Suitable Substrates for Hydrophilic Medical Device Coatings

Hydrophilic coatings are thin, surface treatments applied to the surfaces of medical tools like catheters, stents, and implants. These coatings play a pivotal role in modern medical device engineering, making them safer and more effective. For example, they increase the wettability of medical device surfaces and help them interact better with the body.

Some important functions of hydrophilic medical coatings include:

• Reduced friction: These lubricious coatings help reduce the friction on the surface of the device. This allows them to move smoother and easier, causing less irritation to the body.
• Improved safety: these coatings can lower the risk of immune response and thrombosis, as well as help reduce infections.
• Longer-lasting: Hydrophilic coatings make medical devices more durable.
• Reduce Infections: Some hydrophilic coatings exhibit antimicrobial properties, which help reduce infections.
• Help cells grow: Advanced bioactive coatings can promote tissue growth. This property is important for wound healing.  

Hydrophilic coatings can be extremely beneficial for these devices. However, the success of a hydrophilic coating depends as much on the substrate material, the preparation of the substrate, and the application of the coating as it does on the coating chemistry itself. 

This article explores the key properties of suitable substrates for hydrophilic medical device coatings. We will examine specific examples of substrates for our Hydromer® medical coatings, with insights into how each material performs in real-world medical applications. This article is intended to help you, as a biomedical engineer, select the best substrate and coating for your device.

Properties of Suitable Substrates for Hydrophilic Medical Device Coatings

The performance of these coatings is very much linked to the properties of the underlying substrate it coats. Below we discuss some of the critical characteristics that define an ideal substrate for a hydrophilic medical coating. For instance, we explore how properties, such as surface energy, roughness, chemical compatibility, thermal and mechanical stability, and surface cleanliness together influence coating adhesion and overall device functionality as well as performance.

1. Surface Energy and Wettability

In a hydrophilic coating, surface energy refers to the degree to which the coating’s surface attracts and interacts with water molecules. Hydrophilic surfaces have high surface energy. They readily “wet” and adhere to water, causing it to spread and form a thin film. 

This high surface energy is crucial for applications like self-cleaning, antifogging, and enhancing wetting and adhesion. Materials like metals and glass naturally have higher surface energy, while many polymers (e.g., polyethylene) require pretreatment to make their surfaces more hydrophilic.^1,2

If you want more information on this topic, make sure to read our guide on Wettability and Uniform Moisture Distribution in Medical Coatings.

2. Surface Roughness 

Surface roughness is a key factor in determining how well a liquid (like water) will wet a surface. Roughness can enhance the interaction between a hydrophilic surface and water. This makes it easier for water to spread and adhere. A rough surface provides more surface area for water molecules to interact with, leading to better wetting and adhesion. ^2,3

3. Chemical Compatibility

The substrate surface that is coated needs to be able to “connect” with the coating. This can happen either by the coating adhering directly to it or by modifying the surface in a way that helps the two bond together better. For example, functional groups like hydroxyl (–OH) or amine (–NH₂) promote strong bonding.⁴

4. Thermal and Mechanical Stability

Substrates should be able to withstand the heat or curing processes involved in the application of the coating. Additionally, mechanical stability of the substrate ensures that the coating remains intact during flexing, bending, or long-term use.⁵

5. Surface Cleanliness

Contaminants such as dust, oil, or oxidation layers can compromise adhesion of the coating to the substrate. Cleanroom-friendly substrates or those easily cleaned before coating are ideal.⁶

Proper surface preparation is critical for a coating. You can learn more about Medical Device Surface Preparation For a Hydrophilic Coating.

Common Substrates Used in Hydrophilic Medical Coatings

Hydromer, Inc. is a leader in hydrophilic coating solutions. Our company offers coatings that are compatible with a wide range of medical-grade materials.¹¹

Below are some key substrates that our coatings work with, along with their properties, applications, and benefits:

1. Stainless Steel

• Properties: Strong, corrosion-resistant, high surface energy
• Applications: Guidewires, surgical instruments, stents
• Benefit of Coating: Hydrophilic coatings reduce insertion force and minimize tissue damage during procedures.

2. Titanium and Titanium Alloys

• Properties: Biocompatible, corrosion-resistant, lightweight.
• Applications: Orthopedic and dental implants, cardiovascular implants.
• Benefit of Coating: Enhanced hemocompatibility and reduced protein adhesion, lowering the risk of thrombosis.

3. Polyether Block Amide (PEBA / Pebax®)

• Properties: Thermoplastic elastomer, flexible, good fatigue resistance.
• Applications: Catheter shafts, balloon catheters, introducers.
• Benefit of Coating: Provides a smooth, low-friction surface for easier vascular navigation. Learn more about the uses and properties of PEBAX in medical devices.

4. Silicone

• Properties: Highly elastic, inert, excellent biocompatibility.
• Applications: Drains, shunts, soft catheters, implantable devices.
• Benefit of Coating: Reduces surface tackiness and insertion trauma; supports long-term use.

5. Teflon (PTFE)

• Properties: Chemically inert, low surface energy, non-stick.
• Applications: Tubing, vascular grafts, liners.
• Benefit of Coating: Specialized primers allow hydrophilic coatings to adhere effectively, improving surface lubricity.

6. Polycarbonate (PC)

• Properties: Transparent, impact-resistant, thermally stable.
• Applications: Diagnostic housings, connectors, optical components.
• Benefit of Coating: Improves fog resistance and moisture control; useful in vision-critical devices.

7. Polyvinyl Chloride (PVC)

• Properties: Cost-effective, flexible, chemically resistant.
• Applications: IV tubing, fluid bags, cannulae.
• Benefit of Coating: Enhanced patient comfort and reduced friction during insertion.

Considerations for Applying a Hydrophilic Coating to a Substrate

There is more than one requirement for a hydrophilic coating to function optimally. It must be chemically compatible with the substrate. It also must be uniformly and securely applied using an application method suited to the device’s material, geometry, and intended use.

Hydromer, Inc. employs multiple coating techniques to ensure high-performance coverage across diverse medical substrates. These application techniques include, but are not limited to the ones discussed below.

If want to learn more detail about these you can read our guide on Common Coating Methods.  

1. Dip Coating

• Description: Dip coating is a method used to cover a substrate, with a liquid coating solution. In this process, the substrate is dipped into the liquid and then pulled out. As it comes out, a thin layer of the coating sticks to the surface of the object.
• Best For: Tubing, catheters, guidewires, and devices with uniform geometries.
• Benefits: Ensures complete and even coverage, scalable for high-volume production.

2. Spray Coating

• Description: Spray coating is a method used to apply a layer of material onto a surface. In this process, the coating material is broken down into tiny droplets and then sprayed onto a surface, known as a substrate. This technique is often used to create a protective layer or to add a decorative finish to the surface.
• Best For: Irregularly shaped or complex parts such as connectors, housings, and implant components.
• Benefits: Precise control over coating thickness; ideal for selective or localized coverage.

3. Spin Coating

• Description: Spin coating is an easy and commonly used method for applying thin, even layers of materials onto a surface. In this process, a liquid solution is placed on a spinning surface. The spinning creates a force that spreads the liquid out evenly, resulting in a smooth, thin coating. This technique is important in various fields, including medical sciences, because it helps create uniform layers that are essential for many applications.
• Best For: Discs, lenses, and small planar diagnostic components.
• Benefits: Produces ultra-thin, highly uniform coatings with controlled thickness.

4. Meniscus Coating

• Description: Meniscus coating is a way to apply thin layers of materials, such as liquid polymers, onto surfaces. This method works by flowing the coating material over a sloped, permeable surface. As the liquid moves, it creates a smooth downward flow along the slope, helping to spread the material evenly.
• Best For: Wire-like or cylindrical substrates such as guidewires, stylets, or electrodes.
• Benefits: Enables precise, reproducible coating on narrow or high-tolerance components.

5. Hydromer Proprietary Meniscus Coating Method 

Hydromer®, Inc. has developed a proprietary meniscus coating technique that meets the unique requirements of certain medical devices. This application technique is a good match for applying coatings to devices that are long (100+ cm) and have a small diameter, such as guidewires and long, small diameter catheters. Cardiovascular and neurovascular devices are good candidates for this specialty coating technique.

Find out more about Hydromer’s contract coating services.

Why Selecting a Suitable Substrate is Important For Hydrophilic Medical Device Coatings

In hydrophilic medical coatings, choosing the right substrate is just as important as the coating itself. It’s crucial to understand how the physical, chemical, and mechanical properties of the substrate interact with the hydrophilic surface layer. This knowledge helps in developing new medical products and improving patient care, while ensuring durability.

Hydromer’s technology works with many types of medical-grade substrates, providing solutions that focus on real-world needs. By selecting the right substrate, developers of medical devices can lower the force needed to insert devices, improve how well they work with the body, and ensure they function effectively for a long time. These benefits are essential in modern medical engineering.

Looking for a custom medical device coating for your application? Contact the coating experts at Hydromer with questions or to start your project. 

References

Click to see the references for this article.

1. Pilotek S, Schmidt HK. Wettability of Microstructured Hydrophobic Sol-Gel Coatings. Journal of Sol-Gel Science and Technology. 2003/01/01 2003;26(1):789-792. doi:10.1023/A:1020779011844

2. Chen J-S, Chao S-Y, Chen C-C. The Effect of Hydrophilic Surface Coating of Fins on the Performance of Fin-and-Tube Heat Exchangers. Applied Sciences. 2023;13(18). doi:10.3390/app131810450 

3. Zhao M-H, Chen X-P, Wang Q. Wetting failure of hydrophilic surfaces promoted by surface roughness. Scientific Reports. 2014/06/20 2014;4(1):5376. doi:10.1038/srep05376

4. Trantidou T, Elani Y, Parsons ES, Ces O. Hydrophilic Surface Modification of PDMS for Droplet Microfluidics Using a Simple, Quick, and Robust Method via PVA Deposition. Microsystems & Nanoengineering. 2017;3(1)doi:10.1038/micronano.2016.91

5. Sidky PS, Hocking MG. Review of inorganic coatings and coating processes for reducing wear and corrosion. British Corrosion Journal. 1999/03/01 1999;34(3):171-183. doi:10.1179/000705999101500815

6. Jean MB. Dos and don’ts in characterizing and cleaning optical surfaces. 2005:596518.

7. Liu Z, Tu P, Ji Y, Cai Z, Wu H, Xu B. An eco-friendly and durable anti-fogging coating based on sulfobetaines and silicones. Progress in Organic Coatings. 2023;177:107413. 

8. Liu J, Qu S, Suo Z, Yang W. Functional hydrogel coatings. Natl Sci Rev. Feb 2021;8(2):nwaa254. doi:10.1093/nsr/nwaa254

9. Abdulhasan AA, Sheng EL, Mustafa AM, Isa MRB. Recent Advancements in Biocompatible Coatings for Metallic and Non-Metallic Biomaterials: A Review. Corrosion Science and Technology. 2024;23(5):449-469. 

10. Zang X, Ni Y, Wang Q, et al. Non-toxic evolution: Advances in multifunctional antifouling coatings. Materials Today. 2024/06/01/ 2024;75:210-243. doi:https://doi.org/10.1016/j.mattod.2024.03.018

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

12. Winters GL, Nutt MJ. Stainless steels for medical and surgical applications. vol 1438. Astm International; 2003.

13. Baltatu MS, Vizureanu P, Sandu AV, Florido-Suarez N, Saceleanu MV, Mirza-Rosca JC. New titanium alloys, promising materials for medical devices. Materials. 2021;14(20):5934. 

14. Inam H, Ali MN, Jameel IR, Awaiz D, Qureshi Z. Development of Robust PEBAX-Based Angiographic Catheter: Design and In Vitro Study. Materials. 2024;17(17):4248. 

15. Poojari Y. Silicones for encapsulation of medical device implants. Silicon. 2017;9(5):645-649. 

16. Choi J. Development of Teflon Coating Equipment Used in Medical Treatment. Journal of the Korea Academia-Industrial cooperation Society. 2018;19(11):344-349. 

17. de Brouwer H, van den Bogerd J, Hoover J. Color stability of polycarbonate for optical applications. European Polymer Journal. 2015;71:558-566. 

18. Ali S, Khan OS, Youssef AM, Saba I, Alfedaih D. Hydrophilic catheters for intermittent catheterization and occurrence of urinary tract infections. A retrospective comparative study in patients with spinal cord Injury. BMC urology. 2024/06/12 2024;24(1):122. doi:10.1186/s12894-024-01510-y

19. Tang X, Yan X. Dip-coating for fibrous materials: mechanism, methods and applications. Journal of Sol-Gel Science and Technology. 2017/02/01 2017;81(2):378-404. doi:10.1007/s10971-016-4197-7

20. Kistler SF, Schweizer PM. Coating Science and Technology: An Overview. In: Kistler SF, Schweizer PM, eds. Liquid Film Coating: Scientific principles and their technological implications. Springer Netherlands; 1997:3-15.

21. Azlin-Hasim S, Cruz-Romero MC, Morris MA, Cummins E, Kerry JP. Spray Coating Application for the Development of Nanocoated Antimicrobial Low-Density Polyethylene Films to Increase the Shelf Life of Chicken Breast Fillets. Food Science and Technology International. 2018;24(8):688-698. doi:10.1177/1082013218789224

22. Schunk PR, Hurd AJ, Brinker CJ. Free-Meniscus Coating Processes. In: Kistler SF, Schweizer PM, eds. Liquid Film Coating: Scientific principles and their technological implications. Springer Netherlands; 1997:673-708.