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Tablets & Capsules May 2017 33 takes place when the tablet is deformed [4]. A high TBI indicates a brittle tablet, and the TBI is easy to deter- mine using force-displacement data gathered during a diametrical breaking test. See Figure 3. It is useful to point out that tablet brittleness depends on tablet poros- ity, and the TBI value generally increases exponentially with increasing porosity [5]. Therefore, for a given material, the more porous the tablet, the more brittle it usually is. To compare the brittleness of different materi- als, the TBI should be normalized by porosity. For exam- ple, a TBI value at zero porosity can be used to compare the brittleness of the material. Earlier work showed that a TBI value of less than 150 is preferred in order to avoid problems associated with excessive brittleness. The TBI of some common excipients follows this order: lactose (monohydrate) > dibasic calcium phosphate (dihydrate) > microcrystalline cellulose > hydroxypropyl cellulose. The addition of lubricating excipients can also increase tablet brittleness [6]. Revised method for quickly determining tablet indentation hardness As discussed above, plasticity is an important material property and must be considered when designing high- quality tablets. Traditionally, the plasticity of powders was indirectly quantified using plasticity parameters derived from analyzing the compressibility data from the Heckel equation, the Kuentz-Leuenberger equation, or others [7]. A more reliable method, however, is to assess plasticity directly using indentation hardness, where low hardness corresponds to high plasticity. Indentation hardness is determined simply by dividing the applied indentation force by area of the permanent indentation. Next, a calibrated-light microscope that rapidly captures digital images was used to determine the precise area, which allows measurement of the indenta- tion hardness [8]. See Figure 4. Using this method, it was shown that tablet hardness decreases exponentially with increasing porosity and that hardness extrapolated to zero porosity correlates well with the expected plasticity of common pharmaceutical excipients. When integrated with routine methods of determining tablet tensile strength, these new techniques pave the way for comprehensive characterization of powder com- pression properties. Information obtained from these studies, in combination with data obtained from a mod- ern compaction simulator, will play an essential role in truly realizing the science-guided design of high-quality tablet products, as a Quality-by-Design philosophy requires. T&C References 1. Paul, S., Chang, S.-Y., and Sun, C.C. The phenom- enon of tablet flashing–its impact on tableting data analy- sis and a method to eliminate it. Powder Technology, 2017. 305: 117-124. 2. Osei-Yeboah, F. and Sun C.C. Validation and appli- cations of an expedited tablet friability method. Int J Pharm, 2015. 484(1-2): 146-155. 3. Osei-Yeboah, F., et al., A formulation strategy for solving the overgranulation problem in high shear wet granulation. J Pharm Sci, 2014. 103(8): 2434-2440. 4. Gong, X. and Sun, C.C. A new tablet brittleness index. Eur J Pharm Biopharm, 2015. 93: 260-266. 5. Gong, X. et al. Dependence of tablet brittleness on tensile strength and porosity. Int J Pharm, 2015. 493(1- 2): 208-213. 6. Paul, S. and Sun, C.C. Lubrication with magnesium stearate increases tablet brittleness. Powder Technology, 2017. 309: 126-132. 7. Sun, C.C. Microstructure of tablet—Pharmaceutical significance, assessment, and engineering. Pharm Res, 2016: 1-11. 8. Patel, S. and Sun, C.C. Macroindentation hardness measurement—Modernization and applications. Int J Pharm, 2016. 506(1-2): 262-267. Changquan Calvin Sun, PhD, is an associate professor and the director of graduate studies in the Department of Pharma- ceutics at the University of Minnesota, WDH 9-177, 308 Harvard Street Southeast, Minneapolis, MN 55455. Tel. 612 624 3722. E-mail: sunx0053@umn.edu. He is an editorial advisor to Tablets & Capsules. New products? Boost your reach with our Quick Web Resource this October. A special feature connecting readers to the top suppliers of solid dosage materials and equipment. Contact tc@cscpub.com