Why Formulation Design Is Critical in Medical Hydrophilic Coatings
Effective hydrophilic coatings are an essential part of medical devices. These coatings are functional surface modifications that are applied in micron scale thicknesses. Despite this they still significantly influence device safety and clinical performance. They are subjected to harsh conditions, such as being continually hydrated, exposed to mechanical stress, sterilized repeatedly, and stored for long periods of time. In other words, the demands these coatings are subjected to are significant. As a result, they need to be formulated with durability, safety, and performance in mind.
Ultraviolet A (UVA) curing has made UV light a preferred method to properly cross-link hydrophilic coatings. This method allows for faster curing, with no solvents, at almost room temperature.
However, the benefits of UVA curing can only be realized when the formulation is deliberately engineered for its specific medical use. Poor UVA coating formulation can result in several issues, such as incomplete curing, high extractables, brittle coatings, poor adhesion, excessive swelling, etc. All of these issues can cause issues with regulatory approvals and more.
In this article we give a high level overview on the process of formulating UVA cured hydrophilic coatings for medical devices. We will look at how each component within the formulation contributes towards the performance, and safety of the coating. If you are a medical device engineer you will want to read to the end!
Core Architecture of UVA Cured Hydrophilic Coatings: Semi-IPNs 1-3
Semi-Interpenetrating Polymer Networks (Semi-IPNs)
The majority of medical-grade hydrophilic coatings are comprised of semi-interpenetrating polymer networks (semi-IPNs). These can be described as:
- A UV curable, covalently cross-linked backbone that encompasses three key properties: adhesion, cohesion, and mechanical integrity
- A hydrophilic polymer phase of the semi-IPN (typically non-reactive) that imparts water retention capability, lubricity, and a low coefficient of friction to the coating
The UV-A curing process creates a reactive polymeric network that entraps the hydrophilic component(s). This allows for individual modification of both lubricity and durability.
Considerations for UVA Coating Formulation & Tradeoffs
Excessively increasing hydrophilic content of the coating will provide swelling and high lubricity. As hydrophilic content increases it will weaken the coating’s adhesion and decrease its resistance to abrasion.
The opposite is true for a high degree of crosslinking density. As the crosslinking density increases it will increase the durability, but it will also decrease the ability of the coating to hydrate and lubricate.
Therefore, the formulation design must find the optimal balance of both extremes to achieve the desired clinical outcomes.
This balance can be achieved through the selection of different formulation components. These are what we discuss below.
1. UV Curable Polymer Backbone Selection
Polymer selection during the UVA coating formulation process is critical. Below are two of the common polymer backbones used for these coatings.
A. Urethane Acrylates 4
Urethane Acrylates are a widely used polymer for coatings, adhesives and sealants. This is because of their outstanding adhesion characteristics, flexibility, and the ability to tailor their mechanical properties. The type of soft segment polymer (polyether or polyester) used will affect the hydrolytic stability and flexibility of the urethane acrylate. And the amount of hard segment used will have an effect on the abrasiveness of the urethane acrylate.
B. Polyether and PEGBased Acrylates 5
Polyethylene glycol (PEG) acrylate polymers are naturally hydrophilic. They are frequently utilized as reactive diluents or network formers. This is because they possess inherent hydrophilicity. The molecular weight selected for a particular PEG acrylate polymer greatly influences its ability to swell as well as the amount of crosslinking that occurs.
2. Hydrophilic Polymer Phase Design
Polyvinylpyrrolidone (PVP)2,6
PVP is the most commonly used hydrophilic polymer in medical coatings. This is due to its excellent water solubility, biocompatibility, and lubricity.
Molecular weight (MW) selection is critical as different MW’s give PVP different characteristics:
- Low MW PVP hydrates rapidly, but it may leach
- High MW PVP improves retention, but it increases viscosity
Considerations for Polymer Retention
UVA coating formulations must demonstrate minimal leaching under simulated use and sterilization conditions.
Retention of the hydrophilic phase is governed by:
- Crosslink density of the UV cured network
- Hydrogen bonding and physical entanglement
- Molecular weight of the hydrophilic polymer
3. Photoinitiator Selection for UVA Medical Coating Formulation7,8
The selection of photoinitiator also has a great affect on the coating, including its curing, as well as its toxicological profile, and regulatory compliance.
Types of Initiators: Type I vs Type II
The types of initiators for UV cure coatings include:
- Type I (cleavage) initiators offer fast cure and high efficiency
- Type II systems may be used with amine synergists, but they can introduce additional extractables, which should be considered and understood
Type I initiators are often preferred for hydrophilic medical coatings.
Spectral Matching of Photoinitiators to UVA LEDs
Photoinitiators must absorb efficiently in the 320–400 nm range (commonly 365, 385, or 395 nm). Efficient absorption ensures deep, uniform curing with minimal initiator loading.
Toxicological and Regulatory Considerations of Photoinitiator Selection
Medical device coating formulations favor photoinitiators with:
- Well characterized toxicological profiles
- Low migration potential
- Minimal, low molecular weight cleavage products
Concentration of the initiator should be minimized while maintaining full conversion.
4. Formulation to Manage Oxygen Inhibition 9-11
Oxygen inhibition affects surface cure, which can compromise a complete curing of the coating. This can cause issues with adhesion, particulate resistance, etc.
Common mitigation strategies to combat oxygen inhibition include:
- Using higher UVA irradiance
- Use of surface active co-initiators
- Carbon dioxide inerting (where required or possible)
- Formulation additives that scavenge oxygen
The chosen strategy must be compatible with medical manufacturing constraints (and regulations).
5. Selection of Additives and Secondary Components in UVA Coating Formulation
Additional ingredients or additives may also be added to the formulation to improve the quality of the coating. While not an exhaustive list, these may include:
A. Adhesion Promoters
Functional monomers or primers may be used to enhance bonding to substrates such as nylon, Pebax®, polyurethane, or metals. However, these must be carefully evaluated for extractables.
B. Stabilizers and Antioxidants
UVA curing minimizes photodegradation. However, stabilizers may be required to ensure shelf life and sterilization stability.
C. Rheology Modifiers
Viscosity control is critical for uniform coating thickness. However, additives must not interfere with curing or increase particulate risk.
6. Formulating UVA Coatings for Optimized Crosslink Density
This optimization typically involves systematic variation of monomer functionality, oligomer content, and UV dose.
Crosslink density governs:
- Swelling ratio and hydration speed
- Lubricity and coefficient of friction
- Abrasion resistance and durability
- Retention of hydrophilic polymers
These optimization steps will assess the quality and suitability of the UV curing formulation.
Hydromer® Custom, UV Cure Hydrophilic Medical Coatings
At Hydromer we have been innovating in the area of hydrophilic medical coating technology for 40+ years. Recently, we have added to our product portfolio with the introduction of HydrUV™, an advanced line of UVA-Cure, custom hydrophilic medical coatings. These coatings offer long lasting hydrophilicity, faster processing time, enhanced durability, and more.
Our medical coatings are trusted by some of the industry’s leading medical device manufacturers. All of our coatings can be custom formulated to meet your device’s specific requirements.
In addition, our company offers a wide range of coating services including contract R&D, custom coating formulation, contract coating services, analytical testing, and more. This means you will get a full-service partner to help you meet your product development goals.
Conclusion
UVA curing allows for flexibility and control when it comes to medical coatings. UVA cured hydrophilic coatings that are designed correctly will provide the user with many benefits. These include consistent lubricity, excellent adhesion, minimal particulate risk, and regulatory compliance, all of which are required of today’s medical devices.
However, their formulation requires multiple disciplines to create a safe and effective coating. These include polymer chemistry, photophysics, surface science, toxicology, and regulatory strategy, which not all coating suppliers may have. In the end the success of a device coating depends upon the careful selection of all components of its formulation as well as their optimal performance characteristics.
References
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