Measuring the Coating Thickness on Medical Stents

There are a number of varieties of stents which are utilized for different medical conditions. Drug-eluting stents (DES) are one class of vascular stent which are designed to minimize in-stent restenosis. This was a key weakness of bare-metal stents. DES is made up of a polymer coating, a standard metallic stent, and an antiproliferative drug which is embedded within a polymer and released over time.

Non-uniform coating can lead to unpredictable drug distribution and release. One of the most important parts of DES manufacturing is quality control of coating uniformity and thickness.

Covered stents are another class. They are utilized for clot extraction, and in cases of aneurism or cancer, restriction of the blood flow. Stent encapsulation is also employed in vascular application to deploy the stent. The covering of the stent is created from PTFE, polyurethane, or similar polymer materials.

It is vital to test the thickness of covering membrane on the struts of the stent and between them. MProbe VisHR-MSP system provides a non-destructive, reliable, measurement solution for quality control in both instances.

1. Vascular DES Stents Coating

DES vascular stent.

Figure 1. DES vascular stent.

The coating was measured directly on the product. DES mesh was created by ~80-120 µm Co-Cr wire which is coated with the Sirolumus eluting polymer. To perform the measurements, a small spot size (~8 µm) was employed and the results were correlated and verified with the SEM measurement. Measurements were performed throughout a number of production runs in order to modify the production line settings to a target thickness of ~ 4.5 µm.

Measurement location – red circle indicates the measurement spot area. The surface is ablated to increase the retention of the medicated polymer.

Figure 2. Measurement location – red circle indicates the measurement spot area. The surface is ablated to increase the retention of the medicated polymer.

Measurement of stent from the 1st production run – thickness 6.69µm ( coating is too thick).

Figure 3. Measurement of stent from the 1st production run – thickness 6.69 µm ( coating is too thick).

Measurement of stent after production line adjustment – thickness 2.55µm (coating is too thin).

Figure 4. Measurement of stent after production line adjustment – thickness 2.55 µm (coating is too thin).

Measurement of the stent coating after second adjustment of production line – thickness 4.35µm (coating thickness is on-target).

Figure 5. Measurement of the stent coating after second adjustment of production line – thickness 4.35µm (coating thickness is on-target).

SEM measurement of the stents coating shows a perfect match to MProbe thickness measurement: 4.35 µm.

Figure 6. SEM measurement of the stents coating shows a perfect match to MProbe thickness measurement: 4.35 µm.

2. Encapsulated/Covered Stents

7 Polyurethane covered/encapsulated stent.

Figure 7. Polyurethane covered/encapsulated stent.

Using the thin (~ 200 µm) nitinol wires the stent scaffolding is built, and polyurethane cover thickness target is ~40 µm in order to decrease the recoil force. The measurement was performed by utilizing 40 µm spot (from membrane between the struts) and 20 µm spot for the measurement on struts.

Membrane thickness measurement.

Figure 8. Membrane thickness measurement.

Enlarged membrane measurement area (reticle indicates spot location).

Figure 9. Enlarged membrane measurement area (reticle indicates spot location).

Polyurethane thickness measurement on a strut. Thickness is reduced due to stretching.

Figure 10. Polyurethane thickness measurement on a strut. Thickness is reduced due to stretching.

Enlarged area – reticle indicates measurement location.

Figure 11. Enlarged area – reticle indicates measurement location.

MProbe MSP system used in the measurement.

Figure 12. MProbe MSP system used in the measurement.

Conclusion

A fast, affordable, non-destructive, and accurate coating thickness measurement with the MProbe MSP system makes it simple to check stent samples and verify that the coating is within tolerances.

This information has been sourced, reviewed and adapted from materials provided by SemiconSoft.

For more information on this source, please visit SemiconSoft.

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