COATING TECHNOLOGY
INCREASING SURFACE ENERGY For a majority of applications, plasma treatments are used to increase the surface free energy of the material. Surface energy is defined as the sum of all intermolecular forces that are on the surface of a material, the degree of attraction or repulsion force of a material surface exerts on another material. When a substrate has a high surface energy, it tends to
attract. For this reason, adhesives and other liquids often spread more easily across the surface. This wettability promotes superior adhesion using chemical adhesives. On the other hand, substrates that have a low
surface energy – such as silicone or PTFE – are difficult to adhere to other materials without first altering the surface to increase the free energy. There are several plasma methods to increase surface energy, including physical and chemical plasmas along with PECVD coating surfaces. In addition, plasma can increase the surface area of bonding by nano-roughening a surface. Surfaces that are highly ordered or very crystalline, tend to have very low surface energies. To disrupt that order, ionised plasma gas is used to bombard the surface. The most common and affordable options are helium,
nitrogen and argon. According to Barden, the selection of the type of gas is determined by the amount of ion momentum required to disrupt the surface order. “To create more of an effect, gases with higher atomic masses can be used. At one end of the spectrum you can use helium for a light impact on the surface, whereas argon, which has 20 times the atomic mass, will impact the surface with much higher force.” This creates a surface with a high dispersive effect (high wettability).
Another method of increasing surface energy is
to create a polarisable group on the surface by using chemical plasma. For example, O2
plasma can be used
to create surface hydroxyls that allow liquids to spread through hydrogen bonding mechanisms.
ADHESION TO NON-STICK COATINGS Plasma technology can also be used to control the surface chemistry of PTFE to improve bonding, not only for adhesives, but also inks, coatings and biomaterials. PTFE and other fluoropolymers are known for
their low coefficients of friction, exceptional chemical resistance and biocompatibility. They also offer high melting points, low dielectric constants, and resistance to flammability. However, the application of PTFE is often limited due to the material’s inability to be adhesively bonded to other materials. It has an inherently low surface energy and poor polarisability. “When a surface is really hydrophobic, such as
Teflon, it’s very difficult to bond to it,” says Barden. “If you apply a liquid or adhesive, it just won’t spread effectively.” Fortunately, adhesion properties of PTFE can be dramatically improved using PECVD techniques. The process creates a coating with polar functional groups on the surface that act as excellent anchors to either hydrogen bond or covalently attach hydrophilic coatings. Although ammonia gas plasma activation is traditionally used for this purpose, PVA TePla has developed an alcohol PECVD process that improves bonding strength 1.5 times over ammonia and 8.5 times over the untreated surface.
ADHESION OF BIOLOGICAL MOLECULES G
as plasma can also provide surface conditioning of in vitro diagnostic platforms prior to the adsorption of biological molecules (protein/antibody, cells, carbohydrate, etc.) or biomimetic polymers. This includes precision cleaning of the substrate at the molecular level, along with raising the surface energy to improve surface assimilation of the intended content. “Microtiter and multiwell plates are often made of polystyrene, which is extremely hydrophobic. Water will bead on it,” says Barden. “However, if you treat polystyrene with oxygen plasma it will become very hydrophilic, so water spreads everywhere. This allows aqueous solutions containing biological content to spread and deliver biomolecules to the surface while providing a hydrogen bonding platform to adhere them.”
However, some in vitro diagnostic substrates require more selective surface chemistry to immobilise a customer’s proprietary molecules. For this, PVA TePla has recently developed methods for chemically functionalising various polymer platforms for the selective adhesion promotion and conjugation of bio-active molecules. This is achieved by providing particular chemical functionality or linker chemistries at the surface, allowing conjugation of a wide variety of molecules ranging from small molecule drugs to peptides to larger biopolymers such as carbohydrates and antibodies. Amino, carboxylic, hydroxyl and epoxy functionalities are important examples of the chemistries that are readily obtainable using a gas plasma surface treatment. l
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