Advances in Surface Treatment Technology

Originally Published MPMN March 2001

PRODUCT UPDATE

Advances in Surface Treatment Technology

A variety of techniques can render devices bondable, radiopaque, lubricious, or antimicrobial.

Implantable or invasive devices must be compatible with the human body as well as cost-effective for the manufacturer. While some materials such as plastics and fluoropolymers may be inexpensive in terms of production costs, in their untreated form they may not be suitable for interaction with the human body. The application of certain surface treatment techniques can provide the desired properties for these devices without raising the cost prohibitively. A sampling of suitable surface modification methods follows. Please see page 56 for a Buyers Guide listing of surface modification providers with complete contact information.

Portable plasma systems are suitable for ambient-air use

Although silicone and fluoropolymers are inexpensive and highly moldable, they are not naturally wettable and pose problems for medical device manufacturers if bonding or adhesion is required. Plasma, an electrically conductive ionized gas, can raise the surface energy of a material to provide adequate surface activation for enhanced wetting and adhesive bonding. The recent trend in plasma treatment has been to make surface treatment equipment smaller and portable in order to provide precise control of the plasma stream in any environment.

Tri-Star Technologies (El Segundo, CA) has developed the PT2000P Plasma Station, a portable benchtop unit that prepares nonconductive surfaces for the application of adhesives and lubricious or other specialized coatings. The unit features a flexible interface between the plasma generator and plasma delivery head. Detachable heads in many sizes and configurations can be customized to treat 60-µm to 5-in. exterior areas and can be attached to a conveyor, indexing table, or robotic assemblies, or used as a handheld device.

According to Tri-Star president Alex Kerner, the system "does not require a vacuum chamber or cleanroom of its own. It works in an open, ambient environment at atmospheric pressure and room temperature, making it much more user-friendly than systems needing special environments."

The PT2000P is equipped with a digital plasma exposure meter, a plasma intensity regulator that sets voltage parameters and determines plasma current, a gas-flow regulator, and other controls that allow users to determine and adjust plasma energy. The unit plugs into a regular 110- or 120-V outlet, is cleanroom-compatible, and uses argon gas or other mixes of gases.

Like the PT2000P, the PlasmaPen from Metroline/IPC (Corona, CA) functions in the ambient environment. The PlasmaPen can be run manually or by robot, and requires only a standard 120-V outlet connection and a compressed-air-system connection. The unit uses compressed air or other gases to treat a range of materials including plastics, metals, and glass. When it is activated, gas flows through the pen and is ejected out of the nozzle. Applications include surface cleaning, surface modification, and localized treatment of catheters. The plasma treatment can be combined with other chemicals to give more properties to materials. Options include an internal pump that eliminates the need for compressed air, multiple plasma jets that operate in parallel, and variable power level and gas flow. "Plasma has been a great technical solution, but hard to justify economically," says vice president of sales and marketing Blake Thompson. "The PlasmaPen dramatically reduces costs by being usable for in-line production and spot treatment of small assemblies without engendering the cost of disposing of wet chemicals."

Radiopaque coatings provide accurate invasive device positioning

When using invasive devices like catheters, guidewires, and stents, it is critical that the physician knows exactly where the device is once it is inserted into the body. The addition of metallic radiopaque markers on these devices allows them to be seen on x-ray film or on a fluoroscope. Spire Corp. (Bedford, MA) has developed a system of attaching radiopaque markers through the use of ion-beam-assisted deposition.

The Ion-Sight process involves a combination of evaporation and concurrent ion-beam bombardment in a high-vacuum environment. Ion bombardment intermixes coating and substrate atoms to create very dense, adherent film structures. Gold, silver, or platinum films can be applied to polymers, metals, and ceramics to provide radiopaque markers suitable for use on catheters, guidewires, stents, and vascular grafts.

Polymerization improves effectiveness of invasive devices

Invasive and extracorporeal devices that come in contact with body fluids often require surface treatment to make them lubricious and biocompatible. Untreated, these devices can cause bacterial infections, blood clots, and other complications. STS Biopolymers Inc. (Henrietta, NY) recently developed two surface modification techniques designed to increase the effectiveness and improve the surface quality of catheters and other in-dwelling medical devices.

Graft-Coat, a reverse-phase graft polymerization technology, allows a variety of polymer layers to be permanently bonded to difficult-to-coat surfaces such as silicone, latex, polyethylene, and fluoropolymers. "The major advantage of Graft-Coat surface modification is the chemistry of attaching free radicals to a substrate that allows monomers to covalently bond and polymerize onto the surface within an aqueous medium to create the desired polymer surface," says Richard Whitbourne, chairman of STS Biopolymers. "A benefit of this technology is the ability to coat medical device surfaces, including catheter lumens and odd-shaped geometries, without the use of plasma, or gamma or UV radiation."

Graft-Coat technology can be used to improve the existing surfaces of devices and to incorporate a range of pharmaceutical agents such as antimicrobials, anticancer drugs, and other substances individually or in combination with the coating. Incorporated drugs are delivered at high concentrations at the device surface, with low systemic concentrations. Varying hydrophilic properties reduce friction and increase the physician's ability to control the device, reducing the risk of patient trauma and complications.

Some catheters and other in-dwelling medical devices may activate the body's coagulation and inflammatory system, causing what is known as thrombosis, when they come into contact with the patient's blood. To prevent thrombosis, device manufacturers can utilize the anticlotting activity of heparin polymer surface coatings.

A common treatment for invasive medical devices is the use of a heparin benzalkonium (HBAK) complex. Benzalkonium allows HBAK to adhere to device surfaces, but is sufficiently soluble in blood that, upon presentation to vasculature, it is quickly washed away. STS Biopolymers has developed heparin-containing Medi-Coat polymer coatings for use on implants and in-dwelling catheters to render them hemocompatible for sustained periods. The Medi-Coat system consists of an inert biocompatible polymer matrix used to entrap heparin complexes on the surfaces of medical devices. The polymer coating acts as a reservoir for the heparin complex, releasing the agent slowly over time upon exposure to blood to increase the duration of anticoagulant activity. A high local heparin concentration at the device surface results in prolonged antithrombogenic protection from less than one hour to more than two weeks, while maintaining low systemic concentration levels.

Silver-based antimicrobial treatment suited for long-term in-dwelling devices

Also for use in catheters and other invasive devices is an antimicrobial compound developed by Agion Technologies Inc. (Wakefield, MA). Its effectiveness has been demonstrated in laboratories against a range of nosocomial, airborne, and waterborne bacteria; yeast; fungi; and molds. The compound is composed of silver ions bonded to a naturally occurring, completely inert ceramic material that releases the silver at a slow and steady rate. Ambient moisture in the air causes low-level release that maintains an antimicrobial surface. As humidity increases and the environment becomes more suited for bacterial growth, more silver is released, up to a maximum release rate. Even under very wet conditions, the silver releases very slowly, ensuring long-term protection. Silver kills microbes by interacting with multiple binding sites on the microbes' surfaces that are unlike the binding sites used by organic antibiotics, reducing the likelihood of bacteria becoming resistant to the compound. The Agion compound does not cause discoloration in products, tolerates high temperatures used in manufacturing, and is effective against a broad spectrum of bacteria. Potential uses include wound care, heart valves, pacemaker leads, suture rings, feeding tubes, orthopedic implants, small-joint replacements, and catheters of all types.

Katherine Sweeny

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