Experts Weigh In: Selecting the Right Surface Modification


William Leventon

August 1, 2008

12 Min Read
Experts Weigh In: Selecting the Right Surface Modification

Parylene coatings are used in a wide range of medical devices including stents, pacemakers, electrosurgical tools, and endoscopic seals. Collage courtesy of Specialty Coating Systems Inc.

When it comes to modifying the surface of a product, medical device firms have no shortage of choices. These include a number of materials that provide many different properties and perform a variety of functions. In addition to evaluating properties and performance, manufacturers must consider critical factors such as cost, application, and longevity before choosing a coating for their medical device.

Then there are the coating suppliers to consider. Do they provide access to a wide range of coating choices? Do they offer help in focusing on the right coating for a product? Can they meet tight production schedules? In this article, surface modification suppliers discuss coating options and key services offered by companies. They also offer advice on how to select a surface modification product, the right coating, and the right supplier for a device.

Coating Functions

When applied to medical devices, coatings perform a variety of important functions, notes Lonny Wolgemuth, medical market specialist for Specialty Coating Systems (Indianapolis).

Isolation and Insulation. Coatings can serve as a barrier that protects a device from its environment. For example, a coating can keep moisture, chemicals, and gases from contacting and damaging a device surface. It can also protect devices by providing thermal and dielectric insulation. Conversely, a coating barrier can protect the environment from a medical product. For instance, applying coatings to the rubber ends of syringe plungers prevents minute traces of metals that reside in the rubber from leaching out, Wolgemuth says.

Changing Surface Properties. Coatings can change a surface from hydrophobic to hydrophilic, or make a surface biocompatible so that a device won't cause harm in the body.

Keeping Device Materials in Place. Consider ferrites, which consist of iron powder that is compressed at very high temperatures and pressures to form a solid object. Coatings are applied to tie down stray metal particles from ferrite surfaces and thereby prevent them from interfering with the operation of medical devices, Wolgemuth says.

Cosmetic Improvement. Coatings can be used for decorative purposes, according to Margaret Palmer, president of Coatings2go LLC (Carlisle, MA).

Surface Modification Options

A sprayer from Boyd Coatings Research applies custom fluoropolymer coatings to a prototype medical part.

Major surface modification materials include polymers and elastomers. Metal coatings are used to make a surface conductive, improve resistance to radiation, and shield enclosure contents from radio-frequency signals. Chemically coating substances adds antibacterial properties to devices or can be used to release drugs in the bodies of patients.

Plasma etching modifies device surfaces and improves the adhesion of a coating to a device. During the process, gas ions bombard a smooth surface to make it rough. A number of different gases can be used to etch a surface, but the right choice depends on the material.

Plasma etching can also be used to coat a product, according to John Wood, a systems engineer for Plasma Etch Inc. (Carson City, NV). Only a few molecules deep, this type of coating is sometimes used to protect an underlying coat or to reduce stiction during manufacturing processes.

Selecting a Coating

There are many factors to consider before choosing a coating for a medical device. Some of the most important factors include the following considerations.

How Well Will the Coating Perform Its Intended Function? Although this might seem obvious, there is more to making such a decision than some people might realize. “Somebody might look at the properties of PTFE and say, ‘I want to use that to protect my instrument from sterilizing solutions,'” says Donald Garcia, president of Boyd Coatings Research Company, Inc. (Hudson, MA). “PTFE will withstand sterilizing solutions, but that particular coating is porous, so a sterilizing solution will go right through the coating and get to the manufacturer's instrument.”

Has the Coating Technology Been Used Before? Potential users need to know whether a coating is a new entity or whether it has a track record, according to Charles Olson, vice president and general manager of hydrophilic technologies for SurModics Inc. (Eden Prairie, MN). Specifically, device manufacturers need to know what types of products a coating has been used on and what experiences others have had with it. This information can reduce risk for users and help them get products to market faster, Olson says.

What Is the Expected Lifetime of the Device or Application? Will the coating be used on a disposable instrument or on a longer-lasting device? If the device is a disposable instrument that will only be used a few times, an immersion coating or a one-coat system may be most appropriate, says Tracey Sherman, president of Donwell Co. (Manchester, CT). If the device will be used repeatedly over a longer period of time, a two- or three-coat system with ceramic reinforcement might be the best choice, Sherman says.

How Does the Coating Affect Device Manufacturing? Olson points out that a coating might need to be removed from consideration if the materials and processing involved can't be integrated into a cost-effective manufacturing strategy. Device companies must also consider how much time it takes to apply and cure a coating because of the effect that these processes will have on production throughput.

Will the Coating Have Unintended Effects? For example, a coating might absorb light of certain wavelengths and thereby prevent a light-generating or light-receiving device from working properly, says Wolgemuth. It could also adversely affect the sensing ability of a transducer.

Hydrophilic coatings from Surface Labs reduce friction during insertion and removal of devices. Obtaining adhesion is often difficult with low-energy surfaces such as silicone.

What Are the Conditions during and after the Coating Process? Many coatings cure at temperatures ranging from 300° to 800°F, according to Sherman. Will a device be deformed or damaged when exposed to such high temperatures? For example, a PTFE coating might cure at 750°F, a temperature high enough to soften a thin aluminum part. On the other hand, some devices may operate at temperatures that are too high for a particular coating to survive, says Wolgemuth. Potential users need to know how much expansion and contraction a coating will experience as a device moves through its functional temperature range.

How Uniform Is the Coating? A coating might be thinner in one location on a device than another, which means it's less capable in some places than in others. Users of brush-on coatings have very little control over coating thickness, according to Wolgemuth.

What Are the Space and Design Constraints? If the fit is tight in a certain spot, there might not be enough room to apply a relatively thick coating. In addition, some coating processes can't be used with certain design configurations. “You can't apply a spray coating down a 1⁄8-in.-diameter hole that's 4 in. deep,” says Sherman. “You can't spray where you can't see.” In a case like this, an immersion process can apply certain fluoropolymer coatings.

What Are the Adhesion Characteristics? Will the coating adhere well to your device? Will the coating stick to itself, or will it start to flake off and disperse particulate?

What Sterilization Method Will Be Used? If the device must be sterilized, the coating must be able to handle the sterilization method that the device will undergo.

(click to enlarge)Parylene coatings are applied via a vapor deposition process, in which components are coated in a vacuum chamber at ambient temperatures.

What Is the Supplier's Response Time? How long will it take a supplier to modify a standard coating to meet the company's needs? According to Palmer, a supplier's response time can be affected by the difficulty of a job, as well as its size. Large jobs sometimes take precedence over smaller ones.

What Does the Coating Cost? Palmer points out that a number of companies might be able to provide a coating that meets certain needs, but they should also provide it at an affordable price. Cost always comes up in discussions between Garcia and his customers. Sometimes it's a critical part of the discussion, so it must be considered when developing a coating. “I could have a coating that's the cat's meow, but it might cost more than the price of the customer's device,” says Garcia. In that case, a company might be able to change a feature of the device or component. Other cost-saving options include changing the tolerance or location of the coating application.

Advice on Coating Selection

When selecting the right coating for an application, top suppliers offer a number of suggestions. It helps to make a list of the pros and cons of the coatings under consideration, Wolgemuth says. In addition, question vendors closely about their coatings, suggests Wood. Even product-touting salespeople will start revealing “chinks in the armor” of their products if they talk long enough, he says.

Antiblocking coatings provide hard slick surfaces to eliminate product loss caused by tacky surfaces adhering to, or sliding against each other. Photo courtesy of Surface Labs.

It's also a good idea to find out about their track record, capabilities, and method of structuring business partnerships with customers, Olson advises. Think twice about doing business with companies that sell proprietary coatings that they claim only they can apply. “I would look to companies that give you the names of the coatings they're putting on, so you're not tied to one particular coater,” Sherman says. Several suppliers also caution device firms about putting too much emphasis on cost. “There's a tendency to overspend” on surface-modification technology, according to Wood. “I think it makes everybody feel better when they have the most expensive name. But sometimes the highest-cost option isn't the best one.”

On the other hand, some device firms gravitate toward the cheapest coating option. Such a coating might work well for a short time but then wear off. A supplier of inexpensive coatings could be “a financially unstable mom-and-pop shop that might not be there the next day,” Olson says. “It all comes back to risk. You can buy some very cheap coatings, but at the end of the day, you're going to see some [downsides] associated with them.”

Along with overemphasizing cost, there are other mistakes made by medical device companies when choosing coatings. According to Wolgemuth, one of these errors is selecting a coating that provides properties that a company is looking for, but not properties that are required by the application. For example, a medical firm might choose a coating that meets a biocompatibility requirement very well, but the coating can't withstand the abrasion that it will be subjected to when the device is in use. Sometimes coatings don't meet application requirements because a company chooses a coating that was used on another device for a different purpose or in a different environment.

This is just one reason that companies unwisely skip the process of investigating surface-modification alternatives for a new product. “I see a lot of people with something carved in stone that they have to use,” Wood says. “Maybe they read an article about it, or maybe a colleague from another company recommended it, and they're not willing to hear about any other options.”

This stent has a heparin-containing coating. The heparin, which is corrosive to steel, is often coated over a passivating primer to protect the steel struts. Heparin can be bonded or quick-releasing depending on its attachment method.

According to Palmer, success with a coating on a previous project sometimes prevents device manufacturers from considering all of the coating alternatives for a new project. In other cases, a contract holds a device manufacturer to a certain coating supplier, so only that supplier's coatings are considered for the company's new products. This can severely reduce the options. “Some coating suppliers have just a few coatings that they feel comfortable with, so no matter what your application is, you're going to get one of those,” says Garcia.

Options are limited when dealing with coating manufacturers, too, Sherman says. “They're going to tell you that their coating is the best,” he says. On the other hand, coating applicators also “look at coatings from many different manufacturers and pick the one that's best for a particular application.”

Help from Suppliers

Device firms can often count on suppliers for assistance in finding the right coatings for their products. Some coating jobs, however, are too small to interest certain suppliers.
“If you're going to sell 10 widgets a year and charge $10 each for them, there are probably suppliers that won't be interested in lending a hand,” Palmer says.

If suppliers are interested in a job, they can be helpful in many ways. “Usually, the client calls us, and either I or one of my senior engineers asks them what they want the coating to do,” Garcia says. “Then we take the properties the client wants and try to marry them to a coating that provides all of those properties. But sometimes clients can't get all the properties they want because of the nature of the [coating] materials.”

To prevent coating selection errors, the coating provider should ask its customers detailed questions before coating selection. “We have to ask these questions up front,” says Wolgemuth. Once coating suppliers have a good idea of a customer's requirements, they should be willing to coat some sample materials, parts, or devices to see whether the coating actually works. Suppliers can apply different coating thicknesses, formulations with slightly different performance characteristics, or entirely different coatings to substrates for evaluation. Samples are often exchanged a number of times between a coating supplier and a device manufacturer, as different coating options are tested, until the customer is finally satisfied. Suppliers can also produce data that show how its coatings compare with competing products.


Medical device companies have a range of choices when it comes to modifying the surface of a product. Not only are material properties important, but also factors such as cost, application, and longevity. It is essential to address all considerations before putting the coating on the product.

Copyright ©2008 Medical Device & Diagnostic Industry

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