Surface Energy Matters More Than You Think
Mechanical precision, structural integrity, and material compatibility are the usual determinants of performance for medical device engineering. However, in practice, a medical device’s ultimate performance and safety are determined at the interface between the device and the human body. And this is predominantly determined by surface science, namely surface energy as opposed to bulk properties.1,2
Surface energy is a fundamental parameter that controls how a medical device interacts with physiological environments. For example it affects:
- How fluids spread and remain on the surface of a device
- The level of friction forces while inserting or navigating the device in the body
- Adsorption of proteins and other biological macromolecules
- Cellular responses and the potential for thrombus (blood clot) formation
Surface energy, under both continuous and dynamic conditions regulates these interfacial interactions. This is true in the use of minimally invasive/intravascular (inside a blood vessel) medical devices.3,4 In many cases, even minimal inefficiencies at the interfacial location will often lead to an increase in insertion force, cause trauma to the vascular system, or compromise the ability to control the device.
Throughout our 40+ year history, the Hydromer® has developed hydrophilic coating technology that is designed to precisely control these interfacial interactions. They do so by controlling surface energy at the molecular level. As a result, these hydrophilic coatings provide an avenue for manufacturers to produce innovative medical devices with improved functionality and performance.
In this article, we will cover surface energy and why it matters so much to optimizing the device-tissue interface. Make sure you read to the end because the information in this article is key to outperforming your competition.
Introduction to Surface Energy?
Surface energy arises because molecules at a material’s surface are not fully surrounded by neighboring molecules. This results in an energetically unstable state. To minimize this energy, surfaces interact with external substances, particularly liquids.5,6
Effects of Surface Energy on Medical Device Coatings
From an engineering perspective, surface energy determines:
- Wettability (how easily a liquid spreads across a surface)
- Adhesion (how well coatings or biological materials attach)
- Interfacial stability under mechanical and fluid stress
Surface Energy of Common Medical Device Polymers
Most common polymers used in medical devices, such as polyethylene, polyurethane, silicone, and PEBAX, exhibit low surface energy. Low surface energy results in:
- Poor wetting by aqueous fluids
- High contact angles
- Limited chemical reactivity at the surface
These characteristics make untreated polymers inherently unsuitable for several applications. Specifically, they are a poor fit for those requiring lubricity, coating adhesion, or controlled bio-interaction.
Transforming Hydrophobic Surfaces with Hydrophilic Medical Coatings
Medical device hydrophilic coatings can significantly alter the surface energy of many different kinds of materials. These coatings are polymers that contain polar/hydrophilic atoms. They are applied or deposited onto non-hydrophilic materials (medical device substrates). The result is a high-energy, wettable/hydrophilic interface.7-9
Some of the advanced chemistries used include:
- Polyvinylpyrrolidone (PVP)
- Polyethylene glycol (PEG)
- Zwitterionic and amphiphilic polymers
When exposed to aqueous environments, these coatings:
- Rapidly hydrate through hydrogen bonding
- Absorb and retain water within the polymer matrix
- Create a physically stable, water-rich boundary layer
This hydrated interface acts as a lubricating film. It dramatically reduces friction between the device and the surrounding tissues.
In clinical applications, such as vascular catheterization or neurovascular navigation, this translates into:
- Lower insertion forces
- Enhanced trackability through tortuous anatomy
- Reduced endothelial damage
The Science of Lubricity: It’s All About Water Retention
The lubrication properties of hydrophilic coatings are dependent on the water interactions occurring at the interfaces of the coating.10-12
The lubrication properties of high surface energy coatings are listed below. They:
- Have a significant attraction to water molecules
- Retain hydration despite being subjected to shear force
- Withstand dehydration due to repeated movement
Why is this important?
It is important because frictional forces acting on or caused by biological systems will be affected by:
- Contact pressure
- Relative motion
- Fluid in the area of contact
Hydrophilic medical device coatings that are formulated correctly allow for a stable hydration layer to be maintained. This prevents direct contact between the device’s surface and the biological tissue at all times.
Advanced medical device hydrophilic coatings, such as those by Hydromer® are designed to:
- Retain water under dynamic flow conditions
- Provide consistent lubricity over extended use
- Minimize performance variability between procedures
The Adhesion Challenge: Overcoming the Surface Energy Mismatch
In order to achieve durable coating adhesion, a formulation must address the fundamental mismatch between:
- Low-energy polymer device substrates, and
- Higher-energy hydrophilic coatings
If not addressed through formulation this mismatch leads to weak interfacial bonding between the two materials and an increased risk of:
- Coating delamination
- Surface defects
- Particulate shedding during use
To mitigate these potential issues, surface activation techniques are employed to temporarily elevate substrate surface energy.
Surface Activation Methods Used to Overcome the Surface Energy Mismatch
The following surface preparation and activation methods are used for surface activation:
- Plasma treatment to create polar functional groups and increase surface roughness on a nanoscale basis
- Corona discharge to oxidize surfaces for better wettability
- Chemical priming to generate adhesion-promoting layers that connect the two parts (substrate & coating).
These surface activation treatments enhance:
- Chemical bonding potential
- Mechanical interlocking
- Coating uniformity
Hydromer’s coating systems utilize adhesion strategies that are integrated into the formulation. They provide a durable, long-lasting solution even when applied to extreme conditions, such as PTFE (polytetrafluoroethylene) or hyper-elastic loose polymers.
How Surface Energy Effects Medical Device Biocompatibility
The surface energy of a material is one of the key determinants in how a device will respond in a biological environment after implantation or insertion. 4,13
When blood or tissue comes into contact with a device, protein adsorption is typically the first event that occurs. This results in subsequent biologic processes, such as:
- Platelet adhesion
- Coagulation cascade activation
- Cellular attachment
Biocompatibility of Hydrophilic (Higher Energy) vs Hydrophobic (Low Energy) Surfaces
The biocompatibility of different materials has affects on their biocompatibility. Below we compare the biocompatibility differences between hydrophobic and hydrophilic materials.
Low-Energy Hydrophobic Surfaces
Low surface energy, hydrophobic materials and films tend to:
- Promote nonspecific protein adsorption
- Induce conformational changes in proteins
- Increase thrombogenic potential
Higher-Energy Hydrophilic Surfaces
In contrast, hydrophilic surfaces:
- Resist protein adsorption through hydration barriers
- Maintain protein structure and function
- Reduce platelet activation and adhesion
These abilities make hydrophilic coatings particularly valuable for:
- Cardiovascular devices
- Neurovascular systems
- Long-dwell catheters
Balancing Higher-Surface Energy Performance with Durability
Increasing surface energy with hydrophilic coatings enhances wettability and lubricity. However, the increase can introduce challenges related to mechanical and chemical stability if the coating is not formulated correctly.
Hydrophilic coatings need to be durable. Specifically, they must be able to withstand:
- Repeated mechanical deformation
- Shear forces during device manipulation
- Prolonged exposure to aqueous environments
Formulation and engineering strategies to ensure adequate durability of the coating include:
- Crosslinking: Stabilizes the polymer network and reduces excessive swelling
- Covalent bonding: Anchors the coating to the substrate for improved adhesion
- Multilayer coating systems: Separate adhesion, mechanical strength, and lubricity functions
The objective is to create coatings that are not only highly functional but also robust so they can be effective throughout the device’s life.
How to Measure and Optimize Surface Energy
Surface energy cannot be observed directly. However, it can be quantified through well-established analytical techniques:14,15 These include:
- Contact angle measurement is an evaluation of wettability and surface hydrophilicity
- Surface free energy calculations provide quantitative insight into polar and dispersive components
- Tribological testing measures friction and lubricity under simulated use conditions
These techniques allow coating engineers to:
- Fine-tune coating formulations
- Validate performance across substrates
- Ensure reproducibility in manufacturing
Why Effective Surface Energy Engineering Is a Competitive Advantage
For medical device manufacturers (OEMs and start-ups alike), effective surface energy engineering is important. It is a powerful factor for improving product performance and differentiation.16-18
Optimized surface energy of medical devices leads to:
- Enhanced device handling and control
- Reduced insertion force and tissue trauma
- Improved patient comfort and safety
- Greater consistency across clinical use cases
In increasingly competitive and regulated medical device markets, these advantages can significantly impact:
- Product adoption
- Clinical outcomes
- Regulatory approval pathways
In turn, being able to optimize your devices’ surface energy can be a key differentiation factor.
Hydromer® Approach to Surface Energy Engineering
At Hydromer, surface energy engineering is central to our custom medical device hydrophilic coatings formulas.
Our scientists utilize the following:
- Advanced polymer chemistries
- Substrate-specific adhesion strategies
- Optimized hydration behavior
Hydromer provides hydrophilic coating solutions for medical devices that are formulated specifically for:
- Different types of devices (e.g., vascular & orthopedic devices)
- The ability to maintain lubricity under “real world” conditions
- Meet or exceed high durability & safety standards
Final Thoughts
You cannot see surface energy, but it significantly defines the way in which medical devices interact with the body. Many medical devices use polymer substrates that are not suitable for use with human tissue. Advanced engineered hydrophilic coatings can be applied to existing material surfaces. These coatings help create high-performance, biocompatible interfaces. They do this by altering the substrates’ surface properties. For device manufacturers seeking to enhance functionality, minimize risk, and improve outcomes, mastering surface energy is not just an advantage, it is an absolute necessity.
Ready to formulate a custom, advanced medical device hydrophilic coatings for your product? Contact our coating experts to get samples and a free consultation.
References
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