Institute of Bioanalytical Sciences


Microscopic measurement of layer thickness of coated Pellets


  • Mario Scharmer
  • Stefanie Bartsch
  • Jürgen Kodura
  • Dimitri Wiegel
  • Betram Wolf
  • Ingo Schellenberg

Veröffentlich in: Vortrag zum 7th international Granulation Workshop Sheffield, 3. Juli 2015

There is a number of methods for measuring the particle size of pellets, such as sieve analysis, light scattering and diffraction and calculations based on the facutal density of the components. Yet there is a lack of direct measuring of the thickness of coated layers. For the implementation of a microscopical method, numerous pellets of microcrystalline cellulose were coated in a laboratory on a fluidized bed coater GPCG 1.1 (Glatt Ingenieurtechnik GmbH, Weimar, Germany) by using a model drug substance and a polymer. The coated pellets were embedded in light-curing resin and sanded with a grinding machine Smart Lam 2.0 (Heraeus Kulzer GmbH, Wehrheim, Germany) at a thin section. The radii of the cores and the coated pellets were measured with a transmitted-light microscope AxioPlan Imaging 2 (Carl Zeiss Microscopy GmbH, Jena, Germany) and its three different tools (circle-point, circle-radius and ruler-tool) and evaluated using the software bundle AxioVision. The difference between the radius of the core and the radius of the coated pellet indicates the thickness of the coating layer.

A pellet size of more than 1000 µm was too large for the microscopic image section. Therefore, the panorama function of the software had to be used. To improve low depth focus, an extended depth focus based on a z-stack was calculated for each sample. The particular data and the microscopic data were compared statistically. All measuring tools could show a linear correlation with the calculated layer thickness. The circle-point measuring tool is easy to handle, in good correlation to the calculated coating thickness and also without interference from the operator. The described microscopic method is a useful means for the calibration of in-line measurement systems like NIR (Near-infrared) or HIS (Hyper Spectral Imaging).