Balloon catheters are usually made of inflated extruded tubes where the tubes are stretched in axial and radial directions in a balloon forming machine. The main properties of balloons are surface smoothness, puncture resistance, and burst strength. These properties are controlled by polymer selection and processing.1
It is important to characterize and study the effects of nylon 12 and Pebax within physical and mechanical balloon properties, especially considering the extent of application of these materials in the bio-medical field. This article explores the differences between the crystalline structure of these two materials, as well as the materials’ physical and mechanical performance when used for balloon catheters.2
Balloon diameters range from 2 to 10 mm and their length ranges from 2 to 10 cm. Diameter varies depending on the material and application. Nylon 12 and Pebax are widely used in balloon applications because of their mechanical properties and biocompatibility.3,4 The properties of formed balloons are dependent on the type of material, extrusion process, and balloon forming program.
Extruding the material into polymer tubes is the first step for manufacturing the balloon. The rheology, molecular structure, and extrusion processing parameters play important roles in producing tubes with different physical and mechanical properties. The tubes are then stretched axially and radially in a balloon forming machine using a specific recipe that includes combinations of temperature and air pressure to make the final balloon.
The room temperature aging of nylon 12 increases the glass transition temperature (Tg), making the samples stiffer due to the cold crystallization during the amorphous phase. There are two crystalline phases of α and γ for nylon 12 for both oriented and nonoriented samples. The crystalline structure of nylon, in addition to the orientation of crystals, contributes to the mechanical properties of polyamide. The γ form is more ductile than the α form; however, a transformation of the γ form to α will occur for nylon 6 if extruded samples are stretched.6
For Pebax, the soft-to-hard segment ratio plays an important role in controlling the physical and mechanical properties of the balloon. The hard segments have been reported to be (50 ± 20) nm wide by (300 ± 150) nm long. The extrusion process is responsible for hard segment orientation along the axial direction. During the balloon forming process, the crystalline structure is broken and rearranged into smaller crystals and becomes more oriented. The γ crystals transform to α crystals during stretching.8
Grilamid L25 (nylon 12) from EMS and Pebax® 6333 SA01 MED (a thermoplastic elastomer from Arkema) tubes were produced using a one-inch extruder. Balloons with dimensions of 6 × 100 mm were produced using a Confluent Medical Technologies balloon forming machine. Dynamic mechanical analyzer (DMA) tests were performed in tension mode.
To evaluate orientation and characterize phase formation, FTIR experiments were carried out by recording infrared spectra with a resolution of 4 cm–1 and an accumulation of 16 scans. Thermal properties were analyzed using a differential scanning calorimeter (DSC).
The samples were heated from –30 to 200 °C at a heating rate of 10 °C/min, then cooled at the same rate to –30 °C and heated again to 200 °C. Tensile tests were performed with a crosshead speed of 127 mm/min. Puncture tests were performed based on ASTM F1306 with a probe size of 1.6 mm diameter.
The viscosity data was extracted from the data provided by the suppliers, and the viscosity curves (viscosity versus shear rate at 230 °C) are plotted in Figure 1. The polymer viscosity is an important criterion for the extrusion process because it correlates with pressure applied at the die. The pressure at the die determines the consistency and stability of the melt flow and is important because it maintains the dimensions for the extruded tube.
The extrusion process for tubes is usually designed for a shear rate range of 100–1000 s –1. For this study, the range is between 400 and 800 s–1 at the die, where shear thinning behavior is more pronounced for nylon than for Pebax.9
In the extrusion process, materials are melted and then enter a water bath where they crystallize and form a morphology. The morphology and orientation within each tube determines the physical and mechanical properties. The Pebax as is a copolymer of nylon 12 (PA) and polyether (PEO). The morphology of the Pebax balloon surface was recorded using polarized optical microscopy (see Figure 2). The extruded tube is expanded axially and radially (blow molding) to form the balloon. One of the effects of the expansion is the orientation of the polymer, which improves mechanical strength. Tensile tests in the axial direction were performed on the tubes and the balloons (see Figure 3).
Stretching significantly increases the stress at break for both nylon and Pebax. The greatest impact of stretching during the balloon-forming process is the change in the behavior of the material. Yielding disappears, and instead a sharp increase in stress is seen. Similar results were observed for nylon 6 when uniaxially stretched.
The other noticeable change is the yielding behavior of nylon 12, which disappears upon stretching. To evaluate the tensile strength of the balloons in axial and radial directions, a rectangular sample was cut from the balloons, and tensile tests were performed along axial and radial directions (see Figure 4). The balloon is stronger overall in radial direction when compared to the axial direction. This is more pronounced for nylon than Pebax balloon, which is advantageous for biomedical applications because radial burst increases risk of difficulty in removal and is therefore considered a higher risk failure mode compared to longitudinal burst. The tensile behavior is less sensitive to direction for Pebax balloons. This is likely a result of the presence of soft segments that reduce the effect of crystallinity and normalize crystal orientation for the Pebax balloon. Both the flexibility and stiffness of balloons are important in their applications.
A DMA was used to evaluate the balloon stiffness. Increasing tan α indicates that the material has more energy dissipation potential so the greater the tan α, the more dissipative the material is. On the other hand, decreasing tan α means that the material acts more elastic, and by applying a load, it has more potential to store the load rather than dissipating it.
Nylon tubes are more elastic than Pebax at room temperature, but as temperature increases, nylon shows a Tg around 51 °C where the elastic modulus drops, and a maximum in tan α is observed. This means the nylon tube absorbs the highest energy at this temperature. The situation changes when the nylon tubes are made into balloons. The nylon balloons become stiffer and more elastic than tube and the Tg peak shifts to a higher temperature (82°C). This is most likely due to the molecular orientation for the balloon. The Pebax balloon is stiffer than the tube whereas the Pebax balloon has more capability to dissipate energy. However, this capability decreases with temperature. The DSC was used to analyze the thermal behavior of the samples. The presence of Tg (at 50 °C) is confirmed for nylon tubes; however, for the tube and the balloon, Tg is less pronounced in the DSC experiments. The melting point of Pebax is lower than nylon for both tube and balloon: 168 °C for Pebax and 176 °C for nylon. The effects of balloon formation (stretching) on shifting the melting point are more pronounced for Pebax than nylon. The DSC of crystallization from the melt reveals a faster and sharper crystallization for nylon than Pebax where a 12 °C difference in crystallization temperature (Tc) can be observed for both materials.
Three peaks at wavenumbers of 904, 936, and 946 cm–1 were selected for FTIR characterizations. The peaks at 904 and 946 cm–1 correspond to the γ-crystalline phase, and the 936 cm–1 peak is associated with the α-crystalline phase of the polyamide. The measurements were performed along axial (0°) and radial (90°) directions to evaluate the effect of orientation using a polarized beam.
The crystals in nylon tubes mostly consist of γ phases, and some convert to α phase when the tube is formed into the balloon (see Figure 5). The Pebax tube crystals have more α phases when compared to nylon, and this becomes more so when forming into a balloon. The nylon 12 samples are more sensitive to orientation when compared to Pebax. For Pebax, the soft segments can be identified using the peak at wavenumber 1100 cm–1.
Nylon exhibits a strong resistance to puncture in comparison to other polymers, especially when it is biaxially stretched. The puncture strength (penetration force) is greater for Pebax balloons. The nylon balloon sample showed a good yield against puncture but underperformed against the Pebax sample both in penetration force and deflection distance (see Figure 6).
Burst pressure is an important parameter in medical applications of balloon catheters and represents the strength of balloon when pressurized.8 Burst pressure tests were performed on the tube and balloon samples in a ramp test (see Figure 7).
Nylon tubes with a wall thickness of 0.355 mm passed the maximum capacity of the burst pressure machine. The lower burst pressure for Pebax tubes is likely due to the presence of soft segments. This contributes more to compliance rather than strength.
The melt rheology and crystalline structure of the nylon 12 and Pebax 6333 tubes and balloons were evaluated using DMA, DSC, and FTIR measurements. Among the physical and mechanical properties noted, nylon 12 shows a greater shear thinning behavior in a melt state when compared to Pebax and therefore the nylon tube dimension is more sensitive to extrusion processing parameters such as puller speed. The Pebax balloon morphology revealed a hybrid structure consisting of hard segments dispersed with soft segments and amorphous phases.
The glass transition temperature of the nylon tube shifted to higher temperatures (51–82 °C), which means nylon becomes stiffer and less flexible when turned into a balloon. The melting temperature shifted to higher temperatures when the tubes were formed into balloons; this shift was more pronounced for the Pebax balloon than the nylon. The Pebax balloons showed a higher puncture resistance than the nylon balloon. However, the burst pressure was lower for Pebax (half of a nylon balloon), which is in accordance with the balloon tensile data.
- Ro A. J. and Davé V., Mater. Sci. Eng. 2013, C33, 909–915.
- Sadeghi, F. and Le, D., “Characterization of polymeric biomedical balloon: physical and mechanical properties,” Journal of Polymer Engineering, (2021): 000010151520210203.
- Saab M. A., Med. Device Diagn. Ind. Mag. 2000, 86–96.
- Warner J. A., et al., J. Biomed. Mater. Res. B Appl. Biomater. 2016, 104b, 470–475.
- Sadeghi F. and Ajji A., Int. Polym. Process. XXVII 2012, 5, 565–573.
- Dencheva N., et al., J. Appl. Polym. Sci. 2006, 32, 2242–2252.
- Warner J. A., et al., J. Biomed. Mater. Res. B Appl. Biomater. 2016, 104, 470–475.
- Zhen L., et al., Appl. Mech. Mater. 2014, 528, 153–161.
- Wagner J. R., et al., The Definitive Processing Guide and Handbook (Plastics Design Library), 2nd ed.; Elsevier: MA, USA, 2014.
- Money B.K. and Swenson J., Macromolecules 2013, 46, 6949–6954.
This article was written by Farhad Sadeghi, Senior Staff Process Development Engineer, and David Le, Process Development Engineer, Confluent Medical, Laguna Niguel, CA. It is a summary abstract of a paper that appeared in the Journal of Polymer Engineering. The authors wish to thank Frank Duran for help in the fabrication of puncture test fixture, and Sayeh Ghazyani with assisting in plotting the graphs. For more information, visit here .