ENGINEERING INSIGHT
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One of the drawbacks of the conventional method for measuring catheter-balloon wall thickness is that it offers limited precision. The technique prescribes that a technician take a deflated balloon, compress it with a micrometer, and divide the resulting measurement by two to obtain the thickness of the individual walls. The unfortunate consequence of this method is that there is no way to determine whether the balloon’s walls are uneven. The accuracy of the measurement also depends on the competence of the operator, who must clamp the balloon in the gauge properly and determine both the measurement position and compression force accurately. Finally, the accuracy of the measurement can be compromised if the micrometer is dirty or has otherwise been misused. Though the gauge normally used for obtaining catheter-balloon wall thickness is designed to measure down to 0.001 mm, the accuracy of such devices often varies substantially from that figure. The amount of potential inaccuracy is particularly problematic considering that the absolute wall thickness of balloons typically ranges from 0.015 to 0.060 mm.
Accuracy Is Everything
To provide greater precision than manual measurement allows, Medical Production Technology Europe B.V. (Leek, Netherlands; www.mpteurope.nl [6]) has developed a noncontact system that has been shown to offer repeatability better than 0.0005 mm. Unlike a micrometer, the device can gauge the width and other geometrical measurements of both inflated and deflated balloons.
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Scanning the external geometry of an inflated balloon with a high-resolution vision system, the system can test balloons at pressures of up to 30 bar. The device uses custom software to calculate the position of the balloon and geometry features such as diameter, working length, transition points, and cone angles. The geometry data are used to calculate the position of the probing points for the wall thickness analysis. Sensor and balloon motion are servo controlled to ensure that the balloon, even after reclamping it in the fixture, is measured at the same location each time.
A spectrometer with a resolution of approximately 0.0001 mm that is capable of analyzing a broad spectrum of light is used to analyze the wall thickness. A probe sends a light ray to the balloon surface. The resulting interference between light reflecting from the outer and inner surfaces of the balloon used to determine the thickness of the balloon wall. Because the optical properties of the material determine the phase shift of the light rays, the system is calibrated according to the materials used in the balloon. The used spectrum can be adjusted for optimizing the analysis, depending on the balloon materials.
Put to the Test
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The company, which has more than a decade of experience producing tooling to support catheter manufacturing, has tested the system on various balloon shapes and materials. Figures 1–3 represent the results of a few of these tests. In Figure 1 [7], a deflated polyurethane 3 × 20-mm balloon was measured six times. After the first three measurements, the balloon was rotated 90°. This test showed that the wall thickness there is lower than the wall thickness at the middle of the balloon at points number 0 and 20, which represent the proximal and distal cone, respectively. In addition, the figure demonstrates an average variation of 0.0017 mm in the balloon wall thickness.
Figures 2 [8] and 3 [9] reflect a measurement test performed on a nylon balloon, which was inflated to 6 bar. Once measuring began, the balloon was allowed to continue inflating for approximately 10 minutes. A number of geometry measurements were made for this duration that reflect the growth of the balloon over time. Starting at 4.01 mm, the diameter increased to 4.05 mm after 10 minutes of inflation. Figure 2 shows the decrease in wall thickness accompanying the expansion of the balloon.