The use of X-ray microtomography to investigate the microstructure of pharmaceutical tablets: Potentials and comparison to common physical methods

Within this study, tablets microstructure was investigated by X-ray microtomgraphy. The aim was to gain information about their microstructure, and thus, derive deeper interpretation of tablet properties (mechanical strength, component distribution) and qualified property functions. Challenges in image processing are discussed for the correct identification of solids and voids. Furthermore, XMT measurements are critically compared with complementary physical methods for characterizing active pharmaceutical ingredient (API) content and porosity and its distribution (mercury porosimetry, calculated tablet porosity, Focused Ion Beam-Scanning Electron Microscopy (FIB-SEM)).

The derived porosity by XMT is generally lower than the calculated porosity based on geometrical data due to the resolution of the XMT in relation to the pore sizes in tablets. With rising compactions stress and API concentration, deviations between the actual and the calculated API decrease. XMT showed that API clusters are present for all tablets containing >1 wt% of ibuprofen. The 3D orientation of the components is assessable by deriving cord lengths along all dimensions of the tablets.

An increasing compaction stress leads to rising cord lengths, showing higher connectivity of the respective material. Its lesser extent in the z-direction illustrates the anisotropy of the compaction process. Additionally, cracks in the fabric are identified in tablets without visible macroscopic damage. Finally, the application of XMT provides valuable structural insights if its limitations are taken into account and its strengths are fostered by advanced pre- and post-processing.

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Materials: The pharmaceutical excipient microcrystalline cellulose (MCC, x₁₀ = 20 μm, x₅₀ = 63 μm, x₉₀ = 139 μm, Vivapur® 101, JRS Pharma, Rosenberg, Germany) and the active pharmaceutical ingredient ibuprofen (IBU, x₁₀ = 9 μm, x₅₀ = 31 μm, x₉₀ = 34 μm Novartis Pharma, Basel, Switzerland) were selected as materials. MCC mainly consists of elongated primary particles and of approximately spherical agglomerates, while the particle shape of IBU is approximately rectangular (Fig. 1). The solid density determined by helium pycnometry is 1.544 g/cm³ for MCC and 1.112 g/cm³ for IBU. Additionally, magnesium stearate (Faci, Carasco GE, Italy) was used as lubricant.

Article information: A.K. Schomberg, A. Diener, I. Wünsch, J.H. Finke, A. Kwade, The use of X-ray microtomography to investigate the microstructure of pharmaceutical tablets: Potentials and comparison to common physical methods, International Journal of Pharmaceutics: X, 2021. https://doi.org/10.1016/j.ijpx.2021.100090.