PRODUCT UPDATE

Developments in Surface Treatment Technologies

Norbert Sparrow

Suppliers offer ways to improve a material's physical and mechanical characteristics as well as its biocompatibility.

A variety of surface treatment processes and materials are available to device manufacturers to achieve a range of product enhancements. In this issue's Product Update section, we present a selection of companies that have developed technologies to limit nosocomial infections, enhance ultrasound visualization, and improve lubricity, among other material improvements.

The most recent development at STS Biopolymers (Henrietta, NY, USA) involves a coating that permits low-cost placement and tracking of devices used with transesophageal ultrasound probes. According to company chairman Richard Whitbourne, the ultrasound echogenic coating maximizes the advantages of ultrasound while overcoming the contrast-resolution limitation of medical devices in vivo.

Several companies described in the following pages provide plasma-based treatment services and equipment that are designed to enhance the surface properties of polymers. According to Anthony Vanlandeghem of Europlasma (Oudenaarde, Belgium), one advantage of this method is that it enables manufacturers to optimize the compatibility of a device with other groups or layers without altering its structural and mechanical properties.

Nova Tran Ltd. (Northampton, UK) specializes in the deposition of parylene films by means of vapour condensation. The polymer coating is, in fact, grown one molecule at a time, according to marketing manager Chris Hardy, and therefore it provides complete and uniform coverage. Ample mechanical and barrier protection is ensured at layers as thin as 15 µm, he adds.

Infection resistance is a major concern among manufacturers and end-users, and a number of options exist to endow devices with antimicrobial properties. Spire Corp. (Bedford, MA, USA) offers a proprietary process optimized for each customer's design that produces an infection-resistant and thromboresistant surface. At Creavis (Marl, Germany) a coating has been developed that reduces bacteria and fungus adhesion by 90% compared to untreated surfaces, according to R&D manager Peter Ottersbach. The coating is compatible with numerous materials including silicone. And SurModics (Eden Prairie, MN, USA) offers three different antimicrobial coating options based on its PhotoLink technology that integrates easily into existing manufacturing systems.

Detailed information on these and other firms supplying surface treatment services and equipment can be found in the Product Update profiles that follow.

Maret S.A.

Initially, polishing was one of several services offered by a supplier of synthetic ruby, sapphire, and metal components to the watchmaking industry. The polishing technique continues to be a benchmark of quality for watchmakers, but it has also been adopted by medical device companies. Process flexibility and the fact that a minimal amount of material can be removed while respecting tight dimensional tolerances are among factors contributing to the technology's success.

"People often assume that what we do is a form of electropolishing, but that is not an accurate description," stresses Christian Farine of Maret S.A. (Bôle, Switzerland). "Maret polishing is a mechanical process that uses a chemical recipe that we have developed and that we can adapt to specific application requirements."

The polishing technique is suited for most materials used in medical devices, according to Farine, including plastics, stainless steels, and titanium. "Titanium, as you probably know, can be difficult to treat, but it poses no problem for us," says Farine.

Some of the functional benefits that the company's surface treatment technology can impart to a material include an improved coefficient of friction as well as corrosion resistance. Farine also points out that the company has substantial experience processing small delicate parts for the watchmaking industry: "We are accustomed to removing material in the 1- to 5-µm range to very precise tolerances," he says.

Maret polishing is not inexpensive, concedes Farine, but to achieve a comparable finish on all the surfaces of a small part would essentially require doing it manually. Seen from that perspective, Farine adds, Maret polishing is, in fact, a bargain.

Wesley Coe (Cambridge) Ltd.

A contract manufacturer specializing in the manufacture, assembly, and packaging of medical devices also offers surface treatment services. "Most of our surface treatment work involves chemical etching, chemical treatment, and blasting with various media, either to improve adhesion or to prevent it," says Simon J. W. Coe, managing director at Wesley Coe (Cambridge) Ltd. (Cambridge, UK). "We specialize in what you might call engineering finishing, which involves a product's functional qualities."

Gold, silver, nickel and electroless nickel, tin, tin-lead alloys, dip solder coatings, and dull and bright copper are among the finishes typically provided by the company. Plating can be performed on a variety of substrates including ceramics, aluminium alloys, titanium, molybdenum, and metal matrix composites. Other surface treatments available include chemical processes, electropolishing, etching, and precision bead blasting.

Wesley Coe's competitive advantage, according to Coe, is its experience working with small to medium- size batches. "That has served us well with the medical device industry," he says, "because smaller companies tend to feel more comfortable working with us than they might with larger, more impersonal contract manufacturers." Wesley Coe also does a substantial amount of business with electronic OEMs, adds Coe, "so, overall, we are really quite used to dealing with delicate, small-batch, high-value components."

The company is equally at ease working to client specifications or to BS, ISO, MIL, and other standards. Full traceability and certificates of compliance are available; coating thickness is measured by means of x-ray fluorescence equipment. The company, which is US FDA registered, is certified to ISO 9002 and EN 46002 and is equipped with a Class 10,000 cleanroom.

STS Biopolymers

A company specializing in surface modification of medical devices has announced the development of an ultrasound echogenic coating that will allow low-cost placement and tracking of devices such as stents, biopsy needles, and catheters. According to Richard J. Whitbourne, chairman of STS Biopolymers (Henrietta, NY, USA), the coating maximizes the advantages of ultrasound imaging while overcoming the contrast resolution limitation that has plagued in vivo medical devices.

"The coating contains discrete gas bubbles and pores that provide acoustically reflective interfaces within the coated device surfaces," explains Whitbourne. "The acoustical irregularities in the surface create a significant difference in the impedance between the substrate material, the coating, and the surrounding tissue, so you get a good strong signal that reflects in multiple directions," he says.

In a typical biopsy procedure, for example, the signal will travel from the transducer to the surface of the needle and then bounce off into a direction where it is difficult for the transducer to pick up. Treated with Echo-Coat, the needle can be visualized at almost any angle, according to Whitbourne. STS Biopolymers plans to launch a line of coated biopsy needles before the end of 1998 and has made the technology available for licensing. The coating can be applied to most polymeric and metal surfaces by means of a simple dipping process.

The company has also developed an antiinfective surface treatment that combines nonreactive polymer coatings with antimicrobial and antibiotic agents. The antiinfective agents elute from the Medi-Coat coating, providing high drug concentrations near the device surface. "We are able to control the rate at which water diffuses in and out of the coating as well as the solubility of the drug," says Whitbourne. Consequently, the elusion rate can be programmed to occur rapidly or progressively over a period of weeks or even months, he adds. In addition to its drug-delivery properties, Medi-Coat can also enhance lubricity and resistance to protein adhesion.

Nova Tran Ltd.

A USP Class VI polymer film that can be grown onto complex shapes and substrates is devoid of pinholes at thicknesses down to less than 1 µm. The uniform and complete coverage that characterizes parylene coatings is a product of the deposition technology, called vapour condensation.

"The device is placed in a vacuum chamber at room temperature into which we introduce a monomeric gas at a higher temperature," explains Chris Hardy of Nova Tran Ltd. (Northampton, UK). The gas condenses and instantly polymerizes on the surface that is to be treated. "The principle is condensation," he adds, "and a good analogy is the way that steam condenses onto a mirror or a glass. We grow the coating, literally, one molecule at a time," says Hardy.

Parylene is effective even in ultrathin layers. A 15-µm coating, for example, provides mechanical and barrier protection as well as lubricity, and films less than 0.5 µm thick can be deposited accurately. To demonstrate the material's capability to adapt to almost any surface, Nova Tran has coated a duck feather with a 0.5-µm layer of parylene. "Under high magnification, you can see that the coating deposits uniformly and completely, even on the finest structure," says Hardy. More conventional applications include pacemakers, stents, catheters, guidewires, and seals.

In cases where a company does not wish to send its device to an outside source for surface treatment either because of confidentiality concerns or because it must remain in a clean environment for the duration of the production cycle, Nova Tran can build an in-line deposition system to customer specifications.

Creavis Medical Technology

A company has developed surface modification technologies designed to enhance the antithrombotic and bacteriaphobic properties of polymers while controlling their protein and bacteria adhesion. Creavis Medical Technology (Marl, Germany), a wholly owned subsidiary of Hüls AG, also produces coatings designed to enhance lubricity.

One coating developed by the company to prevent infections reportedly reduces bacteria and fungus adhesion on catheters and similar devices by more than 90% compared to untreated surfaces, according to R&D manager Peter Ottersbach. "The process involves covalently bonding a hydrophilic coating to the polymer's surface," says Ottersbach. "The coating is only about 10 µm thick, so it will not alter the final size of the product, nor will it affect the material's mechanical properties." The coating is compatible with an array of materials including polyurethane, polyamide, polyolefin, and silicone.

To foster lubricity, the company treats products with a hydrogel-based coating. According to the type of material that is being treated and its specific application, hydrogels of varying chemical compositions and physical characteristics are fixed to the surface. The tailored hydrogels can also bring about a significant reduction in bacteria adhesion and colonization, which helps to prevent nosocomial infections and subsequent posttreatment expenses.

Europlasma

Low-pressure or cold plasma systems can clean a substrate, remove moulding residue, activate the substrate, and perform surface etching in one operation, according to a supplier of standard and custom plasma equipment. "All of this is accomplished without altering the structural or mechanical properties of the base material," says Anthony Vanlandeghem of Europlasma (Oudenaarde, Belgium).

A typical plasma installation comprises a vacuum chamber, gas-distribution system, RF generator with impedance network, and process controller. The specific mechanical design and process varies based on the application, according to Vanlandeghem. "Different gases are used depending on the property that the user wants to enhance: oxygen is introduced to create hydroxyl or carbonyl groups when wettability is desired, whereas fluorinated gases are used to promote hydrophobicity."

The process is stable and reproducible, adds Vanlandeghem, because the process parameters can be easily controlled. Wettability, for example, is validated by the use of special solutions in combination with contact-angle tests. "The smaller the contact angle, the greater the wettability," says Vanlandeghem.

Plasma treatment is a clean technology that uses no solvents, produces no fumes, and creates no disposal problems. It is suited for an array of materials and devices to enhance biocompatibility, adhesion, and wettability.

SurModics Inc.

A surface modification technology can produce multiple substrate enhancements. These include lubricity, haemocompatibility, infection resistance, wettability, guided tissue growth, reduced protein and cell adhesion, drug delivery, biomolecule attachment, and antimicrobial properties. PhotoLink technology, developed by SurModics (Eden Prairie, MN, USA), can be applied to a wide range of polymers, according to marketing manager Nancy Hupfer, and the reagent formulations used in the process are nontoxic, are applied under ambient conditions, and are predominantly water or water/alcohol based. "No hazardous solvents are used," Hupfer stresses.

Three separate antimicrobial coating options are available to device manufacuturers using the PhotoLink system. Microbial adherence can be suppressed by depositing a hydrophilic coating on the substrate. For higher levels of antimicrobial activity, agents can be incorporated into covalently bound hydrophilic coatings and progressively released from the coating matrix. A third approach is to photochemically immobilize an antimicrobial agent directly on the surface of the device. The covalent bond between the antimicrobial agent and the device surface is designed to promote long-term antimicrobial activity.

One of the key advantages of PhotoLink compared to other surface modification technologies, says Hupfer, is that it easily integrates into existing manufacturing systems and that it can generate a host of surface characteristics. "The process may change a bit depending on the application—dipping may be fine for catheters but it wouldn't work with stents—but essentially what you're doing is getting the reagent onto the device and illuminating it," she says.

Buck Plasma Electronic GmbH

An ISO 9000–certified company supplies its low-pressure plasma process and systems to device manufacturers. Because the process is dry, cold, and fast, it is not only efficient but ecologically sound, according to Christian Laubner of Buck Plasma Electronic GmbH (Neuenburg, Germany).

Medical device applications of the process include enhanced wettability and adhesion, lubricity, and ultrafine cleaning. In addition, diamond-like carbon coatings can be deposited to protect devices from wear and corrosion and Diacor can be used to protect a device's functional and decorative aspects. Syringes, catheters, tubing, hubs, silicone parts, metal components, and numerous diagnostic devices can be treated using this technology, adds Laubner. The range of systems that are available can meet a variety of client needs, and the company provides complete support services.

Spire Corp.

A proprietary surface treatment process renders temporary and permanent implants resistant to infection and thrombosis. Ioncide antimicrobial surface treatment, developed by Spire Corp. (Bedford, MA, USA), produces a permanent, durable surface that is optimized for each client's specific design. The resistance properties of Ioncide are supported by several scientific studies and through products that have been approved for marketing by US FDA, such as external fixation pins, catheters, and permanent implants.

Spire Corp. supplies other surface treatment services based on ion-beam-assisted deposition that improve the surface characteristics and performance of orthopaedic devices. Ionguard enhances the mechanical and chemical surface properties of titanium-alloy devices; Ionguard II reduces polyethylene wear in articulating cobalt-chromium orthopaedic devices; and Ionguard III is made to enable polymeric cross-linking on the surface of ultra-high-molecular-weight polyethylene.