PDMS Microfluidics Tutorial: Preparing a Test Pattern

Soft lithography is a non-photolithographic patterning technique pioneered by George Whitesides' group at Harvard in the mid-1990s, which enables micro and nano fabrication through self-assembly and replica molding. Unlike photolithography, it can create both topographic structures and chemically patterned regions with different surface energies. The technique is particularly valuable for applications requiring large-area micro/nano structures such as sensors, biological applications, structural color, and textured hydrophobicity, where defect-free quality control is less stringent than in microelectronics. The molding group of soft lithography techniques, including replica molding (RM), MIMIC (Micro Molding in Capillaries), SAMIM (Solvent-Assisted Micro Molding), and CFL (Capillary Force Lithography), achieve pattern replication through capillary-driven flow of polymer solutions. Cross-linked PDMS (polydimethylsiloxane) is the most popular stamp material due to its unique properties: it is an inorganic polymer with a silicon-oxygen backbone, exhibits room temperature elasticity, is optically transparent, biocompatible, non-toxic, chemically resistant, and has low surface energy (approximately 20 mJ/m²). The curing process transforms a viscous liquid into an elastic elastomer through thermal treatment, with the cross-link density determined by the ratio of cross-linker to base polymer.

Silicones are inorganic polymers with silicon-oxygen backbone and pendant organic groups. The Si-O bond polarity and low rotation energy provide exceptional stability and flexibility. Properties range from fluids to elastomers to gels depending on molar mass, cross-linking degree, and pendant groups. PDMS has very low glass transition temperature and high free volume, providing excellent elastomeric behavior and gas permeability. However, hydrophobic methyl groups make PDMS prone to protein adsorption and fibrous capsule formation. Silicone elastomers are 3D covalently cross-linked networks with fillers for reinforcement. Silicone gels are lightly cross-linked networks between fluids and elastomers. Applications include extracorporeal equipment (dialysis, blood oxygenators), catheters, drains, shunts, and reconstructive surgery.

PDMS (polydimethylsiloxane) is a versatile silicone elastomer used for creating microfluidic devices through soft lithography. The material consists of a base polymer and a curing agent that must be mixed in a precise 10:1 weight ratio. The base has a viscous honey-like consistency, while the curing agent is more fluid. Proper preparation requires accurate weighing, thorough mixing for at least one to two minutes, and degassing in a vacuum chamber to eliminate air bubbles that would compromise channel integrity. Understanding these fundamental properties and preparation steps is essential for successful microfluidic device fabrication.

Silicone polymers, primarily polydimethylsiloxane (PDMS), consist of a silicon-oxygen backbone with methyl groups attached to each silicon atom; they can exist as viscous liquids when short-chained or as rubbery solids when cross-linked through various mechanisms such as moisture-curing (producing acetic acid) or platinum-catalyzed hydrosilylation; these polymers have diverse applications including caulk and as supports for palladium nanoparticles used as catalysts in hydrogenation reactions.

Soft lithography revolutionized microfluidics by enabling fabrication using flexible polydimethylsiloxane (PDMS). Master forms are created on silicon substrates using SU-8 photoresist, which allows microstructures with heights up to 200 micrometers and vertical sidewalls. PDMS is prepared as a 10:1 base-to-curing agent mixture, degassed to remove bubbles, and poured onto the master form. After curing at 65°C for 4 hours, the PDMS replica is separated and cut into individual devices. This method produces optically transparent, chemically inert, and biocompatible microfluidic devices.

Soft lithography integrates physics, mathematics, and biology to create microscale patterns for biological applications. The process begins with photolithography: designing patterns in CAD software, printing onto chromium-coated masks, and exposing photoresist on silicon substrates using lasers. Positive/negative photoresist creates holes or pillars corresponding to mask patterns. PDMS (polydimethylsiloxane), a biocompatible polymer, is then cast from these masters. PDMS offers transparency for microscopy, gas permeability for cell culture, and easy sterilization via UV. The two-part polymer (A and B) must be mixed by weight in a standard 10:1 ratio, with bubbles removed through degassing under vacuum. After curing at elevated temperatures (60-80°C) for 1-2 hours, the solid elastomer can be peeled as a negative replica, ready for biological applications.

Creating a two-layer SU8 master mold involves coating and baking photoresist on a silicon wafer. The second layer is applied directly on top of the first layer without developing the first layer first. The soft bake of the first layer serves as the post-exposure bake for the second layer. Photoresist is spun onto the wafer center, covering approximately one-quarter radius, then baked at 65°C followed by 95°C. The wafer is loaded into the pattern writer with consistent orientation, and three alignment marks at coordinates (0,0), (27,0), and (0,18) are used for precise positioning across both layers.

The microfluidic chip is fabricated using photolithography and PDMS glass bonding: (1) SU-8 photoresist (SU-8-075) is spin-coated on a silicon wafer at 1000 rpm to form a 150 micron thick film; (2) A mask is loaded and the wafer is exposed to UV light for 25 seconds; (3) Post-exposure bake is performed at 95°C for 15 minutes; (4) The wafer is developed to create the channel pattern; (5) PDMS is poured onto the SU-8 mold and cured at 120°C for 15 minutes; (6) PDMS is detached from the mold; (7) Holes for inlet/outlet and spheroid loading are created; (8) Oxygen plasma bonding (100 watts, 10 minutes) attaches PDMS to glass. This single-mask process allows efficient fabrication of multiple devices.

Wetting is the ability of a liquid to spread and adhere to a solid surface, determined by the contact angle. Three wetting states exist: no wetting (180° contact angle, strong cohesive forces), poor wetting (90°-180°, cohesive forces dominate), and good wetting (0°-90°, adhesive forces dominate). Surfaces are classified by energy: high surface energy surfaces (metals, glass, ceramics) have strong intermolecular bonds and are easily wetted; low surface energy surfaces (hydrocarbons like polypropylene) have weak bonds and resist wetting; medium surfaces show intermediate behavior. Wetting can be improved through surface treatment or surfactants.

Surface energy is the energy required to create a new surface, and it affects how materials interact with each other. High surface energy materials tend to attract and bond well with other materials, while low surface energy materials resist adhesion. In dentistry, achieving good adhesion between restorative materials and tooth structure requires understanding surface energy relationships. The contact angle between the adhesive and the surface indicates the quality of wetting - a low contact angle indicates good wetting and good adhesion potential.

Wetting is the ability of a liquid to spread on a solid surface, measured by contact angle. Zero contact angle indicates perfect wetting, acute angles indicate good wetting, and obtuse angles indicate poor wetting. Surface energy is the energy of a solid surface, with surface atoms having higher energy than bulk atoms. High surface energy materials attract and bond with other materials. Surface tension causes liquids to bead up rather than spread. For proper dental bonding, the adhesive must have low surface tension to spread properly, and the substrate must have high surface energy to attract the adhesive.

Polymers typically have low surface energy and limited chemical reactivity, making it difficult for paints and adhesives to adhere properly; two common surface treatment methods address this issue: corona treatment uses electron bombardment to create plasma that modifies the polymer surface, increasing its energy and enabling better paint spreading, while flame treatment uses controlled heat exposure to create micro-roughness and reactive sites on the polymer surface, improving both wetting and adhesion properties; both treatments enhance the interaction between the polymer substrate and applied coatings by either increasing surface energy or creating chemical bonding sites.

The principle of minimum energy governs adhesion: liquids spread on solids when their surface energy is lower than the solid's, decreasing total system energy. High-surface-energy materials like metals and ceramics bond easily, while low-surface-energy plastics like Teflon resist bonding. Water droplet tests demonstrate this principle—water spreads on glass (high energy) but forms spheres on cabbage leaves (low energy). Users can improve adhesion by cleaning surfaces for high-energy materials or applying treatments for low-energy plastics.