Tablets layered

Multiple-compressed tablets, layered tablets, and press-coated tablets. 

Compressed tablets that are composed of several layers require specially adapted presses designed with several fed hoppers. For a two-layer tablet, one granulation is first fed to a die and partially compressed into a soft tablet. The second granulation is added, and the total die components then are compressed fiiUy. Such procedures are used when the tablet ingredients may be incompatible, which requires separate granulations. If needed, a layer of inert ingredient, eg, lactose, is inserted between the two. 

Layered tablets are also used for a prolonged or sustained therapeutic effect. In this case, one layer disintegrates and dissolves rapidly to provide the initial dosing, whereas the other is designed for controlled release. 

Osmotic Pressure Controlled Oral Tablets. Alza Corp. has developed a system that is dependent on osmotic pressure developed within a tablet. The core of the tablet is the water-soluble dmg encapsulated in a hydrophobic, semipermeable membrane. Water enters the tablet through the membrane and dissolves the dmg creating a greater osmotic pressure within the tablet. The dmg solution exits at a zero-order rate through a laser drilled hole in the membrane. Should the dmg itself be unable to provide sufficient osmotic pressure to create the necessary pressure gradient, other water-soluble salts or a layer of polymer can be added to the dmg layer. The polymer swells and pushes the dmg solution through the orifice in what is known as a push-pull system (Fig. 3). The exhausted dmg unit then passes out of the body in fecal matter. 

Osmotic Control. Several oral osmotic systems (OROS) have been developed by the Alza Corporation to allow controUed deHvery of highly water-soluble dmgs. The elementary osmotic pump (94) consists of an osmotic core containing dmg surrounded by a semi-permeable membrane having a laser-drilled deHvery orifice. The system looks like a conventional tablet, yet the outer layer allows only the diffusion of water into the core of the unit. The rate of water diffusion into the system is controUed by the membrane s permeabUity to water and by the osmotic activity of the core. Because the membrane does not expand as water is absorbed, the dmg solution must leave the interior of the tablet through the smaU orifice at the same rate that water enters by osmosis. The osmotic driving force is constant until aU of the dmg is dissolved thus, the osmotic system maintains a constant deHvery rate of dmg until the time of complete dissolution of the dmg. 

Griseofulvin is adminisfered orally in the form of tablets. It is not totally absorbed when given orally, and one method of increasing absorption is to reduce the particle size of the drug. Griseofulvin is deposited in the deeper layers of the skin and in hair keratin, and is therefore employed in chemotherapy of fungal infections of these areas caused by susceptible organisms. 

Insoluble Lubricants. Lubricants act by interposing an intermediate layer between the tablet constituents and the die wall. The smaller the amount of stress needed to shear the material, the better its lubricant properties will be. Since they are primarily required to act at the tooling/material interface, lubricants should be incorporated in the final mixing step after all granulation and... 

In a search for novelty as much as functionality, tablets have been produced on presses capable of compressing a second (or even third) layer on top of the original material. Indeed, the standard double-rotary machines require little modification in order to achieve this goal. Such tableting procedures facilitate the co-formulation... 

Figure 7 Picture of an asymmetrical membrane coating layered onto a tablet core. (From Ref. 20.)...

The manufacture of this system utilizes essentially the same process as the one inherent in the manufacture of OROS. The complicating factors are that a bilayer tablet press is required and the tablet must be oriented during the laser drilling operation to ensure that the drug-containing layer is directed toward the laser beam. 

Hardness data for only two amino acids were found in the literature. They are glycine and alanine. They are the smallest of the amino acids. Both consist of rather flat tablet-like collections of atoms that form layered crystal structures in which the molecular sub-groups within the layers are held together by hydrogen bonds (Albrecht and Corey, 1939), and the molecules by London forces. Their hardnesses are ... 

The air gas-diffusion electrode developed in this laboratory [5] is a double-layer tablet (thickness ca.1.5 mm), which separates the electrolyte in the cell from the surrounding air. The electrode comprises two layers a porous, from highly hydrophobic, electrically conductive gas layer (from the side of the air) and a catalytic layer (from the side of the electrolyte). The gas layer consists of a carbon-based hydrophobic material produced from acetylene black and PTFE by a special technology [6], The high porosity of the gas layer ensures effective oxygen supply into the reaction zone of the electrode simultaneously the leakage of the electrolyte through the electrode... 

The cause of the indigestion is removed because the acid is consumed. Solid (unreacted) aluminium hydroxide is relatively insoluble in the gut, and does not dissolve to generate an alkaline solution. Rather, the outer layer of the tablet dissolves slowly, with just sufficient entering solution to neutralize the acid. Tablet dissolution stops when the neutralization reaction is complete. 

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