n-Gauge 1: Case Study 2

Thickness & Refractive Index Measurements on a Stent sample using Beam Profile Reflectometry

The Task

n-eos was approached to use n-Gauge 1 to characterise the coating thickness of a single cardiac stent.  The manufacturer was experimenting with new coating techniques and wanted to evaluate coating uniformity and identify areas of potential failure.

Sample Analysis

The n-Gauge 1 system was calibrated with reference to a set of fused silica discs with thicknesses ranging from ~20μm to ~200μm, as measured on the company’s behalf by the UK’s National Physical Laboratory.  Measurement began with locating the laser spot on the correct part of the sample using the system’s built in optical inspection system. Once the spot was focused upon the required point, the 100X lens was used to acquire the raw BPR data.  Figures 1 and 2 show the spot located on the stent, using a 5X and 20X objective lens respectively.  The sample was measured using random points on the stent struts to collect data.  

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Figure 1: Image of Cardiac Stent from 5x Objective Lens
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Figure 2: Image of Cardiac Stent from 20x Objective Lens

Results

Figure 1 shows the shape of the strut and Figure 2 a high-quality image of the coating surface, which in this case is quite smooth, though displaying some evidence of cracking and ‘crazing’.  Analysis showed that the quality of coating across the stent was generally smooth but with some defects in the vicinity of tight curves.

Raw data was modelled using a number of alternative approaches, beginning with the simplest model of a single polymer film on the substrate, and then adding extra effects such as surface roughness, strain, and multiple film layers until the best overall fit to the raw data was obtained. The substrate properties were held fixed at the values typical for medical grade (316) stainless steel, though some surface roughness was allowed for.  In all cases, the refractive index of each film was measured in tandem with its thickness.

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Figure 3: Reflectance as Measured in the ‘S’ Axis
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Figure 4: Reflectance as Measured in the ‘P’ Axis 
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Figure 5: Raw data for a model consisting of 31.52μm of strained polymer with an index of 1.773
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Figure 6: Modelled data for a model consisting of 31.52μm of strained polymer with an index of 1.773

Conclusion and Outcome

The results shown here indicate that the n-Gauge 1 is able to obtain valuable measurement data from cardiac stents.  The technology delivers the necessary information to support rapid and repeatable characterisation of such samples, and software can be developed to streamline the process of data analysis in any given application.

Using the information provided by n-Gauge 1, the manufacturers were able to adjust the stent coating process to achieve enhanced coating uniformity and performance, particularly in the vicinity of tight curves.  As n-Gauge 1 was able to provide quantitative and qualitative data on stent performance, this adjustment could be achieved quickly at minimal cost and with minimal impact of development timescale.

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