Polyether block amide (PEBA) is used successfully in a range of catheter designs. This article describes how PEBA’s compatiblity with laser machining can simplify catheter manufacturing, for example, transforming single durometer extrusions into multi-durometer shafts more easily than with other fabrication methods.
By: C. Carson and R. Sik
Avicenna Technology, Montevideo, Minnesota, USA
Valuable material properties
Polyether block amide (PEBA), known as Pebax, is a thermoplastic elastomer used in medical catheter tubing that combines the advantages of thermoplastic materials with those of rubbery, elastic materials. The thermoplastic qualities of PEBA mean that it can be easily processed, extruded and shaped at elevated temperatures, and upon cooling its elasticity will allow it to return to its original length or shape after being stretched, compressed or deformed.
PEBA is considered a high performer among its thermoplastic elastomer peers because it is lightweight, and because it offers a wider range of flexibility and hardness, a higher energy return, a greater resistance to fatigue, and better dimensional stability under physical and thermal stress (see Table I). The material offers strong resistance to a range of chemicals and is not affected by exposure to saline or sulphuric acid. It is compliant with United States Pharmacopeia Class VI and can be sterilised by a variety of methods including gamma radiation, steam autoclave and ethylene oxide.
Tube extruders and catheter manufacturers in the medical device industry report that PEBA is a model material in many ways. It has a wide thermal working zone with melt temperatures that range from 134 °C to 174 °C, depending on durometer. PEBA can be co-extruded with thermo-plastic polyurethanes to allow the creation of designer tubing compounds. PEBA easily bonds to other materials using heat or epoxies. This quality makes the PEBA an ideal shaft material for composite balloon catheters.
Its stiffness range coupled with a consistent density among its varied durometers and its easy melt-processing qualities also make PEBA a preferred material in the manufacture of multi-durometer catheter shafts constructed of fused sections of PEBA. Catheters of this design offer good push and torque via the hard durometer proximal sections and offer flexible steering in the soft durometer distal sections. They are often reinforced with stainless steel braid to enhance the shaft’s transfer of torsional force to its distal tip.
Advantages of the laser processing
| FIGURE 1: Laser drilling PEBA catheters accommodates fluid aspiration or drug delivery. |
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Many catheter designs require machined features such as holes and slots, and sometimes grooves and tapers. Mechanical methods such as drilling, grinding and blade skiving can be used to achieve these features, but may leave burrs at the feature’s edge and may introduce undesired heat and stress to the catheter material. Furthermore, contact-machining methods that use drill bits or blades may be hindered by the presence of stainless steel braid in the catheter wall. Using laser energy to perform noncontact catheter machining is an attractive alternative because it can remove polymer material from an embedded braid without stress or heat (Figures 1 and 2). In addition, certain forms of laser energy can remove polymer and braid together, if required.
The process
Laser energy is highly coherent, that is, a laser will emit energy at only one specific wavelength. Laser energy is also highly directional, which means the photons of a laser beam are tightly grouped and travel in parallel. This characteristic allows the laser beam’s energy to be manipulated (masked, focused and redirected) with a beam delivery system of fibre optics or lenses and mirrors.
In the laser machining process, laser ablation vapourises material by matching the material’s absorptive properties to the proper laser wavelength, and then applies this chosen wavelength to the particular material with the correct energy density and duration. When the process variables of power, pulse, focus and dwell are properly set, laser energy breaks the material’s molecular bonds and causes the material to evaporate. True laser ablation does not rely on heat to melt or burn away material and therefore laser ablation produces clean, precisely machined features with no heat effect to the surrounding area.
| FIGURE 2: Laser marking PEBA catheters embeds product data and creates permanent traceability. |
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PEBA is a material that is sensitive to ultraviolet (UV) light and is susceptible to degradation as a result of exposure. These material properties offer a starting point when designing a laser ablation process for PEBA. The UV energy spectrum ranges in wavelength from the near UV of 400 nm approaching violet to the extreme UV of 10 nm approaching X-ray. The UV wavelengths produced by commercial lasers range from the 355 nm of a 3rd harmonic Nd:YAG solid-state laser to the 157 nm of a fluorine gas excimer laser. The shortest practical UV wavelength is generally sought when laser machining PEBA because PEBA does not ablate well when exposed to longer UV wavelengths.
This search for a short UV wavelength commonly leads laser processors to use the 193 nm wavelength of an argon-fluorine gas excimer. However, this choice is not without its challenges. The 193 nm wavelength belongs to the vacuum UV portion of the energy spectrum and it is rapidly absorbed by the oxygen of atmospheric air. As a result, laser-ablation work with 193 nm UV must be performed in a vacuum or in an inert oxygen-free environment.
Putting this infrastructure challenge aside, the argon-fluorine excimer wavelength and all excimer-generated wavelengths offer an attractively large laser beam. An excimer’s raw beam measures 12 mm high × 24 mm wide. After this raw beam is masked and focused it still translates into a large working beam that gives laser processors the ability to apply high levels of energy over widths that exceed 2.5 mm at the work site, and remove significant amounts of PEBA material in a relatively short amount of time. For example, it is possible to remove 0.084 mm3 of PEBA material per minute, which equates to removing a depth of 0.051 mm over a length of 100 mm every minute. Increasing the laser’s pulse rate can further accelerate this bulk removal process.
| TABLE I: Properties of PEBA (Source: Arkema). |
| Property
|
Test |
Value range |
| Hardness |
ISO 868 |
25–72 Shore D |
| Density |
ISO 1183 |
1.00–1.01 g/cm3 |
| Melt Point |
ISO 11357 |
134–174 °C |
| Flexural modulus |
ISO 178 |
12–513 MPa |
| Tensile stress |
ISO 527 1A |
28–54 MPa |
| Tensile strain |
ISO 527 1A |
750–319% |
| Charpy impact |
ISO 179 (at –30 °C and 23 °C) |
No break |
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It is perhaps in this regard that laser machining presents its greatest advantage to manufacturers of PEBA catheters. Laser machining can remove large sections of PEBA from the catheter shaft to create reduced diameter sections that serve as perfect bond sites for softer durometer PEBA. This capability can conceivably simplify catheter manufacturing by allowing single durometer extrusions to be transformed into multi-durometer shafts more easily than fabrication methods that fuse discreet sections of PEBA with the aid of inner mandrels and outer heat-shrink casings.
Precision and efficiency
The qualities of PEBA make it an ideal material for catheters. Further endorsement of the material for medical device applications is its compatibility with laser machining. This avoids introducing undesired flaws, which is common with mechanical machining, and allows significant amounts of material to be removed efficiently and precisely for certain catheter components.
Chad Carson
is President and
Ronald Sik