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Gas Plasma: A Green Alternative for Surface Engineering

Wed, 07/05/2006 - 11:02am
There are many methods with which to affect the surface of a material or device to achieve a desired effect. Therefore, factors related to the process such as environmental considerations or temperature concerns will reduce the number of possible options. This article examines gas plasma processing and how it can be used by medical device manufacturers.
By Gerhard Winter
AT A GLANCE
  • Explanation of gas plasma
  • Benefits of use
  • Environmental factors
  • Non-polymerizing/polymerizable gases
Graph shows the surface of untreated polystyrene under XPS analysis. The surface consists primarily of carbon as represented by the tall peak. (Click here to enlarge)
After a short exposure to an oxygen-containing plasma, the surface chemistry is significantly altered. In the case of lab ware, adding these functional groups increases the polarity of the polystyrene surface to permit cell attachment. (Click here to enlarge)

Relative peak areas from curve fitting analysis. (Click here to enlarge)

Plasma processing is primarily a technique for surface modification of materials, either through cleaning, functionalization of an existing surface, or creation of a new surface by depositing a thin film. Materials that can be treated include polymers, metals, ceramics, and glass, while applications range from the treatment of polymers for increased wettability to deposition of polymerized coatings for lubricity or bondability. Although not as widely used as more conventional surface treatment methods, this environment- and worker-friendly technology is worth considering when precisely controlled processing and “green” manufacturing techniques are of particular concern.
What Is Gas Plasma?
Very simply stated and in its most basic form, plasma is a partially ionized gas generated by applying an electrical field to a gas under at least partial vacuum. When used for surface engineering, the plasma is generated by introducing gas into a vacuum chamber and exposing it to an electromagnetic field. The resultant plasma consists of ions and free electrons, free radicals, excited state species, and neutrals. When a gas is ionized in this manner, both the ions and electrons experience the same force and are accelerated. Collisions occur between these particles which transfers kinetic energy from one to the other. Since energy transfer in two body collisions favors the lighter particle (electrons in the case of plasma), the electrons soon have much greater velocity (i.e. temperature) than the ions.

Vacuum plasma processes are performed at low temperatures—typically between ambient and 50°C. To obtain comparable reactivity at atmospheric pressure would require temperatures of hundreds of degrees Celsius. This phenomenon is due to the fact that despite low gas temperature, high electron temperatures are present.

The first few atomic layers of a surface exposed to vacuum plasma chemically react and combine with the plasma species but the visual and bulk material properties are not affected. The process gas is continually refreshed and evacuated out the chamber to optimize the interactions and also to eliminate any cross contamination issues that could be encountered in cleaning applications.
Significant Factors
Compared to other surface modification techniques, low pressure plasma technology has several distinct advantages. Since treatment occurs under vacuum in a precisely controlled environment, variable results that can be experienced with atmospheric discharge treatment such as corona or so-called “air plasma” is not an issue. Use of automated and precision metering components such as mass flow controllers, PLC control, error monitoring, and batch process data recording can ensure absolute process repeatability.

Since plasma is a dry process, there are no disposal or personnel safety issues commonly associated with conventional wet chemistry surface treatments. The vacuum pump exhaust generally consists of minute amounts of gases such as H2O and CO2 and is easily vented outside the work area. Inherent cleanliness and the ability to automate and control all critical functions of the processing make the technology especially attractive for cleanroom manufacturing environments. In addition, low process temperatures permit treatment of very heat sensitive parts such as balloon catheter components.

Depending on volume requirements, manufacturing logistics, etc., companies may choose to purchase equipment for in-house processing or can outsource to companies offering contract processing services. Equipment purchase cost varies widely depending on chamber size, types of processes to be performed, size of vacuum pump, and level of parts handling automation. Small table top systems designed for R&D or low volume, small part processing typically start in the mid-twenty thousand dollar range.
Types of Plasma Processes
Almost any gas can be used to generate a plasma with specific gas chemistry dependent upon the material to be treated and the desired surface characteristics to be achieved. In the case of non-polymerizing gases such as oxygen or argon, the exposed surface may be cleaned at an atomic level as the active plasma species react with organic contaminants resulting in them being “burned off” at low temperatures and evacuated via the vacuum pump. This process is often used as a final cleaning step on metal and ceramic components to remove residual hydrocarbons. Polymers can be modified by rearrangement of the upper atomic levels by incorporation of polar chemical functional groups onto the surface. This results in increased surface energy (i.e., improved wetting) and is beneficial as a pre-treatment prior to bonding, printing, or coating.

The Challenge to Go “Green”

Another polymerization application is creation of a tie or interface layer to permit bonding of over molding on metal components. For example, suppliers of over molded metal components often encounter issues with silicone or elastomer adhesion to the base metal part. Surface preparation may require caustic chemical etchants or toxic conversion coatings such as chromium or phosphoric acid to ensure a good bond to the base metal substrate. Such was the situation when Technical Services for Electronics (TSE) was faced with a requirement to eliminate the existing chemical surface preparation and utilize only “environmentally-friendly” processes during assembly of a cable used in a hospital operating room environment.


Technical Services for Engineering offers alternatives to medical electronics and instrumentation companies looking to provide cable assemblies produced with environmentally-green technologies.
TSE (Arlington, MN) is a supplier of cable assembly and precision interconnection systems for the medical electronics and instrumentation industries. “Eliminating conventional surface prep on the plated metal back nut connector without changing material or sacrificing bond strength of the silicone over mold was a challenge. Although we had over 20 years’ experience in design and manufacture of custom engineered assemblies, this was a first for us,” according to Steve Sundberg, TSE’s executive vice-president.The solution came in the form of a thin film polymerized coating deposited directly onto the plated back nut. The tie layer provides a surface that is highly receptive to silicone bonding and again, without any additional chemical or mechanical surface preparation to the base part. The resultant bond performance is excellent with results that meet or exceed those achieved with the previous surface preparation methods. “The thin film coating allowed us to give our customer what they wanted without going through extensive re-engineering of the existing product. The results have been excellent with no bond failures in the six years that we’ve been supplying the assembly,” states Sundberg.
A common application in the medical device industry is treatment of polystyrene lab ware for increased hydrophilicity. Typical of polymers with a carbon backbone, polystyrene has innately low surface energy making the surface hydrophobic or non-wetting. A short plasma treatment results in a surface that is spontaneously wettable. While there can be shelf-life issues caused by the underlying base material migrating back to the surface, polystyrene treated with low pressure plasma typically remains hydrophilic for many months.

Rather than simply modifying the existing surface of a material, polymerizable gases can be used to create thin films. During plasma enhanced chemical vapor deposition (PECVD), the active species in the plasma react with themselves as well as with the surface to form a thin film coating. In contrast to conventional methods, plasma polymerization is an atomic, non-molecular process providing excellent adhesion of the coating to most metal and polymer substrates. Since coating thickness is typically well under a micron, even very tight tolerances are not affected. Surface characteristics that can be achieved include tailored wettability (hydrophilic or hydrophobic) and lubricity/reduced coefficient of friction. While surface modification with non-polymerizable gases can have shelf-life constraints resulting from labile molecules in the bulk material migrating back to the surface, plasma deposited coatings are permanent.
Conclusion
Low pressure gas plasma is a very versatile technology that has many uses in the medical device industry both as a manufacturing step as well as for finished disposables. Plasma cleaning, activation, and polymerized coatings are valuable tools for surface engineering of polymers, metals, ceramics, and glass when consistent, precisely controlled processing is required while utilizing only environmentally-friendly techniques.
ONLINE
For additional information on the technologies and products discussed in this article, see the following websites:Gerhard Winter has been involved in low pressure plasma equipment design and plasma process development for over 15 years, with particular emphasis on PECVD thin film polymerization technology. Since 1996, he has been president of PLASMAtech Inc., 1895 Airport Exchange Blvd., Suite 190, Erlanger, KY 41018. The company is a supplier of plasma equipment, contract processing, and process development services to the medical, electronics, and automotive industries. Winter can be reached at 859-647-0730 or gwinter@plasmatechnology.com.
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