Medical Device Link.

Originally Published MDDI May 2002

MATERIALS TESTING

Examining Elliptical Surface Defects on Angioplasty Balloons

Defects in angioplasty balloon material, which result in product discards, may represent a significant portion of production.

Abbas Tcharkhtchi and Erik Andersen

Angioplasty balloons are critical devices for treating blockages. (click to enlarge)

In order to reduce surgical interventions, a procedure was developed in the 1980s to recanalize coronary vessels by using balloon dilation. The technique, percutaneous transluminal coronary angioplasty (PTCA), uses a catheter with a balloon at the distal end. The balloon is pushed forward into the stenosis and inflated to dilate the blockage.^1,2 The calcified deposits are thus pressed into the vessel walls under high pressure to create a wider lumen, improving blood flow.³ This complex application requires sometimes-contradictory properties of the catheter and balloon, and production compromises are a challenge.

Sidebar: Ensuring the Safety of Balloon Catheters

Because of the critical nature of angioplasty, the balloon catheter must have excellent performance characteristics, and defects that may impair its mechanical properties must be eliminated. As can be imagined, however, microscopic defects may be formed on the balloon during the manufacturing process. These defects have different origins, such as:

• Impurities coming from dirt or residue in the processing pathway.
• Gels formed during polymerization or extrusion.
• Marks induced by rough edges of metallic parts in process pathway.
• Elliptical defects formed during balloon manufacturing.

Figure 1. Image of the surface of the balloon near the sleeve observed using optical microscopy (50x).

The aim of this study is to characterize this last type of defect and to identify the causes of its formation. These defects, or "fish-eyes," are generally elliptical with different dimensions. Because such defects can diminish the mechanical properties of the balloon, the defective balloons must be discarded. Scrap may represent more than 95% of production in some situations.

To identify the cause of these elliptical defects, it is necessary to pay special attention to the polymer under study and its properties, and to consider all pertinent process parameters. It is then necessary to study the polymer in its initial state (granules), as well as during processing.

Figure 2. Impurities on the surface of balloon (50x).

Parameters related to the polymer. Generally the mechanical properties of polymers are governed by rigidity of molecular chains, molecular weight distribution, morphology, and physical state. For a given polymer and at a given temperature, then, the material's properties are affected by its molecular weight distribution and its morphology.

Parameters related to the process. The polymer used in balloon manufacturing undergoes the following processes: heating and cooling, mixing, drying, stretching, and inflating. During these operations, different parameters may affect the polymer properties. These may include:

• Temperature during grinding operation, extrusion, storage, thermoforming, and other parts of the processs.
• Humidity of polymer (granules, tube, or balloon).
• Pressure during thermoforming.
• Weight of the mold that is used during thermoforming.
• Cooling rate at the end of extrusion.
• Period of stabilization before the thermoforming process.
• Metallic parts that are in contact with the polymer from extrusion to thermoforming, and the state of their surface.

These parameters are interrelated. The influence of one of them on polymer properties is related to the influence of the others. What is important, then, is to find an optimal condition in which all of these parameters are properly defined.

MATERIAL

Melting Point Tm	171-173°C
Density at 23°C	1.02 g/cm-3
Glass transition temperature	34°C
Tensile strain at break	300%
Tensile stress at break	46 MPa
Shear modulus (at 20°C)	350 MPa
Table I. Performace characteristics of the unoriented polymer material.

Figure 3. Image of a continuous and discontinuous scratch "impact" in center of balloon (50x).

The polymer under study is a copolyamide, identified as PA8020 and supplied by Atofina (Paris). Table I identifies certain characteristics of the unoriented polymer as given by the supplier.

A preliminary study has been performed to determine the molecular weight of this polymer by viscosimetry in solution (using metacresol as solvent). The result shows an intrinsic viscosity in solution of 1.1 dl/g. The molecular weight was then obtained using the Mark-Houwink equation:

The molecular weight was estimated to be approximately 16,000–18,000 g/mole. This molecular weight is relatively low and not far from the critical molecular weight of polyamides, which is generally near 15,000 g/mole.

It should be emphasized that near the critical molecular weight there is a transition zone in which a slight variation of molecular weight may have a critical influence on the mechanical properties of the polymer, particularly on its properties at break.

METHODS OF INVESTIGATION

Figure 4. Photo illustrating a continuous scratch on the surface of balloon (600x).

For this study, four experimental methods were used. The first was viscosimetry, which was used to determine the viscosity of the polymer (inherent viscosity) in solution. By this method, the molecular weight was calculated by means of the Mark-Houwink equation. Second was infrared microspectrophotometry, which was used for the monitoring of eventual changes in the chemical structure of the polymer during processing (i.e., degradation or postpolycondensation), and for characterizing defects on catheter balloons. The third method included both optical and electronic microscopy.

These tests were performed in order to characterize the morphology of impacts and to determine their dimensions. Finally, confocal microscopy provided a high-resolution method that gave information in three dimensions concerning the geometry of defects on the balloon surface.

RESULTS

Figure 5. Image showing the presence of elliptical defect located in center of balloon.

Microscopic observation. As illustrated in Figure 1, direct observation by optical and electronic microscopy shows that the surface of the balloon is not smooth, particularly in the zone of the sleeve, which is perhaps caused by roughness of the metallic parts in the process pathway.

Various defects may be distinguished by direct observation of impurities and gels (Figure 2), scratches created by the surface of the mold (Figures 3 and 4), and isolated fish-eyes (Figure 5).

Characterization of fish-eye defects. The characterization of microscopic defects gives different information. The defects do not arise at all times. They appear when either initial polymer or the processing conditions are not appropriate. The defects, when they exist, always have an elliptical geometry, but with differing dimensions. Some of the defects can reach dimensions of 10 µm x 30 µm. A tensile test on a balloon sample with elliptical defects showed that an impact could initiate the break.

The results of IR spectromicroscopy showed that the balloon is not degraded during extrusion or balloon manufacturing. Indeed the IR spectrum of the polymer before extrusion and after balloon manufacturing is the same. Also, the IR spectrum of the defective zone is the same as that of the initial polymer. This means that the polymer and the defective zone have the same origin, and that the polymer is not degraded in the zone of fish-eyes.

Figure 6. Fish-eyes before and after melting (100x); a = (145°C), b = (173°C), and c = (180°C). (click to enlarge)

To determine if these defects are impurities or gels, a sample displaying fish-eyes was heated to its melting point, under observation by optical microscope (Figure 6). The results show that when the temperature reaches the melting point of the polymer (173°–175°C), the defects disappear. This suggests that the elliptical defects are neither impurities nor gels. In fact, impurities have melting points different than that of the polymer, and gels are cross-linked molecules that cannot be melted.

Elliptical defect thickness. To estimate the thickness of the balloon in the zone of fish-eyes, we used the following methods.

• IR spectromicroscopy in mapping mode.
• Confocal microscopy.

Figure 7. Successive IR spectra of the impact zone in a range of 100 m, using mapping method. (click to enlarge)

IR microspectrophotometry (mapping). Figure 7 shows the result of a mapping test on a balloon in the elliptical defect zone. The experiment was conducted with a 100-µm scanning range, and a 5-µm interval between scans.

Because optical density (peak intensity) is related to the film thickness, according to the Beer-Lambert law, following the optical density of the different chemical bonds in the polymer gives information about thickness changes in the fish-eye zone. This is expressed by the equation:

Figure 8. Variation of the 1278 cm–1 peak intensity in the damaged zone. (click to enlarge)

Figure 8 represents the variation of optical density at 1278 cm^–1 (CN groups). It can be seen that the intensity of the peak decreases from the edges to the center of the fish-eyes.

Near the edge of the defective zone, the thickness of elliptical defects is the same as the balloon. In the center, the elliptical defect zone is very thin, probably as a consequence of the inflating pressure applied for balloon manufacturing.

Figure 9. Roughness near an elliptical defect as determined by confocal microscopy.

Confocal microscopy. As shown in Figure 9, the results of this experiment show that the balloon thickness is not always on the surface of fish-eyes.

It seems that a very thin layer on the surface of the balloon is cracked. By the effect of inflating, a defect is formed and then the polymer builds up at the edges of the defect as a result of elastic deformation. In the case of semicrystalline polymers, this phenomenon is known as skin effect.

This occurrence can be explained as follows. When liquid polymer enters the cooling water during extrusion, the surface of the tube, which contacts the water directly, cools rapidly and the polymer at the surface crystallizes.

Figure 10. Effect of cooling rate on the morphology of an extruded polymer. The top figure shows the cooling rate gradient along the thickness of the tube, the bottom figure shows the formation of a thin layer with different morphology at the tube surface. (click to enlarge)

The cooling rate is slower across the internal portion of the tube than at the surface. The polymer thus crystallizes at a slower rate, with a morphology different from that at the surface. This phenomenon is illustrated in Figure 10.

The figure demonstrates how a very thin layer with a different morphology is formed at the surface of the tube. The differences in morphologies between the surface and other parts of the tube are indeed not very large; however, because the polymer properties are critical during balloon manufacturing, the differences become sufficient to induce significant variations in defect formation during processing.

 

Figure 11. Schematic view of the effect of processing conditions (curves 1 and 2) on stress-strain curves and defect size. (click to enlarge)

As illustrated in Figure 11, processing conditions at the chosen strain (Σp) for balloon manufacturing may be critical regarding defect size.

The critical question is what mechanism of fracture occurs at the surface of the balloon?

Because the processing temperature is about 90°C, the polymer is in its rubbery state. But as the rate of stretching is relatively high (about 20 m/sec), the time-temperature superposition hypothesis suggests that one may consider the mechanism of failure to involve the formation of crazes at points of stress concentration, and the subsequent growth and fracture of the craze at the balloon's surface.

CONCLUSION

During this study, it has been shown that the elliptical surface defects are neither impurities nor gels, that there is no degradation during extrusion, and that the thickness of the elliptical defect zone is less than in other parts of the balloon.

The cause of fish-eye formation is likely to be the skin effect phenomenon. Indeed, elliptical defects, or fish-eyes, are formed on the surface of the balloon because of the morphological heterogeneity of the tube material. During balloon manufacturing, when the polymer is at its ultimate mechanical properties, the defects are formed and their dimensions quickly increase as a result of small morphological changes.

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REFERENCES

1. KL Gould, "Coronary Artery Stenosis" (London: Elsevier, 1991).
2. B Haridas and CA Haynes, "Predictive Analysis at the Forefront of Medical Product Development," Medical Device & Diagnostic Industry 21, no. 10 (1999): 112–119.
3. K Sauerteig and M Giese, "The Effect of Extrusion and Blow Molding Parameters on Angioplasty Balloon Production," Medical Plastics and Biomaterials, (May/June 1998): 46–49.

Abbas Tcharkhtchi, PhD, is an associate professor of mechanical systems at Leonardo da Vinci University (Paris). Erik Andersen is managing director of CathNet-Science (Paris).

Copyright ©2002 Medical Device & Diagnostic Industry