Medical-device materials are typically selected for specific mechanical or chemical properties, but problems can occur integrating them into the medical device. These problems appear in the form of reliability issues often due to a lack of adhesion caused by material incompatibility, or contamination, or both.

A solution comes from modifying a material's surface with plasma. It's an environmentally friendly, cost-effective process that can tailor surfaces to overcome the performance and reliability challenges of manufacturing medical devices.

What plasma does

Plasma works well in medical product manufacturing by modifying surfaces of materials including various metals, ceramics, plastics, and elastomers. It is a consistent and controllable method for improving bonds between incompatible materials, increasing a surface's wettability, rendering it hydrophilic or hydrophobic, and enhancing the biocompatibility of materials. Plasma is also an efficient way to remove a wide variety of organic and inorganic contaminants, introduce specific functional groups, and apply biocompatible coatings.

Plasma, the fourth state of matter, is an electrically neutral gas mixture consisting of various active species including ions, radicals, and chemically active byproducts. These plasma-generated species are used to perform physical, chemical, or physical-chemical surface modification work.

A positive characteristic of plasma processing is that it's typically an environmentally friendly manufacturing process. Many plasma treatments use common environmental or inert gases such as oxygen and argon. Interactions are on an atomic or molecular level so byproducts, which can be in the parts-per-billion, are harmless. These consist of compounds such as carbon dioxide and water. Wet chemistries, on the other hand, are solvent based, and require special handling and waste-stream management of its byproducts.

Plasma can be used to clean, coat, crosslink, functionalize, and etch surfaces. The selection of a specific process is determined by the medical device along with its performance and reliability requirements. Cleaning, coating, and functionalizing surfaces are plasma processes that help improve a product's performance. A few devices that use plasma during manufacture include catheters, contact lenses, pacemakers and defibrillators, and medical filters.

A few applications

Cleaning with plasma is a dry, environmentally benign process that removes contaminants, including organic residues, polymer coatings or build-ups, waxes, metals and metal oxides, fluorine, and other halogens. Additional contaminants that can be removed include handling and packaging residues, and processing materials such as coatings and polishing residues. Plasma cleaning can permit rework, maintain multiple-use fixtures, and process components by eliminating build-up that comes from repetitive use. Plasma is also increasingly used to clean electrical leads to improve their bonding and performance, treat metal stents for biocompatibility and adhesion of polymers and drugs, and process contact-lens-manufacturing fixtures to restore their function and reduce contamination. Plasma surface modifications speed manufacturing, reduce waste streams and their elimination costs, and save valuable production space by reducing a required system's footprint.

Coatings applied with assistance from plasma include biocompatible materials that help create surfaces that are lubricious and low-friction, chemical and moisture barriers, bondable, wear resistant, hydrophilic, hydrophobic, protein resistant, static resistant, or insulative. The resultant films are nanometer thin, strongly adhered conformal coatings capable of being deposited to a wide variety of surfaces. This developing technology offers designers unprecedented capability to tailor surfaces for specific functions. Examples include tie-layers for strong metal to plastic bonds, coatings for contact or intraocular lenses and filter media to improve wettability or alter gas and liquid permeation rates, sealing or capping layers, and layers that can be functionalized for a specific molecular attachment.

Functionalization involves the reaction of plasma-generated radicals with a sample surface, resulting in the addition of specific chemical functions to the surface. For example, an amino (NH₂) function can be added to the surface by using ammonia as a plasma-process gas. Other chemistries can produce oxides, hydroxides, and carboxyl groups. Functionalizing a surface with plasma improves bonding between incompatible materials and helps facilitate strong chemical bonds between the adhesive and bonding materials. Emerging uses of this plasma capability include improving diagnostic sensors and sensitizing blood filters for particular compounds. Functionalization also allows attaching an anticoagulant such as heparin and antiviral or antimicrobial molecules to surfaces. Contact lenses benefit with treatments that make them hydrophilic and resistant to protein deposition.

In a few other applications, plasma ultra-cleans surfaces or activates them so as to increase bonds between a wide variety of materials. Recently, applications can take advantage of plasma's ability to apply coatings and tailor surfaces so as to improve product performance. In catheter manufacturing, for example, plasma strengthens the balloon-to-catheter bond and improves the uniformity and adhesion of hydrophilic and lubricious coatings to various catheters. Plasma cleans mold release and activates the surface of pacemaker and defibrillator headers to improve the bond between the header and can. Plasma can also be used to remove titanium oxide from laser welds or remove polishing residues from implantable metal stents. As a final cleaning step, it helps improve the biocompatibility and bondability of many medical products or components.

Selecting a system

The keys to successfully implementing a plasma treatment include thorough knowledge of the substrate to be processed, the potential machine configuration, and the goal of the application. Plasma technology and configuration, process gases, power, pressure, temperature, and time are key components of the plasma process that must be optimized for success.

Today's rapidly changing manufacturing environment calls for flexible equipment that consistently produces a required result. Plasma systems meet this need with a uniform treatment that is highly controlled and provides repeatable processing. Plasma systems are also easy to maintain, suitable for cleanroom operation, and consume significantly less manufacturing floor space than competing technologies.

Furthermore, plasma equipment should have local and remote monitoring capabilities, it should track key process parameters, and track product processing to include lots and operators.

WHERE THE GASSES WORK WELL

SOURCE GAS	SURFACE MODIFICATION	ELECTRONIC APPLICATIONS
Argon	Removes contaminants by ablation	Wirebonds and die attachments
	Cross linking	Metal adhesion to a polymer
Oxygen	Remove chemical contaminations	From wirebonds
	Remove organics by oxidation	Die attachments
Surface activation	Mold and encapsulant adhesion
	Etching	Remove photoresists
Nitrogen	Surface activation	Mold and encapsulant adhesion
Hydrogen	Remove chemical contamination	From wirebonds
	Reduction process (oxides)	Eutectic die attach
Carbon tetrafluoride and oxygen Or and oxygen	Etch fiber Remove photoresists	Etching polymers and stripping Sulfur hexafluoride
		Thin film etching with oxides and nitrides