Advanced surface treatment techniques such as air plasma, flame plasma, and chemical plasma discharges are playing key roles in the design and development of medical devices comprised of such polymers as high molecular weight polyethylene.

These techniques will alter the first few atomic layers of the polymer to render surfaces wettable so that adhesive bonding can be achieved to troublesome materials such as polyolefins, silicones, and fluoropolymers.

Plasma treatment defined

Plasma treatment usually refers to a plasma reaction that either results in modification of the molecular structure of the surface or atomic substitution. Even with benign gases such as oxygen or nitrogen, plasma treatment can create highly reactive species at low temperatures. High energy ultraviolet light is emitted in the process which, along with high energy ions and electrons, provide the energy necessary to fracture polymer bonds and initiate chemical reactions at the surface.

Only a few atomic layers on the surface are involved in the process, so bulk properties of polymer remain unaltered by the chemistry while the low process temperature eliminates concerns about thermal modification or distortion of the bulk. Unique reactions can be promoted by appropriate choice of reactant gases, and unusual polymer byproducts and structures can be formed. In many instances, plasma cleaning with benign gases such as oxygen or nitrogen provides adequate surface activation for enhanced wetting and adhesive bonding.

Medical device applications

Plasma treatment of polyethylene or polypropylene greatly enhances wetting. Contact angles as low as 22° have been demonstrated on these materials after brief oxygen plasma exposure (see Table 1). When these parts are properly packaged after treatment, the contact angle remains stable for years.

Conversely, medical polymers can be made extremely hydrophobic. Teflon-like films and other similar surface treatments can be easily accomplished on most polymers using fluorinated gases. For example, small diameter tubes can be treated so that when immersed in aqueous solutions they do not draw fluid by capillary action. A simple technique for evaluating plasma surface treatment is a wetting angle test using a contact goniometer. Surface roughness and substrate cleanliness need to be tightly controlled to obtain quantitative data. Standard wetting solutions are used to obtain accurate surface energy values. Most untreated polymers are hardly wettable. Initial contact angles may vary from 60 to 100°. Table 1 shows a sample of some typical contact angle measurements:

Many intravascular devices, such as balloon catheters, are assembled by adhesive bonding of polyethylene components. Chemical surface activation or mechanical surface roughening techniques provide only modest bonding performance, with bond failures noted after as few as eight repetitive inflations. With plasma treatment, up to 40 repetitions are achievable. Typical bond strength data are shown in Table 2.

An oxygen plasma not only removes organic residues, it reacts chemically with the surface to form strong covalent carbon-oxygen bonds, which are more polar and more reactive than the initial carbon-hydrogen bonds. Increased polarity of the surface accounts for substantial increases in wettability and adds a degree of covalent bonding to the surface adhesive interface. (Other gases may be used to attain similar results in instances where oxidizing species may be harmful to components of the assembly.) The bond strength ultimately realized will certainly be affected by:

• Initial cleanliness of the surface(s)

• Wetting of the surface by the adhesive

• Cross-linking effects

• Chemical interaction of the adhesive with the surface.

Any mold release compounds, ‘unpolymerized’ monomers, plasticizers, or additives that migrate to the surface must be removed either by plasma cleaning or washing before surface modification is attempted. Immediate assembly is advised after the surface has been prepared. Once the surface is optimized and bonded, the bond is permanent and does not degrade over time.

Release agents and other contaminations on molded and formed medical parts, however, can impede the performance of these adhesives dramatically if they are not addressed with surface pretreatments such as air plasma, flame plasma, or atmospheric chemical plasma.

Systems defined

An atmospheric air plasma treating system consists of two major components, a power supply and treatment station. The power supply accepts standard utility electrical power and converts it into single phase, higher frequency power that is supplied to the treating device. The treating device applies this power to the surface of medical plastics through an air gap, via an electrode design.

When air is exposed to different voltages, an electrical discharge develops. When this occurs, neutral molecules and electrically charged molecules collide. These collisions cause neutral molecules to become electrically charged, resulting in filamentary discharges or “streamers.” Such filamentary discharges create a cloud of ionized air or “air plasma.” When a medical plastic surface is placed under an air plasma discharge, electrons bombard the treatment surface with energies two or three times that necessary to break molecular bonds on the surface of most substrates. The resulting free radicals react rapidly with other free radicals on the same or different molecular chain, resulting in cross-linking. Oxidative effects on treated surfaces increases surface energy as a result of polar groups created on the surface, primarily in the form of hydroxyl groups, carbonyl groups, amide groups, and carboxylic acid.

Atmospheric flame plasma systems are comprised of a combustion-electrical station and a burner assembly, manufactured with two primary burner configurations - ribbon and enhanced velocity. A flame plasma is formed when a flammable gas and atmospheric air are combined and combusted to form an intense blue flame. The surfaces of medical plastics are made polar as species in the flame plasma affect the electron distribution and density on the surface. Polar functional groups such as ether, ester, carbonyl, carboxyl, and hydroxyl are contained in a flame plasma. These groups are incorporated into the surface and affect the electron density of the polymer material. This polarization and functionalization is made through reactive oxidation of a surface. ESCA analysis shows that oxidation depth through flame treatment is 5 to 10nm. This is generally less in depth than air plasma treatment, where oxidation depth is believed to be over 10nm. However, flame plasma treatment's extensive oxidation, due to reactions with hyrdroxyl (OH) radicals in the flame, results in a cleaned and highly wettable surface, which is relatively stable upon aging.

An atmospheric chemical plasma system is also composed of a power supply and treatment station whereby the system generates an electrically charged atmosphere similar to air plasma, but it uses chemical atmospheres in place of air to introduce a wide range of surface modifications to a substrate. The systems are characterized by their generation of high-density reactive species for low-temperature material processing.

The chemical plasma process can involve surface preparation via the breakdown of low molecular weight organic materials (LMWOM) and surface decontamination, fine etching of the surface to create new topographies, grafting of new functional groups or chemical species on the surface, and the deposition of coatings on the surface.

The treatment process is designed to allow the interchange of gas chemistries relative to the application requirements. In the case of medical plastic parts, loose surface oligomers and other residues are repetitively cleaved and degraded until they are removed largely by a combination of bombardment by ions and electrons. These organic residues are converted into water vapor, carbon dioxide, and other nontoxic gases or volatilized materials.

The medical plastic is optimally treated by atmospheric chemical plasma when it is positioned several millimeters downstream from the source. Line speed, power level, chemistry, chemistry mixtures, and material composition primarily determine levels of etching and functionalization that can be achieved.

Application of equipmentand processes

The application of atmospheric air plasma and flame plasma surface pretreatment sufficiently clean the surface of polypropylene, polyethylene, and polyurethane medical parts of LMWOM following molding and forming processes. This surface improvement is identified through surface tension testing using ethyl cellosolve and formamide (dyne) solutions, and subsequently through adhesive adhesion testing. Table 3 illustrates typical treatment results for several mainstream medical polymers and their respective treatment protocols.

The use of low polarity polyolefins in the manufacturing of medical device assemblies such as catheters, syringes, tubing, and other components may be suitably cleaned of molding and forming organic contaminations and functionalized using atmospheric air and flame plasmas for improved adhesive adhesion.

Table 1. Typical wetting angles, before and after oxygen plasma treatment

Medical Polymer	Before	After
Polypropylene	87°	22°
Polyethylene	87°	22°
Polyamide (nylon)	73°	15°
Polyimide	79°	10°
Polycarbonate	75°	33°
Tefzel	92°	53°
PFPE	96°	68°

Table 2. Lap-shear bond strengths (psi), without and with plasma treatment

Medical Polymer	Without	With
Polypropylene	370	1,380
Polyethylene (Low Density)	370	1,450
Polyethylene (High Density)	315	3,125
Nylon	850	4,000
Polystyrene	570	4,000
Polycarbonate (Lexan)	410	928
Tefzel	410	3,200

Table 3. Treatment results

Material	Air Plasma		Flame Plasma
	Initial Dyne	Final Dyne	Initial Dyne	Final Dyne
Polypropylene	30	44	30	46
Polyethylene	31	44	31	46
Polyurethane	31	40	31	42
Treat Width	50.8mm		150mm
Airflow	311 lpm		150 lpm
Treat Gap	6.35 mm		41.3 mm
Ave. Speed	10 fpm		100 fpm