Introduction to PTFE and thin-walled catheter lining

2021-11-13 09:21:01 By : Ms. Ailsa Wang

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Polytetrafluoroethylene (PTFE) is a very popular and versatile fluoropolymer. Teflon has been working with ZEUS since the 1930s, when it was accidentally developed by technicians working for DuPont. PTFE is considered a unique fluoropolymer because it is fully fluorinated and consists only of fluorine and carbon atoms, as shown in Figure 1.

This structure provides many ideal and unique characteristics for PTFE. In fact, according to the latest estimates, by 2020, the global PTFE market is expected to reach 6.44 billion U.S. dollars, with an annual growth rate of 8.1% [1]. This means that the global consumption of PTFE in the next three years is estimated to be 524.1 kilotons [1]! PTFE is mainly referred to as Teflon® and is also widely known as a non-stick coating for cookware, but as this market overview shows, PTFE has indeed become a mainstay in many areas of the modern world.

Figure 1. Classical description of PTFE monomer shown in Lewis structure (left), bond line diagram (middle), and three-dimensional representation (right).

PTFE is a homopolymer of tetrafluoroethylene (TFE), which is integrated by tetrafluoroethylene monomer through free radical reaction (Figure 2). TFE polymerization is catalyzed by the peroxide added in the reaction, and the resulting reaction is self-sustaining until it is quenched or the reactants are exhausted. The result of this reaction is a long chain (high molecular weight) molecule with considerable strength. The PTFE chain length can be widely controlled, so it can be manipulated to suit specific applications. However, unlike most saturated carbon compounds, PTFE polymer chains are generally inflexible; they produce rigid rod-shaped molecules with unique chemical properties.

Figure 2. General reaction scheme for the synthesis of PTFE.

The uniqueness of PTFE comes from its multiple CF bonds. As we all know, of any element in the periodic table, fluorine has the highest electronegativity-its attraction to electrons, including electrons that participate in bonding [2, 3]. Compared with fluorine, carbon contains moderate electronegativity, so the CF bond is greatly polarized toward fluorine. In addition to the covalent sharing of electrons between fluorine and carbon, there is also considerable electrostatic attraction that pulls part of the positive carbon (Cδ) toward the part of the negative fluorine (Fδ-).

Therefore, the covalent nature of the CF bond and its partial electrostatic properties result in an extremely short CF bond, and this effect is compounded with multiple CF bonds in PTFE through multiple dipole interactions. The CF bond length in PTFE is usually about 1.32 Å, and the CF bond length is usually 1.35 Å [4]. This length is shorter than other carbon-halogen bonds, and shorter than CO and CN bonds. (Only the C-Si bond is shorter than the CF bond). The result is that PTFE packs tightly along the CC backbone, but in an unexpected way.

Polytetrafluoroethylene is a close-packed long-chain polymer. However, PTFE does not adopt the typical zigzag pattern observed with most saturated carbon chains, but a spiral shape (Figure 3A). Generally, larger substituents on adjacent carbon atoms along the carbon chain backbone will place themselves in the anti-conformation (Figure 4A). However, due to the large size of fluorine atoms and the high electron density resulting from their electronegativity, the fluorine atoms on adjacent carbons of PTFE cannot form a true staggered and reverse conformation. In contrast, the fluorine atoms are positioned along the carbon skeleton in a helical conformation, and the dihedral angle along the CC atoms is slightly less than 60° in the staggered anti-conformation (Figures 3B and 4).

Figure 3. (A) Typical zigzag conformation of saturated hydrocarbon chain. (B) PTFE spiral geometry with fluorine atoms around the carbon chain skeleton.

This rare phenomenon of PTFE is not due to the gauche effect proposed in the small fluorinated alkanes, but due to the hyperconjugation and interaction between the CC bond and the CF anti-bonding orbital (σCC→σ*CF) of PTFE (Figure 5) [ 5]. The energy loss caused by hyperconjugation exceeds the energy loss caused by the smaller dihedral angles of adjacent CC atoms. With every increase of CF2, these energy benefits will become more prominent. The result is a tighter-packed spiral PTFE with excellent stability. The helical conformation of the PTFE selected in the zigzag conformation is so stable that it has also been observed in perchlorinated alkanes [5]. (It should be noted that based on pressure and temperature, PTFE exists in at least two other phases or transitional conformations of helicity changes, in which the second phase dominates near and below room temperature [6-8]. (These phases I won’t go into details here).

Figure 4. Newman projection: geometric comparison of small fluorinated alkanes and PTFE. The staggered anti-conformation of 1,2-difluoroethane, C2H4F2 (left), the gauche conformation of 1,2-difluoroethane (middle), and the zigzag staggered conformation of PTFE (right).

Figure 5. Schematic diagram of PTFE spiral geometry: the exaggerated (quasi) gauche conformation of PTFE (left), also showing σCC→σ*CF orbital interaction and hyperconjugation (right).

The close-packed nature of polytetrafluoroethylene and its fully bonded carbon chain give this polymer a variety of desirable properties, making it meet today's needs. The three lone pairs of fluorine electrons balance the overall dipole moment regardless of its CF bond and its high electron density. This means that the fluorine atoms of PTFE are not polarizable. The fluorine atom of PTFE also has a complete electron valence shell octet. These two aspects make the fluorine of PTFE a poor electron donor and a poor hydrogen bond acceptor. The result is a non-reactive fluorine sheath formed around the CC backbone of PTFE, which makes PTFE very resistant to chemicals. (However, PTFE may still be affected by specific conditions involving halogenated compounds or alkali metals).

The non-reactive fluorine sheath of PTFE also has other important meanings. The outer dense fluorine "capsule" of PTFE has a high electron density and also repels other PTFE chains. Therefore, the coefficient of friction of the PTFE surface is very low, usually reported in the range of 0.02-0.08. The non-reactivity of PTFE to even small molecules (such as water or other PTFE chains) means that PTFE hardly adheres to anything. These characteristics translate into excellent lubricity for specific applications.

Polytetrafluoroethylene is also considered a rigid molecule. Unlike carbon chain polymers that are less saturated with halogens or simple hydrocarbon chains (such as polyethylene), PTFE does not allow free rotation around the CC backbone single bonds. The electron density and the space required for the electrons of the fluorine atom lead to steric repulsion through the ortho position and the 1,3-position. Despite the existence of hyperconjugation, this repulsion makes the rotational energy barrier of the CC single bond very disadvantageous. The result is a conformationally inflexible PTFE polymer chain.

PTFE can be manufactured in various products. The chemical and mechanical properties of PTFE make these products useful in a variety of applications. PTFE can be developed into molded parts; used as flexible pipe joints, electrical insulators, bearings, valve bodies, gears; and extruded into pipes (Figure 6). PTFE can be processed into precision parts with very fine tolerances, can be made into thin sheets or films, and can also be heat-shrinked. The tight bonding characteristics of PTFE show excellent wear resistance and excellent lubricity.

Figure 6. Examples of PTFE extruded products: (A) uncolored and (B) pigment tube, (C) PTFE heat shrinkable tube with spiral color, (D) PTFE monofilament (close-up view) and (E) Zeus PTFE Sub -Lite -Wall® tube. (The image is not to scale).

With properties such as chemical resistance and lubricity, it is no wonder that PTFE has found its way in medical applications, where these properties are particularly needed. As early as the 1970s, PTFE has a long history of successful and safe use in the medical industry [9]. PTFE can be extruded into a tube with a very thin wall, making it very suitable for vascular catheter components, where uniform and small diameters are the most important. For example, PTFE can be used to guide the inner wall (bottom liner) of the catheter to provide a very smooth inner surface. The smooth PTFE inner diameter (ID) of these catheters reduces friction with different catheter technologies, such as balloons, stents, or plaqueectomy devices, because they are pushed through the tight confinement of the catheter lumen. If the catheter ID does not have sufficient lubricity, devices such as stents may collapse in an accordion-like manner when pushed through the catheter lumen. The lubricity effect of the increase in the inner diameter of the catheter is that the deployment force of the catheter device when passing through the lumen is reduced, thereby increasing the likelihood of a successful operation.

Dip coating is a competing technology to obtain thin polymer walls on components. This process is performed to impart some surface properties or qualities to the coated part, such as increased stiffness or lubricity. In the example described here, dip coating can be performed on the mandrel as part of the catheter construction process. Then, once the coating is cured, the other components of the catheter, including the nylon sheath, braiding, and sheath reflow process steps, will be built on the cured dip-coated mandrel (Figure 7). After completing the catheter construction, the mandrel is then taken out of the newly constructed catheter, leaving the dip coating, which becomes the innermost cavity wall.

Figure 7. Catheter structure: basic catheter components: mandrel, base linear, braided reinforcement, sheath (typical of nylon or Pebax®) and peelable (FEP) heat shrink tubing.

Although dip coating may seem a simple process at first, there are some limitations that prevent it from being widely used in catheter construction. Dip coating may show unevenness similar to the surface of orange peel. Sometimes, there may be many cross-sectional lines called flutter on the dip-coated surface, which are caused by vibration during the coating process. Dip-coated surfaces may also produce pits, depressions and even holes in the cured layer due to contaminants (including moisture) during the coating process. The surface defects of the catheter ID severely hinder the use of the catheter. Although these defects can be resolved in different ways, the superior accuracy of the dip coating process usually requires higher cost and time.

Finally, due in part to the aforementioned shortcomings, dip-coated mandrels used for catheter ID lumen walls may experience poor adhesion. These defects may be in the form of poor adhesion of the lining to the sheath laid on the braid or poor adhesion of the dip coating (liner) to the mandrel itself. Imperfect bonding can lead to delamination defects, which is the most serious defect in catheter linings. This failure can affect basic aspects of the catheter, such as torsional transmittance, deflection, or pushability. The end result of this defect may be the failure of the catheter itself. Although it rarely occurs, delamination defects pose a serious additional risk to the dip coating process.

The demand for smaller equipment has driven the pursuit of dip coating processes for thin-walled extrusions and conduit constructions. These devices belong to the general description of microcatheters and are necessary for minimally invasive surgery (MISP) to navigate very small vasculature. These procedures can be diagnostic, survey, mapping, or interventional in nature. The catheter design strikes a balance between function and structure. The basic goals focus on catheter traceability, flexibility, pushability and torque (transmittance or torsion). Based on the tortuosity of the vasculature being explored, catheter outer diameter (OD) and ID play a role in selecting the appropriate design and equipment for the application. Therefore, the catheter ID wall thickness is a trade-off between the required basic catheter functional properties and the application environment.

Advancing the design of microcatheters to meet the requirements of the cardiac, neurovascular, and peripheral markets is a daunting task. The demand for more functions and smaller devices is ongoing. Based on these requirements, Zeus Industrial Products Inc. is manufacturing their new Sub-Lite-Wall® StreamLiner™ series of thin-walled conduit liners. The wall thickness of Zeus Sub-Lite-Wall® tubing has reached 0.005" (0.127 mm) and below, and it is now made smaller with StreamLiner™ series thin-walled conduit lining extrusions. These special extrusions are made of PTFE and The construction benefits from the long-term success of Zeus’ Sub-Lite-Wall® products. These thin-walled extrusions are produced mainly to allow medical device manufacturers and engineers to better understand the tortuous pathways of the body. Now, with this Market conditions drive access. For smaller and smaller vascular systems, the Sub-Lite-Wall® StreamLiner™ series of PTFE extrusions will become the market standard.

The first product in the Zeus StreamLiner™ series, StreamLiner™ XT, improves on its existing first-class thin-walled conduit lining. StreamLiner™ XT uses Zeus's LoPro™ proprietary technology with a maximum wall thickness of 0.00075 inches (0.01905 mm) and can be developed with IDs as low as 0.013 inches (0.330 mm) (Figure 8). These ultra-small wall catheter linings allow for a smaller outer diameter (OD) of the finished device while also increasing the potential of the catheter lumen. The high lubricity of PTFE also has a special advantage because it leads to an improvement (reduction) in the deployment force in the catheter lumen, which can be used to push the technology through it (fiber optics, cameras, etc.).

Figure 8. Sub-Lite-Wall® StreamLiner™ XT: (A) StreamLiner™ XT squeeze tube, and (B) StreamLiner™ XT schematic, diagram and functional description.

As a free extrudate, StreamLiner™ XT has many advantages over dip coating. Compared with traditional linings, these PTFE linings are tougher and stronger. StreamLiner™ XT will not have pits, depressions or holes that are prone to dip-coated linings. Therefore, the uniformity of StreamLiner™ XT contributes to its ultra-smooth lumen ID surface finish. StreamLiner™ series linings are not prone to delamination associated with dip-coated linings. Together, these improvements enable the Sub-Lite-Wall® StreamLiner™ series to reduce patient risk while providing superior performance and effective dip coating alternatives for successful patient outcomes.

PTFE is a synthetic fluoropolymer that was first discovered in the 1930s and has now developed into a global industry. The fully fluorinated nature of PTFE contributes to the unique helical structure of this polymer to produce a wide range of beneficial properties. Due to its low friction coefficient and excellent chemical resistance, PTFE has entered the medical field and has a long history of successful use in this field. In order to improve PTFE extrusion technology, Zeus Industrial Products Inc. has developed an ultra-thin wall PTFE conduit liner, which is launched as its Sub-Lite-Wall® StreamLiner™ series.

The first product in the series uses Zeus’ proprietary LoPro™ technology StreamLiner™ XT, which has been enhanced on the basis of Zeus’s already first-class wall thickness to create a lining with a wall thickness of <0.00075" (0.01905 mm). As a free extrusion Compression parts, StreamLiner™ series eliminates many competitive risks of dipping technology, such as uneven cavity wall surfaces and delamination. These linings make the finished device stronger or stronger while retaining sufficient functional characteristics such as twistability, Flexibility and pushability. The super-lubricated catheter lumen ID significantly improves the deployment force of successful MISP through the catheter. Sub-Lite-Wall® StreamLiner™ XT allows more access to small blood vessels, including peripheral and neurovascular systems, It also represents a new tool for medical device clinicians and engineers to improve the treatment effect of patients.

This information is derived from materials provided by Zeus Industrial Products, Inc. and has been reviewed and adapted.

For more information on this source, please visit Zeus Industrial Products, Inc.

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