Medical Device Surface Preparation For a Hydrophilic Coating

Hydrophilic coatings are “slippery-when-wet” surface modifications. These coatings have many functions, such as increasing lubricity and reducing friction. In the medical field, they play a crucial role in creating high-performance medical devices. The coatings help reduce complications and improve patient outcomes. But for these coatings to be effective proper coating application is absolutely critical. And proper application requires proper surface preparation. 

This guide covers the importance of surface preparation when applying hydrophilic coatings. We cover how to prepare a medical device surface before applying a hydrophilic coating. If you are in any way involved in coating medical devices this guide should be helpful. 

Hydrophilic Coatings Surface Compatibility

As per a recent market study, the medical device industry has significantly boosted the demand for high-performance hydrophilic medical device coatings. They are used to coat a wide range of medical devices such as catheters, guidewires, balloons, stents, implants and more. 

The formulations can also be applied to a wide range of substrates. These substrates range from metals to plastics to ceramics and beyond. For example, Hydromer®’s custom hydrophilic coating formulations can be applied to a long list of substrates. Here is a quick list of substrates our coatings work with. Feel free to contact us if you don’t see your substrate listed below.

Implications of Inadequate Surface Preparation

Medical devices are exposed to contaminants during every stage. They may become exposed during manufacturing, delivery, handling, and more. The surfaces can become contaminated from manufacturing residues, skin, airborne particles, etc. 

As a result, it is important to properly clean the surfaces before coating them. This is because their effectiveness heavily relies on the quality of the surface.

Here’s how contaminated surfaces can affect the efficacy of the hydrophilic coatings:

• Grease: Poor cleaning or failure to remove greases or residual oils after machining and processing may result in voids, or areas with reduced adhesion during coating. Such voids between the substrate’s surface and hydrophilic coatings may lead to undesirable peeling or flaking. 
• Foreign Material: Similar to grease, the presence of foreign material and particulates leads to voids and irregularities. The device surface gets exposed to foreign materials, such as airborne particles, dust, and chemicals, mainly during handling and transportation.
• Body Fluids: Biological contaminants such as human skin oils and sweat can contaminate the surface of devices during handling. 
• Release Agents: Medical device parts manufactured by injection molding use release agents such as silicon oil to make the surface slippery. Such slippery surfaces significantly reduce coating adhesion.

Contamination may even have an extended impact on the entire batch if you plan to coat multiple devices together in a batch. For instance, if using dip coating the contaminants may leach into the coating bath. This of course will adversely affect the performance of further devices.

There is no shortage of ways that a medical device may become contaminated prior to being coated. It is also important to note that this contamination may happen multiple times. For instance, the raw substrate of the device may become contaminated. This affects the proper adherence of a primer. Or, the primer layer may become contaminated, which affects the proper adherence of the hydrophilic top coat. 

The important thing is to understand that this happens and how to properly prepare them prior to coating. 

How to clean material surface prior to applying hydrophilic coatings

Proper surface cleaning plays a crucial role in obtaining a durable hydrophilic coating finish. It focuses on removing unwanted contaminants such as dust, grease, oils, etc. It is a critical step in the manufacturing process. And proper prep is achieved both chemically and/or mechanically.

Let us discuss various methods and cleaning agents essential for meticulous surface preparation.

1. Assessment

This step includes a quick evaluation of the surface condition. The contaminants that are present will dictate the further processes and type of solvents or cleaning agents required to clean the surface prior to coating. Keep in mind that a thorough examination can save you both time and money in the long run.

2. Removal of Contaminants

Advanced ultrasonic cleaners may be used to remove contaminants from the material surface. The substrate is dipped into a specialized solvent, and contaminants are removed using high-frequency sound waves. For substrates having complex geometries, a few ultrasonic cleaners employ turbulent flow cleaning technology. 

At Hydromer, Inc., we use specialized cleaning agents and solvents to remove different contaminants fully. Below are some of the steps we take for customer utilizing our contract coating services.

• A combination of isopropyl alcohol (IPA), ethanol, and enzymatic cleaners is used to get rid of polar substances, dust, particulate matter, body fluids, etc.
• Hydrocarbons like heptane, hexane, or acetone are aggressive, so they are used to remove nonpolar substances like oils and greases.

We also use specialized cleaning agents and lint-free polyester wipes to remove contaminants. These are often used in the final stage to get rid of the particles and fibers. Wipes are used in a single direction to avoid relocation of the contaminants.

Specialized Surface Preparation Techniques

Extra steps are necessary when it comes to preparing harder to coat substrates. Some examples of these include Polypropylene and PTFE. In these cases it is necessary to activate the surface with preparation techniques like Plasma pretreatment or corona treatment. These both activate the surface as well as further clean the surface as well.

Some of these speciality techniques that we use at our facilities are discussed in more detail below:

• Corona surface treatment is a process used to improve the adhesion of inks, coatings, and adhesives on non-porous materials like plastics, films, and metals. It works by exposing the material to a high-voltage electrical discharge (corona discharge). This increases surface energy and enhances wettability. This treatment is commonly used in industries such as packaging, printing, and automotive manufacturing.
• Plasma pretreatment is a surface modification process that uses ionized gas (plasma) to clean, activate, or coat materials. This improves adhesion for inks, coatings, and adhesives. It alters the surface at a molecular level by removing contaminants, increasing surface energy, and introducing functional chemical groups. This technique is widely used in industries like automotive, electronics, and medical device manufacturing.

3. Drying

This is the final step, where trapped moisture and humidity are removed. The optimal coating adhesion and long life are possible only when the coating is applied on a clean and dry surface.

How to evaluate the performance of Hydrophilic Coatings

After a substrate has been prepared and coated it must be evaluated. The evaluation step is important to ensure the properties of the coating, but also that the coating was applied correctly. 

The quality of the coating is affected by various things such as storage conditions, cleaning process, curing methods, application methods, etc. Here is a quick list of common tests to determine the performance of hydrophilic coatings:

• Lubricity Test: A pinch test is used to measure the coefficient of friction or lubricity of the hydrophilic coatings. It is one of the most beneficial properties that prevent blood clotting, leading to atrial fibrillation, strokes, or even heart attacks. 
• Contact Angle Measurement: The contact angle of a hydrophilic coating should be as small as possible. This shows greater wettability or level of hydrophilicity. Properly coated medical devices exhibit minimal or even zero contact angles.
• Durability Test: The durability of the hydrophilic coatings becomes critical when the medical device is inserted or implanted in the human body. Loose bonding may lead to excessive discomfort and pain for the patient. It is determined using multiple pinch tests. This test is helpful to understand the adhesion of the coating to the substrate, along with how long the coating lasts. 

To learn more about various evaluation tests for hydrophilic coatings, please read our article: Testing the Performance Of Hydrophilic Coatings.

Conclusion

Thoroughly cleaning and preparation of a medical device substrate is the key to success. By eliminating contaminants that can interfere with coating adhesion and functionality, you ensure that the medical device delivers the desired performance. 

At Hydromer, we bring decades of experience in order to properly prepare substrates prior to coating them. This expertise and know-know is employed for all of our contract coating customers. Our goal is to allow our clients to focus on their core strengths and help them bring advanced medical device products to market quickly.

If you have any questions, we’re here to help! Please feel free to contact us.

Phone: +1 704-490-4031
Email: in**@******er.com