It is hard enough to create a successful tablet. But making one that’s robust enough to survive the journey from manufacturing to packaging is another undertaking entirely. And if overlooked, it could result in a tarnished brand reputation at best, and patients taking inaccurate dosages at worst. During this journey, tablets may turn, tumble, chip, and crack. But they should retain no less than 99% of their original weight. The propensity for a tablet to lose a portion of its gross weight due to chipping and cracking is a term called friability. A tablet’s friability should be no more than 1%. This is most likely to happen during coating. To consider a tablet robust enough for packaging, there are two things to measure: manufacturability (the ability of a tablet to be manufactured) and tabletability (the ability of a powder to be compressed into a tablet form). In this article, we’ll focus on tabletability.
WHAT IS TABLETABILITY?
Tabletability describes the correlation between the tablet’s mechanical strength with increases in compression pressure under normal conditions. Mechanical strength of a material is the maximum amount of compression force that a material can bear without breaking or becoming permanently deformed. Tabletability may begin decreasing after reaching a specific compression pressure value. This represents over-compression.
Why is Tabletability Important?
In Figure 1, Profile A is an example of good tabletability. Profile B is an example where tabletability is acceptable at lower compaction pressures but deteriorates at higher pressures, and Profile C is an example of poor tabletability. Many factors can impact the tabletability curve including the powder deformation characteristics, particle size, shape, moisture content, and amount of powder fines (see Figure 2). Every product behaves differently on a tablet press even when run on a different day. Variation is due to changes in the properties of the raw materials, API, excipients from batch to batch, and atmospheric conditions. Therefore, it is crucial that a product has a robust tabletability profile. Profile B is an example of where tablet attributes might be acceptable at the research level or even scale up level but fails at the manufacturing level. Operating too close to the peak of the curve or on the descending side does not give any flexibility to the press operator when issues are found.
What’s the Difference between Tabletability and Compactibility?
The two are related, but not interchangeable terms. Mechanical strength is the amount of force required to cause a tablet to fracture or break. Mathematically, it factors in the tablet’s surface area and thickness. Tabletability is checking the ability of a material to form a tablet at a given compression pressure.
Similarly, compactibility is a plot of tablet mechanical strength against a tablet’s solid fraction. Solid fraction is a ratio of the tablet density relative to the powder’s true density. Suppose that there are two materials containing the same amount of solid fraction (let’s say 0.7, since it is the ratio of tablet bulk density to a material’s true density).
If there were two materials compressed at 100 MPa. If Material A and B have solid fractions equal to 0.5 and 0.7, and their respective tablet mechanical strengths 50 MPa and 100 MPa, then we could conclude that Material B has better tabletability and compactibility. As a result, Material B is good for tableting since it requires less work, and forms a less porous compact tablet that holds its shape, despite the forces and other impacts it may receive during coating and packaging.
What’s the Difference between Tabletability and Compressibility?
Tabletability is the relationship between a tablet’s mechanical strength when under tension at any given compression pressure. Compressibility measures a tablet’s solid fraction (a ratio of 0, where the tablet is entirely porous (which is only theoretically possible), and 1, where the tablet is entirely solid and has no air pockets within it), against any given compaction pressure (measured in MPa). Figure 2 shows an example of this relationship with three different types of powders. Note how different powders are more (or less) amenable to compression based on their material makeup (API, excipient, binders, fines, etc.)
How is Tablet Breaking Force Measured?
Tablet breaking force is measured with an instrument called a tablet “hardness” tester. The measurement of breaking force (which is at times confused with hardness) is measured as a kilopond, Newtons, and gram force. When converting the breaking force to mechanical strength by normalizing for the tablet geometry, Megapascals (MPa) is the industry standard. A target mechanical strength of 1 to 2MPa is a representative value for a robust tablet that will withstand handling, friability and coating operations.
It’s also worth mentioning that, while USP is a U.S. standard, the measurement units are the same, regardless of one’s territory on the globe.
Tablet breaking force is very closely related to a tablet’s propensity for defects such as picking, sticking, and capping (the three most common tableting issues). By measuring hardness, one can determine how likely issues like capping in late-stage development are to occur.
However, there wasn’t always a universally accepted standard or index for measuring hardness. The Young’s moduli of tested metals ranges between copper (117 GPa) and tungsten (400-410 GPa). These materials are extremely hard compared to typical pharmaceutical powders. Moreover, the applied compression pressure was in the range of 6.87 MPa to 827.37 MPa. This is an extremely large compression pressure range when compared to pharmaceutical processes, which are typically operated between ~50 to ~250 MPa compression pressure (depending on the formulation composition). Today, scientists usually rely on the Heckel equation for characterizing material deformation properties and how this translates into tabletability. The Heckel equation measures the relative density of a powder at any given compression pressure.
Why is Tablet Breaking Force Measured?
Tablet capping is one of the most prominent tablet defects in the tableting industry. There have been no standardized procedures to determine the capping tendency of the material. That’s why Natoli has developed an in-house technique called “Capping Index” to determine the capping propensity of the material. This profile can help to determine the best materials for (re)formulation, and to optimize the tablet compression conditions needed to achieve robust tableting.
Conclusion
Natoli Scientific, a division of Natoli Engineering Company Inc., we can generate a USP <1062> compactibility profile for your materials or formulations so you can ensure that your tablets are manufactured within specification. Check out our friability and harness testers. And, for a growing operation, consider adding Natoli AIM™ to your tablet presses. Our training courses and suite of R&D software and tablet presses can help you understand how to gather your USP <1062> data, analyze it properly to mine rich insights on your formulation’s scalability.
A strong tablet going into a package will create a strong impression on consumers removing them. Ensuring a tablet’s integrity doesn’t require luck, but science. At Natoli Scientific, we don’t just help your tablets look great for patients. We empower you with the science behind how to make robust tablets. Join us at this year’s annual PACK Expo International 2022 trade show! We will be showing industry professionals, like you, how to achieve the solid science behind solid tablets.
