The performance of a medical device often depends on a simple yet important factor: Friction. For example, friction greatly determines the ability of a catheter to move through blood vessels or a guidewire to pass through tortuous pathways. Both medical devices greatly benefit from reduced surface friction. Reduced friction can make the difference between a smooth procedure and a difficult, painful one.
That is why lubricious, hydrophilic medical device coatings are applied onto medical devices. They greatly reduce friction. This is particularly important for medical devices that come in contact with body tissues during surgical procedures. But how can we measure and understand the level of friction of a device’s surface accurately? The answer is through analytical testing, specifically friction testing, These tests measure the friction coefficient using tribological methods.
In this article, we will look at how friction is measured. They we will discuss how these results are important for designing medical devices. We also cover how hydrophilic coatings contribute to the low-friction performance of these devices.
Why Friction Reduction Is Critical For Medical Devices
Medical devices that come into contact with tissue or are inserted into the body must have minimum surface friction. Examples of devices where low friction is critical include catheters, sheaths, cannulas, or implants.
Friction reduction in these devices is critical for several key reasons, which include the following:
- Patient safety: Lower friction reduces the risk of tissue damage, or inflammation.
- Surgeon control: Slippery, low-friction devices offer surgeons easier insertion, retraction, and maneuverability. In other words, they make the surgeon’s job easier.
- Device integrity: Lower friction equates to less stress on the device’s surface during movement.
- Infection control: Friction-induced microtrauma can compromise tissue barriers.
Hydrophilic coatings are special surface treatments that are applied to the surface of medical devices. These lubricious coatings can significantly lower the coefficient of friction. This is critically important for medical devices that come into contact with body tissues, such as catheters and stents.
For example, applying hydrophilic coatings can reduce friction by up to 80% compared to natural materials. This significance decrease makes it easier to insert and move these devices smoothly and helps reduce damage to tissues.1
Learn more about Hydrophilic Coatings for Medical Devices: Uses & Benefits
Understanding Coefficient of Friction
The friction of a surface is measured by the coefficient of friction. Below we discuss what this is, and what a good coefficient is for a medical device application.
What is coefficient of friction?
It is the ratio of the resistance force between two surfaces to the normal force pressing them together.2 The coefficient of friction is a critical measurement when it comes to understanding surface friction. It characterizes the interaction between two surfaces in contact. For example, the level of friction between a medical device and human tissue.
What is a “good” coefficient of friction?
A “good” or acceptable coefficient of friction largely depends on the specific requirements of a device used for a specific application. For instance, a lower coefficient of friction is typically desirable in biomedical applications (medical devices).
Hydrophilic coatings can help reduce the friction of a device’s surface. Below you can see a high-level comparison of coated vs uncoated (natural) materials.
- Uncoated polymers (like nylon, PU): Coefficient of friction ~ 0.3–0.6 in wet conditions.3
- Hydrophilic coated surfaces: Coefficient of friction as low as 0.02–0.05 under hydration.4
- FDA submission documents and ISO standards (e.g., ISO 11070 for catheters) often include coefficient of friction data as supporting evidence of safety and performance.
As you can imagine, being able to accurately measure and understand the coefficient of friction is very important when designing and qualifying biomedical devices. It affects how well the devices work and how safe they are for patients. It also may hep determine how the devices will be received and perceived by the healthcare providers using them.
So how can medical device manufacturers accurately and reliably measure the friction of their devices?
Quantifying Lubricity: Testing Coefficient of Friction
To move beyond subjective descriptors like “slick” or “smooth,” engineers rely on tribological testing. This is the study of understanding friction, lubrication, and wear phenomena for interacting surfaces. They use these tests to measure the coefficient of friction.
The following are the testing methods that are used to measure lubricity or coefficient of friction.
1. Pin-on-Disk Friction Testing 5
Pin-on-disk testing is a fundamental method for evaluating tribological properties, particularly friction and wear. This testing is used to test various coatings that are used in numerous applications, including medical devices.
This method entails a stationary pin pressed against a rotating disk, allowing for consistent measurement of the coefficient of friction and wear resistance under controlled conditions.
- How it works: A coated surface (disk) is rotated against a small probe or pin under controlled pressure and speed.
- What it measures: Dynamic and static friction over time.
- Output: Friction vs. time curves, showing wear or degradation of the coating under motion.
2. Tribometers and Linear Friction Testers 5
Tribometers and linear friction testers are essential tools when it comes to evaluating tribological performance. They allow researchers and engineers to assess the friction and wear properties of materials and coatings under various conditions.
The methodology typically allows for precise control of operational parameters. These include load, velocity, and environmental conditions. This is important as they can significantly influence the frictional characteristics of a material during testing.
- Modern tribometers (rotary or reciprocating) can simulate linear motion like catheter insertion.
- Samples can be tested under wet (saline, plasma) or dry conditions to mimic in vivo environments.
- Measures lubricity retention over multiple cycles, simulating device insertion/retraction during real-world use.
3. In-Situ Simulations Friction Testing
In-situ simulations are another type of friction testing. These are essential for assessing tribological properties under conditions meant to mimic real-world use.
- In-situ simulations:
- Study how polymer parts in medical devices, such as retractable syringes and catheters perform under friction. These simulation studies offer real-time insights into the safety and reliability of these devices. 6
- Real-time methods and in-situ evaluations are crucial for assessing the efficacy of surface coatings for implants. Friction and wear can be evaluated during cyclic loading under simulated physiological conditions. 7
The Role of Hydrophilic Medical Coatings in Friction Reduction
Hydrophilic coatings are lubricious, surface treatments that reduce friction on the surface of medical devices. Reduced friction is important for medical devices that need to enter and move through wet, narrow, or sensitive areas in the body.
Hydrophilic coatings are water-loving and “slippery-when-wet”. They absorb moisture and create a thin, slippery layer similar to a gel. This low-friction, gel-like layer makes it much easier for the device to insert and slide against the surrounding tissues. This helps in significantly lowering the coefficient of friction between the device and surrounding tissues.
Here are some of the biggest reasons why hydrophilic medical coatings are the go-to solution for friction reduction in medical devices.
Hydrogel Formation for Lubricity 8,9
- Upon hydration, the coating becomes swollen and hydrated. It acts as a lubricious interface between the device surface and the body.
- This reduces shear forces between the device surface and tissue or blood vessel walls.
- Hydrophilic coatings help reduce insertion force and patient discomfort in catheters, introducers, and guidewires.
- Lubricious coating make it easier for devices to navigate through tortuous pathways. This makes surgeons’ jobs easier and can help produce better patient outcomes.
Consistent Low-Friction Performance 9,10
- Well-formulated hydrophilic coatings can maintain low coefficient of friction values over repeated insertion-retraction cycles.
- High-performance coatings show minimal degradation of lubricity under real-use conditions. Examples include saline flow or body temperature exposure.
Hydrophilic Coatings Are Compatible with Wide Range of Substrates
- Hydrophilic coatings can be formulated to bond to many materials, including polymers (like polyurethane, nylon, Pebax) and metals (stainless steel, nitinol). This allows for their use across different device categories using different materials. Learn more about the wide range of suitable substrates for hydrophilic medical coatings.
They Provide Enhanced Clinical Outcomes11
- Reduction in friction translates directly into:
- Shorter procedure times
- Lower risk of trauma or bleeding
- Improved navigability in minimally invasive systems
- Also contributes to greater device control and lower stress on delicate anatomical structures
Help Meet Regulatory Requirements and Provide Market Differentiation
- Reduced friction measurements from hydrophilic coatings can help device manufacturers in multiple key ways. These include:
- Regulatory filings (FDA, CE) and approvals
- Clinical marketing materials (product differentiation)
Hydrophilic coatings can be applied by a wide range of coating application techniques. These include dip-coating, spray-coating, meniscus coating, UV-curable processes and more.
Hydromer®, Inc. Hydrophilic Coatings & Specialized Analytical Testing Services
Hydromer®, Inc. is a manufacturer of custom, hydrophilic medical device coatings. In addition, we offer a range of support services, including contract R&D, contract coating and specialized, analytical testing services.
Our testing services can be used to help you develop new product and improve existing products. In addition, our team can help ensure the effectiveness and safety of your medical devices.
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
Being able to accurately and reliably measure the coefficient of friction of medical devices is critically important. Understanding and controlling friction in these devices is crucial. That is because friction affects not just how they function, but also how well they work within the body. If the friction of a device’s surface is too high or too low, it can cause problems. These include wear and tear of the device as well as infections, or damage to body tissues.
The coefficient of friction can be measured scientifically using different tribological testing methods. Friction testing methods like pin-on-disk systems and tribometers allow engineers to understand the friction and durability of device coatings used in real-life conditions.
Hydromer® hydrophilic medical coatings help reduce friction on the surface of devices. Reduced friction helps produce safer and more effective devices. And this helps produce better patient outcomes.
Advanced testing methods help manufacturers better understand a product’s performance in terms of coefficient of friction. Device companies can use this information to develop products that meet regulatory requirements. In addition, they can use the data to create unique products that stand out in the market.
If you are a medical device manufacturer, then finding a reliable coatings partner is critical. Companies like Hydromer®, Inc. provide custom coatings and many support services in addition to analytical testing.
Contact us if you need help developing, testing, or applying a hydrophilic medical device coating for your product.
References
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