Evaluation of the separation properties of bi-layer tablets.
Compressed tablets have long dominated the pharmaceutical market as the most widely used dosage form due to their convenience in administration, compactness and ease of manufacturing, making up to 80% of administered drugs.
Over the years, advancements in technology and innovation have brought about increasing interest within the pharmaceutical industry in developing tablets containing two or more Active Pharmaceutical Ingredients (API) in a single dosage, form promoting customer convenience, compliance, and marketing.
The development of tablets with different pre-defined drug release profiles has enabled immediate release and extended release active compounds to be contained in one dosage form. However, production of such tablets has faced great difficulties, being prone to fracture along the material interface of the adjacent layers (de-lamination) during compaction due to inherent binding weaknesses. The mechanical integrity of the tablet can also be compromised if the compacted layers are too soft or too hard, weakening the bonding and adhesion at the interface between the compacted layers. Other common challenges include inaccurate individual layer weight ratio, cross contamination between layers and first layer tampering force. These factors have complicated the mechanical structures of this drug delivery system, thereby requiring complex tablet structural design. Understanding the mechanical strengths of these tablets is commercially significant as tablet failure can be very costly to the industry.
The CT3 Texture Analyser can determine failure as a function of layer properties. In the test, the tablet is inserted into a fixture cavity so that the tablet layers lie in the plane of a guillotine-like blade. The guillotine-like blade then exerts a force on the tablet until the two component layers are sheared apart. The shear force is regarded as a measure of adhesion strength. The lower the force to shear the adjacent compacted layers, the more likely the tablet will fail, either in the manufacture process, during packaging and shipping, or when consumed by the customer.
Note: When placing the tablet into the sample carrier, the orientation of sample placement must be kept consistent.
Typical Plot: A comparison of failure properties for shear tests performed on three bi-layer tablets
Figure 1 shows the fracture strengths and brittleness of a bi-layer tablet of 18 x 8 mm. The fracture strengths of three tablets have been overlaid.
Figure 2 shows load versus distance for the fracture strength of three bi-layer tablets of the same batch. The probe penetrates the sample to a target distance of 10 mm during which time the increasing force applied ultimately fractures the two layers apart. Once the specified distance has been attained, the probe returns to its starting position indicated in the lower part of the graph.
When a trigger force of 50 g has been attained at the sample surface, the probe proceeds to shear the sample at a test speed of 1 mm/s to a target distance of 10 mm during which time data is collected. The force is seen to increase as the probe exerts a force on the tablet. The tablet fractures when it can no longer withstand the force. The shear force (hardness) required to separate the layers is regarded as a measure of adhesion force between the two layers. The hardness work done is an indication of the energy required to separate the two layers.
Results obtained from four bilayer tablets give the following mean hardness and hardness work done values shown below: