Tablet:Manufacturing methods/Direct compression

Introduction

In early days, most of the tablets require granulation of the powdered Active Pharmaceutical Ingredient (API) and Excipients. At the availability of new excipients or modified form of old excipients and the invention of new tablet machinery or modification of old tablet machinery provides an ease in manufacturing of tablets by simple procedure of direct compression.
Amongst the techniques used to prepare tablets, direct compression is the most advanced technology. It involves only blending and compression. Thus offering advantage particularly in terms of speedy production. Because it requires fewer unit operations, less machinery, reduced number of personnel and considerably less processing time along with increased product stability.

Definition:

The term “direct compression” is defined as the process by which tablets are compressed directly from powder mixture of API and suitable excipients. No pretreatment of the powder blend by wet or dry granulation procedure is required.

The events that motivates the industry people to use direct compression technique

I.Commercial availability of the directly compressible excipients possessing both good compressibility and good flowability.
For example, Spray dried lactose, Anhydrous lactose, Starch-1500, microcrystalline cellulose, Di-Pac, Sorbitol
II. Major advances in tablet compression machinery,
i) Improved positive die feeding
ii) Precompression of powder blend

Merits

i)Direct compression is more efficient and economical process as compared to other processes, because it involves only dry blending and compaction of API and necessary excipients.
ii)The most important advantage of direct compression is economical process.Reduced processing time, reduced labor costs, fewer manufacturing steps, and less number of equipments are required, less process validation, reduced consumption of power.
iii)Elimination of heat and moisture, thus increasing not only the stability but also the suitability of the process for thermolabile and moisture sensitive API’s.
iv)Particle size uniformity.
v)Prime particle dissolution.
In case of directly compressed tablets after disintegration, each primary drug particle is liberated. While in the case of tablets prepared by compression of granules, small drug particles with a larger surface area adhere together into larger agglomerates; thus decreasing the surface area available for dissolution.
vi)The chances of batch-to-batch variation are negligible, because the unit operations required for manufacturing processes is fewer.
vii)Chemical stability problems for API and excipient would be avoided.
viii)Provides stability against the effect of aging which affects the dissolution rates.

Merits over wet granulation process

The variables faced in the processing of the granules can lead to significant tableting problems. Properties of granules formed can be affected by viscosity of granulating solution, the rate of addition of granulating solution, type of mixer used and duration of mixing, method and rate of dry and wet blending. The above variables can change the density and the particle size of the resulting granules and may have a major influence on fill weight and compaction qualities. Drying can lead to unblending as soluble API migrates to the surface of the drying granules.

Demerits

Excipient Related
i)Problems in the uniform distribution of low dose drugs.
ii)High dose drugs having high bulk volume, poor compressibility and poor flowability are not suitable for direct compression. For example, Aluminium Hydroxide, Magnesium Hydroxide
iii)The choice of excipients for direct compression is extremely critical. Direct compression diluents and binders must possess both good compressibility and good flowability.
iv)Many active ingredients are not compressible either in crystalline or amorphous forms.
v)Direct compression blends may lead to unblending because of difference in particle size or density of drug and excipients. Similarly the lack of moisture may give rise to static charges, which may lead to unblending.
vi)Non-uniform distribution of colour, especially in tablets of deep colours.
Process Related
i)Capping, lamination, splitting, or layering of tablets is sometimes related to air entrapment during direct compression. When air is trapped, the resulting tablets expand when the pressure of tablet is released, resulting in splits or layers in the tablet.
ii)In some cases require greater sophistication in blending and compression equipments.
iii)Direct compression equipments are expensive.

Manufacturing steps for direct compression

Direct compression involves comparatively few steps:
i)Milling of drug and excipients.
ii)Mixing of drug and excipients.
iii)Tablet compression.

Direct compression Excipients

Direct compression excipients mainly include diluents, binders and disintegrants. Generally these are common materials that have been modified during the chemical manufacturing process, in such a way to improve compressibility and flowability of the material. The physicochemical properties of the ingredients such as particle size, flowability and moisture are critical in direct compression tableting. The success of direct compression formulation is highly dependent on functional behavior of excipients.

An ideal direct compression excipient should possess the following attributes

i) It should have good compressibility.
ii) It should possess good hardness after compression, that is material should not possess any deformational properties; otherwise this may lead to capping and lamination of tablets.
iii) It should have good flowability.
iv) It should be physiologically inert.
v) It should be compatible with wide range of API.
vi) It should be stable to various environmental conditions (air, moisture, heat, etc.).
vii) It should not show any physical or chemical change in its properties on aging.
viii) It should have high dilution potential. i.e. Able to incorporate high amount of API.
ix) It should be colourless, odorless and tasteless.
x) It should accept colourants uniformity.
xi) It should possess suitable organoleptic properties according to formulation type, that is in case of chewable tablet diluent should have suitable taste and flavor. For example mannitol produces cooling sensation in mouth and also sweet test.
xii) It should not interfere with bioavailability and biological activity of active ingredients.
xiii) It should be easily available and economical in cost.

Major excipients required in direct compression

I.Diluents
II.Binders
III.Disintegrants

Diluents

Selection of direct compression diluent is extremely critical, because the success or failure of direct compression formulation completely depends on characteristics of diluents. There are number of factors playing key role in selection of optimum diluent. Factors like- Primary properties of API (particle size and shape, bulk density, solubility), the characteristics needed for processing (flowability, compressibity), and factors affecting stability (moisture, light, and other environmental factors), economical approach and availability of material. After all, one can say that raw material specifications should be framed in such a way that they provide an ease in manufacturing procedures and reduce chances of batch to batch variation. This becomes possible only when the raw material specifications reflect most of properties of diluents as mentioned in section 1.5.

Binders

Binders are the agents used to impart cohesive qualities to the powdered material. The quality of binder used has considerable influence on the characteristic of the direct compression tablets. The direct compression method for preparing tablets requires materials which are not only free flowing but also sufficiently cohesive to act as binder.