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Measurement of single and multi layer thin film coatings for medical implants.

Brice, L K W; Morris, S*; Motely, J*; Leach, R K (2008) Measurement of single and multi layer thin film coatings for medical implants. In: euspen 2008, 18-22 May 2008, Zurich.

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There is a large market for implantable medical devices such as cardiac stents and prosthetic orthopaedic joints; all these medical implants are subject to a varying degree of rejection by their host. To reduce this rejection, anti-rejection drugs are administered to the patient. It is common for the implants themselves to be coated with a polymer, which contains the drug. The total dose and dosage rate can be controlled by the thickness of the polymer coating and the concentration of drug. Control and measurement of the coating thickness is, therefore, very important, but is currently difficult to achieve accurately and within the production process.

Nightingale-EOS has developed a bench top device capable of making measurements of the thickness and refractive index of polymer coatings. The system uses beam profile reflectometry to measure film thickness. This measurement technique has been developed from an established technology used to measure planar films but has been adapted to work with highly curved surfaces such as those present on stents.

This paper presents the work carried out to develop and assess the beam profile reflectometer¿s capability and accuracy when measuring different thicknesses of polymer films. Measurements were carried out across a range of stents, of different curvature and coating thickness. Polymer samples were developed as transfer artefacts to validate the beam profile reflectometer when measuring curved surfaces. The artefacts have been developed at the National Physical Laboratory, along with Lombard Medical, to provide a traceable route to national standards. A laser system was used to cut a step through the polymer coating to the substrate. The step in the artefacts allowed the physical thickness of the polymer to be measured. A range of equipment at the National Physical Laboratory was then used to measure accurately the artefact step heights. Results from AFM measurements, scanning white light interferometry and three-dimensional optical microscopes were compared to those of the beam profile reflectometer, to demonstrate the effectiveness of the technique and its application to curved surfaces.

The beam profile reflectometer is fast, compact and does not rely on a priori knowledge of the optical constants of the polymer film. These attributes and the ease of use of the beam profile reflectometer, mean that the system is suitable for measuring large numbers of devices in a production environment.

Item Type: Conference or Workshop Item (UNSPECIFIED)
Subjects: Engineering Measurements
Engineering Measurements > Dimensional
Last Modified: 02 Feb 2018 13:15
URI: http://eprintspublications.npl.co.uk/id/eprint/4150

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