat a clean plastic Petri dish with SO-100cSt, as described in step 1.2.3.
   2. Place the PDMS mold into the coated plastic Petri dish, then pour the degassed PDMS mixture over the mold until the PDMS reaches the desired height (Figure 2E).
      NOTE: In this study, the mixture was filled to a level 0.5 cm above the micropillar tips.
   3. Degas the assembled mold again to remove any air bubbles that may have formed during the pouring step.
   4. Cure the PDMS on a preheated hot plate at 110 °C for 10 min, or longer if needed, until fully solidified.
      NOTE: The curing temperature must be sufficiently high to avoid prolonged curing times. Extended curing at lower temperatures could lead to silicone oil diffusion into the PDMS, thereby increasing adhesion between the mold and the replicated structure. In this study, the curing temperature was set to 110 °C, allowing complete curing within 10 min.
   5. After curing, gently insert clean tweezers between the mold and the replica and carefully demold the final product from the mold (Figure 2F).
   6. Gently wipe the final PDMS product using a wiper soaked in isopropyl alcohol to remove any residual SO-100cSt or dust.
   7. Dry the surface using a N₂ blower.

Results

A silicon mold (M1) featuring a densely packed array of circular holes was used to evaluate the improved double casting method. Figure 3A illustrates the SEM image of the top surface demonstrating a uniform pattern of hole array with a diameter of 143 µm and a period of 150 µm. The side-view image reveals a hole depth of 284 µm in Figure 3B, corresponding to an aspect ratio of approximately 2:1 and confirming a tightly packed mic...

Discussion

This protocol presents a cost-effective and reproducible approach for PDMS double casting that combines thermal aging with silicone oil surface treatment to overcome demolding challenges in microfabrication. Conventional techniques such as chemical surface treatments or thermal aging alone often require specialized equipment or provide inconsistent results, especially when replicating dense, high-aspect-ratio features. By integrating thermal aging with a non-adhesive barrier, our method simplifies implementation whi...

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
Contact Angle measurement equipment	Surface Elecrto Optics(SEO)	Phoenix 300
Curing agent	Dow	Sylgard 184b
Digital camera	SONY	E3ISPM06300KPB
Hexane	Sigma-Aldrich	32293	≥99% (GC), ACS reagent
Hot plate	MTOPS	HSD180
Isopropyl alcohol (IPA)	Raontech	PS2032-001
MR-100 (vacuum pump oil)	Moresco	1802-00005-02	44.6 cSt
Optical microscope	OLYMPUS	BX51
PDMS base	Dow	Sylgard 184a
SEM	Thermo Fisher Scientific	Verios 5 UC
Silicone oil	ShinEtsu	KF-96-100CS	100 cSt
Silicone oil	ShinEtsu	KF-96-1000CS	1000 cSt
Spin coater	DONG AH TRADE CORP	ACE-200
Trichloro(octadecyl)silane (OTS)	Sigma-Aldrich	104817
Ultrasonic cleaner	Branson	5510E-DTH
Vacuum pump	GAST	DOL-701-AA