PDMS BONDING (Microfluidics)
Poly(dimethylsiloxane) (PDMS) is a silcone-based organic polymer used extensively in professional and academic research laboratories for its low cost and high versatility. PDMS is inert, transparent and easily customized via soft lithography, a technique used to mold PDMS and imprint nano-scale features and microchannels into its surface. Harrick Plasma Cleaners remove organic contamination and activate the PDMS surface in preparation for bonding with glass, PDMS or other similarly treated surfaces.
Plasma Surface Modification of PDMS
After patterning a PDMS substrate by replica molding from a master mold, the PDMS is oxidized in air or oxygen (O2) plasma. An air or O2 plasma removes organic, hydrocarbon material by chemical reaction with highly reactive oxygen radicals and ablation by energetic oxygen ions. This leaves silanol (SiOH) groups on the surface, rendering the surface more hydrophilic and increasing surface wettability. Following plasma activation, the PDMS is immediately placed in contact with another oxidized PDMS or glass surface to form bridging Si-O-Si bond at the interface, creating an irreversible seal. This water-tight covalent bond is ideal for microchannel formation and function.
Plasma treatment has been used to facilitate the fabrication of microfluidic devices for applications such as:
- Study of chemical reactions and fluid flow on micron scale
- Detection of biological organism or chemical species
- Clinical diagnostics and drug screening for medical research
- Manipulation of fluid on cellular length scale
- Cell Culture, Tissue Culture, Organoid Studies
- Single Cell Sequencing – Droplet Isolation
Figure 1. Water drop contact angle on a blank PDMS surface as a function of air plasma treatment time using a Harrick Plasma cleaner.
Jiang, X., H. Zheng, S. Gourdin, P. T. Hammond. “Polymer-on-Polymer Stamping: Universal Approaches to Chemically Patterned Surfaces.” Langmuir (2002) 18: 2607-2615; Zheng, H., M. F. Rubner, P. T. Hammond. “Particle Assembly on Patterned “Plus/Minus” Polyelectrolyte Surfaces Via Polymer-On-Polymer Stamping.” Langmuir (2002) 18: 4505-4510.
Plasma treatment introduces polar functional groups that increase the wettability of a substrate. Increased surface wettability enhances fluid flow within microfludic devices and improves PDMS biocompatability.
Immediately following plasma treatment, the PDMS surface begins to undergo hydrophobic recovery as the high energy surface reconfigures towards a lower energy state. It is recommended that PDMS bonding and other subsequent processing steps be performed within 15 minutes to an hour of plasma treatment.
Additionally, alternating hydrophilic-hydrophobic regions may be patterned on microfluidic surfaces by plasma treating devices through a patterned mask.
- Use oxygen (O2) or room air as the process gas
- Pressure: 200 mTorr to 1 Torr
- RF power: Typically HIGH
- Process time: 15-60 seconds
- As is the case with experimental processes and fabrication techniques, plasma process conditions reported by users have varied widely, even when plasma treating similar PDMS materials.
Additional Process Considerations
- After Plasma Treatment, press and hold PDMS components together lightly for 30 sesonds. Do not pull apart and adjust alignment as this will disrupt bond formation. Pressing with too much force may collapse microfluidic channels.
- Heat the assembled device at 80-100 degrees Celsius for 60 seconds in an oven or hot plate. The high temperature provides activation energy for additional bond formation.
- Cleanliness: the presence of particulates or oil can block bonds from forming. Avoid touching the surfaces to be bonded when removing PDMS from the plasma chamber
- Surface Roughness: Smooth surfaces maximize contatct between bonding materials, providing more opportunity for siloxane bond formation. Adjust your soft lithography process to ensure smooth substrates.
- Air vs Oxygen: Oxygen is more efficient than air due to the higher concentration of reactive oxygen species. Additionally, air from an environment prone to daily fluctuations in humidity or particulates can adversly affect PDMS bonding.
- Plasma treatment should not exceed 2 min, as prolonged plasma exposure causes cracking in PDMS and migration of low molecular mass molecules from bulk to surface, decreasing the number of hydrophilic SiOH groups and resulting in weak or incomplete bonding
- Oxidized surfaces should be brought into contact immediately after plasma treatment to achieve strongest bond possible
- PDMS surface recovers hydrophobic properties (aging) with time after plasma treatment (~1 hour).