APPLICATION NOTE

Characterization of coatingson drug-eluting stents‣ Introduction
The introduction of drug-eluting stents (DES) has, over the past decade, revolutionized thefield of interventional cardiology. The DES-type of stent is coated with a thin layer of drugeluting material that dramatically reduces stent restenosis. In most cases this prevents vessel
revascularization.
DES usually comprise three main components:• A metallic stent structure, the stent platform, usually made of stainlesssteel, cobalt-chromium, or other metallic alloys.
• A drug carrier, usually polymer-based, that is used as a coating for themetallic stent and that contains and delivers the drug to the artery overtime.
• A drug embedded in the polymer-based coating, that suppresses the formation of excess tissue.
Manufacturing approaches for DES comprise various types of materials (drugs and polymers)
together with diﬀerent coating processes. Despite the variations, most of the coating methods
eﬀectively consist of spraying the drug-eluting coating over the metallic stent structure.
The characterization of coating uniformity – with thicknesses typically between 5 and 15 µm– has undeniably become an essential necessity in order to reduce therapeutic risks, but thisstep also doubles as an eﬀective control of the coating process and its stability. Nonetheless,the characterization of such coatings is still a challenging application, due in part to the needto perform high-throughput and non-destructive measurements on structures with very
complex morphologies.
It has been common in the industry in recent years to test equipment such as Confocal Laser
Scanning Microscopy (CLSM), Chromatic Confocal Distance Sensors (CCDS), Spectroscopic
Reflectometry or Total Coating Mass as non-destructive techniques for coating characterization. The three former approaches each exhibit diﬀerent limitations when used for coating
characterization on DES, e.g. low working distance, point vs. areal measurements and low
light eﬃciency. Although Total Mass Coating is the approach most frequently used to streamline stent production, it does rely on a qualitative assumption, i.e. on the homogeneity of thecoating.
None of these technologies has proven to be ideal for a fast and reliable characterization ofDES coatings at the rate required for production, due in part to the fact that they require adiﬀerent measurement platform to characterize coatings than the one used to inspect the restof the stent.
Characterization of coatings on drug-eluting stents using the Q six stent inspection system

Insertion, balloon inflation, stent in position,
in-stent restenosis (from left)‣ Coating characterization using the Q six
Q six uses Vertical Scanning Interferometry (VSI) as an embedded technology for the characterization of coating thickness and coating uniformity on DES. This technology provides athickness map of the stent surface with a lateral resolution of 0.5 µm and a verticalresolution of 2 nm.
The main advantage provided by Q six is the fact that coating characterization is performed injust few seconds and on the same platform where dimensional and visual inspection of thestent is performed. This allows use of the same platform for inspection of all critical characteristics of the stent in production.
In order to obtain a three-dimensional map of a surface, Q six performs a vertical scan thatlocates the best contrast of an interferometry signal at every point in the field-of-view (FOV)
acquired through an appropriate objective. The maximum contrast position determines theexact height of every point on the surface within the FOV.

Coating measurement on a DES

In the case of coating measurements, two interference signals are acquired: one corresponding to the air/coating interface, and a second signal corresponding to the coating/stent interface. By applying correction models based on the index of refraction of the coating, a thickness
map of the coating is obtained with nanometer accuracy by simply subtracting one interface
plane from the other.

Interference patterns for the upper (left) andlower (right) interfaces of the coating

Characterizing coatings on DES with the Q six is as easy as focusing on the strut surface, selecting the size of the area to be mapped and entering the index of refraction of the coating
material into the Coating panel of the SensoINSPECT User Interface. A few seconds after
starting the acquisition, SensoINSPECT will then show the thickness map of the coating
inside the selected area on screen.

The results are shown as a 3D map of thecoating thickness. The user can analyze
any transversal section (profile) of thethickness map by simply manipulating
the profile handles shown on screen withthe mouse. It is also possible to evaluate
the coating thickness of the selectedprofile, which provides a highly intuitiveand quantitative characterization ofcoating thickness and uniformity.

Characterization of coatings on drug-eluting stents using the Q six stent inspection system

Together with the quantitative results (3D thickness map and selected profile), a set of surface
parameters and profile parameters to qualitatively complete the set of information is always
displayed. The average value provides useful information for thickness characterization, while
the maximum, minimum and standard deviation values provide the information needed forcoating uniformity characterization.

‣ Conclusions
By using the technology embedded in the Q six stent inspection system, several gains arepossible in a production environment:• Coating characterization in production can be performed using the same measurement
platform used during the device prototyping and development.

• The same platform can be used for dimensional measurements, visual inspection andArea (top) and profile (bottom) parameter tables

coating characterization.
• Coating integrity assessment and coating characterization can be performed within asingle measurement routine, facilitating streamlining of the process.
• Quantitative and reliable values for coating thickness are obtained within a few seconds
with nanometer accuracy and repeatability.
• Coating measurements are performed optically with no contact with the sample, guaranteeing no sample manipulation.

Parameters panel in SensoINSPECT

Sensofar Medical provides state-of-the-art technology for the inspection of implantable medical devices.
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