Choosing a carrier fluid might seem straightforward, but there are many variables to consider.
By Jay Tourigny
Medical device manufacturers need a carrier fluid to dilute concentrated silicone lubricants so a thin film of silicone can be applied to an article, such as a suturing needle, by dipping or spraying.
Coatings are widely used in the medical device industry for a variety of reasons ranging from cosmetic to performance enhancing. In some cases, the coating media is sold only in concentrated form, which requires the user to specify a carrier fluid for coating application. For example, disposable needles used for delivering medications, withdrawing fluids, or suturing are coated with silicone-fluid lubricants. The lubricious coating on the needle surface reduces drag forces, allowing the needle to go through the skin more easily, ultimately making it less painful for the patient. A carrier fluid is needed to dilute the concentrated silicone lubricant so that a thin film of silicone can be applied to the needles by dipping or spraying.
|Learn about future applications and advances in new materials and coatings at the MD&M Minneapolis conference October 29 & 30, 2014.|
In the simplest of terms, a carrier fluid does its job by mixing with and delivering a coating to a substrate by some variation of a dipping, spray, or wipe process. Once applied, the carrier fluid evaporates away, leaving only the desired coating.
There are many different carrier fluids available. On the surface, selecting one may seem straightforward, but many variables should be carefully considered to select the right carrier fluid for an application’s specific needs. There’s an art and science to choosing the carrier fluid that will ensure ease of validation, time and cost savings, and maximum performance.
In many cases, manufacturers are already using carrier fluids but may not be aware of it because they’re included in a prepared coating, like ink. When a unique formulation is needed—for example, to apply a protective coating or pharmaceutical coating—it’s critical to consider multiple variables.
This article examines the key factors to consider when selecting a carrier fluid.
The first critical question to ask is whether a solvent is needed that will act as a carrier fluid by dissolving an ingredient and depositing a coating or simply act as a carrier fluid to deposit a coating. Many lubricant coatings such as silicone, a popular material for coating medical devices, must be dissolved to be applied. For example, the major supplier of medical-grade silicone fluid polydimethylsiloxane lubricant provides it in various viscosities, but only in concentrated form. Medical device manufacturers need a carrier fluid to dilute the concentrated silicone lubricant so a thin film of silicone can be applied to an article.
The other option for carrier fluids is simply deposition of a coating to medical devices. For this kind of application, the media being applied to a surface is held in suspension rather than dissolved in the carrier fluid. In this process, the carrier does not dissolve or otherwise change the physical properties of the coating media. This can be a reliable alternative to electrostatic or thermal coating processes in which the use of heat may damage sensitive substrates or the media being applied. For example, carrier fluids are commonly used to apply dry lubricants such as polytetrafluoroethylene (PTFE), graphite, or even specialized medicine coatings to small parts. This process typically uses a carrier fluid to quickly apply thin coatings or to quickly treat high volumes of parts with complex surface geometries.
If dissolving media such as silicone or a specialized adhesive coating is important, the solvent strength will become a key parameter. For example, silicone oil is a common coating used as a lubricant or as a component in pressure-sensitive adhesives that need to be dissolved in order to adhere or disperse evenly. Alcohol is a widely used solvent, but alcohol won’t dissolve silicone. The use of a well-selected solvent carrier will properly dissolve the lubricant or adhesive without impacting the final physical properties essential to the coating’s performance.
The viscosities of both the carrier fluid and the coating are also important factors to consider. In the medical device world, companies can buy carrier fluids in a range of viscosities—from thinner than water to thicker than pudding. The key is to select a carrier fluid with a viscosity compatible to the coating to maintain good control during application but that will not change the properties of the final coating when combined.
Many medical devices utilize complex geometries with multiple blind holes or multiple surfaced angles. Devices with intricate surfaces require carrier fluids that impart a low surface tension during use to ensure that all angles and holes of the treated substrate surface will be coated evenly.
The carrier fluid often determines the dry rate of a coating. The speed and complete drying of a carrier without residue are important factors that can impact the quality and speed of the production line. Boiling point is an indicator of dry time, so selecting a carrier fluid with a lower boiling point will provide a faster dry rate; a higher boiling point typically indicates a slower dry rate. For a carrier fluid used to apply paint or ink for cosmetic or branding purposes, a design engineer may select a carrier fluid with a higher boiling point so the coating dries slowly to eliminate the appearance of brush strokes. On the other hand, if a carrier fluid is needed to apply a lubricious coating to needles, select a carrier fluid with a lower boiling point so it dries quickly. Carrier dry rates can be enhanced by adding heat, although this adds an additional step and cost to the process. To help aid the decision-making process, set up and review a chart specifying potential carrier fluid boiling points.
Carrier fluids are available in a range of environmental profiles with many subject to regulatory restrictions. Be sure to consult with your carrier fluid supplier to ensure you are selecting a carrier that has environmental properties suitable for long-term use on your production line. For example, for silicone deposition in particular, AK-225 has been a popular option but will no longer be available, as it is being phased out and banned by the U.S. Environmental Protection Agency. Companies currently using an AK-225 carrier fluid will need to find a regulatory-compliant replacement product by December 31, 2014. (For tips on making a smooth transition and a list of regulatory-compliant replacement options, visit www.ak225replacement.com).
Nonflammable carrier fluid options have always been a preferred method of application, providing low risk of fire—which is especially important for high-volume production facilities. While original nonflammable carrier options such as AK-225 were low-risk, they are being regulated out of production due to environmental issues. The rapidly changing regulatory landscape has made it increasingly difficult to specify nonflammable carrier fluids. For silicone deposition, nonflammable options have all but disappeared from the market, and many medical device manufacturers are considering flammable options to replace them.
Aliphatic and aromatic hydrocarbons are two commonly used flammable carrier fluid options, but they can be difficult to use, especially in high-volume production facilities because of the flammability risk. In an environment already prone to static discharge, it takes just one spark to start a fire. The engineering controls required to mitigate risks associated with handling flammable liquids are expensive, and even with controls the use of flammable liquids is often difficult for health and safety officers to support.
With increased focus on changing health and safety concerns, the tide is turning away from flammable options due to their high risk factors. New, nonflammable provide a safe, reliable drop-in replacement to widely used flammable carriers such as hexane or toluene.
Many conditions can cause bioburden, but fundamentally, water is a primary growth medium for bacteria. Therefore, solvents are often preferable to water-based carrier fluids because they present an environment that is hostile to bacteria growth, greatly simplifying process control requirements.
If bioburden is not properly addressed during carrier fluid selection, it can lead to increased difficulty in the validation process. A solvent-based carrier fluid can significantly reduce bioburden issues as well as eliminate the need for recleaning, repackaging, or even scrapping devices that have been affected by bioburden. A well-vetted solvent choice will ultimately improve the validation process as well as reduce time and expense.
The ideal carrier fluid should be of high purity to standards that are well-defined and repeatable, meaning the carrier fluid will evaporate completely, leaving no residue that can compromise the purity of the coating. OEMs should specify a carrier that comes from a supplier with a well-organized quality system and laboratory testing facilities that can provide proper certifications related to carrier fluid purity.
To ensure carrier fluid consistency when applied during production, measure it in weight percent rather than volume percent when mixing and blending. For example, during shipping or when stored in a warehouse, temperature variables can significantly change the volume of a carrier fluid through thermal expansion or contraction. By using weight during blending, carrier fluid content will be less impacted by volume variances caused by temperature changes and therefore provide essential measurement consistency when validating a new production process.
Avoid carrier fluids that will damage or change the substrate of the piece that needs to be coated. For example, if a device is made of plastic, avoid aggressive carrier fluids that may damage plastics or elastomers. Use care to select a carrier fluid that is compatible with the substrate materials to avoid stress fracturing, softening, corrosion, or other damage to treated substrates.
However, in some cases selecting an aggressive carrier fluid may be desirable. For example, if the substrate surface is too smooth for a coating to adhere, you may be able to select a carrier fluid that will etch the surface gently to enhance coating adhesion. In these situations it is important to carefully test and select a carrier fluid and coating to ensure a consistent, controlled process.
Most paint stores will shake a container of paint at the time of purchase to ensure you get the expected color and coating consistency when the paint is applied. A good hang time—meaning the coating particles remain suspended in the carrier fluid for a long time—is an important consideration because it will ensure the coating is applied with consistency during production.
Particles in some carrier fluids will fall out of suspension quickly, which if not addressed can impact coating consistency from part to part or in production batches from one day to another. Maintaining a uniform dispersion of particles in the carrier fluid allows easier control of the process, which is important in validation.
Careful selection of the right carrier fluid is important on many levels, from ease of validation to controlling safety, quality variables, and production costs. For example, choosing the right carrier fluid can also mean that the coating can be applied in-house instead of sending it out to a third party. This helps to achieve cost savings and provides better quality and process control over parts.
When specifying a carrier fluid, look for a supplier that can provide expert advice and help to simplify the selection process, including individualized consulting, lab testing, and multiple options such as off-the-shelf and customized formulations to ensure selection of the right carrier fluid for specific needs.
|Learn about future applications and advances in new materials and coatings at MD&M Minneapolis October 29 & 30, 2014.|
Jay Tourigny is senior vice president at MicroCare Medical. A 25-year veteran of the industry, he holds numerous U.S. patents for cleaning-related products that are used on a daily basis in medical, fiber optic, and precision cleaning applications.