How ToughFlex™ and ElastiFlex™ PTFE Liners Affect Kink Resistance, Pushability, and Torque Response

ToughFlex™ is Hydromer®’s high yield strength series, and ElastiFlex™ is the high elongation series with elongation of 800% or more.

PTFE Liners form the innermost layer of medical devices such as catheters. Its unique combination of mechanical, chemical, and thermal properties makes it the preferred choice for devices navigating the human body. 

PTFE, also known as polytetrafluoroethylene, is a soft fluoropolymer with extremely low coefficient of friction, which enhances pushability, torque response, and kink resistance.

In this article, we will discuss how catheter performance heavily depends on PTFE liners. Our focus is on learning what pushability, torque response, and kink resistance mean, and how the PTFE liner influences each of them. At the end, we have also compared the ToughFlex™ and ElastiFlex™ PTFE liner series by Hydromer®. ToughFlex™ is the high yield strength series, and ElastiFlex™ is the high elongation series with elongation of 800% or more. Both are available with walls as thin as 0.0005 in (0.0127 mm). If you are a biomedical engineer, medical device manufacturer, or procurement professional working with catheters, this guide is for you.

Key Takeaways:

• A PTFE liner forms the innermost layer of medical devices such as catheters.
• ToughFlex™ is the high yield strength series, and ElastiFlex™ is the high elongation series with elongation of 800% or more.
• ToughFlex™’s yield strength serves pushability and torque response.
• ElastiFlex™’s elongation improves kink resistance and trackability.

The Role of PTFE Liners in Catheter Procedures

Catheters must navigate narrow, winding blood vessels to reach their target. Physicians control this movement externally by pushing, pulling, and rotating the hub. Each input must transfer efficiently down the shaft without harming surrounding tissue. The PTFE liner is essential to achieving this performance.

Guidewires, stents, coils, and balloons move along the liner surface inside the catheter. PTFE’s low coefficient of friction allows these devices to advance and retract with minimal force, resulting in smoother exchanges and improved tip control. This enables physicians to focus on the procedure rather than managing catheter resistance.

PTFE Liner Properties that Improve the Catheter Mechanical Performance

PTFE provides a unique set of properties to the catheter’s innermost layer. The following five are most important for mechanical performance, particularly pushability, torque response, and kink resistance.

1. Low Coefficient of Friction

PTFE has one of the lowest coefficients of friction among all solid materials. As such, guidewires and devices slide through the lumen with minimal resistance. Less force is wasted inside the catheter, and more of the physician’s input reaches the tip.

2. Natural Flexibility from a Low Flexural Modulus

PTFE is a soft polymer with a flexural modulus of about 0.5 GPa. That is far below the stiffness of the metal braid and most jacket materials used in the shaft. The liner bends with the catheter instead of fighting it. This leaves the reinforcement layer and the jacket free to set the stiffness profile of the shaft.

3. Extreme Elongation

Standard PTFE stretches between 300 and 550% of its original length before breaking, as per typical published values. The material deforms instead of fracturing when placed under strain.

When the shaft bends tightly, the liner stretches along the outer curve without cracking. Specialized liner series can achieve even greater elongation, which we will compare later in this article.

4. Thermal Stability

PTFE melts at approximately 327 °C and can withstand continuous service temperatures up to 260 °C, both well above the reflow temperatures of common jacket polymers.

During reflow lamination, the jacket melts and flows around the braid while the liner holds its shape. The lumen stays smooth and open through the entire assembly process.

5. Ultra-Thin Wall

A few manufacturers, such as Hydromer®, use modern free extrusion to produce liner walls as thin as 0.0005 in (0.0127 mm). The liner adds almost nothing to the overall wall stack of the catheter.

Thin walls preserve the inner diameter for a given outer profile, leaving more room for devices to pass through. A thinner liner also contributes less bending stiffness, helping ensure the shaft stays trackable in tortuous anatomy.

Typical Properties of PTFE Liners or Tubing

Property	Typical Values	Impact on Catheter Performance
Coefficient of friction (dynamic)	0.05 to 0.10	Smooth guidewire and device passage
Flexural modulus	About 0.5 GPa	Bends with the shaft instead of resisting it
Elongation at break	300 to 550%	Survives tight bends without cracking
Tensile strength	20 to 35 MPa	Withstands axial and processing loads
Melting point	About 327 °C	Holds shape during jacket reflow
Continuous service temperature	Up to 260 °C	Stable through assembly and sterilization heat

The values above are typical published ranges for virgin PTFE and are provided for reference. Actual liner performance varies with resin grade, processing method, and wall thickness, so always confirm the exact specifications with your supplier as per your project.

The Relationship Between Pushability, Torque, and Kink Resistance

These three properties are the main pillars of catheter mechanical performance, each describing how the shaft responds to physician input:

• Pushability: It is the ability of the shaft to transfer axial force from the hub to the tip. Since most catheters advance over a guidewire, part of this force is spent overcoming friction inside the lumen. A catheter with good pushability advances without buckling or wasting energy along the way.
• Torque Response: It is the ability of the shaft to convert rotation at the hub into equal rotation at the tip. This rotation also happens around the guidewire, so the shaft must turn freely without sticking inside the lumen. The goal is to achieve a 1:1 response, so the tip points exactly where the physician steers it.
• Kink Resistance: It is the ability of the shaft to bend around tight curves without the lumen collapsing. A kink-resistant catheter keeps its pathway open even in the most tortuous anatomy.

These three properties are deeply connected, and they often pull against each other. Balancing such trade-offs is one of the core challenges of catheter design. For example,

• A stiffer shaft improves pushability and torque response, but it struggles to follow tight vessel curves.
• A softer shaft tracks easily, but it absorbs push force and twists unevenly under rotation.

The kink resistance is the foundation that protects the other two. Once the shaft kinks, the lumen closes and both push and torque are lost at that point. The catheter must be withdrawn and replaced, which extends the procedure time and increases the risk to the patient.

That said, modern catheters do not rely on a single material to balance these demands. The composite construction splits the job between the liner, the reinforcement layer, and the outer jacket.

How PTFE Liners Affect Catheter Pushability

Pushability is the efficiency with which the axial force travels from the hub to the tip. The physician pushes at one end, and the shaft must carry that force through every curve of the vasculature. Since the catheter advances over a guidewire, the liner surface stays in constant sliding contact with the wire along the entire length.

Below are the three ways the liner preserves the push force inside the lumen:

• Stick-slip Prevention: High lumen friction creates a problem known as stick-slip. The shaft resists movement at first, stores the push force as elastic energy, and then releases it in a sudden jump. Such jumps make the tip motion unpredictable and increase the risk of vessel trauma.
• Compression Resistance: This is the second contribution of the liner. Pushing places the shaft under axial compression, and in tight bends the guidewire presses hard against the inner curve of the lumen. A liner with sufficient yield strength holds its round shape under this local pressure. If the liner flattens, the lumen narrows at that point and the drag multiplies.
• Wall Uniformity: It also matters more than most can expect. A consistent liner wall maintains the same clearance between the wire and the lumen from the hub to the tip. Thick spots create tight zones that act like brakes on the wire, while thin spots risk deformation under load.

How PTFE Liners Affect Torque Response

The catheter does not rotate in open space. It rotates around a guidewire that runs through the entire lumen. If the shaft sticks on the wire, the input does not reach the tip immediately. The shaft winds up, stores the rotation as torsional energy, and then releases it in one sudden spin. This behavior is known as torque whip, and it makes precise steering nearly impossible.

Below are the four ways the liner supports the rotation of the shaft:

• Free Rotation Around the Guidewire: During steering, the shaft must turn around the stationary wire without grabbing it. The low friction of PTFE keeps this interface loose, helping ensure the tip follows the hub input smoothly and immediately.
• A Stable Foundation for the Braid: The braid is wound directly over the liner during manufacturing. A round and dimensionally consistent liner holds the braid wires at their intended angles. The braid transmits torque efficiently only when this geometry stays intact along the shaft.
• Resistance to Wrinkling Under Torsion: Twisting places the liner wall under shear. A liner that wrinkles creates ridges inside the lumen, and these ridges grab the wire during rotation. Sufficient elongation lets the liner follow the twist without surface damage.
• Unified Rotation of the Composite Wall: The liner, braid, and jacket must rotate as a single unit. The etched outer surface of the liner allows it to bond firmly into the composite during reflow. If the layers slip against each other, the tip lags behind the hub, and the 1:1 response is lost.

Note that a perfect braid cannot compensate for a poor liner. The braid sets the torsional strength of the shaft, but the liner keeps the geometry round, the layers coupled, and the wire free.

How PTFE Liners Affect Kink Resistance

Kink resistance is tested every time the catheter enters a tight vessel curve. As the bend radius shrinks, the cross-section of the shaft flattens from a circle into an oval. Past a critical point, the wall buckles inward, and the lumen closes. This is a kink, and in a composite shaft the crease is usually permanent.

The consequences are immediate. The guidewire jams at the kink point, injections stop, and the catheter must be withdrawn and exchanged.

Below are the three ways the liner keeps the lumen open in tight anatomy:

• High Elongation on the Outer Curve: Bending stretches the wall along the outer side of the curve. A liner with high elongation follows this stretch without cracking or tearing.
• Thin Walls that Reduce Bending Strain: The strain in the liner wall grows with its thickness at a given bend radius. A thinner liner experiences less stretch in the same curve, helping ensure the shaft reaches tighter bends before any layer reaches its limit.
• Support Against Ovalization: The lumen flattens before it buckles. The liner works together with the braid or coil to hold the cross-section round, and a liner that resists local deformation delays the flattening that leads to a kink.

ToughFlex™ and ElastiFlex™ by Hydromer for Enhanced Catheter Performance

Hydromer®, Inc., with its manufacturing partner jMedtech, offers free extruded PTFE liners in two proprietary series. Both belong to the MatrixLiner® family and are produced in a full Class 10,000 cleanroom environment. Each series is fully customizable to the needs of the application.

The two series exist because of the trade-off we discussed earlier. A single liner formulation cannot maximize compression resistance and stretchability at the same time. As such, the series lets the design team bias the liner toward the performance pillar that matters most for their device.

ToughFlex™: The High Yield Strength Series

ToughFlex™ is engineered for yield strength. The liner successfully withstands external compression and internal fluid pressure in clinical environments such as vascular interventions and catheter delivery, minimizing deformation or rupture.

In practice, this strength serves pushability and torque response. The liner holds its round shape where the guidewire presses into tight bends, and it gives the braid a stable foundation that keeps the shaft geometry intact.

For shafts where compression resistance decides the performance, ToughFlex™ is the best choice.

ElastiFlex™: The High Elongation Series

ElastiFlex™ is engineered for stretching. The series reaches an elongation of 800% or more, which sits well beyond the 300 to 550% typical of standard PTFE. It sustains bending and tensile loading without brittle failure.

This stretchability improves kink resistance and trackability. The liner accommodates strain along curves without cracking, helping keep the lumen open in complex anatomy.

ElastiFlex™ is recommended for shafts that must withstand tight and repeated bends.

	ToughFlex™ (TF)	ElastiFlex™ (EF)
Purpose	High Yield Strength	High Elongation
Headline property	Yield ratio > 0.65	Elongation at break of 800% or more
Mechanical behavior	Resists compression and internal pressure, minimizing deformation or rupture	Sustains bending and tensile loading without brittle failure
Force-displacement behavior	Reaches high force early and holds it through displacement	Builds force gradually across a much longer displacement range
Primary performance pillars	Pushability and torque response	Kink resistance and trackability
Wall thickness capability	As thin as 0.0005 in (0.0127 mm)	As thin as 0.0005 in (0.0127 mm)
Customization	Fully customizable per application	Fully customizable per application

Conclusion: Selecting the Right Liner Series

Pushability, torque response, and kink resistance are determined by the catheter’s inner layers, particularly the liner. The choice of liner directly affects how much of the physician’s input reaches the tip.

We can help you select the appropriate liner series for your device’s anatomical and performance requirements. Both ToughFlex™ and ElastiFlex™ are fully customizable for your project. 

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