A clean surface with greater surface wettability is frequently advantageous to increase adhesion and improve bonding to other surfaces. This article details the advantages of plasma treatment in varying surface wettability characteristics for adhesion and other uses, plasma processing recommendations, and examples of contact angle measurements on plasma-treated materials.
Advantages of Plasma Treatment for Surface Adhesion
Plasma cleaning eliminates biological contaminants through chemical reaction (O2 or air plasma) or physical ablation (Ar plasma). In addition to this, plasma treatment introduces chemical functional groups (carbonyl, carboxyl, hydroxyl) on the surface, making the majority of surfaces hydrophilic.
Figure 1. Water drop contact angle measurement on 316L stainless steel (a) as received, (b) after chemical clean (ultrasonication in 70% ethanol, acetone, and 40% nitric acid), and (c) after chemical clean and O2 plasma treatment. Data from Mahapatro A, Johnson DM, Patel DN, Feldman MD, Ayon AA and Agrawal CM. "Surface modification of functional self-assembled monolayers on 316L stainless steel via lipase catalysis." Langmuir (2006) 22: 901-905.
This can be seen in a decrease in water contact angle and increased wettability [Figure 1 and Figure 2]. Improved wettability gets the surface ready for ensuing processing (e.g. film deposition or adsorption of molecules) by enhancing surface coverage and the spreading of coatings and improving bonding properties between two surfaces.
Figure 2. Water droplet contact angle measurement on ultrahigh molecular weight polyethylene (UHMWPE) as a function of O2 plasma treatment time. Data from Widmer MR, Heuberger M, Voros J and Spencer ND. "Influence of polymer surface chemistry on frictional properties under protein-lubrication conditions: implications for hip-implant design." Tribol. Lett. (2001) 10: 111-116.
Surfaces can be plasma cleaned for modification 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:
- Clean gold surfaces ahead of self-assembly experiments
- Activate electron microscopy (EM) grids before applying samples for imaging
- Oxidize titanium to improve biocompatibility as a medical implant material
- Treat biomaterial surfaces and polymer scaffolds to increase cell attachment and spreading for cell culturing
- Plasma treat fibers to improve bonding properties to matrix in fiber-reinforced composite materials
- Clean AFM cantilever tips to study bonding characteristics of divergent materials by mechanical testing or AFM force measurements
- Activate carbon surfaces to encourage attachment of silver (Ag) nanoparticles, for use as an antibacterial medium and for water disinfection
Air or oxygen (O2) gas is generally used for surface activation and 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.
As an alternative, an argon plasma may be favored for cleaning in order minimize the 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.
For applications that are susceptible to possible contamination from trace impurities in borosilicate glass, a quartz chamber is advised ahead of the standard Pyrex chamber.
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: 500 mTorr to 1 Torr
- RF power: Typically, HIGH
- Process time: One to three minutes
- Low RF power may be used to minimize surface roughening; the process time may need adjusting to offset the lower power
- 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.