**7. 1 Weight variation, sequence**

Weight variation is generally studied in the case of solid unit dosage forms. The weight variation can be subjected to variation between units and also between the layers. Each layer should possess the predetermined weight without any major deviation in range. Individual layer weight [18]. It is preferable for the layers to have similar weight so that it aids produce required strength, weight distribution, appearance, and similar compression profiles.

#### **7. 2 Mechanical strength**

It is noted that excipients play important role in producing a high-quality layered tablet. Excipients that confer brittleness to the formulation are preferred to provide sufficient mechanical strength to layered tablets such that they can withstand mechanical pressure during production, packing, and transportation [19, 20].

#### **7. 3 Lubrication**

It is studied that increase in lubricant concentration, decreases the strength of interfacial bonding and can deteriorate the interfacial interaction between the layers [21, 22].

### **7. 4 Adhesion strength**

The major challenge is holding the layers together which is dependent on interlayer adhesion strength between two or more layers used. In the production of layered tablets, initially, a central core is prepared by pre-compression, then the upper and lower layers are compressed around the central core. Necessary interlayer adhesion strength is required to hold these layers together as a single unit. Low compression force for the core and high compression force for layers can help in attaining adequate adhesion [23–25].

#### **7. 5 Interlayer cross-contamination**

There is a high possibility of cross-contamination between the adjacent layers. To overcome this, scraper plates are placed around the die fill to remove residual powder [26]. In some cases, suction and dust removers are also being used.

#### **7. 6 Long term integrity and storage**

The layered tablets should display a long-term physical and chemical integrity throughout the shelf life of the dosage form [28]. Temperature and humidity changes during storage have a greater impact on interlayer adhesion [27, 28].
