PTFE is the preferred material for medical device liners. This is due to its low coefficient of friction (CoF) and chemical inertness. PTFE Liners provide a safe, smooth inner lumen for easy passage of devices like guidewires, fluids, and instruments. However, its low surface energy, which creates its lubricity, also creates PTFE liner bonding challenges with other materials. To address this, manufacturers modify the PTFE surface at the microscopic level through processes such as etching and tie layer.
Without surface treatment, outer jacket materials such as Pebax or nylon do not adhere reliably to the liner. This increases the risk of delamination under mechanical stress and repeated use. At Hydromer®, Inc., in partnership with jMedtech, we address this bonding challenge using two surface treatments: etching and tie layers. Both work reliably when it comes to bonding the liner to a wide range of catheter jacket materials.
In this article we use Hydromer’s 40+ years of materials expertise to explore the bonding challenges of PTFE liner to catheter jacket materials. Specifically, we will look at why bonding PTFE liners is so difficult, and the ways that the bonding challenge is overcome. We will cover things such as PTFE’s non-stick behavior, how medical devices like catheters are constructed, and why bonding fails when the surface is left untreated. Our goal is to compare the common jacket materials and understand the available bonding solutions to solve this challenge, such as sodium etching, plasma treatment, and tie layers.
The Role and Benefits of PTFE Liners in Modern Catheters
A PTFE liner is the innermost layer of a catheter. It forms the inner lumen, which is the open channel that runs through the full length of the device. Medical devices, such as guidewires and other instruments, biological fluids, and medications pass through this lumen during an operational procedure.
Due to it’s low coefficient of friction (CoF), the liner creates a highly lubricious inner lumen.
Because of this a PTFE liner ensures an inner lumen with an extremely low coefficient of friction, close to 0.04. This is among the lowest CoF of any solid material. Such a low-friction inner surface lets the guidewire and other components move through the catheter smoothly and with very little resistance.
Manufacturers, such as Hydromer® and jMedtech manufacture ultra-thin wall PTFE liners. These play a big role in being able to create low catheter profiles while maintaining the diameter of the inner lumen. This is because thinner liner walls maximize lumen space without increasing the outer diameter. This is critical in neurovascular and coronary devices where precise tracking through narrow vessels is required.
The Bonding Challenge of PTFE Liners: Why PTFE Is So Difficult to Bond
PTFE liner bonding challenges exist because of the material’s very low surface energy. Surface energy is the property of a material that decides how well a liquid, such as an adhesive or a molten polymer, spreads across a surface and sticks to it. A high surface energy surface pulls the liquid in and lets it wet the surface. PTFE does the opposite. Its surface energy is around 18 to 20 mN/m, which is one of the lowest of any solid material.
The Science Behind PTFE’s Non-Stick Behavior
PTFE consists of a carbon and fluorine backbone, with each carbon atom surrounded by tightly packed fluorine atoms. The carbon-fluorine bond is one of the strongest single bonds in organic chemistry.
This dense fluorine shell shields the backbone, resulting in a smooth, non-polar, and chemically inert surface with few reactive sites for adhesives or other polymers to bond.
All of these factors explain the reason why PTFE liners are difficult to bond to. So why does this create an issue? It is because the PTFE liner is the inner-most layer of the catheter, and the layers on top of it need to be bonded to it.
Construction of a Typical Catheter
Nowadays, catheters are made up of multiple, different layers. Each of them does a specific job.
- Inner Liner: The liner forms the inner lumen and is usually a thin-walled PTFE liner. It gives the lumen a smooth, low-friction surface so that guidewires, fluids, and devices pass through with minimal resistance. The outer layer of the liner is treated to ensure it bonds perfectly to the rest of the wall.
- Reinforcement Layer: It is made up of a braid or coil and sits over the liner to add strength and control. Common braid materials include stainless steel, Nitinol, tungsten, and aramid, with braid density ranging from about 20 to 200 picks per inch. The reinforcement layer plays a crucial role in ensuring optimal kink resistance, pushability, and torque transmission.
- Outer Jacket: The jacket is the outermost polymer layer that gives the catheter its shape, stiffness profile, and feel. It is commonly made from Pebax, nylon, TPU, or similar thermoplastics, often in multiple durometers along the length. The jacket flows around the reinforcement and bonds to the treated liner during assembly.
A catheter that consists of all three layers offers a perfect balance of lubricity, flexibility, strength, and torque control. Please note that a reinforcement layer is embedded between the inner liner and outer jacket; the mechanical integrity of the whole wall depends on the jacket’s bonding to the liner.
Bonding Characteristics of Common Catheter Jacket Materials
A catheter’s outer jacket is bonded directly to the treated PTFE liner. As such, both the jacket material and PTFE liner surface treatment play a critical role in the overall success of the catheter during operational procedures.
Below are the most common jacket materials and how each one bonds to PTFE.
1. Pebax
Pebax is the most common jacket material in modern catheters. It is a polyether block amide that comes in a wide range of durometers, from soft 25D grades to stiff 72D grades. This range lets designers vary the stiffness along the length of a single catheter. It is widely used in neurovascular and coronary devices.
Pebax and PTFE Liner Bonding
Pebax bonds well to PTFE when the liner surface is etched. It softens and flows at a relatively low reflow temperature. In turn, it wraps around the reinforcement and joins the treated liner cleanly.
Without etching, Pebax does not adhere to the PTFE liner, and the layers separate. Pebax-based chemistries are also common in tie layers, which makes it a convenient match for PTFE liners.
2. Nylon
Nylon, commonly known as polyamide, is used where a catheter needs more stiffness and column strength. Common grades include PA12, PA11, and PA6. It is stiffer than most Pebax grades. In turn, it improves pushability and shaft support. Nylon is often used in the proximal sections of a catheter.
Bonding to PTFE Liner
Nylon bonds to etched PTFE. However, it needs a higher reflow temperature than Pebax because it melts at a higher temperature. It should be noted that nylon is hygroscopic, which means it absorbs moisture from the air. As such, the material should be dried before reflow, since trapped moisture has potential to cause voids and weak spots at the bond line.
With proper drying and an etched liner, nylon forms a strong and durable bond.
3. TPU (Thermoplastic Polyurethane)
TPU is a thermoplastic mainly used in catheters due to its flexibility and abrasion resistance. It comes in a wide durometer range and is valued for its toughness and kink resistance. TPU is often used in soft distal segments and in devices that flex repeatedly. It also bonds well to many other materials.
TPU and PTFE Liner Bonding
TPU adheres well to PTFE once the liner is surface-treated. It works with thermal reflow and also responds well to adhesive bonding and tie layers. Its lower processing temperature gives some flexibility in how the bond is formed.
It is one of the easiest jacket materials to bond to an etched PTFE liner.
4. Polyurethane
Polyurethane (PU) is a flexible polymer known for its toughness and biocompatibility. It is used in jackets and soft segments where flexibility matters. PU is generally considered biocompatible under normal operating conditions. This makes it suitable for medical devices in contact with blood and tissue.
It is available in both thermoplastic and thermoset forms.
Polyurethane (PU) and PTFE Liner Bonding
Polyurethane bonds well to etched PTFE. The PU’s chemistry is also helpful in the adhesives and tie layers that join PTFE to other materials, so it has a natural affinity for treated fluoropolymer surfaces. With an etched liner, PU forms a reliable bond through reflow or through an adhesive.
This dual role, as both a jacket and a bonding chemistry, makes it a flexible choice in catheter construction.
Catheter Jacket Materials Comparison Chart
| Material | Key Properties & Durometer Range | Typical Application Areas | PTFE Liner Bonding Performance | Processing & Reflow Requirements |
| Pebax | • Polyether block amide • Wide range: 25D (soft) to 72D (stiff) • Enables variable stiffness shafts | • Neurovascular devices • Coronary devices • Outer jackets & tie layers | • Excellent (with etched PTFE) • Softens and flows cleanly around reinforcement. • Fails completely without etching. | • Relatively low reflow temperature. • Flows easily during thermal processing. |
| Nylon (Polyamide) | • Common grades: PA12, PA11, PA6 • High column strength & stiffness • Hygroscopic (absorbs moisture) | • Proximal catheter sections • High-support shafts • Pushability segments | Strong & Durable Bond • Forms an ultra-reliable bond when the base liner is correctly chemically etched. | • Higher melting point; requires higher reflow temps. • Must be thoroughly pre-dried to avoid voids. |
| TPU (Thermoplastic Polyurethane) | • Wide durometer range • Exceptional flexibility • High kink & abrasion resistance | • Soft distal segments • Repeatedly flexing components | Excellent / Easiest to Bond • Adheres strongly to treated PTFE. • Highly compatible with multiple methods. | • Lower processing temperature. • Supports thermal reflow, adhesive bonding, & tie layers. |
| Polyurethane (PU) | • Thermoplastic or thermoset forms • High physical toughness • Superior biocompatibility | • Jackets & soft segments • Direct blood/tissue contact zones | Highly Reliable Bond • Natural chemical affinity for treated fluoropolymer surfaces. | • Exceptionally versatile. • Functions as both a jacket material and a bonding chemistry. |
PTFE Bonding Challenges During Catheter Manufacturing
There is a major reason why PTFE liner bonding challenges exist. Unlike the jacket material, PTFE does not melt and flow during reflow. It stays solid while the jacket melts around it.
As a result, the bond depends entirely on the etched surface holding up through the heat. The process window is narrow.
- Too little heat and the jacket does not flow into the bond.
- Too much heat and the thin liner wall can distort, or the etched layer can degrade.
Additionally, the bonding of a PTFE liner to a catheter jacket requires specialized processes. It is crucial to maintain different variables within the predefined range during manufacturing. Any problem with any one of them shows up at the bond line.
What to Keep an Eye Out For
Here is a quick list of things that must be carefully monitored during production to overcome bonding challenges of PTFE liners:
- Etch quality and uniformity
- Freshness of the etched surface
- Reflow temperature
- Heating dwell time
- Heat distribution along the shaft
- Jacket moisture content
- Reflow pressure
- Layer concentricity
- Cooling rate
Failure to control the above parameters leads to potential weak bonds, voids, or delamination at the liner-to-jacket interface.
So how are the bonding challenges of PTFE overcome? That is what we will cover next.
Top 3 Methods Used to Bond PTFE Liners to Different Types of Catheter Jacket Materials
If a liquid adhesive is applied to an untreated PTFE surface, it beads up instead of spreading out. As a visualization, think of water on a waxed surface. For a reliable bond, the surface energy of the substrate usually needs to be higher than that of the adhesive, often above 36 to 40 mN/m. PTFE sits far below this range, so the adhesive cannot make proper contact.
As such, manufacturers have to raise the surface energy of the liner in order to overcome PTFE liner bonding challenges. The reason is so the jacket or the adhesive can wet it and form a real bond.
Below are a few methods to do so, and properly bond to the PTFE Liner:
1. Sodium Etching
Sodium etching is the most common method for treating PTFE liners. As such, it is commonly used to overcome PTFE Liner Bonding Challenges. It uses a sodium-based chemistry, most often a sodium naphthalene solution, to change the PTFE’s surface properties. The goal is to strip the fluorine atoms from the surface of the PTFE through a chemical reaction. It leaves behind a carbon-rich layer with reactive sites that an adhesive or a molten jacket can bond to.
Etching affects only the top few angstroms of the surface. In turn, the bulk properties of the liner stay the same. The treated surface turns a brown or tan color, and its surface energy rises significantly.
At Hydromer® and jMedtech, we precisely apply etching to the outer surface of the PTFE liner. This helps ensure that the inner lumen keeps its low-friction surface. Our etched PTFE liners bond reliably to jacket materials such as the Pebax, nylon, TPU, and PU.
2. Plasma Surface Treatment
This method is the dry alternative to chemical etching for PTFE liners. It uses an ionized gas, or plasma, to modify the PTFE surface. The plasma breaks some of the surface bonds and adds reactive functional groups, which raises the surface energy.
The plasma surface treatment alters the PTFE surface without harsh wet chemicals, so it is considered to be a cleaner process.
Such treatment heavily relies on precise control of gas, power, and exposure time. Additionally, the treated surface can lose its activity faster than a sodium-etched surface, so parts are usually bonded soon after treatment.
3. Tie Layers
In this method, a thin intermediate layer known as the tie layer is placed between the PTFE liner and the jacket. It is made from a material that bonds to the PTFE on one side and to the jacket on the other.
The tie layer acts as a bridge so two materials that would not bond directly can be joined through it. This is useful when the jacket chemistry does not match the liner well.
Tie layers are often a thin thermoplastic film. Hydromer®, with jMedtech, offers a tie layer method as one of its two PTFE liner surface treatments. Our MorphoLiner® casted liner takes this further, with a thin thermoplastic tie layer built into the liner during the dip coating process. This improves bonding performance directly.
Comparison of PTFE Liner Bonding Methods
| Bonding Method | Working Mechanism | Key Advantages | Key Considerations | Compatible Materials / Applications |
| 1. Sodium Etching | Uses a sodium naphthalene chemical solution to strip fluorine atoms, leaving a carbon-rich layer with reactive sites. | • Most common method. • Only affects the top few angstroms (bulk properties stay the same). • Inner lumen retains its low-friction surface. | •Wet chemical process. • Changes the treated surface to a brown or tan color. | Bonds reliably to Pebax, nylon, TPU, and PU. |
| 2. Plasma Surface Treatment | Uses an ionized gas (plasma) to break surface bonds and add reactive functional groups. | • Clean, dry alternative to chemical etching. • Eliminates the need for harsh wet chemicals. | • Requires precise control over gas, power, and exposure time. • Surface loses its bonding activity quickly; parts must be bonded soon after treatment. | Ideal when a cleaner, non-chemical process is preferred for jacket materials. |
| 3. Tie Layers | Places a thin intermediate thermoplastic film to act as a physical “bridge” that bonds to the PTFE on one side and the jacket on the other. | • Perfect for when jacket chemistry does not match the liner well. • Can be built directly into the liner during dip coating (e.g., MorphoLiner®). | • Introduces an extra intermediate material layer between the liner and jacket. | Used as a reliable alternative surface treatment when direct bonding fails. |
Key Takeaways: Surface Treatment Options to overcome PTFE Liner Bonding Challenges
- Sodium Etching remains the industry workhorse for standard jackets like Pebax and Nylon
- Plasma Treatment offers a chemical-free alternative, although it loses its activity faster than a sodium-etched surface
- Tie Layers (like MorphoLiner®) provide an elegant, built-in solution for otherwise incompatible jacket materials.
What is Catheter Reflow?
Catheter reflow is the process that fuses the separate layers of a catheter into one continuous device. In this process, the liner, the reinforcement layer, and the jacket are assembled, and then heated until the jacket melts and flows.
The treated PTFE liner is placed over a mandrel, the braid or coil is added over the liner, and the jacket is positioned over the reinforcement. An outer layer of heat-shrink tubing is then placed over the whole assembly. This heat shrink is a temporary processing aid and is removed after the process is complete.
During reflow, the assembly is heated in a controlled way. The jacket softens and melts first. As the heat shrink contracts, it squeezes the molten jacket inward. This pressure forces the jacket to flow around the reinforcement and down onto the etched surface of the liner. The treated liner surface lets the jacket wet it and form a bond.
In short, it is the step where the bond between the PTFE liner and the outer jacket is actually formed.
Hydromer®: Advanced Coatings, Liners, and Tubings Manufacturer
Hydromer®, Inc. is a leading hydrophilic coatings manufacturer and supplier with over 40 years of experience. We began with hydrophilic coatings and contract coating services. Recently, through a strategic partnership with jMedtech, we have expanded our portfolio significantly. We now also offer advanced PTFE liners and tubings as well as automated coating equipment and friction testing equipment.
Our PTFE Liner Products include:
- MatrixLiner® Free Extruded Liner
- MatrixLiner® Extruded OTW Liner
- MorphoLiner® Casted OTW Liner
We are a trusted partner to leading medical device manufacturers. Our PTFE liners are fully customized to meet your project’s specific size, strength, surface, and regulatory requirements.
5 Top Advantages of Choosing Hydromer®, Inc. PTFE Liners
Hydromer uses three advanced processes to manufacture PTFE liners: Free Extrusion, Over-The-Wire (OTW) Extrusion, and Dip Coating (Film Casting). Each process is selected based on the required size, strength, and flexibility of the device. The top five benefits of choosing Hydromer PTFE liners are:
- Industry-Leading Surface Etching: Hydromer treats the liner with a high-quality etch to ensure adhesion and performance at its best.
- Multiple Surface Treatment Options: Both etching and tie layers are available, so the surface preparation can be matched to the jacket chemistry. The MorphoLiner® casted liner also carries a thin thermoplastic tie layer built in during dip coating, which improves bonding directly.
- Three Liner Processes: Free Extrusion, OTW Extrusion, and Dip Coating each offer a different balance of strength, flexibility, and wall thickness.
- Consistent, Clean Production: Liners are produced in a Class 10,000 cleanroom with tight dimensional control. A clean surface and a uniform etch widen the bonding process window and help ensure repeatable results during reflow.
- Vertical Integration and Lumen Lubricity Portfolio: Beyond liners, Hydromer offers hydrophilic coatings and other lumen lubricity options, along with engineering support, material customization, prototyping, testing, and validation.
Hydromer®’s PTFE liners, hydrophilic coatings, coating equipment, and contract services can support your product and production needs.
FAQ: PTFE Liner Bonding
PTFE Liner bonding challenges result from the material’s very low surface energy and chemical inertness. PTFE has a highly, non-stick surface, which causes adhesives and jacket polymers to bead up vs spread out and adhere to.
Manufacturers typically use three types of surface modification, including:
1. Sodium etching: uses a sodium naphthalene chemical solution to strip fluorine atoms, leaving a carbon-rich layer with reactive sites.
2. Plasma surface treatment: uses an ionized gas (plasma) to break surface bonds and add reactive functional groups.
3. Tie layers: places a thin intermediate thermoplastic film to act as a physical “bridge” that bonds to the PTFE on one side and the jacket on the other.
Sodium etching is a wet chemical process that uses a sodium naphthalene chemical solution to strip fluorine atoms, leaving a carbon-rich layer with reactive sites. It provides long-lasting surface activity for manufacturing flexibility. Plasma surface treatment is a clean, dry process that uses an ionized gas (plasma) to break surface bonds and add reactive functional groups. It loses its bonding activity quickly; parts must be bonded soon after treatment.
Outer jacket materials will not adhere to an untreated PTFE liner. Due to PTFE’s low surface energy, the jacket material will not wet the surface. This causes issues such as weak bonding, voids, and the risk of delamination between the liner and jacket.
Pebax, nylon, TPU, and PU all bond reliably to properly treated PTFE liners.
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Please contact Hydromer® to learn more about our PTFE liners or to request samples.
