Microfluidic devices allow for the modeling of large systems or components of such systems at a microscopic scale. This allows for a greater potential for automation and portability, while lowering both expenditure and experimental measurement times.
This article details the advantages of plasma treatment for microfluidic device fabrication, its uses, and some processing recommendations to be followed when using the instruments described.
Benefits of Plasma Treatment for Microfluidic Device Fabrication
Poly (dimethylsiloxane) (PDMS) is the material most frequently used to construct microfluidic devices. Plasma treatment renders PDMS surfaces hydrophilic at a high speed, through plasma oxidation [Figure 1]. After this plasma treatment, the PDMS-PDMS or PDMS-glass surfaces can be bonded and permanently sealed to construct leak-proof channels.
Figure 1. Water drop contact angle on a blank poly(dimethylsiloxane) (PDMS) surface as a function of air plasma treatment time using a Harrick Plasma cleaner. Data from 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.
Another reason to render microfluidic channel surfaces hydrophilic may be to improve fluid flow and wetting of channels. Additionally, interchanging hydrophilic-hydrophobic regions may be patterned on microfluidic surfaces by plasma treating devices via a patterned mask.
It is possible to plasma treat microfluidic surfaces and channels without any impact on the bulk properties of the device. Consequently, plasma treatment has been applied to enable the construction of microfluidic devices for uses such as:
- Analysis of chemical reactions and fluid flow on micron scale
- Detection of biological organisms or chemical species
- Clinical diagnostics and drug screening for medical research
- Manipulation of fluid on cellular length scale (microns) for biological research
- Growth of cell and tissue cultures
Following the patterning of a PDMS substrate by replica molding from a master mold, the PDMS is oxidized in air oxygen (O2) plasma. An air or O2 plasma removes biological, hydrocarbon material through a chemical reaction with highly sensitive oxygen radicals and ablation by energetic oxygen ions.
This leaves behind silanol (SiOH) groups on the surface, which make the surface more hydrophilic and increase surface wettability. After plasma activation, the PDMS is positioned to touch another oxidized PDMS or glass surface straight away, to form bridging Si-O-Si bond at the interface, producing a permanent seal.
Below are proposed process settings for plasma activation of PDMS-PDMS or PDMS-glass in a Harrick Plasma cleaner. Please note that it may be necessary to experiment to determine optimal process conditions.
- Use oxygen (O2) or room air as the process gas
- Pressure: 500 mTorr to 1 Torr
- RF power: Typically, HIGH
- Process time: 15 to 60 seconds
- As is the case with experimental processes and fabrication techniques, there has been wide variation in the plasma process conditions reported by users, even when plasma treating comparable PDMS materials.
In the tests carried out for this article, optimal PDMS bonding was detected with the following process conditions: 900-950 mTorr air or O2; HI RF power; 10-20 seconds.
Additional Processing Guidelines
- Plasma treatment must not be carried out for longer than two minutes, as extended plasma exposure results in cracking in PDMS and migration of low molecular mass molecules from bulk to surface, lowering the number of hydrophilic SiOH groups and causing weak or unfinished bonding .
- Oxidized surfaces should be brought into contact directly following plasma treatment to attain the strongest bond possible.
- PDMS surface recovers hydrophobic properties (aging) with time after plasma treatment (~1 hour) [3,4].
References and Further Reading
 Norred SE, Caveney PM, Retterer ST, Boreyko JB, Fowlkes JD, Collier CP and Simpson ML. "Sealable femtoliter chamber arrays for cell-free biology.” J. Vis. Exp. (2015) 97: e52616.
 Bhattacharya S, Datta A, Berg JM, Gangopadhyay S. "Studies on surface wettability of poly(dimethyl) siloxane (PDMS) and glass under oxygen-plasma treatment and correlation with bond strength." J. Microelectrom. S. (2005) 14(3): 590-597.
 Hillborg H, Ankner JF, Gedde UW, Smith GD, Yasuda HK, Wikström K. "Crosslinked polydimethylsiloxane exposed to oxygen plasma studied by neutron reflectometry and other surface specific techniques." Polymer (2000) 41: 6851-6863.
 Duffy DC, McDonald JC, Schueller OJA and Whitesides GM. "Rapid prototyping of microfluidic systems in poly(dimethylsiloxane)". Anal. Chem. (1998) 70: 4974-4984
This information has been sourced, reviewed and adapted from materials provided by Harrick Plasma.
For more information on this source, please visit Harrick Plasma.