Plasma treatment can be used to modify surface chemistry in materials via functional groups introduced by the plasma gas. This article details the advantages of plasma treatment for managing surface properties, plasma processing recommendations, and illustrations of the effect of surface chemistry and contact angle on plasma-treated materials.
Benefits of Plasma Treatment
Plasma treatment can be used to modify surfaces through the attachment or adsorption of functional groups to change surface properties for precise applications. The functional groups created can be modified depending on the process gas used and, in turn, the surface wettability may be changed to be hydrophilic [Figure 1] or hydrophobic [Figure 2] with the proper gas.
Figure 1. Water droplet contact angle as a function of N2/O2 plasma treatment time, using a Harrick Plasma cleaner, on polyetheretherketone (PEEK). The PEEK surface is rendered hydrophilic after 20 seconds of plasma treatment. Data from Ha SW, Kirch M, Birchler F, Eckert KL, Mayer J, Wintermantel E, Sittig C, Pfund-Klingenfuss I, Textor M, Spencer ND, Guecheva M and Vonmont H. "Surface activation of polyetheretherketone (PEEK) and formation of calcium phosphate coatings by precipitation." J. Mater. Sci.- Mater. Med. (1997) 8: 683-690.
Improved wettability readies the surface for succeeding processing (e.g. film deposition or adsorption of molecules) by enhancing surface coverage and spreading of coatings and enhancing bonding properties between two surfaces. Producing a more hydrophobic surface may also be vital for self-cleaning or where water penetration is unwelcome.
Figure 2. Water droplet contact angle as a function of O2 plasma treatment time, using a Harrick Plasma cleaner, on poly(tetrafluoroethylene) (PTFE), indicating increased hydrophobicity. Plasma treatment produces nanoscale roughness that increases hydrophobicity. Data from Lee S-J, Paik B-G, Kim G-B and Jang Y-G. “Selfcleaning features of plasma-treated surfaces with self-assembled monolayer coating.” Jpn. J. Appl. Phys. (2006) 45: 912-918.
Surfaces can be plasma cleaned without any impact on the main properties of the material. Consequently, plasma treatment can be carried out on a broad range of materials, as well as complex surface geometries. The below list contains applications and samples that have been treated with plasma instruments:
- Make surfaces hydrophilic by oxidation and formation of hydroxyl (OH) groups
- Make surfaces hydrophobic with deposition of fluorine-containing groups (CF, CF2, CF3)
- Pattern alternating hydrophilic or hydrophobic regions on surfaces for self-assembly studies
- Graft functional polymers or end groups onto plasma-activated surfaces [Figure 3]
- Encourage the adhesion of cells and cell proliferation on plasma-modified biomaterials or tissue scaffolds
- Deposit polymer layers by plasma polymerization
Figure 3. Surface density of carboxyl (COOH) groups as a function of air plasma treatment time, using a Harrick Plasma cleaner, on 100 μm thick poly(caprolactone) (PCL) nanofiber mats. The COOH layer facilitates subsequent grafting of gelatin molecules onto the PCL nanofiber mats for potential use as tissue-engineering scaffolds. Data from Ma Z, He W, Yong T and Ramakrishna S. "Grafting of gelatin on electrospun poly(caprolactone) nanofibers to improve endothelial cell spreading and proliferation and to control cell orientation." Tissue Eng. (2005) 11: 1149-1158.
Air or oxygen (O2) gas is generally used for plasma cleaning. An air or O2 plasma eliminates biological contaminants through chemical reaction with highly reactive oxygen radicals and ablation by energetic oxygen ions. The plasma also encourages hydroxylation (OH groups) on the surface, making the surface more hydrophilic and improving surface wettability.
Water vapor (H2O) can also be applied to introduce hydroxyl groups and make surfaces more hydrophilic. Specialist gas delivery equipment and gas handling procedures are needed for use with the plasma system. For samples that are susceptible to reactions to moisture, H2O plasma would not be suggested.
As an alternative, an argon plasma may be favored for cleaning in order minimize additional oxidation of surfaces (e.g. metals). Argon plasma does not react with the surface directly, but instead, cleans through ion bombardment and physical ablation of contaminants off the surface. Argon can also be applied to improve surface hydrophilicity through the reaction of the plasma-activated surface upon exposure to ambient air.
Carbon tetrafluoride (CF4) plasma may be used on surfaces to create a hydrophobic coating of fluorine-containing groups (CF, CF2, CF3). The fluorinated plasma lowers the number of hydrophilic polar end groups on surface and lessens surface wettability. Application of fluorinated gas necessitates replacing the standard Pyrex chamber with a quartz chamber.
In addition, applications that are susceptible to possible contamination from trace impurities in borosilicate glass may also find a quartz chamber substitution to be an advisable option.
Below are recommended process conditions for plasma cleaning in a Harrick Plasma cleaner. Please note that it may be necessary to experiment to determine optimal process conditions.
- Pressure: 100 mTorr to 1 Torr
- RF power: MEDIUM or HIGH
- Process time: One to three minutes
- Surfaces should be utilized immediately following plasma treatment; plasma-treated surfaces may recover their untreated surface properties with extended exposure to air.
This information has been sourced, reviewed and adapted from materials provided by Harrick Plasma.
For more information on this source, please visit Harrick Plasma.