**8. Evaluation of layered tablets**

Most of the pre and post-compression evaluations are the same as that of a conventional tablet.

#### **8.1 Pre-compression evaluations**

Before the compression, the API and other excipients are evaluated to study the powder characteristics and micromeritics. The powder particle size is determined using a laser diffractometer. Various other powder properties like Hausner's ratio, Carr's index, and angle of repose are also determined before manufacturing a tablet. The moisture content of the powders is evaluated thermogravimetrically.

#### **8.2 Post compression evaluations**

#### *8.2.1 Weight uniformity*

The formulation should have a uniform weight within the batch. It is a major quality control test in the formulation of tablets. It is necessary to ensure that all tablets have weights within the tolerated limits to ensure intra and inter-batch uniformity data. About 20 tablets from each batch are evaluated for weight uniformity by determining individual weights.

#### *8.2.2 Thickness*

One serious drawback of layered tablets is the increased thickness. It should possess sufficient thickness to accommodate multiple layers of drug and inert materials. It should also be ensured that it lies within the swallowable limits. The thickness of the tablet is determined by the vernier caliper. It ensures that the tablet lies within the desired range, enabling easy swallowing.

#### *8.2.3 Friability and hardness*

To test tablets to withstand mechanical damage during processing, transport, and storage, friability and hardness are evaluated. About 10 tablets are weighed and loaded into the rotating drum of friability apparatus set at 100 rotations. Then the tablets are reweighed to calculate the weight lost. Hardness is the measure of the maximum pressure that a tablet can withstand. It is measured using instruments like Pfizer/Monsanto hardness tester.

#### *8.2.4 Content uniformity*

In order to assure that each tablet contains a labeled amount of API, they are evaluated using UV Visible spectrophotometer. Tablets are dissolved in suitable solvents and evaluated under a specific wavelength. In the case of more than one API, the simultaneous equation method is generally used.

#### *8.2.5 Disintegration time*

Time taken for a tablet to break down directly confers to the absorption. Faster disintegration promotes faster dissolution and absorption. The disintegration apparatus consists of disintegration vessels made up of mesh, immersed in a disintegration medium, which moves at specified cycles at the rate of 28-32 strokes per minute. The disintegration time is calculated as the time at which the tablet completely disintegrates.
