Importance of Adhesion Testing in Hydrophilic Coatings

Hydrophilic medical device coatings are essential for enhancing the performance of modern medical devices. This is especially true for those devices designed for insertion, navigation, or implantation into the body. Hydrophilic coatings reduce friction and improve biocompatibility. But none of their benefits matter if the coating do not adhere and stay in place. As a result, adhesion testing is critical.

Adhesion reliability is one of the most important factors in medical device coating performance. This is true for all types of medical coatings, including hydrophilic ones. Issues can arise if and when a coating delaminates. Poor adhesion can compromise both the safety and functionality of the device. Failure of medical device coatings can also lead to adverse clinical outcomes.¹ 

In turn, it is critical that a coating’s adhesion meets performance requirements. But how can you test a coating’s adhesion? That is what we cover in this article. 

In this guide, we explore the adhesion testing methods used to evaluate hydrophilic coatings. We will also focus on UV-cured systems and their advantages in bond strength and long-term stability of hydrophilic coatings.

Why Adhesion Matters in Medical Device Coatings

A medical coating needs be highly durable and maintain consistent integrity on the device’s surface. A high-performance hydrophilic coating must maintain its integrity in many environments, including:

• During the sterilization process
• Through handling and packaging
• Under real-clinical use (insertion, retraction, flexing)
• In contact with body fluids (saline, blood, mucus)

Potential Issues Caused by Poor Coating Adhesion

If the coatings adhesion is not adequate, it can leave to a number of problems. Specifically, poor adhesion can lead to issues, such as:

• Surface degradation during use
• Particulate shedding 
• Reduced lubricity or fluid transport
• Increased device rejection rates during quality control

From R&D to Regulatory Approvals: Why Adhesion Testing & Data Matters

Regulatory bodies (like the FDA) expect to see evidence that coatings are safe and durable under use conditions.

Adhesion testing helps:

• Confirm manufacturing consistency
• Support product claims for durability and safety
• Reduce failures during medical device usage 

Factors that Contribute to Adhesion of Hydrophilic Coatings

Achieving strong, durable adhesion between a hydrophilic coating and a medical device substrate is very important. A coating’s adhesion is influenced by number of factors, some of which are detailed below: 

1. Surface Cleanliness and Preparation: Proper surface cleaning and preparation before the coating process is very important. Contaminants like dust, oils, or residual mold release agents can prevent coatings from bonding properly. Learn more about Medical Device Surface Preparation For a Hydrophilic Coating. 
2. Surface Energy Matching: Adhesion is stronger when the coating’s surface energy is compatible with the substrate. High surface energy substrates (like metals) typically bond well. And low-energy polymers (e.g., PTFE) often require pretreatment.
3. Surface Roughness and Texture: Slightly roughened surfaces, and corresponding surface area provide more mechanical anchoring for coatings. Techniques like etching or abrasion can improve adhesion by increasing contact area.
4. Use of Primers: Medical coating primers are used to chemically “bridge” coatings to low-energy substrates (or difficult substrates). These specialized intermediates promote chemical compatibility and boost long-term adhesion performance.
5. Surface Activation Treatments: Surface treatments such as plasma, corona discharge, or UV/ozone exposure modify the substrate’s surface chemistry to increase wettability and reactivity. These surface treatments improve bonding sites and strengthen the coating-substrate interface.
6. Controlled Application and Curing Conditions: Precise control of temperature, humidity, and cure time ensures the coating polymerizes effectively and bonds well to the substrate.

It should be noted that the above are all relevant factors that promote adhesion. If any of them is not present then it will contribute to poor adhesion.

Role of Hydrogen Bonding in Hydrophilic Coatings Adhesion

The science behind adhesion is not only mechanical. It is also molecular. Hydrophilic surfaces can form hydrogen bonds with water and other polar molecules. These hydrogen bonds will significantly improve adhesion to the device surfaces. These hydrogen bonds are strong enough to maintain the coating stability in moist biological environments ^2,3

Why Adhesion Testing Is Critical for Hydrophilic Medical Device Coatings

Product performance testing is critical to ensure that a product will perform as required. Coating adhesion testing is a major component of this. Adhesion testing shows that the coating can perform as intended and withstand real world environments and stresses. 

Here are some reasons why adhesion testing of coatings is important when it comes to medical devices: 

1. Reliable Bonding Across Diverse Substrates: Medical devices are made from diverse materials like metals and polymers. Each with unique surface chemistries. So hydrophilic coatings must be carefully engineered often with primers or surface treatments (e.g., plasma, corona) to ensure strong and reliable adhesion.
2. Withstanding Sterilization and Storage: Hydrophilic coatings must maintain strong adhesion during sterilization process. Sterilization process can degrade weakly bonded layers. Temperature and humidity during storage can also cause separation or flaking if adhesion is inadequate.
3. Resistance to Mechanical Stress: Medical devices frequently experience flexing, coiling, and repeated insertion. So hydrophilic coatings must endure these mechanical stresses. Hydrophilic coating should not crack, delaminate, or shed particles. This could lead to complications like embolism or inflammation.
4. Quality Control and Regulatory Readiness: Strong adhesion helps reduce coating-related device rejections during manufacturing QC inspections. Adhesion data supports regulatory submissions, demonstrating product safety and consistency in accordance with ISO 10993 and FDA guidance documents.

Adhesion Testing: Methods and Metrics

Several standardized methods are used to assess coating adhesion. Each provides insights into how strongly a coating bonds to its substrate and how it behaves under stress. 

Some of the common adhesion tests used for coatings are discussed below. 

1. Crosshatch Adhesion Testing (ASTM D3359)⁴

• Method: A grid of cuts is made through the coating using a blade tool. This is followed by the application and removal of adhesive tape on the coating. Shows a coatings ability to adhere to the substrate and resist separation. 
• Scoring: Based on how much coating is removed from the cut edges (rated 0B to 5B).
• Ideal Result: 5B (no coating removal) indicates strong adhesion.

Most suitable for: Thin coatings on rigid substrates. It is a visual based test useful for fast QC screening.

2. Pull-Off Adhesion Testing (ASTM D4541)⁵

• Method: A stud is glued to the coating and pulled off using a mechanical tester. The test is used to determine the force required to separate it from the substrate.
• Measurement: Adhesion strength in MPa or psi.
• Ideal Result: Higher numbers = stronger bond. Typically, >2 MPa is desired for medical coatings, but it depends on the substrate and specific application.

Most suitable for: Quantitative bond strength analysis, regulatory submission support.

3. Peel Testing (ASTM D903)⁶

• Method: One end of the coating or a laminated film is pulled at a fixed angle (usually 180° or 90°) to determine the force required to peel it from the substrate.
• Measurement: Peel force in N/m or lb/in.
• Ideal Result: A consistent, controlled peel force across test samples.

Most suitable for: Flexible films, tubing, multilayer coatings.

4. Flex Testing (Mandrel Bend, ASTM D522)⁷

• Method: The coated sample is bent around a mandrel to check for cracking, flaking, or delamination.
• Measurement: Pass/fail based on visible damage.
• Ideal Result: No cracking or loss of adhesion after bending.

Most suitable for: Coatings on flexible substrates like Pebax or polyurethane.

Hydromer®’s Role in Advanced Hydrophilic Coatings for Medical Devices

Hydromer®, Inc. develops and supplies custom hydrophilic coatings for medical devices. Hydromer medical coatings, at the most basic level are designed to be:

• Safe for the body (biocompatible)
• Hydrophilic
• Provide a slippery surface (lubricious)

Our coatings help improve how medical devices work, such as reducing friction and allowing for better navigation with less pain. Hydromer coatings can be formulated to adhere to different materials like nylon, Pebax, polyurethanes, silicone, metals, and more. After curing they create strong, low-friction surfaces that make doctors’ jobs easier and patient outcomes better. 

Hydromer also provides the ability to customize the formulations. You can custom formulate a coating with extra functionality, such as:

1. Anti-microbial properties (preventing germs and reducing infections)
2. Anti-thrombogenic (preventing blood clot) 
3. Drug-eluting capabilities (drug-release) 

Hydromer also offers a wide range of support services to our medical device manufacturing partners. These include expert analytical testing services led by Ph.D.-level scientists. Our analytical testing uses advanced equipment and scientifically backed analytical testing methods.

In addition, we offer contract R&D services, including product development as well as contract coating services, technology transfer services, and more. 

Conclusion: Reliable Adhesion is Important

Hydrophilic coatings are as functionally diverse as the materials they cover. These coatings serve many functions, from improving lubricity and maneuverability in catheters to resisting biofouling on marine surfaces. However, their performance depends greatly on how well they adhere to the substrate under specific conditions. Adhesion testing offers essential insights into the reliability and suitability of the coatings for the device. Using industry-standard testing for peel, pull-off, crosshatch, and flex testing allows manufacturers to ensure that the hydrophilic coatings work as intended.  The adhesion testing also helps to ensure that the coating integrity is upheld in the actual biological environment. Hydromer offers custom hydrophilic coatings as well as analytical testing services to help you ensure product performance. 

Contact us today to learn more about our coatings and our support services. 

References

Click to see the references for this article

1. Parillo M, Wawer Matos Reimer RP, Mallio CA, Reimer P, van der Molen AJ. Hydrophilic Polymer Embolism: An Underappreciated Complication of Endovascular Procedures that Physicians Should be Aware Of. CardioVascular and Interventional Radiology. 2025:1-14. 

2. Hydromer. Medical Device Coatings. Hydromer. https://hydromer.com/medical-device-coatings/

3. Edwards PA, Price M, Nimchuk N, Mahon J. Novel Water Loving Coatings (WLC) Lubricious and Durable Guidewires. 2017; Accessed 2/1/2025. https://doi.org/10.1115/DMD2017-3434 2017 Design of Medical Devices Conference

4. Standards A. Standard Test Methods for Rating Adhesion by Tape Test ASTM D3359-23. ASTM Standards. https://store.astm.org/d3359-23.html

5. standards A. Standard Test Method for Pull-Off Strength of Coatings Using Portable Adhesion Testers ASTM D4541-22. ASTM standars. https://store.astm.org/d4541-22.html

6. standards A. Standard Test Method for Peel or Stripping Strength of Adhesive Bonds ASTM D903-98(2017). ASTM standards. https://store.astm.org/d0903-98r17.html

7. Standards A. Standard Test Methods for Mandrel Bend Test of Attached Organic Coatings ASTM D522-93a(2008). ASTM Standards. https://store.astm.org/d0522-93ar08.html

8. Hydromer. UV Curing for Medical Devices: Role of PEG, PVP, Chitosan. Hydromer. https://hydromer.com/uv-curing-for-medical-devices-role-of-peg-pvp-chitosan/#:~:text=Materials%20like%20PEG%2C%20PVP%2C%20and,%2C%20and%20drug%2Dreleasing%20stents.

9. Hwang MY, Yoon SH, Kim M. Ultraviolet Irradiation Surface Treatment to Enhance the Bonding Strength of Polyamide-Based Carbon Fiber-Reinforced Thermoplastic Polymers. Polymers. 2024;16(20). doi:10.3390/polym16202864