Medical devices: Critical factors to consider when choosing a carrier fluid

In the medical device world, coatings are commonly used on many products for purposes ranging from cosmetic to performance enhancing. In cases when a coating media is available in concentrated form only, a carrier fluid is required to dilute and apply the coating. Suturing needles, for example, are coated with silicone-fluid lubricants to reduce drag forces and allow the needle to move easily through the skin. The concentrated silicone lubricant is diluted with carrier fluid and the resulting blend is applied to the needle. As the carrier fluid evaporates, it leaves behind the desired uniform thin-film of silicone on the needle surface.

Here’s how it works: a carrier fluid is blended with and delivers a coating to a substrate through dipping, spraying, or wiping. The specified coating is the only thing that remains after the carrier fluid evaporates completely.

For anyone currently using HCFC-225 as a carrier fluid, commonly used for silicone deposition, time is of the essence in selecting a replacement.

HCFC-225 will no longer be available, as it is being phased out and banned by the U.S. Environmental Protection Agency (EPA). If a company is currently using an HCFC-225 carrier fluid, it will need to find a regulatory-compliant replacement product by Dec. 31, 2014.

Choosing from the wide variety of carrier fluids may sound simple, but there are many important factors to consider. For medical device manufacturers, careful selection of the right carrier fluid for a specific need will help to ensure ease of validation, time and cost savings, and maximum performance. Key questions and points to consider include:
 

To dissolve or not to dissolve?

The first step is to determine what purpose the carrier fluid serves in the process. Is a carrier fluid needed to dissolve a material and deposit that material as a coating, or just deposit a coating? Widely used medical device lubricant coatings such as silicone are typically sold only in concentrated form, requiring dissolution prior to application. For example, MicroCare Universal Carrier Fluid is used to dissolve silicone, in addition to many other materials, because it evaporates quickly leaving behind a smooth, even coating.

Carrier fluids can also be used to deposit solids to coat a surface. In this case, the media being applied to a surface is not dissolved in the carrier fluid, but it is held in suspension. 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 where the presence of heat may damage sensitive substrates or the media being applied. For example, carrier fluids are commonly used to apply dry lubricants such as PTFE, graphite, or even specialized medicine coatings to small parts. This process typically uses a carrier fluid to quickly apply very thin-film coatings, or to quickly apply coatings to high volumes of parts with complex surface geometries.
 

How strong?

The strength of a carrier solvent is important when dissolving materials like silicone or viscous ingredients used in specialized adhesive coatings. Silicone frequently serves as a lubricant, or as an ingredient in pressure-sensitive adhesives. The silicone frequently needs to be dissolved in a carrier to ensure uniform dispersion and adhesion. While isopropyl alcohol is a commonly used solvent, it won’t dissolve silicone. Carefully selecting a solvent that will properly dissolve the coating’s active ingredients without affecting the physical properties of those ingredients is critical to the coating’s overall performance.
 

Environmental suitability for long-term use?

A range of carrier fluids spans the environmental spectrum, with many subject to regulatory restrictions. Consult with your carrier fluid vendor to ensure you are selecting a carrier that has environmental properties suitable for long-term application in your facility.
 

Flammable or nonflammable?

Thanks to their low risk of fire, nonflammable carrier fluid options have always been the preferred option, especially for high-volume production facilities. While some nonflammable carrier options such as HCFC-225 present low-risk, they are being regulated out of production due to environmental concerns. The changing regulatory landscape has made it difficult to select nonflammable carrier fluids.

For silicone deposition, nonflammable options have nearly disappeared from the market and many medical device manufacturers are considering flammable options to replace them. Common flammable carrier options – including aliphatic and aromatic hydrocarbons – can be difficult to use, especially in high-volume production facilities, due to the flammability risk. In an environment that may be prone to unexpected static discharge, it takes just one spark to start a fire.

The engineering controls required to mitigate risks associated with handling flammable liquids are costly, and even with proper controls, the use of flammable liquids as a carrier fluid is often difficult for corporate health and safety officers to support.

Changing health and safety concerns are under the microscope like never before, causing many companies to abandon flammable options due to their high risk factors. New, nonflammable options such as the MicroCare Universal Carrier Fluid provide a safe and reliable replacement for commonly used flammable solvents such as hexane or toluene.
 

Are bioburden risks reduced?

While many factors can cause bioburden during the manufacture of medical devices, water is a necessary growth medium for bacteria. Solvents are often more suitable than water-based carrier fluids because they present an environment that does not support bacteria growth, and as a result greatly simplify process control requirements.

If bioburden is not considered during carrier fluid selection, it can result in problems within the validation process. Solvent-based carrier fluids can minimize bioburden control problems, as well as eliminate the need for re-cleaning, re-packaging, or worst case, scrapping devices that have unresolvable bioburden issues. Carefully choosing a solvent that does not sustain bacteria growth will ultimately simplify the validation process and reduce time and expense in the production process.
 

How does it handle complex surfaces?

Complex geometries or assemblies with blind holes or multiple surfaced angles are common to many medical device designs. These kinds of assemblies require carrier fluids with low surface tension properties to ensure the coating uniformly covers all angles and surfaces of the treated area.
 

Fast or slow dry time?

The dry rate of a coating is often determined by the dry rate of a carrier fluid, and the boiling point indicates dry time. A lower boiling point will provide a faster dry rate; a higher boiling point typically indicates a slower dry rate.

For paint or ink coatings applied for cosmetic or marking purposes, a design engineer may want to select a carrier fluid with a higher boiling point to ensure the coating dries slowly, therefore eliminating the appearance of brush strokes. For lubricant coating applications, select a carrier fluid with a lower boiling point to ensure rapid drying.
 

Is there a consistent hang time?

A steady hang time is important because it will ensure a coating is applied with consistency during production. A consistent hang time means coating particles remain uniformly suspended and dispersed in the carrier fluid for a long time. In certain carrier fluids, particles will fall out of suspension too quickly. If this is not addressed, it can affect coating consistency from part to part, or in production batches from one day to the next. Ensuring uniform suspension of particles in the carrier fluid allows easier control of the process, which is important in process validation and ongoing production.
 

Is it compatible?

Test a potential carrier fluid to ensure it is compatible with the substrate of the piece that needs coating. An immersion test will confirm the carrier fluid does not damage the treated surface by causing stress fracturing, softening, corrosion, or other damage. Stay clear of aggressive solvents that damage or otherwise change the substrate being treated.

In certain applications, an aggressive carrier fluid may be advantageous. For example, if the substrate surface is too smooth for a coating to adhere, a carrier fluid may be specified that will etch the surface lightly to improve coating adhesion. In these instances, it is especially important to test and select a carrier fluid and coating to ensure they maintain a consistent, controlled process.
 

Are purity standards repeatable?

It’s important to select a carrier fluid of high purity to standards that are well defined and reproducible. This means the carrier fluid will always evaporate completely leaving no residues that can impact the purity of the coating. Select a carrier supplier with a well-organized quality system and lab testing that can provide carrier fluid purity certifications.
 

Can it maintain consistency?

To maintain product consistency when mixing and blending the carrier fluid with active ingredients, measure all ingredients in weight percent versus volume percent. Temperature swings can significantly change the volume of most liquid ingredients through thermal expansion or contraction. Mixing by weight during blending reduces the chance that carrier fluid content will be impacted by volume variances caused by temperature changes. This provides critical measurement consistency when validating a new production process.
 

Conclusion

Evaluating critical factors and asking the right questions in the carrier fluid selection process is important because choosing the right fluid can impact ease of validation, ensure worker safety, and minimize production costs. Furthermore, selecting a carrier fluid that can be applied in-house instead of outsourcing it to a third party can reduce costs and provide improved quality and process control over parts.

During the selection process, also consider the source and long-term support available. Look for a supplier that lends expertise and advice to simplify the process, including consulting, lab testing, and product options such as off-the-shelf or customized formulations to ensure selection of the right carrier fluid.

MicroCare Medical
www.microcaremedical.com

About the author: Jay Tourigny is senior vice president at MicroCare Medical. and can be reached at jaytourigny@microcare.com or 860.515.3011.