Zoe Chu¹ ², Christopher Windows-Yule¹, Ian Gabbott², Gavin Reynolds², Rachael Shinebaum² and Andy Ingram¹

1. School of Chemical Engineering, University of Birmingham, Edgbaston, B15 2TT, 2. Oral Product Development, Pharmaceutical Technology & Development, Operations, AstraZeneca, Macclesfield, UK

A tablet consists of the drug, the active pharmaceutical ingredients (API), and the other components, excipients. The tensile strength of a tablet is one of the most important outcome parameters of compression as it will need to withstand coating, packing, and shipping stresses but also not affect the dissolution time. Currently, there is a lack of understanding of why excipients improve the bulk powder properties and the bonding mechanisms of the powder during compression. This makes tablet formulation development an inefficient trial-and-error process that is needed for every new API (1). Therefore, there is a need to understand the fundamentals of the bonding mechanisms of the powder during compaction. In literature, surface energy of powders contributes a significant amount towards the tensile strength (2) as hydrophobic powders, which have a lower surface energy (3), are seen to disrupt the tablet strength significantly (4,5). This was seen in author’s previous study where the more hydrophobic (measured by sessile drop method surface energy calculations) the powders, the lower the surface energy.

For that previous experiment, measuring the surface energy of all the powders is important for analysing the tensile strength data and to confirm the hypothesis that the surface energy really has an impact on tablet tensile strength. However, there are many different ways to measure surface energy and all of them from literature seem to have their obstacles. The ‘industry standard way’ to measure surface energy is deemed as inverse gas chromatography (IGC). When comparing results from the IGC to the sessile drop contact angle test, these presented very contrasting results and had different surface energy descending order. Therefore, a comparison study has been completed where the sessile drop method, IGC, dynamic vapor sorption, kinetic adhesion test and atomic force microscopy have all been compared. This study aims to highlight these limitations and address the gaps in getting an accurate surface energy measurement of powders.

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2. Sun CC. 4 Role of Surface Free Energy in Powder Behavior and Tablet Strength. In: Adhesion in Pharmaceutical, Biomedical and Dental Fields. 2017. p. 75–88.

3. Sunkara D, Capece M. Influence of Material Properties on the Effectiveness of Glidants Used to Improve the Flowability of Cohesive Pharmaceutical Powders. AAPS PharmSciTech. 2018;19(4):1920–30.

4. Zuurman K, Maarschalk KVDV, Bolhuis GK. Effect of magnesium stearate on bonding and porosity expansion of tablets produced from materials with different consolidation properties. 1999;179:107–15.

5. Mishra SM, Rohera BD. Mechanics of tablet formation: a comparative evaluation of percolation theory with classical concepts. Pharm Dev Technol [Internet]. 2019;24(8):954–66.