Plasma Modifications of Polymers' Surfaces

The field of biomedical applications needs polymers that, besides satisfying the physical requirements of their application, show the so called “biocompatibility”, reacting properly with the biological environment in which they are employed. Since biocompatibility involves the interface between the device and the biological environment, surface modification techniques can be of a great help to solve this problem avoiding costly changes of materials.

Among the techniques of surface modification the “cold plasma” is seen as one of the best and of the most promising thanks to its characteristics:

  • FAST AND FLEXIBLE
  • ECONOMIC

Plasma treatments are suitable for:

  • preparing hydrophilic or hydrophobic surfaces
  • suitable for biomedical and industrial applications
  • improve the adhesion of inks, paints and glues
  • surface cleaning / etching

A wide range of articles have shown an improvement of their performance related to the biological activity thanks to the plasma treatment

  • Cuvettes
  • Filters
  • Membranes
  • Biochips

Plasma Process

Plasma equipment and process can be so briefly described:

The equipment consists in a vacuum chamber with one or more gas inlet (equipped with proper dosing systems), two electrodes connected to a RF (or different frequency) generator are located within the chamber in order to supply to the gas that flows between them the energy that excitate its molecules generating the plasma.

The process runs in this way:

  • articles to be treated are placed within the chamber
  • a fixed degree of vacuum is reached
  • a fixed amount of the chosen gas(es) is made to flow through the chamber and excited by the electrical discharge creating the PLASMA
  • the reactive species of plasma react with the surface of the device modifying its properties

The process works in short times (from seconds to minutes) practically at room temperature affecting only the surface of the parts that are treated.

The features of the plasma process:

  • Action limited to a thin surface layer, without affecting the bulk properties of the material
  • Short times usual treatments on polymers range within minutes
  • Low temperature: the process temperature is near to the room temperature, which avoids unwanted effects on the polymers
  • Flexibility: it works on a wide range of materials and applications: from degreasing of a metallic surface to deposition of a new polymeric layer
  • Uniformity and reproducibility: in comparison with other surface modification techniques such as chemical or corona, the plasma process, being run under strictly controlled conditions (vacuum) and parameters warrants a constancy of results
  • Environment friendly the dry nature of the process means no pollution on the treated parts and no chemical effluents, after the treatment the parts need no any additional process
  • Cost effective: the above listed features mean low operating cost

Plasma applications

Depending upon the type of gas or vapour that is employed in the plasma process it is possible to obtain different effects:

  • Plasma cleaning
    plasma removes every organic contaminant from the surface leaving a surface cleaned on a molecular scale
  • Surface activation
    employs a reactive gas (e.g. O2, N2) which creates different functional groups modifying the hydrophilicity and the biochemical reactivity of the polymer’s surface plasma induced grafting employs inert gases (e.g. Ar,He) which remove some atomic species from the basic polymer generating reactive and cross-linked surfaces
  • Plasma deposition
    some gases (e.g. Methane) can be plasma polymerised, coating the device’s surface with a thin layer of a different polymer

One of the most used applications of the COLD PLASMA technology is the modification of the wettability of the polymers. Polymers usually show an unwettable surface (contact angle > 90°).

This property means that, whenever we need to have a good adhesion of anything to the surface, we encounter a poor response and we need to treat the surface in order to have an acceptable behaviour. Examples of application where a good wettability is important are, for example:

  • adhesion of biological molecules binding proteins for immunoassays application – Tissue Culture surfaces
  • glueing
  • inking
  • painting

Since every type of polymer shows different properties, the treatment must be studied and applied for the specific material, previously taking into consideration such features as :

  • the raw material
  • the shape of the part
  • the application

and therefore applying to the process the specific parameters:

  • TYPE OF GAS(ES)
  • TIME
  • POWER

and, after the treatment, testing the treated parts for

  • efficiency of the treatment
  • uniformity
  • durability of its effects during storage

and cooperating with the user in order to find the best treatment both for technical and economical features.

Biomat studies, develops and applies surface treatments with its plasma reactor installed in its facility.