Many medical devices, especially those used in minimally invasive procedures, rely on hydrophilic coatings. Hydrophilic coatings are applied to many devices. Examples include guidewires, vascular and urinary catheters, introducer sheaths, endoscopes, disposable diagnostic devices, and microfluidic devices. These surface coatings significantly reduce friction, enhance tracking ability, and lessen trauma caused by insertion. They allow the devices to be inserted and navigated easier and allows the clinician to control them better.1 This results in better device performance and better patient outcomes.
With that being said, the method used to cure the hydrophilic coating plays a decisive role in achieving and maintaining the coatings’ beneficial properties. Ultraviolet (UV) curing has emerged as the preferred manufacturing approach for these coatings. That is because it enables rapid, solvent-free polymerization with exceptional process control.2,3 But what UV spectrum should the coatings be cured in for the best results?
In this article we present a new view on UV curing technology, specifically from the perspective of curing hydrophilic medical device coatings. We look specifically at the attributes of UV-A (315–400 nm) curing, with special regard to the attributes of HydrUV™, as a unique example of curing efficiency, coating performance, substrate protection, and occupational safety. So if you are interested in UV coatings for medical devices you will want to be sure to read this article to the end!
Requirements of Hydrophilic Coatings in Medical Device Applications
Before looking at the specifics of UV curing, it is important to understand the requirements of hydrophilic medical device coatings.
Why?
It is because the curing method can play a role in achieving these critical requirements.
From a medical device standpoint, hydrophilic coatings must simultaneously meet several demanding criteria. These coatings must have:
- Extremely low coefficient of friction under wet conditions
- Rapid and consistent hydration
- Strong adhesion to multiple substrates, including polymeric, metallic, and ceramic
- Resistance to delamination, cracking, and particulate shedding
- Stability after sterilization and during shelf life
- Demonstrated biocompatibility and hemocompatibility
Now that we understand the requirements of the coatings, we explore why and how the curing method plays a role in if they can achieve them.
Why Curing Strategy Matters for Medical Device Safety
Unlike most decorative or industrial coatings, hydrophilic medical device coatings are regulated materials. Their chemical composition, residuals, and mechanical integrity directly affect patient safety.
Incomplete or inconsistent curing of the coating can lead to:
- Residual monomers or oligomers that may leach into the body
- Weak crosslink density resulting in coating sloughing or particulates
- Variability in lubricity and hydration behavior
- Adhesion loss during simulated or actual clinical use
UV curing offers a high level of control over crosslinking reactions. This allows manufacturers to design coatings that meet strict performance and regulatory expectations. At the same time they can still meet high production throughput.
Fundamentals of UV Curing in Hydrophilic Coatings
The photochemical process of UV curing occurs when photoinitiators in the coating formula absorb energy from UV radiation (light). This forms a broken-down component that creates free radicals. These free radicals then react with the oligomer chains of the polymer. They combine to create a linear or branched structure through a polymerization and crosslinking reaction.4
In hydrophilic medical coatings, UV curing typically stabilizes a polymer network. This either contains or entraps highly hydrophilic polymers, such as polyvinylpyrrolidone (PVP) or polyethylene glycol (PEG). The resulting structure is often described as a semi-interpenetrating polymer network (semi-IPN). This combines mechanical integrity with high water uptake and lubricity.5,6
Key photochemical parameters governing UV curing performance include:
- Photoinitiator absorption spectrum, which must align with the emission wavelength of the UV source
- Quantum efficiency and radical yield, influencing cure speed and conversion
- Penetration depth, determined by wavelength and formulation absorbance
- Oxygen inhibition, which can suppress surface polymerization in radical systems
The UV Spectrum and Material Interactions
Ultraviolet (UV) radiation is typically categorized into three classes: UV-A, UV-B, and UV-C.
The energy of the photons increases as the wavelength decreases, which significantly impacts polymer curing and compatibility with other materials.
UV-A (315–400 nm)
Ultraviolet (UV)-A provides an excellent balance of activation energy and penetration depth into the polymer film. This enables uniform curing of polymer films through thin to moderately hydrophilic coatings, typically a few microns thick. This property is a significant advantage for medical devices constructed of heat- and radiation-sensitive polymers, including polyurethane, Pebax®, nylon, and silicone.
UV-B (280–315 nm)
Ultraviolet (UV)-B has very similar issues as UV-C, although to a lower extent. These include chain scission, discoloration, and substrate degradation. They are useful for industrial applications where faster processing and higher resistance is needed.
UV-C (200–280 nm)
Ultraviolet (UV)-C radiation is characterized by a high degree of energy and a high ability to be absorbed at the surface of organic materials. This property may enable the curing of surfaces quickly. However, UV-C also presents a greater chance of:
- over-curing (too many cross-links created between polymer chains)
- chain scission (destroying the polymer chain)
- discoloration (the formation of color on an excessive basis)
- substrate degradation (damage due to the use of excessive amounts of UV-C)
Advantages of UV-A Curing for Hydrophilic Medical Coatings 2-8
Curing in the UV-A spectrum provides several benefits vs other UV spectrums. We discuss these in detail below.
1. Deep, Uniform Cure and Low Extractables
It is critical to ensure uniform, complete curing throughout the thickness of a coated surface. The UV-A spectrum of radiation is able to penetrate to help support uniform crosslinking processes and provide homogeneous conversion and crosslink densities across all surfaces of the coated product. This results in a decreased level of extractable and leachable substances.
2. Preservation of Substrate Integrity
UV-A uses lower energy levels and lower curing temperature (which is nearly ambient) compared to either UV-B or UV-C. This provides a reduction in heat distortion, reduced or eliminated brittleness, and reduced or eliminated oxidative degradation of the polymeric substrate. This helps retain mechanical properties, such as flexibility, torque response, and tensile strength.
3. Enhanced Adhesion and Mechanical Durability
UV-A allows for controlled curing of the coating. This allows the applicator to precisely tune the degree of crosslink density formed at the interface between the coating and substrate. This leads to an improvement in strength and integrity of the interface. And that leads to a higher degree of adhesion, a lower risk of delamination, and improved performance under repeated hydration and dehydration as experienced with medical devices.
4. Reduced Particulate Generation
Particulate shedding from hydrophilic coatings is a major clinical and regulatory concern. Rapid UV-A curing minimizes the time coatings remain in a semi-cured state, which is where debris formation and contamination are most likely. A well-crosslinked network further resists abrasion during device navigation when in use.
5. Manufacturing Efficiency and Process Integration
The capabilities of UV-A LED systems include:
- Rapid turn-on and turn-off times
- Consistent UV light output throughout the entire length of a coating cycle
- Long times between events
- Low levels of heat.
These features allow for UV-A LED systems to provide inline curing of hydrophilic coatings with high throughput and excellent repeatability. This is important as these are the two primary requirements of mass-produced medical devices.
6. Occupational and Environmental Safety
UV-A curing systems carry much lower safety hazards compared to UV-C curing systems. This reduces the chances of an acute injury occurring due to UV light exposure. As a result, it reduces the need for engineering controls and providing a simpler shield barrier. In addition, mercury and ozone are no longer present in LED-based UV-A systems. This increases the overall sustainability of these technologies.
Why Hydromer® HydrUV™ UV Hydrophilic Coatings are a Great Option?
Hydromer® HydrUV™ Hydrophilic Coatings were developed specifically for use on Medical Devices that require fast, strong, and consistent performance. They have been engineered to provide maximum lubricity over extended periods of time. At the same time, they provide a high level of adhesion and reliability in clinical settings that demand rigorous conditions. 9
Key Advantages of Hydromer® HydrUV™ Coatings for Medical Device Applications
- Enhanced durability: Maintains mechanical strength and lubricity under repeated stress.
- Fast, safer curing: UV-A curing shortens processing time, boosts productivity, and reduces manufacturing delays.
- Long-lasting lubrication: Ensures smooth device performance and minimizes tissue trauma during sensitive procedures.
- Broad material compatibility: Proven adhesion on a range of materials such as metals and polymers.
- Precise and safe curing: UV-A light minimizes thermal stress, improves worker safety, and ensures uniform curing on complex geometries.
HydrUV™: A Versatile Coating Solution
HydrUV™ has been proven to adhere well and sustain performance through numerous mechanical cycles. This makes it suitable for use in catheters and guidewires where there is a high degree of movement and stress placed on the device.10
Our UV coatings are provide versatility. That is because HydrUV™ coatings adhere to a wide range of substrates without compromising the quality of the coatings themselves. Some of the many suitable substrates include:
- Nylon
- PETG (Polyethylene Terephthalate Glycol)
- TPU (Thermoplastic Polyurethane)
- Polyether Block Amide (PEBAX)
- Polypropylene (PP)
- Polycarbonate (PC)
Suitable Applications
HydrUV™ is perfect for devices that require long-lasting hydrophilicity and lubricity, frequent movement, and biocompatibility. Example medical device applications include:
- Urological and cardiovascular catheters
- Central venous access devices
- Minimally invasive surgical tools
- Interventional radiology instruments
- Diagnostic tubing and sensors
Hydromer, Your Trusted Coatings Innovation Partner
Hydromer, Inc. has been pioneering hydrophilic medical coating technology for over 40 years and continues to develop new, innovative coatings today. The development of HydrUV™ supports the demands of OEMs by providing advanced performance and practical/scalable manufacturing options. In addition, we provide a full-range of coating services to support you no matter what stage in the product cycle you are in.
To assist manufacturers in testing and optimizing HydrUV™ for their unique end-user application, Hydromer provides sample kits and technical assistance, along with contract R&D and regulatory consulting services.
Samples of our coatings or consultations are available by contacting in**@******er.com.
Conclusion
The wavelength at which a hydrophilic medical device coating cures is foundational to the overall design of the coating. As such, HydrUV™ has been developed with a wavelength in the “UV-A” region to achieve an optimal balance of precision, protection, and performance. UV-A curing results in a deep and uniform polymerization of the hydrophilic coating material. It also protects sensitive substrates while simplifying biocompatibility compliance and increasing occupational safety. As medical device manufacturers continue to evolve devices with higher performance and greater regulatory scrutiny, UV-A curing represents a scientifically supported and “future-ready” technology option for next-generation hydrophilic coatings.
References:
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7. Gaston A, Khokhar AZ, Bilbao L, et al. Nanopatterned UV curable hydrogels for biomedical applications. Microelectronic engineering. 2010;87(5-8):1057-1061.
8. Zhang P, Qin B, Xia J. UV Curable Robust Durable Hydrophobic Coating Based on Epoxy Polyhedral Oligomeric Silsesquioxanes (EP-POSS) and Their Derivatives. Acs Omega. 2022;7(20):17108-17118. doi:10.1021/acsomega.2c00534
9. Hydromer. Medical Device Coatings. Hydromer. https://hydromer.com/medical-device-coatings/
10. Hydromer. HydrUV™: Revolutionizing UV Medical Device Coatings. https://hydromer.com/hydruv-revolutionizing-uv-medical-device-coatings/
