Surface science is very important when it comes to developing medical devices. This is because a device’s surface greatly affects how it interacts with and works inside the human body. One key surface characteristic of a medical device is hydrophilicity. This determines how a surface attracts and interacts with water. This property is especially important for medical devices that come into contact with blood and fluids, as well as those used in minimally invasive procedures. That is why medical devices are commonly coated with hydrophilic medical device coatings.
But how can a device manufacturer ensure that the hydrophilic coating will meet performance and regulatory requirements? Specialized analytical and quality tests are used for measuring hydrophilicity. Techniques like surface energy measurements and contact angle analysis help biomedical engineers better understand coatings. For instance, these methods help determine how well a coating attracts water, which is key to ensuring that the product works effectively.
In this article, we will discuss why hydrophilicity matters for medical devices. Then we will dive into how to measure hydrophilicity in medical device coatings. This includes an overview of the common tests that are used for measuring hydrophilicity of a coating.
Why Hydrophilicity Matters in Medical Devices
Medical devices need to be able to move smoothly throughout the body, over soft tissues and through tortuous pathways. Not all materials naturally allow for this. As a result, hydrophilic coatings are commonly applied to medical device surfaces. These coatings allow the device’s surface to attract water, which makes the surface slippery and reduces friction. This reduces the force needed for insertion of the device and reduces trauma. These coatings make the doctor’s job easier. They also make the experience more comfortable for the patient.
Beyond lubricity, hydrophilic coatings offer other benefits for medical devices. For instance, they can:
- Reduce protein and bacterial adhesion
- Improve anti-thrombogenic (anti-clot) properties in vascular devices.
- Ensure even fluid distribution in diagnostic tools and wound dressings
- Provide antimicrobial properties
- And more
However, achieving these benefits consistently requires adequate hydrophilicity. Ensuring this requires precise measurement and quality control.
So how can device manufacturers’ ensure that the coatings work as intended on their devices? There are key techniques used to accomplish this, which we discuss in more detail below.
Measuring Hydrophilicity: Key Analytical Techniques
1. Contact Angle Measurement
Contact angle measurement is an important method for analyzing hydrophilic coatings. It helps individuals understand how well these coatings can be wet by water (their wettability). This is important when it comes to measuring hydrophilicity.
Imagine a water droplet on a coated surface. The way it interacts with that surface shows how hydrophilic (water-attracting) it is. Contact angle (CA) is used to measure this property.
A lower contact angle means that the surface is more wettable, and the water will spread on the surface better, maintaining contact. Wettability is important for many applications. These include biomedical devices as well as anti-fogging surfaces, and self-cleaning materials. 1-3
Contact Angle Testing For Measuring Hydrophilicity
Contact angle testing is a common way to measure how much a surface likes water (the surface’s hydrophilicity). In this test, a drop of water is put on a coated surface. The angle created between the edge of the water droplet and the surface is then measured with a tool called a goniometer.
Relationship Between Contact Angle and Hydrophilicity
Below you can see the relationship between the measured contact angle and the surface’s hydrophilicity.
- Lower contact angles = higher hydrophilicity.
- General benchmarks:
- > 90° → Hydrophobic surface
- 30–90° → Moderately hydrophilic
- < 30° → Highly hydrophilic
In medical coatings, a contact angle below 30° is often preferred to ensure optimal wetting and lubricity 1,3
Why Contact Angle Matters in Medical Devices:
- Ensures consistency in product performance
- Ensures wettability and improved biocompatibility with biological fluids
- A lower contact angle enhances lubricity and reduces friction between the device and the body
- Contact angle directly correlates to insertion force reduction in medical applications
- This is essential data for regulatory submissions (FDA, ISO standards)
2. Surface Energy Analysis
Surface energy analysis is another important component of hydrophilic coatings. This property greatly affects how well a coating can attract water and stick to surfaces, as well as how it will perform overall.
The interaction between hydrophilic surfaces and water is largely governed by surface energy. So to enhance the water-attracting properties of these coatings, researchers link surface energy to the degree of hydrophilicity.
By understanding how surface structure and energy work together, scientists can create coatings that make the most of these properties. This leads to better water-attracting characteristics.4 As a result, the surface energy needs to be measured.
How Surface Energy is Measured
Surface energy is determined by measuring the contact angles of different test liquids (usually a polar and a non-polar liquid) on the coating. These values are used to calculate surface energy using models like the Owens-Wendt method. It involves measuring the contact angles of at least two liquids on the solid surface. Then these measurements are used to calculate the polar and dispersive components of the solid’s surface free energy.
Relation Between Surface Energy and Hydrophilicity
- Higher surface energy → hydrophilic and better wettability
- Direct correlation with protein resistance and fluid spread behavior.
Why Surface Energy Matters in Medical Devices:
Surface energy is important when designing devices and understanding how they will work with the body. Below are some of the reasons why a device’s surface energy matters.
- Ensures wettability and improved biocompatibility with biological fluids
- Ensures predictable device behavior in clinical environments
- Important for medical applications, such as wound dressings, drug delivery surfaces, and biosensors
3. Dynamic Contact Angles: Advancing vs. Receding
Medical devices, like catheters and wearable patches, are often in motion. Their surfaces go through dynamic conditions.
We can study how a coating behaves while in motion. This is done by measuring the angles at which a liquid starts to stick and then pulls away. These measurements tell us how the coating reacts when a liquid moves across it. These include:
- The advancing contact angle (θA) – the biggest angle when a droplet starts to spread out on a surface.
- The receding contact angle (θR) – the smallest angle when the droplet is pulled away.
Dynamic contact angles are the angles formed between a liquid and a surface. They can change a lot based on how a surface is modified.
When surfaces are designed at the nanoscale, they can help achieve higher advancing angles while keeping the receding angles lower. This means we can better control how liquids behave on device surfaces.
One study found that special coatings can be made to enhance super-hydrophilicity, allowing contact angles to drop as low as 5°.1
4. Wettability Assessment For Measuring Hydrophilicity
Performing a wettability assessment of hydrophilic coatings is essential. It helps with understanding and optimizing the behavior of the coatings when in contact with liquids. It provides information on how well the coating performs in real-world applications where fluid interaction is critical. Apart from surface energy measurement and contact angle Wilhelmy Plate Method is also considered for the wettability assessment.5
Understanding Wettability Using the Wilhelmy Plate Method
Wilhelmy Plate Method involves dipping a coated plate into a liquid and measuring the force as it contacts and leaves the liquid surface. The key parameters that are determined from this method are:
- Contact angle
- Surface tension
- Wetting force
The advantages of using the Wilhelmy Plate Method for measuring hydrophilicity include the following:
- High precision and repeatability
- Dynamic measurement: detects real-time wetting behavior
- Suitable for small coating samples and non-flat surfaces
Challenges and Considerations When Measuring Hydrophilicity
Contact angle and surface energy measurements are powerful tools when it comes to measuring hydrophilicity. However, these measurements must be performed under controlled environmental conditions.
Below are some critical items to keep in mind before and during taking these measurements.
- Humidity and temperature can affect results
- Surface contamination can skew data, making proper cleaning essential
- For devices undergoing sterilization, pre- and post-sterilization testing is recommended to confirm long-term performance
Regulatory Importance of Analytical Testing
There are some core reasons why properly measuring hydrophilicity is important from both a performance and regulatory standpoint. From a regulatory standpoint, here are some of the biggest reasons:
- ISO 10993-18 highlights the importance of chemical and surface characterization in medical device testing.
- Demonstrating hydrophilic properties through validated analytical methods supports smoother regulatory approvals for FDA and CE-marked products.
Hydromer® Medical Coatings Analytical Testing Services
As a medical device manufacturer you know that finding a qualified analytical testing lab is critical to your product’s success. A qualified lab can help your company ensure patient safety, navigate stringent regulatory requirements, and protect your brand reputation.
Hydromer®, Inc. offers a full line of support services to medical device manufacturers. This includes a range of specialized, analytical testing services. All our tests are performed by a skilled team of scientists with Ph.D. degrees in analytical, bio-, and polymer chemistry. Our advanced testing services can be used to help you ensure the effectiveness and safety of your materials. In addition, our testing services can be utilized to support custom research and development for your products.
Hydromer’s laboratory is equipped capable of performing a wide range of analytical testing techniques, including (but not limited to):
- FTIR (Fourier Transform Infrared Spectroscopy)
- GC/MS (Gas Chromatography-Mass Spectrometry)
- HPLC (High-Performance Liquid Chromatography)
- UV/Visible Spectroscopy
- Rheology
- Goniometry (to measure contact angles)
- Wettability assessments
At Hydromer, our clients receive personalized product development supported by strong scientific knowledge and reliable testing methods. We can help you meet your product and regulatory requirements, no matter where you are in the product development process.
Conclusion: Analytical Testing Ensures Confidence and Compliance
Analytical techniques, such as contact angle analysis and surface energy testing provide important information. This information helps ensure that devices work as intended. They are important to understand the quality, uniformity, and effectiveness of hydrophilic coatings. Contact angle and surface energy measurement can help in the selection of materials for specific applications. And they can enhance the performance of coatings intended for use in medical contexts.
Hydromer, Inc. is dedicated to helping our clients ensure the safety and performance of their medical devices. We offer a wide range of expert services to clients to help them meet their product requirements. This includes personalized R&D services supported by strong scientific knowledge and reliable, analytical testing capabilities.
Contact our medical coating experts today to learn more about our custom medical coatings and contract services.
References
Click here to see the references for this article.
3. Huang K-T, Yeh S-B, Huang CJ. Surface Modification for Superhydrophilicity and Underwater Superoleophobicity: Applications in Antifog, Underwater Self-Cleaning, and Oil–Water Separation. Acs Applied Materials & Interfaces. 2015;7(38):21021-21029. doi:10.1021/acsami.5b07362
4. Fleckenstein BS, Sterr J, Langowski HC. The Influence of High Pressure Treatment and Thermal Pasteurization on the Surface of Polymeric Packaging Films. Packaging Technology and Science. 2016;29(6):323-336. doi:10.1002/pts.2213
5. Park J, Pasaogullari U, Bonville L. Wettability measurements of irregular shapes with Wilhelmy plate method. Applied Surface Science. 2018/01/01/ 2018;427:273-280. doi:https://doi.org/10.1016/j.apsusc.2017.08.186
