Dry Coating Techniques
The main techniques for the manufacturing of coated solid dosage forms are based on the deposition of different materials onto substrate cores from solutions or suspensions. Therefore, the evaporation of large amounts of liquids (no less than 70% w/w with respect to coating material) is required. the need for water removal also necessitates time- and energy-consuming drying phases and could cause drug instability issues.
To overcome the problems associated , a number of new innovative applications of have been proposed and are generally referred to as dry coating techniques.
The dusting method for the application of sugar coatings or the powder layering technique for the manufacturing of drug-loaded pellets represents early attempts to lessen the amount of water employed in coating or coating-like processes. The role of water is limited to that of a solvent for wetting/binding formulations that are sprayed onto inert seeds only to promote the adhesion of a drug powder blend.
Variety of dry coating approaches:
Techniques are divided into liquid-based and powder-based techniques, depending on the physical status of coat forming agents when layered onto the substrate.
A second classification tier is based on how coat formation/consolidation is achieved.
1.Liquid-based dry coating techniques
Liquid-based dry coating processes involve the deposition of non aqueous liquid materials and their subsequent consolidation in a continuous layer. Such liquids may be molten formulations that are expected to solidify by cooling (hot melt coating technique), or liquid precursors that undergo polymerization directly on the core surface (polymerization coating technique).
The polymerization coating technique appears to have very limited potential for use in the pharmaceutical field because of technical and toxicological reasons.
2.Powder-based dry coating techniques
The formation of a layer around a core, starting from solid particulate materials, can be performed by means of compaction, exploiting well known tableting methods, or by heat curing following powder layering.
Recently proposed powder-based dry coating techniques exploit heating treatments that induce particle modifications (i.e., melting and glassy-rubbery transition) and promote the formation of the final coating layer. These kinds of techniques can be further classified — based on whether liquid systems are added to help particle deposition during the layering phase and/or achieve the final coating formation/consolidation during the curing step.
When powder formulations are involved in a dry coating process, two common steps need to be faced — the distribution of a powder blend onto the substrate and its turning into a continuous coating.
The large number and diversity of dry coating approaches reviewed confirm the increasing interest towards avoiding the use of water within the coating of solid cores. Advantageous applications have been described with respect to process time, overall manufacturing costs and ability to overcome water-induced degradation of active ingredients. Indepth knowledge of the mechanisms of coating formation and, in some cases, availability of suitable coat-forming agents and industrial-scale equipment should be regarded as the main issues for the consolidation of dry coating technology in the pharmaceutical field.
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