Performance, manufacturability, safety, and overall cost are crucial when selecting coating technologies, while ease of application, use of hazardous solvents, cycle time, and scrap rates are key when evaluating a coating’s manufacturing process.

AT A GLANCE

• Enhancements explored
• Processes defined
• Advice and guidelines

By Robert Hergenrother and Sara Macklin
In recent years, several surface modification technology platforms have been developed in an attempt to improve the function of medical device surfaces by adding or enhancing surface characteristics. Such enhancements can include lubricity, hemocompatibility, infection resistance, tissue integration, and most recently, the ability to deliver drugs from the surface of a device. Coatings modify surface characteristics by either passivation to prevent undesirable biological responses or by activation to incorporate a specific functionality or functionalities into the device/environment interface.

Figure 1: Typical manufacturing process for coating a medical device

In determining which coating technology to select, device manufacturers must not only consider which technology delivers the surface-enhancing characteristics they desire but also which manufacturing process will best meet their needs. Ease of application, use of hazardous solvents, cycle time, and scrap rates must all be taken into consideration. Figure 1 illustrates a typical manufacturing process for coating a medical device.

Preparing the Surface

Mixing Coating Reagents
Coating reagents are usually packaged and shipped in dry form and must be dissolved in a liquid before use. Most coating reagents must be dissolved in harsh solvents such as methyl ethyl ketone (MEK), tetrahydrofuran (THF), or chloroform while some may simply be dissolved in water or water/isopropyl alcohol. Improper mixing conditions or mix ratios can result in poor performance. When harsh solvents are used, it is very important to provide proper ventilation and follow all safety protocols for proper storage, handling, and disposal.

Cleaning the Device
It is important to start with a clean surface. Prior to coating a substrate, make sure any contaminants from handling and prior manufacturing steps have been removed. Wiping with a clean-wipe saturated in isopropyl alcohol or using sonication with a cleaning solution can be effective cleaning methods.

Pre-treating Non-hydrocarbon-based Substrates
Metals, glass, and ceramics may require a pre-treatment to achieve a durable coating bond. The pre-treatment can be as simple as adding a tie-layer of hydrocarbon-based material—such as parylene or silane—to the surface. The tie-layer acts as an intermediary between the device surface and the coating, bonding to both. Without a tie-layer on non-hydrocarbon-based substrates, low durability and flaking of the coating may occur.Masking the Device
It is not always desirable to coat certain areas of a device. For example, it is best not to coat the balloon portion of a stent delivery catheter with a lubricious coating because a slippery surface could cause a stent to slide off the balloon during deployment. Catheter side holes and pacing lead electrodes are other examples of areas that may be negatively affected by a coating. Selective coating, or masking, can prevent the reagent solution from coming into contact with the part, or the device can be mechanically reworked after the coating application to unblock side holes.

Manufacturing Steps

Step 1: Applying the Coating Solution

Table 1: Comparison of Application Methods

Coating solutions can be applied to devices in the same ways that a liquid can be applied. The most common method is to dip the device into the coating solution. Dipping is an affordable, simple, and reliable coating method when done with appropriate equipment. It’s best to use equipment capable of controlling the dip speed, the distance and dwell time in the coating solution, and the speed at which the part is withdrawn from the dip tank—the most critical dipping factor.

It may seem counterintuitive, but the faster the part is withdrawn from the dip tank, the thicker the coating. During a fast withdrawal, the coating solution has less time to drain off the device, resulting in a thicker coating. Conversely, the coating solution has more time to drain off when the device is withdrawn slowly, resulting in a thinner coating. Performance requirements of the device should be taken into consideration when determining the withdrawal speed.

In general, thick coatings are more lubricious and thin coatings are more durable. When designing or purchasing coating equipment, it is also important to consider capacity, available floor space, and cost. When using coating solutions that contain harsh solvents, it’s key to think about storage space, adequate ventilation, explosion prevention, and hazardous waste disposal.

Spray coating is usually reserved for devices and surfaces where a consistent coating thickness is especially important. Stents and woven baskets are commonly spray-coated to minimize pooling and drip spots that could occur with a dip process. Spray coating generally requires more expensive equipment and uses more coating solution than dip coating. Because spraying generates particulate matter in the air—and depending on the solvent system used—ventilation and explosion prevention are critical factors to consider.

A simple but less effective method to coat devices is to manually apply the reagent solution with a saturated brush, clean-wipe, or sponge. Although brushing can be faster than dipping or spraying, it is not a recommended coating process because it cannot be precisely controlled and often produces uneven coatings and inconsistent results. Table 1 offers a comparison of application methods.

Step 2: Curing the Coating Solution

Figure 2: Semi-automated test machine used to check lubricity and coating durability

Most solvent-based coatings require heat to adequately bond to the surface of a device, while other coating technologies do not require heat but rather exposure to UV light. Time and temperature requirements should be taken into consideration when selecting a coating. Heat curing is typically a batch process accomplished by hanging several parts on a rack and then placing the rack in an oven. Belt or turn-style oven systems can be employed to “un-batch” the heat curing process for better throughput. Cure times can have a significant impact on the manufacturing floor in terms of throughput and cost. For obvious reasons, long cure times are disruptive to continuous-flow processing and can potentially produce substantial amounts of scrap if the parts are batched in large quantities. Again, floor space, ventilation, explosion prevention, and waste disposal must be considered when handling coatings containing harsh solvents.

Step 3: Monitoring Performance/Quality Control
Quality assurance is an important part of any manufacturing process. Performance, durability, surface finish, and any other factors that could affect the safety and efficacy of the device should be monitored. Ideally, quality assurance feedback occurs soon after the coating is applied so that any problems can be quickly resolved. Processes requiring long cure times run the risk of producing large amounts of scrap if problems are not discovered until several hours after the parts have been made.

Figure 3: Graph of lubricity versus durability

Lubricity (frictional force) can be measured by pinching a post-cured device between two pinch-pads that are compressed at a constant force and then measuring the force required to pull the device through the pinch-pads. Durability is measured by repeating the lubricity test several times and measuring any change in lubricity. An example of a semi-automated test machine used to check lubricity and coating durability is shown in Figure 2. A graph of lubricity versus durability can be made from the generated data to provide visual feedback of performance as illustrated in Figure 3.

Coating Options

Depending on performance requirements, it may be necessary to coat a device more than one time. Generally, multiple coatings will enhance performance; however, the law of diminishing return comes into play—two coats may not perform twice as well as one coat, and three coats may not perform half again as well as two. The importance of performance improvement versus additional cost should be evaluated when determining the number of coats necessary. A single coat performs more than adequately for most applications. A few coating manufacturers have the capability to combine coatings for enhanced performance with some offering coatings for lubricity, hemocompatibility, infection resistance, tissue integration, and drug delivery.

Robert Hergenrother, Ph.D., is the director of R&D technology development and Sara Macklin is the manager of hydrophilic technologies at SurModics Inc., 9924 W. 74th St., Eden Prairie, MN 55344, a leading provider of surface modification and drug-delivery solutions for the healthcare industry. They can be reached at 952-829-2700.

ONLINE

For additional information on the technologies discussed in this article, see Medical Design Technology online at www.mdtmag.com or SurModics Inc. at www.surmodics.com.